GTA
Airguns by Make and Model => Crosman Airguns => Topic started by: aidave on May 29, 2013, 12:18:16 AM
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I know there's a lot of theory about a lot of things in life -- with fire arms it's reducing mass to lessen lockup time. With motorcycles, it's about lightening the rotating mass.
Since I have the rifle torn down; I was polishing the ram (not the gas spring, but the ram with the seal on it). It dawned on me that if it was lightened up, either by drilling or machining, as long as it was done evenly; that this would allow the gas spring to expand faster. Day dreaming more; I thought about taking measurements and having my local machinist or one of the online prototype builders do one in T3 aluminum.
Has any one tried this? if so, did it improve anything?
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I decreased the weight of the piston in search on higher fps. Well, I didn't get any more fps. The fps stayed the same. What it did was reduce the recoil.
I used a cut off wheel on an air grinder. That piston metal is pretty "dang" hard. A drill bit ain't gonna do it.
work was done on trail np .22.
Iam still running in the 800's (fps) with RWS hobby wad cutters 12 grain and 680 to 690 fps with H and N barracuda hunter extreme HP 19.09 grain.
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I don't think you would get anything better from lighenting as far as a better FPS.
A Piston or spring only does one stroke. So it's the swept volume that produces power.
And that's from a dead stop to dead stop for the most part.
In an enternal combustion engine you have multi strokes at VERY high rates and the lighter the load on things like bearings and rods help produce the extra power.
Mostly from reduced friction and the rotation force being reduced on the offset pistons in such a design.
Remember, we need to move all that dead weight once the piston is done doing it's job in that application.
You may get a better acceleration at firing time with lighter parts but it is probably so small as to make the effort not worth the time as the stroke is so small.
That's just my opinion and hope to see the real experts give us the scoop.
:D
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First post here, I actually joined to research this exact question. Does removing a small amount of weight from a Nitro ram gun's piston skirt improve performance and how? My plan is to machine material out of my xl1100's piston skirt, but it still needs to be stable enough to not deform. I am looking to mostly smooth out the overall firing sequence/reduce recoil without loosing velocity. If I gain velocity great, time for a heavier pellet. Thanks guys, I am really enjoying the forum thus far. -Craig
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Hi Craig,
I hope the experts join in soon. (I'm new myself)
But I have a background in High output engines.
Given you have a single stroke with a limited pressure behind it?
I doubt you would gain much from chopping weight from the piston and moving parts.
I'd tend to think reduceing any friction would gain you more power.
That may involve buttoning the piston (Old Drag racer trick) to doing a total re-bore-hone depending on how tight you want to get.
I still doubt that is worth the effort for the hobby shooter.
Now if you have a semi-auto or full auto? That may be a different thing.
I know on full auto it is something to consider depending on the Gun.
I look at it this way.
IF a lighter piston made any real world difference? There would be a gazillion people out there selling them to you!
Find me one with the data to prove it.
:D
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You wont gain any increases in either FPS or FPE. Lightening the piston is usually done to reduce recoil. Since it also reduces the mass of the piston, it reduces the momentum as well. Would you rather get hit by a golf ball going 50 mph, or a ping pong ball going 50 mph? Same thing. The lighter weight of the piston reduces its ability to overcome the pressure of the air compressed in the cylinder, so usually if you lighten the piston, you will also need to go to a slightly lighter pellet to maintain velocity and hopefully FPE. Typically though, you lose FPE regardless.
This is less a problem with gas piston rifles though because they do not work in the same manner as a spring and maintain a higher force through their full stroke rather than a gradual decrease in force as they extend as happens with a spring. It is still basic physics though, so it still applies.
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Most people add weight to the piston to gain energy from the shot cycle, this increases the felt recoil since you are moving more weight over a distance you are doing more work this work is then transfered to the air being compressed which is then transfered to the pellet, the pellet has to be tight enough not to move until most of the energy is transfered yet be out of the barrel before you start to get deminishing returns, lots of math in there but yes the mass of the piston affects both the power and shot cycle of the gun....
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I couldn't agree with Paul 68 anymore than what he said. He commented to me about this whole fps/fpe increase idea I had a few months ago. Unless your ready to buy new parts that you screwed up, don't do it.
Fortunately for me, I did some things to my trail thinking it would do something I wanted it to do (increase fps) and what it really did was reduce the recoil. No loss in fps yet no gain. Unless someone else can tell me their standard trail np (not xl) is pushing 12 grain pellets at 820fps and h and N ftt's pushing 750fps. Heck, the barracudas 19.09 grain are hitting 20.18 fpe.
I also should have listened to Paul68 and the rest of the guys about the transfer port hole size too. Stock is .125" and I opened it to .14". I definitely lost fps there (approximately 1.7% on light pellets but leaned out with heavy pellets-same fps). Positive thing, even less recoil (weird). the strangest thing when I opened the transfer port hole was when I put a .125" drill bit through the transfer port hole I would have expected it to go straight into the combustion chamber. It didn't. It went in about 3/4 the way and stopped like there was something blocking it on the combustion chamber side. I first drilled with the .125" bit all the way through ( should of stopped and checked there) but I got the other bit out and opened her up.
For what its worth, I think the lighter piston in a gas ram is better.2 reasons:
1) no loss in fps
2) reduced recoil
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Excerpt from the latest in a long series of "technical" articles published monthly in the current issue of Airgun World:
"Reducing the length of the piston stroke is but one of the numerous modifications that people carry out to improve their spring air rifles, along with varying the mainspring and preload, varying the effective piston mass, altering the diameter of the transfer port, and more. Every modification has an effect on the shot cycle, but any perceived improvement comes at the price of less than desirable side effects."
The author and his cronies have taken up where the Cardews left off. Pretty good reading for shade tree physicists.
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So do we agree that reduced recoil is an overall improvement? The force that a ram exerts is most likely consistent throughout its stroke, the piston is less likely to bounce off the cushion of air when lightend because the ram is still exerting significant force. Aren't we all ultimately looking for a smoother shooting cycle? -Craig
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So do we agree that reduced recoil is an overall improvement? The force that a ram exerts is most likely consistent throughout its stroke, the piston is less likely to bounce off the cushion of air when lightend because the ram is still exerting significant force. Aren't we all ultimately looking for a smoother shooting cycle? -Craig
I'd agree to that statement due to the fact a ram maintains a higher constant extended force in all states.
A spring has variable amounts of pressure and is highly prone to rebound.
Take the shocks off the front of your car once to see this effect first hand if you need a good example.
Now using the shocks on a car is a queer way to explain what I'm thinking as those are actually 2 way rams.
They have a some what neutral point and reduce rebound in both directions.
I can see a reason to mod weights in a springer due to the rebound effect.
In a gas piston? I can't see any reason other then just taking weight off the Gun.
And that is probably a waste of time for what you gain.
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So do we agree that reduced recoil is an overall improvement? The force that a ram exerts is most likely consistent throughout its stroke, the piston is less likely to bounce off the cushion of air when lightend because the ram is still exerting significant force. Aren't we all ultimately looking for a smoother shooting cycle? -Craig
I'd agree to that statement due to the fact a ram maintains a higher constant extended force in all states.
A spring has variable amounts of pressure and is highly prone to rebound.
Take the shocks off the front of your car once to see this effect first hand if you need a good example.
Now using the shocks on a car is a queer way to explain what I'm thinking as those are actually 2 way rams.
They have a some what neutral point and reduce rebound in both directions.
I can see a reason to mod weights in a springer due to the rebound effect.
In a gas piston? I can't see any reason other then just taking weight off the Gun.
And that is probably a waste of time for what you gain.
Not everyone is looking for a smoother shot cycle per say some want more power regardless of the recoil, by lightening the piston you reduce the energy transferred to the pellet...
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I hadn't considered less moving mass for less felt recoil -- but that sounds worth it to me, during the winter when I won't be shooting much; take my time to cut out some mass, and chamfer/bevel everything nice and smooth. even a slight drop in fps wouldn't bug me, since it would probably help with lighter pellets.
Something to think about...
thank you for your input.
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If the force exerted from the ram is constant, and enough weight remains in the modified piston, there will probably be little to no loss in Velocity. There will be a loss in felt recoil, less mass needs to slow down/speed up. Again I am going to modify and confirm over the summer months. -Craig
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They are easy enough to get from Crosman and less than $20.00 with a new seal already installed do go for it and let us know how it worked...
