Brace Height and Arrow Speed

[Knoll]

Topic or comments re impact of brace height on arrow speed comes up regularly.
I have not seen the subject tested. This fella did.

Impact . . .  virtually zippo.

https://www.youtube.com/watch?v=yvjuZL_fdw0

[Del the cat]

I call foul!
IMO it's meaningless for a couple of reasons reasons.

1. It's a modern bow with huge amounts of deflex reflex using F/glass. The high preload/deflection on the limbs at brace (and the shape of the force draw curve) probably means the small change in brace height makes little difference.
2. The brace height is much higher than with a wooden bow and a 1" change in a 7.5" brace is a much smaller percentage change than it would be in a 5.5" brace, which is more typical for a selfbow . Eg 1" in 7.5 is about 13% where from a 5.5" brace it is over 18%

I'm not saying the results are necessarilly different for a self bow... just that this test is doesn't prove the case for a self bow.
Del

[DC]

I'm with Del but I'll stick a self bow in my shooting machine and try it at more like our brace heights. Maybe tomorrow when I get a bunch of crap off my bench

[PatM]

What do you guys think of the statement that no  bow pushes the arrow more than 19 inches?

[IrishJay]

I'd say that's not true Pat, my f/g recurve for example, braces at 8", measured to the handle of course, it's just a hair over 9.5" to the back of the bow. My draw is 29.5", so it's pushing the arrow 20" from my anchor back to brace.     

[Scyth]

Try these two procedures to establish maximum cast of arrows :

1a.) Start off with a measured to the target (I use 18 meters) . . .
2a.) Start off with “0” point of nock height then mark the place on the target 3a.) Then incremental (1/8” used) to raised of nock height to 1” . . .
4a.) This should give you is a “sweet spot” of maximum cast of the arrows . . .

Using the new nock point:

1b.) Start off with a measured to the target (I use 18 meters) . . .
2b.) Start off with 5” point (“0” point) of brace height then mark the place on the target  . .
3b.) Then incremental (1/8” used) to raised of brace height to 8.5” . . .
4b.) This should give you is a “sweet spot” of maximum cast of the arrows . . .

. . . and this is all she wrote . . .

regards,

Scyth

[PatM]

Oh I know that math but the statement was in reference to bows of all brace heights and draw lengths.

[DC]

Well I cleaned off the bench. I used a 40#@28" ELB and a 400 gr arrow. The brace height was 6.25". I shot 7 shots- 175,174,175,174,174,174 and 175. I dropped the brace height to 5.25 and shot another 7-177,176,177,176,176,176 and 176. I thought that was pretty convincing but to check I put the brace height back to 6.25 and shot 3 176's in a row. Didn't prove anything. I'll have to do it again. Somebody have any idea what might have gone wrong? When I put the brace height back to 6.25" the speed should have gone down. I'm puzzled.

I'll try my other chrono.

[IrishJay]

Pat are you suggesting that the arrow/string/limbs reach maximum velocity within the first 19" and that even if there is more than 19" of travel the arrow doesn't continue to accelerate,  hence no more push?

[DC]

We need more than that Pat. There has to be some kind of conditions for that statement. An 84" longbow with a 36" draw length will obviously(to me at least) have more than a 19" power stroke.

[PatM]

It wasn't my idea.  There were no conditions attached to the statement  AFAIK.

[DC]

OK second try, different chrono. New battery. 6.25 brace height 177,177,177,177,176,177 and 176. 5.25 BH 176,178,178,177,177,177,178. Then I put the BH back To 6.25 and got 177 and got called in for supper. There is a slight difference but it's only an average of .5 fps. That 176 on the second group of numbers bothers me as it was the first shot after I lengthened the string but  even if I changed it to 178 the average hardly changed. So 1" in BH doesn't make a lot of difference. Maybe changing fron 5" to 8" might be more noticeable but nobody uses an 8" BH do they

[bassman]

Larry Hatfield did a lot of studies on this very subject, and that 19 inches was the number he came up with.Brace the bow were it best suits you as velocity is marginal at best.

[Bayou Ben]

Thanks for doing that experiment Don.  I thought it would make more of a difference than that.

[Del the cat]

What do you guys think of the statement that no  bow pushes the arrow more than 19 inches?

That statement is compete and utter nonsense!
Take a warbow (or a Yumi!) for example at 32" draw ... the arrow doesn't leave the string after 19".
Maximum acceleration is probably pretty much as soon as it is loosed, BUT it is still accelerating.
If the bow stopped pushing after 19 inches we would see the arrow leave the string after that!
I think the bloke doesn't understand the difference between velocity and acceleration.

As an example if you are in a car and yo floor it. Maximum acceleration is pretty much once it gets some grip and overcomes the initial inertia. Then it accelerates smoothly, pretty smooth the rate of acceleration starts to drop off BUT it is still accelerating.... it may still be accelerating when it hits 70mph, but only slowly ... doesn't mean it's stopped being pushed.
Del