Brace height adustment effects on arrow spine

[olddogrib]

I raised this question on another site and I'm still not sure I understand. I think I can get a general concensus that a lower brace height does, in fact, have a slightly longer associated power stroke. This is easiest for me to think about as the linear inches the arrow will travel from anchor to brace height, when the arrow will leave the string after a bit of over-travel. If this is true, one would expect that if you had a slightly stiff arrow that you needed to make act weaker, that this could be accomplished by decreasing brace height. I was told that this is backwards, one should increase brace height and it has to do with the release point of the arrow from the string relative to a high or low brace. I've tinkered around trying to prove it one way or the other, but I think the differences are so small(or my form is not consistent enough) to tell. Supposedly this theory is covered in the Bowyer's Bible extensively. Somebody please explain "how brace height affects arrow spine" for dummies! Lastly, if you are succesfull in tweaking brace height to optimize arrow flight, will that necessarily be the point where the bow is quietest? Inquiring minds want to know!

[Stumpkiller]

This from the Three Rivers Archery site:

   

If you brace a bow too low, the feathers will hit the shelf before the nock leaves the string. The arrow actually stays on the string past the brace height measurement. It travels forward a bit before pulling itself loose. If the feathers come into contact with the shelf before the nock clears the string, your arrow flight will be erratic. You'll be prone to having the string slap your wrist with ultra-low brace heights too. The bow will be a bit smoother and pull a little less at the lower brace heights and conversely if you short-string your bow, the weight will increase slightly and the angle of string pinch will increase. You can't hurt a bow with a high brace height, but you can hurt performance. The bow will pull harder and the short string will force the limbs to stop short in their travel path, robbing you of energy. You should be looking for the "sweet spot" - that special brace height where the bow feels good during the draw and release, and your arrow flight is crisp, clean, and straight.

   http://www.3riversarchery.com/longbow-recurve.asp  

When I was shooting target archery in college we had a different approach.  With a Martin Ventura and single pin sight we would shoot an arrow and adjust the brace height.  The brace height that threw the highest arrow in the target (using the same sight setting) was the most efficient.  High brace heights rob you of energy (and so your arrow begins to act as if overspined) and low brace heights slap your arm and the and arrow does funny things if it holds the string too long; not to mention the bow starts to make lots of noise.

[Don Stokes]

One factor to keep in mind is the weight of the string. Heavier strings will tune best to a higher brace height than skinny, low-stretch strings.

[olddogrib]

Stumpkiller,
That's an interesting trick that ought to have potential, although I'm not sure I trust my form enough to get a clear read with it. It might be even better with a shooting machine and/or mechanical release, but I suppose if you take all the human variables out of it, the only thing you end up proving is that you found the best brace height for the machine.  I originally didn't agree with those that argue that brace height will slightly affect poundage at full draw.  I argued that poundage at a 28" draw (or any other) will be the same regardless of whether it starts at a "preload" of 7", with an additional 21" drawn or 8", with an incrmental pull of 20". But the more I think about it, I realize that as you shorten the string to achieve a higher brace, at some point you probably have to flex the limbs an an additional slight amount to get to that full draw of 28". At some ridiculously high brace height(limb pre-stress from shortened string) you would probably negate the ability to even pull to desired draw due to the potential limb flex being used up in the brace.  I guess my point is that within narrow bowyer recommended brace ranges, I would suspect that one could easily compensate by lowering the head to anchor from "pulling to conditioned resistance" and completely obscuring what your're trying to prove with the short-draw.  I'm rambling only to admit that my logic may be flawed.  I'm ready for a PhD of trad bow engineering to explain this to the simple-minded.  Maybe I've just had too much caffeine today,  I'll counteract it with some "Budweezer" and maybe it will all make sense!

[Stumpkiller]

And that was with a target bow using (I confess, shudder) a pin sight, so it was steady and repeatable.  On the other hand, I went through a lot more nocks then that I do now.  ;-)

It was just to illustrate that there is an "optimal" brace height for bow efficiency.  I do not use that system with instinctive shooting.  I start on an unfamiar bow with a nock point at 3/8" high and a brace height of 1/8 of the bow's AMO length and try moving the brace height a half inch at a time either way, then I move the nock 1/8" up and down and repeat with both.  Eventually I find a "sweet spot" based on arrow flight and bow noise.  