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As far as my experiences go with the lighter piston, It did reduce recoil but did not increase fps and shot cycle appeared faster. Like I said earlier, I used a cut off wheel on my air grinder. No drill bit of mine would touch the metal. I say go ahead. Hack it up. a piston is fairly cheap to replace. I think the way I cut mine also reduced metal to metal contact (less friction) because I hacked off the top of the piston in a large area.
Its kind of funny how that "dang" gas piston works. Completely different than a spring.
What fun would this be if there wasn't people hacking up the stuff to make things better or adversely improving something we didn't expect. Just be safe doing it and think it through.
Everyone has crazy ideas and some work. If you have never done it " the lightening of the piston on a gas ram" all comments are theory only. Theory's are good and are very welcome. It really helps making that first cut decision.
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If the force exerted from the ram is constant, and enough weight remains in the modified piston, there will probably be little to no loss in Velocity.
What would be fascinating is instrumenting the compression chamber right at the port into the barrel and recording the pressure rise rate and amplitude. They do this for firearms all the time when proofing guns and new ammo chamberings.
Would think that would answer most questions about what the effect is of less piston mass.
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If the force exerted from the ram is constant, and enough weight remains in the modified piston, there will probably be little to no loss in Velocity.
What would be fascinating is instrumenting the compression chamber right at the port into the barrel and recording the pressure rise rate and amplitude. They do this for firearms all the time when proofing guns and new ammo chamberings.
Would think that would answer most questions about what the effect is of less piston mass.
It's easy enough to gauge the effect using a chrony. You simply shoot a ten shot string using the guns favorite pellet, then make your mods, then shoot another string and compare the numbers.
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Robtmc has a good point to measure the psi at the transfer port hole. It would be great to measure BUT, too many dynamic things going on. I don't think Paul 68's idea of the chrony would be accurate either. When the piston comes forward and begins its compression, the pellet moves forward through the barrel before maximum psi is achieved. This in turn changes the volumetric area of the total combustion chamber (not measureable now). Its almost the same reason why two different 14 grain pellets can have a 50 -70 fps difference. More resistance in the chamber.
Finding out how much psi is generated in the combustion chamber is interesting though and would be cool to know what affects it. there must be some kind of way of doing it.
What would be even better than changing piston weight would be changing compressable air in the combustion chamber before the piston even moves. What I mean is compressing a small amount of air in the combustion chamber when the piston is in the "ready to fire" position. this would be like a supercharger or turbo increasing allowable compressable air without increasing the mechanical area of the combustion chamber. Way high fps "I think". Just day dreaming
so it would be like a cross between a pump and gas ram rifle. Pretty cool idea.
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Robtmc has a good point to measure the psi at the transfer port hole. It would be great to measure BUT, too many dynamic things going on. I don't think Paul 68's idea of the chrony would be accurate either. When the piston comes forward and begins its compression, the pellet moves forward through the barrel before maximum psi is achieved. This in turn changes the volumetric area of the total combustion chamber (not measureable now). Its almost the same reason why two different 14 grain pellets can have a 50 -70 fps difference. More resistance in the chamber.
Finding out how much psi is generated in the combustion chamber is interesting though and would be cool to know what affects it. there must be some kind of way of doing it.
What would be even better than changing piston weight would be changing compressable air in the combustion chamber before the piston even moves. What I mean is compressing a small amount of air in the combustion chamber when the piston is in the "ready to fire" position. this would be like a supercharger or turbo increasing allowable compressable air without increasing the mechanical area of the combustion chamber. Way high fps "I think". Just day dreaming
so it would be like a cross between a pump and gas ram rifle. Pretty cool idea.
The point is to measure the effect lightening has on the power produced by the rifle in a realistic and practical manner. Shooting the same pellets in each test, once before, and once after, will give a VERY accurate account of how your modifications have effected the actual performance of the rifle. Since the only variable changed would be the piston weight, any effect on performance would be directly attributable to this change.
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An interesting post about this subject.
http://www.gatewaytoairguns.com/airguns/index.php?topic=2055.0 (http://www.gatewaytoairguns.com/airguns/index.php?topic=2055.0)
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Gun shows up tomorrow morning, chrony a day later. I should have about 4hrs Sunday to log initial shots. I will have to break it in, and hopefully before July 4th I can get the O.E. piston out and modify it. I will try to reuse any swaggy China goo still hanging around the chamber and piston seal. Literally I would like to dissambe, rig the piston, drill it in my press, chamfer, regrease, assemble, shoot. Changing as little as possible, I have a feeling this gun is going to be apart a few times this summer. -Craig
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I would take it apart, clean, degrease, deburr and lube it first then start the testing...
After you get you preliminary data then start hacking at the piston...
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It's easy enough to gauge the effect using a chrony.
All well and good if velocity change is all you are looking for. Did I mention velocity?
Robtmc has a good point to measure the psi at the transfer port hole. It would be great to measure BUT, too many dynamic things going on.
Maybe I did not spell out what I was interested in, very well. Shorty may have hit on it here.
All mentions of markmanship problems with airguns seem to center around the relatively slow transit time from piston release to the pellet leaving the barrel.
No idea at what sort of pressure in the chamber a pellet of any given weight starts to move, but high school physics is enough to know that once that starts, the force (pressure) that remains will continue to act on the pellet. How long does that force act, or to the analogy to powder burners, does is continue through the event?
Benchrest shooter reloaders seeking ultimate precision spend many hours on something called cartridge neck tension, which affects bullet release, part of what is whirling around my aging brain as I think about airgun design and this light piston thing.
Point is, not velocity at all, but how fast, and at what rate, does the pressure from the piston act on the pellet. This may not matter at all, or it may matter a lot with different pellet weights. Does a lighter piston get the pellet out of the barrel faster and minimize the hold and vibration issues?
This is the idea of a pressure curve, and a correlation with pellet weights and accuracy. Spank the pellet with perhaps a fast moving light piston, or give it a slower shove from a slower moving heavy piston?
Not trying to over-think things, but I was an engineer for decades, and was paid to do that. Sorry if this is too much, I'll shut up.
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Fair enough.
It's well known that the time it takes a pellet to begin moving affects accuracy. In every spring gun there is a movement caused by the action of the piston release. This is why follow through on a shot it so important. Unlike a powder burner where the length of time between trigger pull and projectile movement is extremely fast, a spring gun is abysmally slow, relatively speaking. Although the point at which the pellet begins moving does play a role however, the greater problem is the movement caused by the release of the piston. The problem with spring guns is that you have to take into account all of the variables if you are going to achieve extreme accuracy.
For an example consider just lightening the piston. You will have reduced the mass of the moving assembly. Obviously, momentum will be reduced and the amount of force needed to overcome the inertia of that piston mass will also be reduced, so you get a faster cycle. Great, this should reduce recoil and improve "lock time". BUT, you also affect when peak pressure takes place and how high that pressure will be, and you have not changed the amount of force needed to break the pellet loose and begin its movement. So although you might have reduced the violence of the shot cycle, your peak pressure can be lower, and occur later in the shot cycle. You end up with a pellet that begins moving later, moves slower, and thus stays in the barrel longer. The longer it takes the pellets to travel the length of the barrel and finally exit, the more it is affected by the movement of the rifle and the harder it can become to maintain follow through. You then have to change the pellet to compensate, which generally means using a lighter pellet that requires less force to begin moving. So in effect, everything is dependent upon balance.
In effect, the best bet is to produce a gun that is performing as efficiently as possible, with as little movement as possible. If we were to assume accuracy were the only deciding factor in a spring gun, then obviously we would want something lightweight, low powered, and shooting light pellets. This is why the most accurate rifles are those with very low velocity and FPE numbers. If you really want short lock times, you could also reduce stroke, which of course would also further reduce energies and thus require yet more attention to balance.
There is a reason tuners and smiths don't pay much attention to trying to determine pressures and locktime, and instead focus on achieving a balance of power versus smoothness. Usually the best accuracy comes when a gun is producing it's peak power with a given pellet. From a magnum down to a low power target rifle, when you maximize efficiency and smoothness of the shot cycle, you improve accuracy. You get less movement to affect pellet trajectory, less energy lost to friction and unwanted movement and thus faster pellet launch, a pellet that leaves the barrel sooner, and typically a gun that is easier to shoot and more accurate. This is one reason why chrony testing is important. When you find the peak energies produced by the rifle, not velocity, and compare them to the energies produced by your alterations, you learn whether efficency has improved or been negatively impacted.
