Why is R7 preload spacing giving less power?

[DanD]

Hi all,
I'm playing with an old, pre-safety R7. It came with a sacked spring and was shooting in the high 480 fps neighborhood with 8.44s.
I yanked a used spring out of a late model HW30S that was shooting 600fps with 8.44s. This spring, unspaced, does about 530 in the old R7. I read on the ARH site that the old R7s can take .4" of spacing,  so I cut a plastic piston spacer about that big, and velocity dropped to the mid 4s. I thought maybe my spacer was too soft and somehow inhibiting spring motion, so I tried stacking a washer behind the guide to space the back side of the spring in combination with a thinner, harder spacer in the piston and still saw loss of velocity, although less drastic. I've had this gun apart about 10 times double checking everything, and I can't figure out what's going on with it.
The piston seal is a relatively fresh looking OEM HW. It moves smoothly through its entire stroke, and holds air when I cover the transfer port and push it by hand. The breech seal is new. The shot cycle is mild.
I'd appreciate any insight anyone has about this.
Thanks for reading!


 

[cobalt327]

Hi all,
I'm playing with an old, pre-safety R7. It came with a sacked spring and was shooting in the high 480 fps neighborhood with 8.44s.
I yanked a used spring out of a late model HW30S that was shooting 600fps with 8.44s. This spring, unspaced, does about 530 in the old R7. I read on the ARH site that the old R7s can take .4" of spacing,  so I cut a plastic piston spacer about that big, and velocity dropped to the mid 4s. I thought maybe my spacer was too soft and somehow inhibiting spring motion, so I tried stacking a washer behind the guide to space the back side of the spring in combination with a thinner, harder spacer in the piston and still saw loss of velocity, although less drastic. I've had this gun apart about 10 times double checking everything, and I can't figure out what's going on with it.
The piston seal is a relatively fresh looking OEM HW. It moves smoothly through its entire stroke, and holds air when I cover the transfer port and push it by hand. The breech seal is new. The shot cycle is mild.
I'd appreciate any insight anyone has about this.
Thanks for reading!

The spring needs to be able to rotate at the end, so the preload spacer setup may be inhibiting that.

[DomingoT]

Baffling situation.
When you inhibit the free rotation of the ends of a helical spring, the spring compliance goes down, which means the spring stiffness goes up. A spring whose ends are not allowed to rotate stores "more" energy than a spring free to rotate. By itself, inhibiting the rotation would tend to increase the pellet velocity.

[wimpie swart]

Maybe the spacer pushed the spring beyond free space movement, meaning; now some of the coils are coilbinding, this would bring down the velocity. I have found that almost all factory fitted springs are just too long. While the rifle is out of the stock; you have to cock the rifle slowly to be able to see how the new or other spring reacts to being compressed in the rifle.
If you need to keep most of the velocity, then you cut a coil or 2, redress the end.
If yiu need to bring velocity down a bit, then you collapse a coil or 2.

[eeler1]

Whatever. 

Spring guns are notorious for not behaving the way they should.  And there's a balance between the spring and piston movement and the 'cushion' of air that the piston creates, kind of a timing thing.  Maybe not timed quite right.  You might try different spacers, not necessarily the max spacer, to get what you are looking for. 

Pellets can influence the 'timing' as well, depending on fit and weight.

R7's are nice, accurate, easy to shoot, gentle pellet rifles.  If it is power you want, maybe an R7 isn't the gun for you.

[DanD]

This old R7 seems to have sealing issues. I surmise the extra preload and piston velocity was overpowering a stiff old seal and causing it to skip past some wider parts of the tube,  whereas the slower piston velocity of the unspaced spring was giving the seal lip more time to expand into the tube...
 

[DanD]

Just an update:
I installed a new ARH seal and sized i the skirt down as it was very tight. I also installed a new ARH tune kit built with extra spacing for pre-safety guns.
Short story is its very consistent with JSB 8.44 @ 555 fps, JSB 7.87 @ 572 fps, and FTT Green at 662 fps.
I may order a fresh OEM seal and/or another aftermarket seal to compare with my sizing attempt. If I do, I'll update this thread with the results.

[SpiralGroove]

Hey Dan,
Have you tried AA Falcons (7.3) or JSB 7.3's?
They seem to be the best shooters in many newer HW30's, plus more velocity  .

[DanD]

Hey Dan,
Have you tried AA Falcons (7.3) or JSB 7.3's?
They seem to be the best shooters in many newer HW30's, plus more velocity  .

Hi Kirk,
The 7.3s are a bit erratic in this old R7 so I left the results out, but they peaked out with barely more velocity than the 7.87s, and many shots were lower than the lowest 7.87 readings.

[SpiralGroove]

Hey Dan,
Have you tried AA Falcons (7.3) or JSB 7.3's?
They seem to be the best shooters in many newer HW30's, plus more velocity  .

Hi Kirk,
The 7.3s are a bit erratic in this old R7 so I left the results out, but they peaked out with barely more velocity than the 7.87s, and many shots were lower than the lowest 7.87 readings.

Yeah, I almost assumed you'd used them, but thought I'd mention it ... just in case .

[eeler1]

This old R7 seems to have sealing issues. I surmise the extra preload and piston velocity was overpowering a stiff old seal and causing it to skip past some wider parts of the tube,  whereas the slower piston velocity of the unspaced spring was giving the seal lip more time to expand into the tube...

Good catch!  Hadn’t thought about that issue. 

[Artie]

Too caught up in hurricane prep currently but I've been working/testing my early/pre-safety R7 with synthetic seal. I get the notion that piston seal fit and spring rate/spacing are a tightrope walk where if one is changed the other is affected. Could be compression tube, probably is, but it seems I battle between the two to achieve max fps. Adjust one, have to tweak the other..

[Bryan Heimann]

Every springer has a limit.  Oversprung past a point, the power will decrease.  I have read a lot of stuff in the forums over the years, a lot of opinions but I can't give you an exact reason why.
I experienced it for myself experimenting with a B19 (stoeger x20s), too much preload or poston weight, or not enough of either, can decrease the power.  That's why good tuners approach it with a goal of improving the shooting characteristics and accuracy, with complete disregard for the numbers.

To little spring and etc. can also sabotage the accuracy and shooting characteristics.