Lightening the ram/piston?

[Privateer]

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?

[Paul68]

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


Chris from CAP Airguns has also attested to the same thing.

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?

[Paul68]

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.

[Laser-Fist]

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

[Scotchmo]

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.

[Paul68]

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.

[shorty]

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.

[Scotchmo]

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.

[Paul68]

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.

[Scotchmo]

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.

[Paul68]

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?

[Scotchmo]

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.

[aidave]

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.

[Bryan Heimann]

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.

[Privateer]

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.

[robtmc]

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. 

[shorty]

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 ?

[robtmc]

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.

[Paul68]

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.

[shorty]

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.