Guns are designed around a desired set of parameters and performance goals. Trying to take a gun designed to produce high energies and work well within hunting applications as well as plinking and turn it into a target rifle is a only going to meet with limited results. The best bet is to work towards achieving most balanced and efficient platform possible. Just lightening a piston is only a small part of the equation, and really just a step in the wrong direction with a given rifle imo because you are moving away from that rifles design parameters and trying to make a square peg fit a round hole. The best you can do is maximize a given platforms' potential. If you are after sheer accuracy then, better to start with a platform designed around producing accuracy.
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I love busting paul68's butt. Your great man. I bet if we were neighbors, competition would never stop and many beers would be drunk in our garages or shops.
I say lighten the "dang" piston. I have proof it works. I have a chrony. I have chrony numbers. I ran a string of chrony tests on some new pellets I received yesterday that never have been shot out of my trail.
Impressively I must say. Baracuda hunter extreme 19.09 grain averaged 690fps giving 20.18 fpe. All the posts I have read about the trail np, I have never seen fpe numbers over 18 fpe.
This trail np or b18/19 platform is so much fun to work with.
Accuracy ? I am now convinced that the accuracy from this gun does not come from its engine. It comes from the barrel assembly, lock up, and barrel I/D. The barrel assembly or barrel itself is the accuracy problem.
for example: I just received FTT's 5.53 ,5.54,JSB jumbo's, crow mag's, all the good shooting trail pellets. I figured well, I'll tighten the barrel pivot screw along with the rest of the screws, give her a few drops of PFPE oil down the transfer port hole and all should be awesome. Well Chrony testing showed great numbers, but accuracy was *(&^. It took atleast 100 pellets to calm down then a thorough barrel cleaning to get back the accuracy after another 100 pellets. It was that "dang" barrel pivot screw that messed up everything. Stupid lock up system.
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For an example consider just lightening the piston. You will have reduced the mass of the moving assembly. Obviously, momentum will be reduced and the amount of force needed to overcome the inertia of that piston mass will also be reduced, so you get a faster cycle. Great, this should reduce recoil and improve "lock time". BUT, you also affect when peak pressure takes place and how high that pressure will be, and you have not changed the amount of force needed to break the pellet loose and begin its movement. So although you might have reduced the violence of the shot cycle, your peak pressure can be lower, and occur later in the shot cycle. You end up with a pellet that begins moving later, moves slower, and thus stays in the barrel longer.
So to take this in another direction, you're saying if we increase the cycle time (heavier piston) we'd develop higher pressure? Sorry, but the way I see it -- if the piston is lighter, it moves faster, develops pressure sooner (not later).
Just imagine if the weight were 5# the gas spring may push it, but it will move very slow and more pressure will 'escape' (or be lost) around the pellet
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Okay, think of it this way.
You have a 2 lbs hammer. You drop it on a nail from 4 feet. It smacks the nail in 1/4"
Now drop a 4lbs hammer on a nail from 4 feet. What do you think will happen and why?
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I love busting paul68's butt. Your great man. I bet if we were neighbors, competition would never stop and many beers would be drunk in our garages or shops.
I say lighten the "dang" piston. I have proof it works. I have a chrony. I have chrony numbers. I ran a string of chrony tests on some new pellets I received yesterday that never have been shot out of my trail.
Impressively I must say. Baracuda hunter extreme 19.09 grain averaged 690fps giving 20.18 fpe. All the posts I have read about the trail np, I have never seen fpe numbers over 18 fpe.
This trail np or b18/19 platform is so much fun to work with.
Accuracy ? I am now convinced that the accuracy from this gun does not come from its engine. It comes from the barrel assembly, lock up, and barrel I/D. The barrel assembly or barrel itself is the accuracy problem.
for example: I just received FTT's 5.53 ,5.54,JSB jumbo's, crow mag's, all the good shooting trail pellets. I figured well, I'll tighten the barrel pivot screw along with the rest of the screws, give her a few drops of PFPE oil down the transfer port hole and all should be awesome. Well Chrony testing showed great numbers, but accuracy was *(&^. It took atleast 100 pellets to calm down then a thorough barrel cleaning to get back the accuracy after another 100 pellets. It was that "dang" barrel pivot screw that messed up everything. Stupid lock up system.
That's fine and it's what I enjoy about this forum. I'm far from knowing everything and been wrong before, and learned because of it.
However, I have to say your results run contrary to what is usually found by lightening a piston, which makes me wonder what else is at play if your numbers are right and went up.
For instance, you mention introducing oil into the compression chamber. That alone suggests heavy deisling, which can easily boost FPE numbers unnaturally.
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PFPE oil. Look it up. It is purchased from Solvay. M60 is the trade name. Non hyrdrocarbon lubricant. Zero deizeling.Its a type of flouro polymer lubricant. Sounds like your calling BS on me.
What's it gonna take... I' ll prove it anyway you way you want. I got no reason to lie or B/S. Say it and I'll do it. No prob moustache. Now that was funny.
The results are contrary to springs. Maybe not gas rams..... They are different.
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Good writeup Paul. Thanks for taking the time.
What bothers me a bit about what may be the tuners' process sounds like a lot of trial and error with parts swapping rather than any sort of formal engineering approach to things.
Doubt that is what happens in the labs at RWS and all. Then again, they may have a known set of basics that they stay close to. In other words, the guts of the guns really does not change that much over time other than materials improvements and such.
I have noted on GTA that the pros never offer much info. It is their bread and butter, not surprising they do not share any knowledge I suppose.
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Okay, think of it this way.
You have a 2 lbs hammer. You drop it on a nail from 4 feet. It smacks the nail in 1/4"
Now drop a 4lbs hammer on a nail from 4 feet. What do you think will happen and why?
I don't think that applies to AGs myself.
Your talking about gravity and the effects of.
AG's with springs or pistons use a linier device to launch the mass that compresses the given swept volume.
That compressed air then launches the pellet. How tight that pellet is in the breech, and it's weight, most probably affects how soon it moves?
I do see that the Pros do offer advice.
I'd no sooner have a new Guy that read a few books on modding a 350 Chevy engine rebuild my engine then I'd sent a Gun to a new guy doing tunes.
They (like the masters at doing 350 Chevy engines) say right up front it takes time and many things to learn by hands on to get to thier level.
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Here's what a chopped piston looks like. It's whats in my trail np hardwood right now.
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Okay, think of it this way.
You have a 2 lbs hammer. You drop it on a nail from 4 feet. It smacks the nail in 1/4"
Now drop a 4lbs hammer on a nail from 4 feet. What do you think will happen and why?
I don't think that applies to AGs myself.
Your talking about gravity and the effects of.
AG's with springs or pistons use a linier device to launch the mass that compresses the given swept volume.
That compressed air then launches the pellet. How tight that pellet is in the breech, and it's weight, most probably affects how soon it moves?
Doh!
No, not about gravity at all. It's about mass, kinetic energy, and inertia. Drop a ping pong ball on your foot from four feet, it wont hurt at all. Drop a 5lb weight from four feet, you'll probably break a couple toes. So what is different? They both drop from the same height and have the same exact force working on them.
The difference is the mass of the object that is falling. More mass, heavier object, means more inertia. Inertia is the tendency of a body to resist changes to it current state ie resting or motion.
How this applies to airguns and piston weight is simple.
The heavier piston will be better able to compress a given volume of air once it has begun moving and will resist the opposing force of high pressure better than a lighter one. It builds speed more slowly, but due to its larger mass absorbs more kinetic energy and achieves greater inertia.
A lighter piston has less mass and will not build as much inertia, thus it will not be able to compress a given volume of air to as high a pressure unless a lot more energy is applied to it.
This is why rifles with heavier pistons tend to be more efficient with heavier pellets and rifles with lighter pistons work better with lighter pellets.
Put a heavy pellet in a rifle with a light piston and the piston will bounce sooner and energy will be lost because the piston cannot overcome the pressure enough to efficiently launch the pellet.
Basically, force equals mass times acceleration. Lighten the piston and you lose mass and only gain very little acceleration, so you end up with a net loss in force. You'd have to increase the force (stronger spring for higher acceleration) in order to achieve the same amount of force as you would with a heavier piston.
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;D
OK. I got you now.
Thanks for explaining it better.
My brain don't work to good at times now days.
I'm trying to learn so if I'm off? Just whack me up side the noggin when needed.
The Wife does.
