Hold sensitivity.. Jim's AGW article...

[Shed tuner]

Interesting read Jim, as always The vibration graphs were especially interesting....

Quick rewind.. Whilst I'm sure plenty of folks might have considered that hold sensitivity was very much a function of stroke length, as you I'm sure have more conversation than I about such things with a far wider range of people (I live in a cave), personally I had always thought that hold sensitivity was more a function of piston bounce and it's timing (relative to pellet exit).

Obvious example is an export spec long stroke throttled back to 12 FP with a weedy spring; lots of piston bounce. Shorten the stroke back to std 12 FP specs, less hold sensitivity, stronger spring job done

But both this and the vibration theory come back to the same, pleasingly simple hypothesis.. that what you ideally want is for the pellet to be exiting the muzzle when the muzzle itself is not subject to much movement... now whether that movement is related to rifle displacement or the more subtle affect of vibrations (presumably themselves due to aggressive piston bounce - which would certainly explain why some very light pistons can be hold sensitive without piston inertia to calm things down) is a more interesting question. Am going to dig out some of Jamie accelerometer graphs in a sec and see what can be deduced...

[Shed tuner]

See here for my 24mm HW80



Also the 22mm TX



In both cases the velocity is (near) constant, with low acceleration, at the time of pellet exit (the green dotted line)

[TonyL]

Lovely stuff.

I haven't managed to read the article yet but, as ever, looking forward to it.

Cheers for sharing those graphs and information, Jon.

[TonyL]

What do the "Y" and "Z" lines correspond to, Mr B?

[Shed tuner]

What do the "Y" and "Z" lines correspond to, Mr B?

X= along
Y = up and down
Z = side to side

See the "Y" muzzle flip in the last graph...

[BTDT]

Glad you enjoyed the article, Jon.

With hold sensitivity, as with so many aspects of the springer shot cycle, we tend to hamper ourselves by seeking a single reason for any phenomenon, when there could be more than one cause, depending on a variety of factors, including how the rifle is set up, and how it is used. People find a simple relationship between recoil displacement and hold sensitivity appealing, because it’s easily understood, and we can alter recoil displacement at will. It is equally likely that the cause is recoil or surge acceleration, vibrations at some point in the recoil cycle, or a combination of these plus displacements. We just don’t yet know.

Jamie’s graphs are based on acceleration measurements. To find velocity, we have to integrate the acceleration data, averaging each pair of figures, which can mask large differences between adjacent figures, smoothing out vibrations. If you note the fluctuations in Jamie’s x axis accelerometer graph, you’ll see they’ve largely disappeared from the integrated velocity graph. Sorry, Jon, but when the pellet exits, the vibration is there, but you can’t see it because it’s been integrated out. The graphs in my article were direct measurements of velocities, and showed all vibration, but had I measured accelerations and integrated the data, the vibrations would be greatly diminished, if they showed at all.

We might know more about hold sensitivity next year. Nomads are hosting the inaugural UKAHFT 2019 Recoil HFT Championship on 26^th May, and there will be plenty of rifles with short strokes, light pistons, reduced diameter cylinders and the like. I just need to convince the owners to let me test their rifles…

[Shed tuner]

Glad you enjoyed the article, Jon.

If you note the fluctuations in Jamie’s x axis accelerometer graph, you’ll see they’ve largely disappeared from the integrated velocity graph. Sorry, Jon, but when the pellet exits, the vibration is there, but you can’t see it because it’s been integrated out.

Yeah, agreed in the vibration being masked... although the mass of the rifle will also help with that...

I was more making the point about the veolcity of the muzzle being relatively constant, rather than rapidly changing (for example when excessive bounce is occuring), and that being a good starting point for why stroke reductions (to reduce that excessive bounce) can be a good idea...

But for sure, as I said at the beginning, the vibration detail from your article is very interesting as another factor - and possibly a more significant one too... especially if that translates to the muzzle, or indeed is magnified at the muzzle.... where did you stick your accelerometer ?

[BTDT]

Yeah, agreed in the vibration being masked... although the mass of the rifle will also help with that...

I was more making the point about the veolcity of the muzzle being relatively constant, rather than rapidly changing (for example when excessive bounce is occuring), and that being a good starting point for why stroke reductions (to reduce that excessive bounce) can be a good idea...

But for sure, as I said at the beginning, the vibration detail from your article is very interesting as another factor - and possibly a more significant one too... especially if that translates to the muzzle, or indeed is magnified at the muzzle.... where did you stick your accelerometer ?

I only measure the X axis, Jon, and always from around three inches forward of the rear of the scope grooves.

[Terry D]

To find velocity, we have to integrate the acceleration data, averaging each pair of figures, which can mask large differences between adjacent figures, smoothing out vibrations. If you note the fluctuations in Jamie’s x axis accelerometer graph, you’ll see they’ve largely disappeared from the integrated velocity graph. Sorry, Jon, but when the pellet exits, the vibration is there, but you can’t see it because it’s been integrated out.

Sista! Haven't I been saying EXACTLY this for bleddy years? Well, haven't I? Eh?













No, I haven't, but still. Springers, eh?

[RichardH]

When I was a lad the cure for hold sensitivity was called practice

[cooper_dan]

Nomads on the 26th will be the perfect place to test theories about hold sensitivity and feed the debate of which tune works best. Not only because the bottomless well of jim's knowledge will be present, but if this years course is anything to go by, the terrain and course layout will highlight any poor technique or particularly hold sensitive setup. Going up the hill this year there were elevated shots requiring high holds up the peg, and the ground you were lying on had camber, together it really messed up your hold if you weren't conscious of it.

The best modification I made to my TX was putting a foam pad on the forend to rest my hand against. Couple that with the 'light fingered' technique I borrowed from Terry D (I think from an article about the blue laminate hw97?) and hold sensitivity can be removed through technique rather than tuning.

I find the technical articles fascinating, but the best advice Jim has ever given me still stands - buy lots of pellets and practice!

[BTDT]

Couple that with the 'light fingered' technique I borrowed from Terry D...

and

[Shed tuner]

I only measure the X axis, Jon, and always from around three inches forward of the rear of the scope grooves.

that old single axis technology Jim, come on, get with the times (don't let using Terry as a benchmark for technological evolution keep you behind the times )

[Murphy]

When I was a lad the cure for hold sensitivity was called practice

I always assumed it was invented after HFT started.

[ballisticboy]

While barrel vibration is going to be significant in hold sensitivity, it is the effect it has on the group size at the target which is important and this is also dependent on the pellet reaction to barrel movement.

As the pellet leaves the barrel, when the head leaves the barrel the pellet CG will start to move sideways in the direction of the vibration. However, the rear of the pellet is still held in the barrel and its sideways movement will not be the same as the pellet CG which will cause a yawing rate. If there is one thing diabolo pellets as a design do not like it is yawing rates due to their basic ballistic properties but the result, in terms of group size, will depend on the ballistic properties of each pellet type. The effects can be minimised by trying to match the pellet exit timing to the barrel movement but the pellet reaction to barrel exit conditions will still be important and you could easily end up with a pellet which has ideal timing but lousy ballistic properties giving apparent high sensitivity.

Hence, hold sensitivity will depend on the pellet design as well as what the barrel is doing.