Discussion in 'Piston & Spring' started by Tinks, Oct 24, 2018.
what is the function of your tube ?
why no rear guide ?
The function, to try and minimise the possibility of the piston going off axis during it's travel and binding, sapping power (as suggested earlier by Rob and Nick), although my piston and guide arrangement seems ok there is still something wrong.
No rear guide in the same way HW didn't fit one, also with the rotary piston a closely fitting skirt to bore should hopefully be enough of a rear guide to work, it also makes the built a bit easier.
Should be ok, I have used oven baked epoxy for joining ally skirts to piston heads in the past and its plenty strong enough, prior to that i tried araldite and they came apart , any idea how hot you can get carbon fibre before it melts .
I like full piston with rear bearing as the surface area contacting the cylinder is small, though when doing 21mm pistons the clearance between the skirt and comp tube is small so as to keep the wall thickness big enough.
So you intend to use the whole length of the carbon piston skirt as a bearing
And have no support for the spring or means of guiding the rod into the trigger block at the back end ?
Not quite, I intend to use the full length of the skirt as the bearing but retain the standard rear guide and tophat to support the spring.
Nick, carbon fibre is ok up to around 300C if it is the high temperature stuff, the curing temp for the epoxy is 80C for 1 hour so I don't foresee a problem. The biggest concern is the epoxy used in making the tube as some will soften around 100C and I don't want any deformation during curing.
More friction there surely than a couple mm wide bearing
Will there be enough of it in the comp tube to keep it straight?
Think CF looks polished because it's in resin which comes out of the mould like that, so you're polishing the resin.
You could well be right, this idea is still on my 'to try' list as the more pressing issue is getting this skirtless piston to work at 760fps, currently reworking the piston head but it's very cold in the garage today.
Bear in mind that whilst carbon is strong it's not very hard, you can mark it with a finger nail. The surface is predominantly resin, either phenolic or epoxy, that's what gives it a nice shiny finish. For things like car and bike bits they either apply a clear lacquer (or additional resin) and top it off to get it smooth or use a low density filler and sand it smooth then apply paint. You always see the weave on carbon parts unless you add something to the surface and sand it. Vacuum bagging in an autoclave does give a better finish, but it's not smooth and to tight tolerances like machined parts are.
If it's tube it'll probably be phenolic prepreg so you're running it on cured 'glue' if it's used as a bearing surface. It's been a while since I had to know but got a pretty good idea you're looking at sub 200 deg c before the resin starts to degrade but it'll abrade pretty fast. As soon as the carbon is exposed it'll start to fray.
I think so, it will be approx 120mm long.
As for the surface finish, don't know until I have tried it........might be a couple of weeks before I can get round to this.
Thanks for that information, I don't expect any part of this end of the piston getting close to a detrimental temperature in normal use.
I found this very interesting, http://www.tribology.fink.rs/journals/2005/3-4/5.pdf
Check this to see what happens when you run piston tolerances too tight.
Not necessary either .
I think he is running an o ring, in which case the clearance needs to be tighter than with a seal, to avoid the o ring trying to squeeze into the gap .
Has that metal bit been scuffing? I'd have thought that would be well away from touching surely?
A bit of an update, made a new piston head with closer tolerance, slightly less crush on the o ring, piston now moves more freely in the comp tube.
Added 40fps with JSB using my lightest spring with 4mm preload, all other springs have produced a reduction..........out of interest spring rate is 7.1N/mm, 24 coils and 215mm long, piston stroke is 82mm. Do these numbers correspond to what is working or am I oversprung (if so I will have a delay getting new vortek springs).
Try the "oversprung Test "to your chrony with superdomes and Jsb 8,44.
Thanks for that, I have a strange set of results when testing different springs with Superdomes and JSB.
A stiff spring will give me a max fps of 730 superdome and 700 jsb, any more preload and both go down.
A more standard spring will give max fps 710 superdome and 720 jsb, more preload and fps goes down.
A soft spring will give max fps 680 superdome and 730 jsb, again more preload and fps goes down.
I believe my problem lies elsewhere, and am working my way through the suggestions above.
The oversprung test relies on a large difference between superdomes and jsb, which I have never been able to achieve, hence my question regarding spring rate etc.
A big thank you to everyone above who has contributed and I will post more when I have some good news.....
With the added 40 fps from the new piston
You are now around 770 ?