;)
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force equals mass time acceleration Basic???? Not that basic with numb skulls. There's also static and dynamic friction occurring with the piston along with an increasing pressure. Iam not smart enough or have the equipment to measure. Lets look at that basic F=MxA formula.
Force = Mass X Acceleration
100 = 100 grams (piston weight) X 1 inch a second ( fake number but Ok ) for giggles, let's just say this is whats in our trail.
Now lets lighten up that piston. Don't forget that the acceleration will increase.
110 = 80 grams (piston weight) X 1.375 inch a sec ( fake number but Ok ).
Could this be what is really happening ? The math says so. But, unless it can be measured, it doesn't mean poop. Just like privateer said. I would rather have a hands on engine builder build my 350 than some dude that just read a book about how to do it.
Something to think about huh.
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force equals mass time acceleration Basic???? Not that basic with numb skulls. There's also static and dynamic friction occurring with the piston along with an increasing pressure. Iam not smart enough or have the equipment to measure. Lets look at that basic F=MxA formula.
Force = Mass X Acceleration
100 = 100 grams (piston weight) X 1 inch a second ( fake number but Ok ) for giggles, let's just say this is whats in our trail.
Now lets lighten up that piston. Don't forget that the acceleration will increase.
110 = 80 grams (piston weight) X 1.375 inch a sec ( fake number but Ok ).
Could this be what is really happening ? The math says so. But, unless it can be measured, it doesn't mean poop. Just like privateer said. I would rather have a hands on engine builder build my 350 than some dude that just read a book about how to do it.
Something to think about huh.
This is why someone a page or two ago said to get out the chrony and get some numbers, then chop away and retest the gun with the same ammo over the same chrony....
In God We Trust, all others bring data....
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Unfortinately, I got my chrony after I did the mod. I am not sure if more fps with lighter piston was even the main subject. Kind of rolled into that. I think the more positive thing to reducing the piston weight was the reduced recoil.
For me, I can say my recoil feels less. But I also say that my car feels faster when I changed my air filter. I also increase the transfer port size from .125" to .14". Even though this did reduce fps with light pellets (maintained heavy pellet weight fps) I can surely feel a lighter recoil with the larger transfer port hole. I did have the chrony for this testing and did test out 8 different pellets ranging in different grains from 12 up to 26.2 grains.
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This is why someone a page or two ago said to get out the chrony and get some numbers, then chop away and retest the gun with the same ammo over the same chrony....
It would be interesting to test several weights -- I'm sure there is a diminishing return with regards to the weight of a piston. I don't buy the "it would be too light to push the air" -- I bet if the gas-spring only pushed it 1/2 thru the stroke; that the heavier piston would go significantly farther.
Since the spring pushes it all the way; the spring is doing "most of the work" -- given the same size piston, a rifle that requires 20# to cock and a rifle that requires 30# to cock will get different results -- it's the spring pushing, not the weight of the piston...
I am not a physicist nor do I play one, but it seems that lighting it to "some degree" would help recoil w/o damaging velocity or energy -- but go beyond that; it could.
If someone has a chronometer,tools to cut, scale, and the desire to experiment, I'd send the money for a new piston & seal. That way we could see what a 5% 10%, 20%, 25% etc weight reduction yields with regards to pellet out the barrel... just arm chair quarterbacking, but I bet about 40-50% weight reduction wouldn't harm end results... Especially seeing how the quality of mfr'ing is -- why reduce the weight (more cutting, etc) when they don't have to? They probably wouldn't reduce the weight, even if there was an improvement, since it would require more time in the build process....
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aidave, Now that's an idea there. I would definitely take you up on that but Iam sure my other mods would void the test. But, I already have a cut piston and I wouldn't mind sharing that piston with someone. As long as I get it back or if someone gives me a replacement.
I am good with that.
Since I already have numbers with the cut piston and someone trades me with a stock one, the numbers can then be compared.
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I'd like to see true world results with data to back it all.
I still think if dropping/adding weight to a piston had any major effects?
Every SOB out there doing parts would have them for sale and have the data to back them up.
I'm still waiting for that link to just one Guy that can do all that!
It's not like AG's just rolled down the pike yesterday.
From past links? I'd think CDT and others are just tired of the whole debate and have better things to do then answer questions no one listens to anyway.
Mac1 is a great example of a master in his area. I can't find a lightened piston for springers on his site? So he's just a hack?
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Not much I can tell you guys except lightening a piston has been done before, and is almost always a failure in regards to providing improvements. The best most report is a slight reduction in recoil and a loss of power.
There are too many variables involved and it takes a comprehensive or balanced approach to if you are going to get any noticeable improvement from lightening the piston. Everything is a tradeoff with AG's and there aint no free lunch.
Take some time to go through the thread linked below, and look closely at Scotchmo's numbers on pages 4 and 5 when working on the efficiency and power of the rifle. His findings jibe very well with what I've tried to explain. I think it should be particularly interesting how he managed to almost maintain FPE by cutting acceleration and increasing piston weight.
http://www.gatewaytoairguns.org/GTA/index.php?topic=29301.60 (http://www.gatewaytoairguns.org/GTA/index.php?topic=29301.60)
Chris from CAP Airguns has also attested to the same thing.
http://www.network54.com/Forum/79537/thread/1297824001/piston+weight (http://www.network54.com/Forum/79537/thread/1297824001/piston+weight)
BTW, did you guys know that it really isnt the spring that is compressing the air?
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This is why someone a page or two ago said to get out the chrony and get some numbers, then chop away and retest the gun with the same ammo over the same chrony....
It would be interesting to test several weights -- I'm sure there is a diminishing return with regards to the weight of a piston. I don't buy the "it would be too light to push the air" -- I bet if the gas-spring only pushed it 1/2 thru the stroke; that the heavier piston would go significantly farther.
Since the spring pushes it all the way; the spring is doing "most of the work" -- given the same size piston, a rifle that requires 20# to cock and a rifle that requires 30# to cock will get different results -- it's the spring pushing, not the weight of the piston...
I am not a physicist nor do I play one, but it seems that lighting it to "some degree" would help recoil w/o damaging velocity or energy -- but go beyond that; it could.
If someone has a chronometer,tools to cut, scale, and the desire to experiment, I'd send the money for a new piston & seal. That way we could see what a 5% 10%, 20%, 25% etc weight reduction yields with regards to pellet out the barrel... just arm chair quarterbacking, but I bet about 40-50% weight reduction wouldn't harm end results... Especially seeing how the quality of mfr'ing is -- why reduce the weight (more cutting, etc) when they don't have to? They probably wouldn't reduce the weight, even if there was an improvement, since it would require more time in the build process....
It isn't the spring that is compressing the air and doing the work. The function of the spring is only to acellerate that piston. It is the mass and inertia of the piston that does the actual work.
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Consider this, if a metal spring starts the piston moving and transmits less energy as it decompresses, a heavier piston is needed to conserve the energy until the pellet starts to move. I am getting confused because a gas ram releases energy in a linear manner, the piston does not have to maintain as much energy because the ram is working almost as hard at the end of its stroke as it is in the beginning. -Craig
Edit: Maybe Crosman just doesn't know what we do, maybe they do not want to change manufacturing processes, maybe the tooling is cost prohibitive. A streamlined manufacturing platform that can pump out pistons for all of its springers by just varying length of cuts is probably what we are dealing with. The gun is made in China for a reason. I need to go to bed, just wanted to put some thoughts down. -Craig
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I read through the post and wanted to reply to some of it. I have tested piston weight as a variable in gas spring guns.
The amount of energy available is a function of the gas spring, not piston weight.
A fixed amount of energy will compress a fixed volume of air to a certain pressure. You can do it quickly with a light piston, or more slowly with a heavier piston. The maximum pressure achieved is the same. It is all about timing in a piston gun.
Neglecting friction and transfer port restrictions, the light piston can reach exactly the same pressure as a heavy piston in a gas spring gun. The timing of that pressure pulse is just different.
In the light piston gun, the pressure pulse climbs faster and diminishes sooner. In order to take advantage of that, a lighter pellet will accelerate at a faster rate and prevent piston bounce.
A heavy pellet in a light piston gun can cause bounce which reduces fpe. Some of the kinetic energy in the piston is then going toward re-compressing the gas spring.
A light pellet in a heavy piston gun causes slam which also reduces the resulting fpe. Some of the kinetic energy is then transferred into the front wall of the chamber rather than compressing the air column.
A heavy piston with a slight transfer port restriction allows a broader range of suitable pellet weights.
