inertia arrow rest

[Max]

Originally Posted by Adam

Same as custard powder, and it's called a thixotropic substance if my memory of school science is correct.
Adam

This must be the only board where you start with Archey and end up debating custard! However, to fuel the debate further - thixotropic is the opposite of the effect seen with cornstarch - thixotropics substances decrease viscosity with shear force, where as cornstarch increases viscosity with shear force.

My prediction for 2007 - Hoyt bring out the custard cam, the harder you pull the faster it goes - its very messy to adjust, but can provide a light snack between ends.

[geoffretired]

Hi, Anyone still reading this for the archery bits?
Love custard cams especially with a nice cup of tea. I like to lick the custard out first then eat the biscuit at the end.
To the boffins out there, I've just realised there has to be something inside the shock absorber apart from custard. Am I right there has to be some sort of compressible material,like air?This is better trapped in some flexible enclosure?

[Max]

Sorry all - got a bit carried away. I have been giving the rest idea some thought. One concept would be to get the rest to drop away quickly under light pressure, but return (relatively) slowly. So to keep it simple, a one way drag device would be the best option. OK lets assume that the rest will drop away purely under forces from the arrow - a bit like a spring rest. I got as far (in my head) as coupling a one way clutch (most simple version a small coil spring acting on a shaft - twist shaft one way spring opens and allows free rotation - twist the other and it locks), with a viscous drag disc to slow the return, then attaching a light tension spring to the rest to power the return and support the arrow.

In action the rest drops quickly freewheeling on the little spring clutch, but storing some energy in the return spring. After the arrow clears the rest it slowly returns as the drag disc slips under the force of the return spring. Seems simple, uses reliable technology and might just work and no custrad in sight. The benfit would be very little force to depress the rest and a dwell long enough to allow the fletchings to clear. I bet you could make one like that Geoff?

[geoffretired]

Hi Max,
I think I follow the basic idea of drop soon and fast but rise later and slow.
As for the technology, I'm not an engineer so if you say it is well known then I accept that. By "coil spring opening" I can only imagine the coil is unwound so the diameter gets bigger allowing it to slip round on an axle inside the coils.
Viscous drag disc??? I understand each word on its own,put them together to make a well known phrase or saying and I'm out of my depth again.
So I have to say that at this present moment in time, I can't make one. It does sound like it should work.
I was thinking of a normal launcher arm and pivot with a shock absorber fitted under the arm. When the arrow presses the arm down it moves lower. When the arrow lifts off, the arm returns slowly as the shock absorber would be slow one way by design. A bit like the door closers but at right angles.
I'm still struggling with my shock absorber for the trigger of the release aid. I'm having to work on small parts that my machine is not really designed for. Hey ho.

[Max]

Sorry Geoff - it's my background with photocopiers and fax machines - little spring clutches are used in just about every paper handling machine, from FAX's to printers.

OK my last idea then I will leave you to your happy retirement. A shock absorber does just that, usually stiffens up on compression and uses a thick damping liquid - like oil. What you want is a damper then only works on the return stroke, not the forward. So - a very thin liquid (low forward inertia) coupled to a one way valve. A little piston pushes a thin (low viscosity) liquid down a tube which allows it to do so because a little ball (a check valve) is open on the way down. On the way back the little ball closes against a seat (perhaps helped by a tiny spring) and restricts the flow, so the piston returns slowly. Very fiddly to make, but a challenge!

OK, this is getting too technical for me....I know, I know, I like technical, but only if it's for good reason. I don't like over complicated. You can go out and buy a decent "simple" reliable launcher for less than £50, drop away if you like. It will work really well for years. Now if you want to build a complex new launcher just because you can and because you find it a challenge, great. But I do feel in this case two cliche's come to mind. "Why re-invent the wheel?" and "if it ain't broke, why try and fix it?" Having said all that I wish you well with your "ACME" MKI custard launcher.

[bluebow]

How about no contact with the arrow - magnetic levitation? Noticed this on the 2012 Olympic Competitpor Blog site mentioned in another AIUK forum:

http://2012olympiccompetitor.wordpre...hery-gadgetry/

Seems to be patented too - so no copying!

http://appft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PG01&p=1&u=%2Fnet ahtml%2FPTO%2Fsrchnum.html&r=1&f=G&l=50&s1=%222005 0188972%22.PGNR.&OS=DN/20050188972&RS=DN/20050188972

[Max]

Nothing wrong with healthy debate even if it is completely off the wall and seemingly pointless - great new ideas have come from such debate. Why not just roll over and buy what the manufacturers offer - ie. not re-invent the wheel?

Well, as an engineer who has worked at solving R&D problems for nearly 40 years I can put it in one word - 'compromise'. Now for the detective work, because as punters with no knowledge of what goes on inside the design houses of the manufacturers we have to look for evidence. With compound arrow rests there are two basic approaches which come at the problem from opposite ends of the spectrum. Both can't be right, but must meet somewhere in the middle at the 'sweet spot'.

One rest (the spring rest) appears to keep the rest in light contact with the arrow (well more or less - without high speed video I can't say). The other, the drop away, does exactly what it says on the tin and drops completely out of the way. OK - so what does the arrow actually need? does it need to be lightly guided or only held up until the string can shove it forward - if so for how long?.

If only one answer is the right one, why are we not all using one type of rest? There must be a compromise somewhere - each design approach will deal with a major compromise of the other, while introducing a compromise of its own - some of us will prefer on compromise over the other. Now - we can go on trying to solve such problems by trial and error, but in the end you need to get to the fundamentals - what does the arrow actually need. Identify that correctly and you can design a rest that is sweet, forgiving and tolerant of variations in setup. Unfortunately the equipment needed to identify that is not available to the majority of us, as is the knowledge of aerodynamics needed to inerpret the data.

No doubt there have been many hours of high speed video shot, but a lot of that will be locked away as commercial secrets. No - we amateurs need to keep dreaming of custard, becuase one day at 3am in the morning someone is going to jerk awake and have a better answer inside his (or her) fevered head. Then they will make shed loads of cash and we will get better kit.

[geoffretired]

Max, that was a very good explanation. I can see both sides on this one but each to his /her own. I can't resist the challenges. Like you, I would love access to the truth about what the arrow wants.
Two simple ideas came to me last night when much needed beauty sleep would have paid dividends?????
One; let the rest stay up while the arrow is at rest. Any downward deflection pushes the arm down towards a magnet that holds it down until separated by the archer before the next shot.
Two; use the toggle switch principle. initially the switch is in the up position. Slight downward movement presses it to the down position.
Both of these should be very simple and use well known technology.

[Rik]

I was a little disappointed to see that that magnetic rest uses a plain old magnetic insert, and not anything more sophisticated (like diamagnetism). Oh well. The manufacturers website, for anyone interested, is here.

On the Barner rest: I haven't seen it, but I'm told there was some video footage taken which showed that the rest didn't actually trigger until after the arrow was past it. That's probably not the effect which was wanted...