I'm really starting to think that these 21/22mm bores with their longer strokes...

[Shed tuner]

...are actually more hold sensitive that the 25s

Shooting today with a 22mm TX, around 90mm stroke, benched. Was getting an inch of vertical stringing. Then realised it was because I was getting lazy with my technique, letting my RH pull down on the pistol grip, the buttstock being supported by the rear bag.

I then shot few a strings much more attentive to position, and the stringing disappeared to give some lovely groups...

Now, the short stroke 25s (78mm), simply would never have that issue.

Going to test the 23 (86mm) tomorrow, but I think that's OK too...

I guess that whilst the longer, smaller bore strokes are more efficient, there comes a point when the extra stroke simply can't be made up for in efficiency and the overall "lock time" suffers ? Going to look at a rather funky recoil simulator and see what it thinks...

[tinbum]

Its my understanding that lock time and hold sensitivity are inextricably linked, so a longer stroke must be that much faster to compensate.

Imo too much has been concentrated on efficiency to the exclusion of repeatable accuracy.
It's still interesting and brings many ideas to the fore!

[slugger]

i love all the new ideas and experiments. and untill i get to try them for myself i cant conclude anything obviously..
but i will say that my home-brew tx [25mm still] is the easyest of all springers ive ever tried to be accurate with.
i cant enphasise enough just how rubish MY shooting technique is, i mean seriously, i cant shoot for shizzle unless rested, and even then i catch myself doing a bad-hold, not balancing the rifle, changing hand position, pulling into my shoulder, forgetting follow-through, you name it, i forget it
but......
this tx is so forgiving that the only time i miss any of my targets is if its windy/gusty ..... and me not guestimating windage correctly
point being, maybe being too efficient is ultimately going to show-up minute differences more...... for example pellet fit....... if you are operating to the very n-th degree of efficiency, then a sudden tight pellet will massivly effect pellet-release-time a lot lot more than an "alround tune" so to speak ?
or a change in temperature even ?
i dunno, im liking the idea of experimenting further with the tx, but im so loathed to mess-up what works so well

[TonyL]

Yep, with you both on this one.

Just going by "simple logic" here, and no hard and fast scientific measurements. The "action time" will necessarily be longer with a longer stroke. Depending on the piston weight, recoil may be higher or lower (obviously lower with the lighter piston), but the duration of the forward stroke, and hence the first recoil phase, is longer. Springing it up more would speed things up but, if it's very, very efficient, the power may creep too high.

What we need is for Jim to get some of these on his rig, where he measures recoil, surge, vertical deflection of muzzle, pellet exit point in relation to where in the cycle that will be (generally in the early stages of surge). On the recoil and surge fronts, he records the deflection and duration.

Quick thought.....with a longer stroke, one would expect the measured rearwards recoil to be of a longer time duration (but maybe less deflection due to lighter piston), so "longer but softer", but do we think the piston bounce may also be longer (and softer)? If so, this is where the extra hold sensitivity has got to come in?

[slugger]

surely the second recoil [rearward bounce] is dictated by pellet release.. so the shorter the barrel / lighter the pellet .....the less distance the piston can actually travel backwards ?
so in theory there is an optimal barrel length for a given tune........
ideally pellet release could be 'timed' to happen as quickly as possible after the piston comes to its first forward 'rest' .......making for little rearward travel, and eliminating almost the second stage recoil........
or even try a tune with a deliberately large lost volume so that the piston has zero rearward travel........just one forward stroke with a soft landing ? i realise this would require seriously accurate volume/weight/piston speed etc calcs , but with enough logging it should be possible ?

[Shed tuner]

Just going by "simple logic" here, and no hard and fast scientific measurements. The "action time" will necessarily be longer with a longer stroke.

Not really, as the small bore has less air to compress, and that air presses back over s smaller surface area, reducing the decelerative force, so the piston is decelerated less, so it's overall journey time is actually shorter... however, see below...