Personally, I like to try for the lowest brace height I can get away with figuring the "piston throw" effect of moving the arrow with the longest push is going to get it moving out with the least paradox.  If you think about it - even if the weight of the bow doesn't change - the shorter throw has to act on the arrow in less time so the momentum will increase the paradox (arrow bending at the riser) as compared to a longer time (lower brace height).  Smooth push vs. a kick.  Am I kidding myself?  Probably.  But my bowhunting mentor taught me to work on arriving at the lowest possible brace height, all else being equal.

He could throw five arrows into a touching group at 20 yards when he was 55 years old, was a former State Champion and could hit squirrels with a bow as well as I could a .22 rifle - I listen when those guys talk.

[hvyhitter]

Sometimes when your brace height is to low, your arrows may "tick" against the riser like a "weak" shaft. You go to a slightly stiffer shaft and the "tick" goes away and your arrow flight looks good........until you try and shoot a broadhead and it doesnt hit with your fieldpoints and shows its a "stiff" shaft. Back to the first shaft and brace height to get your broadheads hitting right. Some bows more likely than others to be this way. Just one more thing to consider cause "been there-done that". Your milage may vary........

[Ragnarok Forge]

Go the the OL Adcock site and read thru the bow tuning process he uses.  It works great and resolves all effort most folks spend on tuning.  If you get bare shaft and fletched shafts to impact properly everything else seems to be covered.  You get maximum efficiency, repeatable shots, a quiet bow, etc......

There are a ton of factors that impact how brace height will impact spine of the arrow.  In general your surmise is correct.  Within reason, a higher brace height gives a stiffer spine and a lower brace height gives a weaker spine.

[Greg Skinner]

Actually using the brace height for making slight adjustments for arrow spine has nothing to do with length of power stroke or changing draw weight.  It is all about the angle the arrow is making with the strike plate.  Increasing brace height decreases that angle, hence the arrow has to bend less to get around it.  Therefore, if your arrow is showing slightly stiff, you need to increase brace height (even though that lessens the power stroke) in order to weaken the dynamic arrow spine.  That is what Viper says in his book and I have found it to be true.  It does seem backwards in a way.

[LostNation_Larry]

Of course the bow design makes a difference.  If the bow is cut past center then the arrow will not be at an angle to the riser as the string approaches brace height.

[kennyb]

Charlie P-the info you provide, along with the others, is very enlightening and helpful. I do have one question and that is: on my new BW, there is a mark my arrows have made on the back side of the rest, closest to me. It is obviously hitting there as the arrow leaves the bow. that being the case, does that mean my brace height is too low? It seems my arrows are sometimes flying erratically, but not always! Thanks for your help.
Kennyb

[Kip l Hoffman]

I suspect that striking the back of the shelf is an indication of a knock point that is too low.  However it could also be an indication of a change in your finger pressure which will change the string angle and thus change arrow flight.  I say this because you mentioned that your arrows fly irratic "soemtimes".  I have found that 3 fingers under can cause variations in string angle.  The change in string angle changes the height above the rest even though the knock point is measured the same At Rest.  The shorter the stroke the less power put to the arrow. This is why cross bows need to be 175# to get the same arrow speed as a 50# bow.  The distance the arrow sets off center to the string at rest makes a huge difference in the proper brace height.  The closer your arrow is to center shot the less critical spine is and the more variation you can have in your release and still attain proper arrow flight.  I do not believe that changing the brace height changes the spine of the arrow, it only changes where the arrow and the string seperate.  I have never measured the change in arrow speed in relation to the brace height.  I suspect it is marginal for our purposes.  Don't know as I helped much on this subject but it sure is one you can get your head all wraped up in, that is for sure.

[gjarcher]

Interesting question ... Since I have a draw board, bow scale, string jig, and chronograph, I decided to test the theories. Here is what I got:

1958 Bear KS 60@28, DL 28¼", 618 grain arrow.

BH: 6¾"
DW@28¼ = 61.0 lbs
Powerstroke = 19.75"
5-shot avg. speed = 166 fps

BH: 8"
DW@28¼" = 62.3 lbs
Powerstroke = 18.5"
5-shot avg. speed = 166 fps

From this I conclude that increasing BH requires a slightly stiffer spine because the arrow is accelerated to the same speed in a shorter distance, ie. more G-force.

This might be peculiar to the bow, but that's how it plays out for my setup.

[SlowBowinMO]

On a side note, I did some testing a couple of years ago that was consistent with what gjarcher just posted.  My focus was on brace height versus performance.  I found lowering the brace height on my Firefly a full inch only yielded 0 to 1 fps increase in speed, but the shooting qualities took a dramatic downturn.

[olddogrib]

Thanks to all, some excellent feedback here and you've helped my understanding greatly.  Always willing to change my opinion when a better one comes along!