It is possible to open up the transfer port and lighten the piston with a resulting improvement in the shot cycle and maximum fpe for a given pellet. But it will be at the expense of pellet selection.
Before making any internal changes to a gas spring gun, you can run some tests that will give you an idea if any changes will help your situation:
Get a variety of similar pellets in different weights - 7.33gr, 7.87gr, 8.44gr, 10.34gr.
Shoot them over the chronograph and record the velocity and determine the fpe of each.
Ask yourself which pellet you want to shoot in the end.
If the lighter pellets are producing higher fpe and you prefer to shoot the heavier pellets, than increase the piston weight until the 10.34gr maxes out. But then you will be getting some piston slam with the lighter pellets.
If the heavier pellets are producing higher fpe and you prefer to shoot the lighter pellets, than reduce the piston weight until the 7.33gr maxes out. But then you will be getting some piston bounce with the heavier pellets. You may also need to open the transfer port a little.
Some people might prefer to shoot all weight pellets at about the same fpe. That can be achieved with the right piston weight and transfer port size. That is how most production guns are setup.
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Thanks for adding some info Scotchmo, I was hoping you'd have a look given the work you did with your custom gun design.
Some of your interpretations are I guess you could say counterintuitive, at least to me.
You said the amount of energy available is a function of the spring, not piston weight. But we know that mass acellerated can have different energies depending on the amount of acelleration. All things being equal, a lighter weight acted on by the same force might achieve higher speed, but its momentum will be lesser than a heavier mass acted on by the same force but moving slightly slower. That says to me that the spring is not doing the actual work of compressing the air, but rather imparting acelleration to the piston, which produces momentum, kinetic energy, and it is that which compresses the air.
If energy available were a function of the spring, then we wouldn't see much if any change in pellet energies by lightening a piston, but we do.
The light piston bounces because it has less momentum and kinetic energy, and thus can't as easily overcome the higher pressures created by a heavier pellet.
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With everything said here. Lightening the piston in a gas ram is Ok to do to achieve a few things.
1) Reduce recoil
2) Dial in the trail for a selected pellet grain range for maximum FPE.
Transfer port sizing may be needed also to achieve maximum efficiency of said pellet grain weight.
Here is what Iam running and what is fact, not theory.....
Trail np .22 hardwood. Engine: Full deburr and combustion chamber hone. Stock piston seal, PFPE grease (krytox), lighten piston, and transfer port opened to .14". All garage work done.
Real numbers:
JSB exact jumbo's 18.1 grain - max fps 691/19.19 fpe
H&N FTT 14.66 grain - max fps 780 /19.8 fpe
Crow magnum 18.21 grain - max fps 694/19.47 fpe
Baracuda hunter extreme 19.09 grain - max fps 692/20.3 fpe
RWs wadcutter 14 grain - max fps 773/18.47 fpe
RWS wadcutter 12 grain - max fps 828 /18.27 fpe.
Skenco 26.2 grain - max fps 524/ 15.97 fpe
CPHP 14.1 grain - max fps 768 /18.47 fpe
From the number above, you can only guess what I like to shoot. Baracuda's for hunting and HTT's for target and sometimes hunting.
My trail's gas ram and piston seal have over 1500 pellets fired through.
I think the trail still has room for improvement for the FPS/FPE. In my case, it's no longer in the engine, but in the barrel. Shortening the barrel and choking it will and should increase the fps further without sacrificing the accuracy.
You can sure bet I'll post if I ever get the right tools to chop,choke,and refit the barrel shroud. I'll tell you a 13 inch barrel is very attractive for the gas ram trail.
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Paul68,
It does not seem counter intuitive to me.
The building up of pressure in the chamber causes the piston to come to a stop at some point. If the pressure does not fall off at that same instant, the piston will rebound backwards (bounce). Move the pellet further down the barrel to reduce the pressure in the chamber (lighter pellet). Or slow the piston down in order to give the pellet more time to get moving (heavier piston).
For any given gas spring, the energy delivered to the system is the same. It does not matter what the mass of the piston is.
Kinetic energy = 1/2 mass x velocity x velocity
Momentum = mass x velocity
Increasing the piston mass will always increase the momentum, but it will not increase the available energy.
You said: "If energy available were a function of the spring, then we wouldn't see much if any change in pellet energies by lightening a piston, but we do."
I often see an increase or at least the same fpe when I combine a lighter piston with a lighter pellet. My tests were run with low friction o-ring seals. The friction in some standard parachute seals increases as you try to drive them too fast, and that can cut your fpe.
Shorty,
You have a handle on what matters. But it looks like you prefer to shoot slightly heavier pellets. A lighter piston may be detrimental. I would suggest taking your favorite pellet. File the nose flatter on a few in order to lighten them. Get the fpe for both the modified and unmodified pellets. You will need a pellet scale and chronograph. If the lightened pellets produce less fpe, than you won't want to lighten the piston. You may even want to increase the piston weight if you are trying to achieve maximum fpe with your heavy pellets. However, you may already be at an optimum piston weight for your chosen pellets. And even if you do get a little more fpe, the heavier piston will also increase the amount of recoil.
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I agree with you for the most part Scotchmo. Timing is indeed important, and I understand how it can affect pellet velocity in regards to whether the pellet moves before peak pressure, during peak pressure, or after, which is part of why I keep stressing balance.
All things being equal though, even though I'm not a number cruncher I do know that greater mass acelerrated carries greater momentum and kinetic energy than a lighter mass, even when that larger mass is moving slower than the lighter mass. Of course, there are limits to this.
I also know that greater mass requires greater force to overcome its inertia, (ignoring breakaway friction and such in a pellet gun). Because of this, a heavier pellet can not only cause the initial movement to take place later in the cycle, but increase the overall pressures as well. It has greater resistance to movement, so pressures increase. Again within limits since there is only so much pressure that can be built with a given volume of air.
So it is not just a matter of reaching a fixed pressure point at a certain time, but also meeting the actual pressure needed to get the pellet moving acellerate it. Pressure doesnt stay the same from one pellet weight to another.
Take a tube and apply a given amount of air instantly and pressure is great. Put a pinhole in that tube and apply pressure instantly and pressure will not be as great. Make the hole bigger and the pressure will be even less. Same thing with pellet weight. Increasing resistance increases pressure.
The lighter piston can reach the needed pressure to begin movement within limits, and once movement begins the pellet also carries momentum so less force is needed to continue that movement and add accleration, so it will send the pellet out the barrel, but, the pressure drop is much more extreme and much energy is lost at initial breakaway since it is harder for the lighter piston to compress the air, and so it expends more of its energy reaching that point, it bounces.
Since a heavier piston carries greater momentum, it can apply greater force to a given volume of air in the chamber.
If the pressure remained the same I would agree with you.
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Paul68,
I am a number cruncher.
I disagree with a few of your statements. Calculations as well as tests prove who is correct.
Piston weight has no affect on peak pressure in a closed chamber. The lighter piston has a shorter dwell time. The pressure builds faster and diminishes (bounces) sooner but it achieves the same peak pressure as with the heavy piston.
You are correct in that the heavy piston has more momentum. But you are wrong when you say that it has more kinetic energy.
The potential energy in the spring is the same, the kinetic energy achieved by either piston is the same, the resulting potential energy in the compressed air is the same. The only difference is that your window of opportunity to utilize that air pressure is reduced with the light piston. Hence the need for a lighter pellet.
If you still disagree, I'll leave it at that and agree to disagree.
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Paul68,
I am a number cruncher.
I disagree with a few of your statements. Calculations as well as tests prove who is correct.
Piston weight has no affect on peak pressure in a closed chamber. The lighter piston has a shorter dwell time. The pressure builds faster and diminishes (bounces) sooner but it achieves the same peak pressure as with the heavy piston.
You are correct in that the heavy piston has more momentum. But you are wrong when you say that it has more kinetic energy.
The potential energy in the spring is the same, the kinetic energy achieved by either piston is the same, the resulting potential energy in the compressed air is the same. The only difference is that your window of opportunity to utilize that air pressure is reduced with the light piston. Hence the need for a lighter pellet.
If you still disagree, I'll leave it at that and agree to disagree.
Yeah, we'll probably not see eye to eye on this. If I'm wrong, I'l probably figure it out eventually. You're right in that I was wrong to say kinetic energy, since that is energy transferred from the spring and it doesnt change. But momentum does with mass and velocity.
Do you agree that a heavier mass has greater inertia? If so, wouldn't weight affect the effort needed to move the pellet, and thus how high pressure can rise in the chamber?
