Springer anti bounce experiment

[NickG]

My 21 mm tx tune has evolved to the point where I have the stroke / piston weight / transfer port/ spring ratios optimized, and when utilising an O or X ring the cycle is soft.
The last improvement to the tune which I use in my target gun has a synthetic piston ring to seal, and offers so much less friction that it makes 0.5 fpe more power which is great as I can use less spring , however the cycle becomes harsher. In an effort to combat this , I remember reading the theory that parachute seals make a softer cycle due to their braking effect when the body of the seal expands at the point of bounce, so this got me thinking if a similar set up would work for my tune.
I have constructed two pistons, the first is based on the BSA Idea of a sliding nose with a buffer behind, in my case the buffer is two O rings that expand to grip the cylinder walls and hopefully slow the bounce down.
The second is a dead blow hammer style affair, with a sliding weigh , or filled with lead shot.
https://photos.app.goo.gl/h36jH3XxA9Wg32nL9
https://photos.app.goo.gl/uashiGtHeDqxB19N7
https://photos.app.goo.gl/iwLgrHRddcizSxxZ7
I have two actions built with the same spec Making the same power for back to back testing so, watch this space .

[TonyL]

And we will eagerly await your results and observations, Nick.

[Shed tuner]

i did the meteor style experiment too.. I still have the head somewhere... it was liek an inverted top hat, with a sliding piston head holding the seal, and a rubber buffer behind it. Unfortunately it had the problem that it expanded during the firing cycle, robbing power, not just at the point of bounce. but maybe I was just impatient and should have tried more fine tuning.

I think the floating inertia design has more promise... maybe add strength by boring a 10mm hole clear through the weight and letting the piston rod pass all the way thru to the "real" nose ?

[NickG]

That's a good idea John .
Did some testing with the sliding nose piston today as it's easier to alter than the inertia weight idea .
Firstly I built it with a spacer instead of a buffer and set the power ( 11.4 fpe) to give a base line.
I then put two o rings in that were sized to just slide in the comp tube power went down by. 1 fpe ,increasing the pre load sent the power lower , I believe this meant the o rings were expanding too easily on the power stroke slowing it all down , it did soften the cycle, probably because the power was down.
Secondly I put two X rings in smaller in diameter and more rigid due to the square section, had the same result, so reasoned the pressure involved must be high enough to compress them too easily.
Thirdly made a spacer up and used a single o ring of a smaller diameter (actually a 20 mm stretched to fit ), and the power shot up, to 11.9 so a useful increase I reduced the pre load to bring it back to 11.4 .
There is a definite improvement to the cycle over the piston without the buffer in , not very scientific but I shoot it indoors over the chrono and have to use ear plugs normally, with this I don't need them .
So now need to shoot it alongside the other gun , weather is too bad today, but plan to bring both guns to the bash for my chief testers to have a go , should be interesting.
One downside is there is a larger shot to shot spread than I like, I think it's due to some movement between the nose and the piston body, so going to button the front to keep it all in line. Hopefully that will sort it out.

[flyingfish]

I volunteer to test if required!!

[Shed tuner]

Did some testing with the sliding nose piston today as it's easier to alter than the inertia weight idea .
Firstly I built it with a spacer instead of a buffer and set the power ( 11.4 fpe) to give a base line.
I then put two o rings in that were sized to just slide in the comp tube power went down by. 1 fpe ,increasing the pre load sent the power lower , I believe this meant the o rings were expanding too easily on the power stroke slowing it all down , it did soften the cycle, probably because the power was down.
Secondly I put two X rings in smaller in diameter and more rigid due to the square section, had the same result, so reasoned the pressure involved must be high enough to compress them too easily.

that pretty much mimics my results... pity I didn't have the patience to go further as you did - take#3 sounds promising - keep us informed

One downside is there is a larger shot to shot spread than I like, I think it's due to some movement between the nose and the piston body, so going to button the front to keep it all in line. Hopefully that will sort it out.

The other concern here is how consitent the expansion timing will be - tiny variations, lets say in pellet fit and therefore peak pressure - may have a large affect on piston braking point and therefore fps variation. Also as that "buffer" oring ring wears through repeated pressurisation and relaxation - it's quite a dynamic, tough environment.

That's why I like the floating inertia option - much less to go wrong

[NickG]

Will try and test this further for now, then get the inertia idea in the gun , problem is going to be getting the spring rate sorted, it's also going to need some sort of buffer in there, and that too will wear.

