effect of changing brace height?

[Spike]

Sorry I am just an engineer, I will defer to those who obviously know integral calculus better than I and its application. Also if compounds only hold at 15# at 30 draw where does the energy come from, must be FM :-)

[bigcountry]

You can harm a bow by raising a BH too much and shooting it.  Most Mfg will ok a bow up to 31.5" of draw.  So you can figure from that whats safe and whats not.  

An easy way to do this is to draw a force curve at 4 points, starting at 0 draw, and ending at 28".  Take that and put it in excel sheet and find the linear equation that goes with it thru excel.  If its not linear than use a 2nd order polynomial to get a curve fit. Then integrate that (sorry for the calculus) linear equation from 0 to whatever your max stroke is at 28" draw.  Thats your total energy.

[bigcountry]

Originally posted by Spike:
Sorry I am just an engineer, I will defer to those who obviously know integral calculus better than I and its application. Also if compounds only hold at 15# at 30 draw where does the energy come from, must be FM :-)

Nope, thier energy curve is not linear like ours.  Actually ours is not perfectly linear either (recurves for instance).  Thier energy curve starts low, and within 2-10", shoots up to 70lbs (depending on how extreme the cam is) and stays at that high energy until 2-4 of full draw and comes down to 15lbs.  Thats why they can capture so much energy.  Its hard for me to draw a 70lb compound now, because you are holding so much wieght at 10" of draw.  Where our (trad bows) wieght is not near that until we almost get to full draw.

If your an engineer, the intregation stuff like this is freshman calculus.

[Brian Krebs]

The Z bow compnay says that Byron has two different bows that he shoots; one he has a 6 1/4 to 6 3/4 inch BH ;  and on the 'Radical' bow he shoots he has a 6 3/4 to 7 1/4 inch BH.

 How does that compare with the BH you have ?

[Spike]

Okay, just did a test, 20# @24" solid glass bow 6 3/4" brace. Twisted up to 9 3/4" (3 inch higher brace) guess what still at 20# @ 24". Draw is measured from depression of grip +1.3/4" per amo standards ie 28" draw is 26.25" from the grip depression.

[Spike]

Longbows and Recurves are fairly linear with a bit of a rise early on on fast designs. Old round wheels stored energy looking like a parabola, the fastest compounds have energy storage curves that approximate a square wave.
Class Dismissed!!! :-)

[bigcountry]

Originally posted by Spike:
Longbows and Recurves are fairly linear with a bit of a rise early on on fast designs. Old round wheels stored energy looking like a parabola, the fastest compounds have energy storage curves that approximate a square wave.
Class Dismissed!!! :-)

I agree whole heartly as I just told you this answer, but if you truely believe that you wouldn't have asked that question a few posts back when you said it must be FM if compounds are only holding 15lbs.  Did you change your mind or was it FM    

Now class is dismissed.

[bigcountry]

Originally posted by Spike:
Okay, just did a test, 20# @24" solid glass bow 6 3/4" brace. Twisted up to 9 3/4" (3 inch higher brace) guess what still at 20# @ 24". Draw is measured from depression of grip +1.3/4" per amo standards ie 28" draw is 26.25" from the grip depression.

Ok, I just did a test, and my selfbow I am building right now, I put on a long string, with 0BH, and drew a total of 28" and got 45lbs, I then put the original string on and set it to 6" and got 55lbs.  

I too am an engineer, and would like to know what FM physics you studied in school.

[Don Stokes]

My head hurts. If you lower brace height, it puts more energy into the arrow. Raise it, and you get less. For me it's a moot point, because I will shoot my bow at the best brace height for the string that's on it. That's how it will perform best.

I'm no engineer, but I used to have one of those striped hats.

[bigcountry]

Don, I agree, if people on here stays within the Mfg recommended BH they most likely will weaken an arrow at its lowest BH.  But we are just having a conversation about what is really happening.  If you stay withing manufactures recommendations, then the energy gained from power stroke increase of 1" will be more than the starting preload and wieght increase.  

It was just a friendly conversation.

[Spike]

Bigcountry, do a test on the Easton Bow Mapper device if you have a shop near you that has one. Let me know what you see. I just tried a widow PLV 47"@28 with 7.25", lowered to 6.25" brace and saw no detectable change in # at the measured 28".

[bigcountry]

Originally posted by Spike:
Bigcountry, do a test on the Easton Bow Mapper device if you have a shop near you that has one. Let me know what you see. I just tried a widow PLV 47"@28 with 7.25", lowered to 6.25" brace and saw no detectable change in # at the measured 28".