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You asked: "Do you agree that a heavier mass has greater inertia?"
My answer: yes, I agree.
You asked: "If so, wouldn't weight affect the effort needed to move the pellet, and thus how high pressure can rise in the chamber?"
My answer: no and no.
OK. I guess we have to do some math. If anyone has an aversion to math, look away. But it does come in handy when trying to figure something out.
We can simplify the model to get at the problem.
Take out the compressing of the air.
We end up with a gas spring that accelerates a weight (piston).
Let's say that we have a 100lbf gas spring that has a 3 inch stroke.
That gas spring has 25fpe of potential energy when cocked. (100lbf x 3in/12in per foot)=25fpe
We are going to individually launch different weight pistons with the gas spring.
Piston-1 is 1lbm
Piston-2 is 2lbm
Final velocity of Piston-1:
sqrt(25fpe/(1lbm x 1/2)) = 7.07fps
Final velocity of Piston-2:
sqrt(25fpe/(2lbm x 1/2)) = 5.00fps
Momentum of Piston-1:
1lbm x 7.07fps = 7.07 lb ft/s
Momentum of Piston-2:
2lbm x 5.00fps = 10 lb ft/s
It is obvious that the heavy piston has more momentum. So what? Do you really think that it can deliver any more energy than the lighter piston just because it has more momentum? They have the same kinetic energy of 25fpe.
Let's look at an extreme example to get the point across.
Let's use a 1000lbm piston with that same spring:
Final velocity of Piston-1000:
sqrt(25fpe/(1000lbm x 1/2)) = .224fps
Momentum of Piston-1000:
1000lbm x .224fps = 224 lb ft/s
That 1000lbm piston gives us over 20 times the momentum of either of the pistons that we used earlier. Can that 224 lb ft/s of momentum deliver any more energy? No.
That would mean free energy exists and it does not.
There is an optimum piston weight for each gun/pellet system. Heavier pistons tend to get used more often because they are the most forgiving when shooting a variety of pellets. But the heavier piston is rarely the optimum when a single specific pellet is to be used.
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Let's look at an extreme example to get the point across.
Let's use a 1000lbm piston with that same spring:
Final velocity of Piston-1000:
sqrt(25fpe/(1000lbm x 1/2)) = .224fps
Momentum of Piston-1000:
1000lbm x .224fps = 224 lb ft/s
That 1000lbm piston gives us over 20 times the momentum of either of the pistons that we used earlier. Can that 224 lb ft/s of momentum deliver any more energy? No.
That would mean free energy exists and it does not.
There is an optimum piston weight for each gun/pellet system. Heavier pistons tend to get used more often because they are the most forgiving when shooting a variety of pellets. But the heavier piston is rarely the optimum when a single specific pellet is to be used.
That's a better way of explaining it - I tried to do the "extreme" piston weight example; but it didn't go over well.
I don't have a chronograph, but, probably, late summer, I'll do some "seat of the pants" experiments on lightening it. I'll order a new piston (or two) when I order some other parts, that way; if it doesn't work out; I can just toss the &^^& ones...
Objectives:
- Does the rifle shoot better? (tighter groupings)
- Does the rifle shoot flatter?
- Does the rifle have less felt recoil?
- Can I shoot lighter pellets w/o going super or trans sonic?
I only target shoot, normally at about 20 yards, out to 40 yards occasionally.. so those will be my test distances...
I will bring a scale to measure each time I cut a bit away, and record results -- it will be while we're at the cabin (no phone) for 2 weeks; so I won't have results to post until late August, or early September.
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A lighter piston will only increase the power of a gun with a long stroke and swept volume, and very heavy piston to start with. Usually a tuner will increase the weight of the piston for more power. This helps hold the piston down against the air pressure long enough to get the mass in the barrel moving before the piston bounces off the pressure thereby reducing the pressure before the pellet exits the muzzle and decreasing power. Some of the long stroke guns will send some pellets away before the piston even comes to the end of the chamber, creating the same condition that is created when the piston rebounds from being too light. In such rare cases, a lighter piston may increase power. Also, in conjunction with a heavier spring or ram, it may be more powerful with a lighter piston. It's not about the heavier or lighter piston, it's about the balance between all the factors. You have length of stroke, strength of the spring or ram, weight of the piston, caliber of the bore, size of the transfer port, pellet weight and fit and composition, how smooth the bore is, etc. That's why you pay good money for a good tune, and why at least to me there's a big difference between a true tune and just a de-bur and lube job with a drop in kit. Tuners are artisans, even fitting a custom guide and seal takes trial and error and practice o get it right.
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All comes down to why no one sells a lighter or heavier piston. More powerful or less powerful springs yes.
It's the Tuners that make it all work together. Math and all that is great as a starting point.
What works on paper don't aways work in real life everytime.
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OK. I guess we have to do some math. If anyone has an aversion to math, look away. But it does come in handy when trying to figure something out.
Very Good Scotchmo. Nothing like math to send panic through a crowd.
Will chew on this later, looks like good stuff.
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I think Iam pretty much done with this too.
Thanks for the extra math skills Scotchmo.
For the trail np .22 (non xl) I can say it's not optimized from the factory. The claims of 950 fps and or 21fpe can not be achieved out of the box. They can be achieved with modifications to the piston and or transfer port hole. I am almost there if not there yet with meeting the said so numbers.
With all this talk about the engine in this b18/19 platform, I cant see why none of the tuners don't sell an "optimized" engine platform for the B18/19.
Let's face it, the trail np is very popular. It sells.... If you could get a tuned upper receiver and piston (with a full debur and hone) for say 50 bucks with the exchange of your upper receiver and piston, would you do it ?????
I think some would. Not everyone has the honing, cutting, or lubricant to do a full engine tune. Consider it a DIY B18/19 tune for half the price.
Are there any tuners reading ?
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If you could get a tuned upper receiver and piston (with a full debur and hone) for say 50 bucks with the exchange of your upper receiver and piston, would you do it ?????
From reading GTA for several months with particular interest in the Trail NP saga, I'd wager the vast majority are far more interested in accuracy than velocity.
The reality seems to be there are those of us that like to take apart our toys and tinker, and those that just want to shoot. The latter might be interested in a tuner package if it supplied the accuracy most of us expect from an air gun.
I prefer reading and learning, and the pleasure of doing things myself.
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I can see that you’re right and we aren’t going to agree Scotchmo. You’ve done a good job of explaining with math the momentum energy gained by an object when accelerated. But that’s all you did. Although the math does show mass times acceleration = momentum, it entirely misses the rest of the issue. Kind of disappointing because if I'm wrong I really want to know why and how.
“You asked: "If so, wouldn't weight affect the effort needed to move the pellet, and thus how high pressure can rise in the chamber?"
My answer: no and no.”
You agree that a heavier object (pellet) has more resistance to movement, then you disagree that this would mean higher force is needed to move it. It can’t be harder to move, yet not require more force to move it. The force potential (momentum) of a moving mass is the sum of acceleration times mass. Not just the initial energy applied. Your own math shows this. Yet even though you show the larger mass has greater momentum, you insist it can’t impart more force, that only the initial amount applied to achieve acceleration counts as kinetic energy. Basically, you are just throwing away all the energy gained by mass x acceleration, ignoring that a heavier pellet will have greater resistance, and assuming only kinetic energy matters.
A mass DOES need force to move as it resists changes to its current state of either rest or movement. A heavier mass has greater resistance. Thus, a heavier pellet will be harder to move than a lighter one, and greater force will be needed to accelerate it. In a spring gun, the force acting on the pellet is represented as air pressure produced by compression. It’s also important to remember when considering the piston that mass doesn’t just resist movement, it also resists stopping once it has started moving – momentum.
In the closed chamber of a pellet rifle, we assume for our purposes here everything is immovable except the pellet, so force represented by compressed air acts upon everything and has nowhere to go until something begins moving, which will be the pellet since it is the path of least resistance. If the chamber had no path for force to follow or act upon, complete compression would take place given enough force behind the piston.
Since none of us has deliberately sealed an air rifles compression chamber and measured pressure, we really don’t know if the maximum possible amount of compression is taking place. But, we can assume it isn’t because the goal is to launch the pellet without bottoming the piston. This means producing as much pressure as possible before pellet launch, but leaving enough room for gradual deceleration of the piston as pressure decreases to avoid damage. This left over room means the air has not been totally compressed. Hence the need for a balanced system in an air rifle.