[Shed tuner]

Will try and test this further for now, then get the inertia idea in the gun , problem is going to be getting the spring rate sorted, it's also going to need some sort of buffer in there, and that too will wear.

yes agreed, but wear in that internal buffer won't make much difference to piston timing - it's just to avoid a hard metal-on-metal inertia weight crash. Maybe use something like a PU ring in the end ? O-rings aren't the answer to everything

[NickG]

No , but they are convenient 😉, going to face the inertia weight vwith some thin neoprene, space is at a premium in there.

[TonyL]

I'd also have a wild guess that the inertia weight platform will be less sensitive to wear issues. Once you've decided on a base, then will commence the fine tuning and balancing of spring rate & preload, pellet release pressure etc......all those inter-relating little facets affecting one another. It's going to be an interesting and enlightening journey.

[Loki_79]

Awesome stuff!

I once tried to model how much friction would be needed in the last inch of travel to make a difference to the peak recoil, and I forget the numbers now, but it was a lot!!

I'm fairly convinced that the parachute seal feeling softer is probably just because it is creating a better seal, rather than adding enough extra friction to slow down the piston at peak pressure.

I wondered about adding a tiny choke/taper at the end of the comp tube, but when it came down to it I think to have any noticeable effect the forces involved were so large that the piston would have just got stuck there and made it imposible to cock the rifle!

I've always fancied the sliding weight idea. I'll try to add that into my model to see how much mass you need and how critical the spring rate is in terms of timing. I think one issue will be that to have a significant effect the added weight will need to be comparible to the mass of the piston, so kind of like doubling the piston mass. You can't get away from the fact that more moving mass has to equal more movement/recoil, but if you can delay the movement until the pellet has gone then who cares!

[Shed tuner]

I've always fancied the sliding weight idea. I'll try to add that into my model to see how much mass you need and how critical the spring rate is in terms of timing. I think one issue will be that to have a significant effect the added weight will need to be comparible to the mass of the piston, so kind of like doubling the piston mass. You can't get away from the fact that more moving mass has to equal more movement/recoil, but if you can delay the movement until the pellet has gone then who cares!

Long time mate

I think the point of the anti bounce mechanism is to slow down the acceleration at the point the piston changes direction, giving more time for the peak air pressure to flow purposefully... but if more of the % of the piston weight is inertia, all that will happen is the piston stalls earlier in the stroke, then is held a fraction longer by the arriving weight before reversing... But in that scenario maybe if would have been better to just have the weight added as static and the piston would stall later in the first place ?

[NickG]

Hi Loki, I don't think parachutes actually seal better, I have compared like for like tunes , with parachute o ring X ring and piston ring, the piston ring makes the most power, followed by the X ring ,

[Loki_79]

Long time mate

I think the point of the anti bounce mechanism is to slow down the acceleration at the point the piston changes direction, giving more time for the peak air pressure to flow purposefully... but if more of the % of the piston weight is inertia, all that will happen is the piston stalls earlier in the stroke, then is held a fraction longer by the arriving weight before reversing... But in that scenario maybe if would have been better to just have the weight added as static and the piston would stall later in the first place ?

Yeah - it has been a while! I also can't make this Bash as it is my wife's birthday on Sunday.

I'd like to know from Nick what sort of weights are achieveable for both the piston and the sliding mass, and how they compare to the mass of his 'standard' 21mm piston. It still seems to me like you would need to be approaching double the overall weight, assuming the main piston is a similar weight, but that is just gut feeling, no actual basis at all.

I think it is the same argument for both parachute seals or for sliding weights - we can potentially increase the time that the pressure is applied, but always at the expense of a lower peak pressure.

Definitely watching Nick's results with interest!

[Loki_79]

Hi Loki, I don't think parachutes actually seal better, I have compared like for like tunes , with parachute o ring X ring and piston ring, the piston ring makes the most power, followed by the X ring ,

If parachutes don't seal any better, then I don't see how they can have greater friction either? To increase the friction enough to slow down the piston must surely improve the seal? Not convinced either way, just seems logical to me.

It would be interesting to measure the actual air volumes (e.g. captured in a balloon / water tank) to confirm that any difference in power correlates to leaking air rather than friction, or rather dynamic friction.

This is all from memory, but taking a chamber pressure around 700psi, 25mm piston seal and 2mm lip, the peak piston friction force for nylon on steel is about 50N, so barely anything compared to the spring forces involved. I think it needs to be around 500N to start having any noticeable effect on piston position and compression volume. Could all be completely different of course with your lighter springs!