I don't need a mapper.  I can just use a bow scale and excel and some calculus. Thats all that machine is doing.  We are both engineers here.  We don't need no stinking mapper.

I doubt you would see a detectable change in lbs with only 1" of change.  

To see a huge change go from 1" BH and then go to 9"BH.  YOu will see it.  As I said to Don, for 95% of the folks on tradgang, its a moot issue if they stay within Mfg specs.  

Ok, can we agree on this Spike?  If you shorten your string, your limbs are bending more?  Can we start there?  Think about an exaggerated 50" string put on a bow that requires a 60" string.

[Spike]

I will concede that you may see a small increase even though my home tests with a calibrated Hanson analog bow scale and steel tape don't show it over a range of useful interest. I did the Easton bow mapper on a Kanati LB I had and it gives really great resoultion of data, no parallax reading errors, uses a calibrated digital scale etc. but unfortunately I did not change the brace and repeat the curve.It outputs a Excel data file. I may still have it around if you would like to see it I could send it by email attachment. My point is I don't believe any increase in # will overcome the energy lost in power stroke, at least over the range of interest. Guess us engineer types are good with numbers but don't do well with communicating and spelling!

[katman]

Here is a quote from SHOOTING THE STICKBOW, by Anthony Camera 2008, page 76,
"To correct for a stiff arrow...Increase the brace height by adding twists to the bowstring. This slightly increases the draw weight of the bow, typically one to three pounds per inch, and decreases the angle of offset as the arrow leaves the string, thus making the arrow weaker."

He also later states decreasing brace corrects for a weak arrow.

page 79,
"As can be imagined, the shorter the string, the higher the brace height and the heavier the draw weight at any given length. That is because with the shorter string, the limbs will need to bend into a tighter arc...most home and Pro Shop bow scales aren't accurate enough to show that amount of weight change...The lower the brace(longer string length) the faster the arrow leaves the bow. This is due to the longer time the arrow is in contact with the motive force-the string."

I certainly agree with these statements and will add that a higher brace will normally yield a more forgiving bow as there is less time for the archer to torque the bow throwing the arrow off target. The trick is to find the correct balance for you, play around with it, it is easy to adjust and observe the results.

[champ38]

What Katman says....thats the way Ive always understood it.

[Friends call me Pac]

Dang now I don't know if I got it backwards or not.  When I know for sure I think I'll write it down on a cheat sheet.

Maybe something like tighty righty, loosey left or right loose, left tight whichever is the correct way.  

[David Lewis]

Perfect explanation Spike

[Old York]

Short version -
Arrow shows too stiff, raise BH.
Arrows shows too weak, lower BH.

Long version -

Easton Tuning Guide, 2nd edition

Brace Height
For recurve bows, another way of altering arrow spine is
with the brace height. By increasing or decreasing the
distance from the bowstring to the pivot point of the grip,
the dynamic spine of the arrow can be made slightly
weaker or stiffer. Increasing brace height will make the
arrow shoot weaker, and decreasing brace height will
make the arrow shoot stiffer.

Brace height affects arrow spine by increasing or
decreasing the amount of energy delivered to the arrow
at the moment of release. Raising the brace height
(shortening the bowstring) compresses the limbs,
increasing stress (prestress or preload) in the limb material.
The more preloading of the limbs, the greater the
actual bow poundage at full draw. The reverse is true
when lowering brace height. A lower brace height
(lengthening the bowstring) reduces the prestress in the
limbs and reduces bow weight at full draw.

 However , raising brace height produces some small loss
in arrow velocity as the slight increase in draw weight
does not equally compensate for the reduction in the
bow's "power stroke". When the power stroke is reduced,
the amount of time the arrow stays on the bowstring is
also reduced, in turn, decreasing the length of time the
arrow has to absorb the bow's energy.
----------------------------------------
It does not mention the offset effect, IMHO this also has a significant influence.
Might not see much change in arrow flight when changing BH on centre or past-centre cut riser.

[bigcountry]

Yep, the easton guide says that.  Lots of people here posting stuff they read. I am more of a experimenter.  My old grandfather told me once, don't believe nothing you hear and half what you see.  It was good advise.

But has anyone actually tried to go to the extremes to find out what is happening?  I am not talkig about reading what another man has done.  I am talking about trying it yourself? I have.  I know one thing, you take a straight longbow down to 3" BH and you just lost 5-9lbs.  No stroke is going to make that up.  There will be a sweet spot for every bow where where speed is optimal or noise is lowest, or both.

[Boom Stick]

Power stroke,  schmower stroke.

Since raising the BH increases poundage, the reason an arrow "acts" weaker is because it actually becomes weaker to that bow.  It's not playing pretend.