If the maximum potential compression is not taking place, then there is potential for more compression and thus higher pressure. With that out of the way.
Momentum equals the amount of force needed to stop a moving mass. Your math shows this.
“Momentum of Piston-1:
1lbm x 7.07fps = 7.07 lb ft/s
Momentum of Piston-2:
2lbm x 5.00fps = 10 lb ft/s”
Since a heavier piston has more momentum, it will take more force to stop it than a lighter piston with less momentum. So yes, it WILL deliver more energy. It goes back to my earlier examples of a golf ball and a ping pong ball moving at 50 MPH. Which would you rather get hit with? The ping pong ball will have the same velocity, but it will not deliver as much energy, hence if it hits you in the head no damage. If the golf ball hits you, it will deliver more energy, hence a cracked skull.
If we put the heavier piston into a rifle and use light pellets, the greater momentum of the piston more easily overcomes the opposing force represented by the compression of air. Since the lighter pellet requires less force to achieve movement, it moves not earlier in stroke, but earlier before maximum compression takes place, and as the pellet begins moving the force opposing the piston represented by compression also drops, which means a lower overall pressure is achieved. Pressure doesn't rise as high and recedes faster before the greater force of the heavier piston, enough so given a heavy enough piston that it ends up crashing into the cylinder floor.
If we then introduce a heavier pellet, we have changed the situation and increased the force needed to move it. That same heavier piston now has a pellet requiring more force to contend with, so pressure will spike closer to maximum compression before the pellet begins to move, causing more of the piston momentum to be expended against an opposing force rather than crashing into the bottom of the cylinder. The heavier piston will have compressed the air more due to higher resistance, thus higher pressure.
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Don't get me wrong, I am sure a lot of people want to tinker. Heck, I have been a process engineer for 20 years. Its in my blood. But, some people lack the skills and tools to achieve these types of things. Hence, aftermarket parts.
I would also agree that the accuracy would be a big ticket item as well for the trail np. I have yet to see any barrel "swap" aftermarket items besides the barrel bushings. I guess that's why my next quest for my benji is barrel work besides the typical breech polishing and re-crown.
You tell me who wouldn't want a direct replacement barrel that gave higher fps and improved accuracy while reducing weight and making the benji less pellet sensitive ???
I am not seeing the trail go away for awhile. I see atleast 2 more years on this platform before things change again.
I might be wrong, but the trail does not have many aftermarket goodies that address our current issues out of the box... Our only solution is to return it, get a real tune >$100, or tinker until we get so frustrated that we buy something that cost twice as much.
Just my opinion.
Ohh yeah, did I say chop the "dang" piston for the 3rd time.
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Paul68
I believe you are still missing the point that the spring is going to transmit the same amount of energy to both pistons, regardless of their respective masses. The pellet is going to absorb the same amount of that energy, namely most of it, regardless of which piston provides that amount of energy. In other words, both pistons are going to deliver equal amounts of their total energy to the pellet. The heavy piston is going to have more energy left over which will cause it to rebound further and/or harder. Vice versa for the light piston which will have less energy left over and not rebound as much .
We are of course assuming both pistons have the same physical dimensions, namely the same length and, hence, the same cocking stroke length and, hence, the same stored energy in the compressed spring.
This cracked me up today.
Question: What is worse, ignorance or apathy?
Answer: I don't know and I don't care. ;D
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john,
You got the point. But I would have reworded your response slightly:
The appropriate pellet is going to absorb the same amount of that energy, namely most of it, regardless of which piston provides that amount of energy.
And, in a good design, neither piston will rebound a substantial amount.
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Paul68
I believe you are still missing the point that the spring is going to transmit the same amount of energy to both pistons, regardless of their respective masses. The pellet is going to absorb the same amount of that energy, namely most of it, regardless of which piston provides that amount of energy. In other words, both pistons are going to deliver equal amounts of their total energy to the pellet. The heavy piston is going to have more energy left over which will cause it to rebound further and/or harder. Vice versa for the light piston which will have less energy left over and not rebound as much .
We are of course assuming both pistons have the same physical dimensions, namely the same length and, hence, the same cocking stroke length and, hence, the same stored energy in the compressed spring.
This cracked me up today.
Question: What is worse, ignorance or apathy?
Answer: I don't know and I don't care. ;D
Naa, I get that entirely. I already explained the problem is that it is not only the force of the spring working on the piston that decides the end force acting on the air, which in turn applies force to the pellet. It is mass x velocity = momentum. Greater momentum = greater force.
Lets try this a different way..
You have a sealed tube with a piston and seal in it on the ground. The piston is positioned at the top of the tube and protrudes out of it.
Drop a 5 lb brick on the end of the piston from a specific height and it will push the piston a certain distance into the tube and compress the air within it.
Now do the same thing with a 10 lb brick, and the brick would force the piston in farther, and compress the air within the tube more.
Why? The force acting on the piston is exactly the same, but for some reason the heavier brick will produce higher compression. It is because although the force acting on the bricks is the same, the heavier brick has more mass and thus more momentum, so it will be harder to stop it.
Something else to consider.
If you have two pistons, and one is heavier than the other, but both have the same momentum, which is going to stop first when it encounters the resistance of air compression? Obviously the lighter piston because although initial momentum is the same, greater mass still has greater resistance to changes in its current state. However, in order for the lighter piston to have the same momentum, the spring would have to move it faster ie applying more kinetic energy. So, that would also mean that the same amount of intial kinetic energy does not produce an equal end force provided by pistons of different weights.
And again, the mass of the pellet is also being ignored. A heavier pellet will resist movement more than a lighter one. Hence a heavier piston is more effective at moving heavier pellets. And the pellet does not just "absorb" the energy. It is acclerated, which in turn imparts momentum to it , which we measure in FPE. FOOT POUNDS OF ENERGY. Increase the force applied to the pellet, you increase velocity and increase momentum, and thus gain FPE. Reduce the force applied to the pellet, you reduce FPE. Keeping pellet weights the same, a lighter piston typically results in lower pellet velocities and lower FPE and vice versa.
You can alter pellet weight to gain efficiency and increase FPE with a light piston, but only to a point, beyond which the lighter piston just wont produce the necessary force to achieve the same pellet energies as a heavier piston without adding a lot more intial kinetic energy.
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Paul68,
Things would be easier if we were on the metric system. Or we could use the proper imperial unit of mass - slugs.
I think that you are confusing pounds mass (lbm) with pounds force (lbf).
The gas spring generates the same force regardless of the mass of the piston that it is propelling.
A falling brick is accelerated by gravity. In which case the force on the 10 lb. brick is twice as much as the force on the 5 lb. brick.
The force of gravity is dependent on the mass of the object. The force of a gas spring is not.
You said that we measure momentum in FPE. That makes about as much sense as saying that we measure distance in MPH. Sorry for the sarcasm. I should have just said that it makes no sense.
You stated that "you increase velocity and increase momentum and thus gain FPE."
However, you must realize that when you increase the piston mass, you will actually have a reduced piston velocity and still have greater piston momentum. That heavier piston has more momentum, but the energy is unchanged.
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Something to read.
http://www.google.com/url?sa=t&rct=j&q=piston%20airgun%20drawings&source=web&cd=11&ved=0CGwQFjAK&url=http%3A%2F%2Fhome2.fvcc.edu%2F~dhicketh%2FDiffEqns%2Fspring07projects%2FStephenCompton%2FSpringAirModel.pdf&ei=pMitUdnMM4jS9ATf6YCQBQ&usg=AFQjCNGadG9oAywk8aUGePWRYBkchyahnQ (http://www.google.com/url?sa=t&rct=j&q=piston%20airgun%20drawings&source=web&cd=11&ved=0CGwQFjAK&url=http%3A%2F%2Fhome2.fvcc.edu%2F~dhicketh%2FDiffEqns%2Fspring07projects%2FStephenCompton%2FSpringAirModel.pdf&ei=pMitUdnMM4jS9ATf6YCQBQ&usg=AFQjCNGadG9oAywk8aUGePWRYBkchyahnQ)
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Paul68,
Things would be easier if we were on the metric system. Or we could use the proper imperial unit of mass - slugs.
I think that you are confusing pounds mass (lbm) with pounds force (lbf).
The gas spring generates the same force regardless of the mass of the piston that it is propelling.
A falling brick is accelerated by gravity. In which case the force on the 10 lb. brick is twice as much as the force on the 5 lb. brick.
The force of gravity is dependent on the mass of the object. The force of a gas spring is not.