Been running some numbers on the top secret funky recoil simulator:

Bore & Stroke / exit time / piston bounce dist / action dist / max piston bounce
25mm & 78mm / 10 / 5.9 / 4 / 28.4
22mm & 92mm / 9.7 / 7.7 / 3.8 / 36.9

lock time (pellet exit time) is actually a fraction quicker on the 22, but the difference is tiny.
the action dist recoil is 0.2mm less, but also a tiny difference.

The far more significant numbers, I'd suggest, are the piston bounce distance (at pellet exit) - the 22s piston has bounced back 7.7mm, the 25s only 5.9 Still not a huge difference, but definitely the most significant so far. Max piston bounce (ie after the pellet has left) is also a fair bit great too...



Maybe this is why ? Maybe we should make our 22mm pistons heavier....

[TonyL]

And/or reduce the stroke a little and add some preload to compensate?

[Shed tuner]

ideally pellet release could be 'timed' to happen as quickly as possible after the piston comes to its first forward 'rest' .......making for little rearward travel, and eliminating almost the second stage recoil........
or even try a tune with a deliberately large lost volume so that the piston has zero rearward travel........just one forward stroke with a soft landing ? i realise this would require seriously accurate volume/weight/piston speed etc calcs , but with enough logging it should be possible ?

So this is possible, but it would shoot horribly I think. Even if it wouldn't, whilst you could set it up to shoot perfectly (no bounce for a given pellet weight, if you were to put a heavier pellet in, it would go over the limit. Other ways to achieve the zero-bounce (or reduce it) - Inertia weights / anti piston bounce devices. Another possibility might be to have a dump valve to dump all the pressure the moment the piston stalls, but it would still waste a lot of energy...

[tinbum]

Maybe we should make our 22mm pistons heavier....

Or increase TP? I assume you've factored this and worked it out, but does the SCR requirement change as diameter/volume/pressure changes?

[Shed tuner]

And/or reduce the stroke a little and add some preload to compensate?

It's hard enough to find a soft enough spring to not over-spring these 22s. As it is I'm running 3mm wire and only 10 mm preload. shortening stroke / adding preload will make it worse (although I'm sure it would address the bounce), but it will shoot nasty and be down on power...

What would help is an even softer spring so I could get more preload - say 2.8 or 2.9mm dia, and around 20-30mm preload...

[Shed tuner]

Or increase TP? I assume you've factored this and worked it out, but does the SCR requirement change as diameter/volume/pressure changes?

nah - all these 22s are perfectly tuned in terms of power output with different pellet weights either side of it's preference - say JSB 7.3s, 7.9s 8.4
So it;s not that kind of bouncing problem (easily sorted with piston weights, larger TPs etc). Exactly the same in that regard to the 25s (although obviously the actual specs will be different to achieve it).

BUT it looks like this "balanced" setup gives more piston bounce in 22s than it does in 25s. Maybe we should tune them for more "slam" ? The 22s are very tricky, as they are so easy to over spring (see above).

[TonyL]

Or increase TP? I assume you've factored this and worked it out, but does the SCR requirement change as diameter/volume/pressure changes?

Another good point, Nick.

Pressure, however, wouldn't affect the SCR, as the SCR relates to the ratio of static parameters. Basically, swept volume vs "lost volume". Usually, this lost volume relates to the TP volume and any lost volume in the piston seal arrangement employed. But I think we ought to also factor in the volume of the hollow section of the pellet, introducing lots of mind-boggling, adjustable factors!!

[TonyL]

On one of the threads last night, I think NickG was also experiencing similar.

Good accuracy, but less forgiving? Think he was going to swap around actions next (full length to HC).

[slugger]

surely reducing stroke would increase slam and reduce power [enable more spring] so could you not simply shorten stroke a mm at a time untill you hit the sweet-spot

[Shed tuner]

On one of the threads last night, I think NickG was also experiencing similar.

Good accuracy, but less forgiving? Think he was going to swap around actions next (full length to HC).

Yes it was on one of my threads I think - the HW80 (24mm) one...

Now that thing is shooting nice - I can watch the pellets with ease I also cold blued it, so it's a little easier on the eye