You said that we measure momentum in FPE. That makes about as much sense as saying that we measure distance in MPH. Sorry for the sarcasm. I should have just said that it makes no sense.
You stated that "you increase velocity and increase momentum and thus gain FPE."
However, you must realize that when you increase the piston mass, you will actually have a reduced piston velocity and still have greater piston momentum. That heavier piston has more momentum, but the energy is unchanged.
When I refered to momentum and FPE with the pellet, I was trying to demonstrate that a faster moving pellet of the same weight has higher energy because that is more well known among air gunners.
I understand what you are saying about gravity. But I'll concede that the force of gravity is greater. It doesn't matter anyway.
If the force of gravity isn't a good way to demonstrate the force produced by momentum, then try hitting a nail with a basic household hammer, then use a small sledge hammer and swing it with the same force and hit the same nail. The heavier hammer will drive the nail into the wood farther.
I also understand about gas springs versus coil springs. Thats why I understand that the constant force applied by the gas spring can affect the end result. BUT, it doesnt negate the effect of momentum, only masks it to an extent because the piston is being accelerated at a constant rate rather than the variable rate supplied by a spring. Momentum still applies. You are still accelerating a greater mass.
Since you are a number cruncher, why not calculate the amount of force needed to compress a given volume of air, say the amount in a popular rifle we know the spring specs for, then calculate the force supplied by the spring only? Would that not provide something useful?
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I think Paul's post #68 give a good summary of why the extra weight produces higher pressure, given the timing of the "pellet launch". By introducing more time in breech/barrel with a heavier pellet, the heavier piston's momentum will yield higher pressures. It's momentum that aids the piston in "driving home", against the building pressures in the sealed system.
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I think Paul's post #68 give a good summary of why the extra weight produces higher pressure, given the timing of the "pellet launch". By introducing more time in breech/barrel with a heavier pellet, the heavier piston's momentum will yield higher pressures. It's momentum that aids the piston in "driving home", against the building pressures in the sealed system.
That pretty succinctly sums up what I've been trying to say in an extremely long winded way ;D
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The link that shorty gives shows the most comprehensive spring gun model that you will find.
I read it when I was doing my spring gun testing last year.
The math involved is much more thorough than used in the Cardew books.
No where is momentum even mentioned. Momentum is a symptom of the process. Energy is the cause. Adding mass to a closed process can indeed result in more momentum. But it does not increase the energy available.
You can add a heavier flywheel to a car engine. You have increased the angular momentum of the engine. It does not increase the power output. Using heavier pistons in the engine will not increase the power either.
I'll say this:
I can cut the piston weight in half and guarantee that I can maintain the same FPE out of any gas spring rifle. It will require using a lighter pellet and sometimes a less restrictive transfer port.
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I thought we were talking "apples to apples" with the same spring, and the same pellet, and the same bore/stroke? The only variable being the piston mass. Staying within those parameters,,, it's a slam dunk.
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I thought we were talking "apples to apples" with the same spring, and the same pellet, and the same bore/stroke? The only variable being the piston mass. Staying within those parameters,,, it's a slam dunk.
“same spring, and the same pellet, and the same bore/stroke”
The only thing that I changed was the piston mass.
Here are the actual results:
7.9oz piston-ram – yields 4.7fpe of muzzle energy
6.6oz piston-ram – yields 8.4fpe of muzzle energy
4.4oz piston-ram – yields 9.5fpe of muzzle energy
What do you mean by “slam dunk”?
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I thought we were talking "apples to apples" with the same spring, and the same pellet, and the same bore/stroke? The only variable being the piston mass. Staying within those parameters,,, it's a slam dunk.
“same spring, and the same pellet, and the same bore/stroke”
The only thing that I changed was the piston mass.
Here are the actual results:
7.9oz piston-ram – yields 4.7fpe of muzzle energy
6.6oz piston-ram – yields 8.4fpe of muzzle energy
4.4oz piston-ram – yields 9.5fpe of muzzle energy
What do you mean by “slam dunk”?
That's a bit misleading don't you think?
You also have results which show a heavier piston producing more FPE. What you have here looks like what is expected with too light a pellet with a heavy piston. That would fit with what I've been saying because the light pellet doesn't provide enough resistance. We both agree, balance is important as well.
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Well gentleman, I think that's it. This post can go on and on. I know what I did and I know it's works for me. Do whatever you want because there's pro's and con's to any modification.
Scotchmo, thank you for your input and info. Your an asset to GTA. I hope we can work on a project one day. Paul68, your still cool in my book too.
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light piston likes lighter pellets, heavy piston likes heavier ones. Long stroke= slow stroke, heavy piston= even slower stroke. Long stroke lighter piston mid weight pellet with strong enough spring (and swept volume) is going to give you the optimum power. Chris at C.A.P. pretty much proved that. With a long stroke, you either gotta have enough weight and friction in the bore to keep that pellet from budging until the perfect time every time, or a light enough piston and strong enough ram to close the distance before the middle weight pellet moves.
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I think I'll join Shorty in bowing out now. This thread to be honest is getting redundant and I'd rather let some bigger brains pick it up if they want to continue it. Thanks for all the great discussion, particularly Scotchmo.
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All the math stuff makes my head hurt.
I still say if it was a proven fact a lighter or heavier piston made a big difference?
There would a billion people out there ready to take your money.
I still can't find one selling such a product.
Now I have a couple tubes of special Snake Oil derived from a little known serpent only found in Central America that will do the job.
It also cures cancers of the skin, removes ticks, repels elephants, makes hair grow on bald heads, and stops all computer viruses!
Both Einstein and Steven Hawkings were proven wrong in thier math. Not by ACTUALLY doing what they proved by math.
But just doing marks on a chalkboard.
I see the same thing here. It's all marks on a chalkboard and only one person offers numbers which are not backed up with further proven results.
To prove those results means one must duplicate what he did and verify every step.
All I see is fuzzy theory only applied to AG's.
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I think I'll join Shorty in bowing out now. This thread to be honest is getting redundant and I'd rather let some bigger brains pick it up if they want to continue it. Thanks for all the great discussion, particularly Scotchmo.
Thanks Paul68 and shorty.
I did have to correct one thing.
Paul68, you said:
"...You also have results which show a heavier piston producing more FPE...."
If you look back at my results, you will see that the heavier piston never achieved as high an FPE as the light piston. The heavy pellets performed best with the heavy piston but I never got quite as much energy as I did with the light pellets and light piston. I'm not saying that a heavy piston can't produce just as much FPE when everything is balanced correctly.
At least we agree that it is all about balancing the variables.
Some day I plan on getting back to experimenting. The Scotchmo Express #2 has been sitting in the shop half done for the last 7 months. I have been working on my "Big (airgun) Year". I have been competing in as many Grand Prix field target matches as I can. I plan on doing every one west of the Mississippi, from Texas to Oregon until the Nationals in Texas in October.
I have also been dedicating time and effort to getting airgun competitions started at our local range. I found a great club with available range time for matches. We have already had two matches. I just started a web site to help promote the smallbore club as well as the new airgun shooting. It is a great shooting spot. I invite anyone within a reasonable distance to attend one of our monthly matches.
www.slsba.org (http://www.slsba.org)
Scott Hull
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As much as I have enjoyed y'all thrashing this out, I think all has been covered adequately. Big thanks for the links to documents for further reading.
With that:
(http://awesomegifs.com/wp-content/uploads/dead-horse.gif)
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Trail xl1100
Piston and seal weight: 414 grams
Nitro ram: 166 grams, weight appears to be evenly distributed, balance point near middle
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Trail xl1100
Piston and seal weight: 414 grams
Nitro ram: 166 grams, weight appears to be evenly distributed, balance point near middle
@Laser
When we talk about balance, we are talking about the relationship between spring, piston weight, air volume, and pellet weight and how it affects the shot cycle and transfer of energy.
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I was specifically referring to the overall weight distribution throughout the length of the nitrogen filled gas ram. The "piston" of the completed ram appears to weight the same as the nitrogen filled body. When balanced on my finger "see-sawed", the balance point appears to be near the center, slightly biased to the body end. Do we discard the weight of the ram's moving piston? Sorry to bother you.
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In that regard, the balance of the ram really doesn't mean anything. The mass of the rod, in my opinion of course, could have a direct relationship to total piston momentum though. Since the rod is moving and the body is not, only the rod will have momentum. Momentum is a vector force, so only direction of rod movement matters, not balance of the rams parts.