Brace height vs limb stress

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Masboy
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Brace height vs limb stress

Post by Masboy »

On one of my matrix limb bows on a vixen riser the first string I made was short an the string set 1/2 inch above the high mark . I measured the width an about 1 inch less than what I read matrix limb bows should be cocked. I then put on a longer string an when cocked it was about the width of a matrix when cocked. don,t that put more pulling weight an pressure on your limbs with a high brace height? so think a lower brace height would put less lb pull an stress on your limbs than a high brace height . looks to me even if the lower brace height, longer powerstroke is faster it would be less stress ,lb pull on your limbs? wrong thinking????
Michiganhunter
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Re: Brace height vs limb stress

Post by Michiganhunter »

I agree and have thought the same thing... the shorter you twist the string to achieve greater brace height the further back the limbs must be stressed to complete the cocking of the bow. In other words shorter string = longer draw cycle for the limbs... One reason I don't agree with the lower brace height being the cause of the limb failure. However a lower brace height may allow the limbs to over flex by allowing them to travel further forward flexing in the opposite direction...? I don't know if that would even be detrimental to limb integrity.
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Re: Brace height vs limb stress

Post by 8ptbuk »

Great question Frank ! I wish you would shoot both Strings thru the chrono to check the speed . I would think the shorter string would be slightly faster !
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BrotherRon
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Re: Brace height vs limb stress

Post by BrotherRon »

Not sure it will make much difference in speed, but I believe the limbs will not travel as far forward on the shot due to the pre-load????
Interesting question though.
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Re: Brace height vs limb stress

Post by SEW »

My thoughts. The shorter the brace heighth, the longer the power stroke and the faster the arrow. I'd think the greater the limb stress.
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wildcatter
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Re: Brace height vs limb stress

Post by wildcatter »

As you shorten the string the limbs not only get more load but the draw cycle gets shorter, giving you less power stroke,

when you start twisting up the string you are more or less pre cocking the bow, loosing power stroke as you increase BH, when you draw the bow the string is already partially cocked and as you draw the bow at full cock the limbs are now drawn back less because it was rested closer to the trigger claws, I find no real difference in speed from touching the string stops on my 380 which is aprox. 3/4" from riser to rail, than I get at 1" from riser to rail,,

I am starting with more stress on the limbs, and when the string is released, I think this keep's the limbs from rebounding farther forward, I believe this forward rebound under stress shocks the limb tips more and as the way they are built the string is hitting the limbs themselves between the tips and the recurve back of the limb, and with isolation creating a double slam at the ends of the limbs and the tips. I know putting velcro fur on the ends of the limbs seem to help quite the shot,,, this just seems to be what causes this damage to the mag tips to me,,

Image

This is why I keep my BH above 3/4" on my Lynx and Mad Max, at 1" BH I am just where that Business card will just slide in between the limb and the string, the lower you let the BH get the more pressure that is placed against those few inches of limb, I believe when oscillation starts,, the limb tips are actually stressing between the limb itself, it is actually allowing the string to slam against back of the limb while the tips recoil forward of that portion being slamed also buy the string,,, I really dont know how to explain it but that is allowing the tips to break past center, and back again violently,,,,, does that make sense??? Not real good at explaining what I am trying to convey,, but this isn't good!!!
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Re: Brace height vs limb stress

Post by SEW »

Wildcatter, you did a great job of expressing yourself; and you made sense. I'm relying some on the Air Brakes and the Dr Stirrup string suppressors(I'm about 1/24" in to the stops) to help take stress off the limbs.
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Re: Brace height vs limb stress

Post by bob1961 »

I keep my string right at the farther line from the riser on my exocet, there it keeps the power stroke/vibration lower for quieter shooting my 500gr arrows....
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Masboy
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Re: Brace height vs limb stress

Post by Masboy »

From my measuring on two of our bows for every 1/2 inch you add to the brace height you narrow your limb width by the same when cocked. from what I read a 380 limb width cocked is 25.37 an the 405 is 24.75 so if you have a .62 higher brace height than normal on your 380 you have the same stress on your limbs cocked as a normal brace height 405. I,m just thinking if you run a high brace height your putting your limbs an limb tips under more stress as the width is narrower under more stress.

I know when I changed from a high brace height to a 5/8 inch lower one on one of our powerful bows it got easier to cock an the limb width increased a little over 1/2 inch. don,t know anything about limb slap,over flexing when they come front but only lost tips on dry fires so far. I always ran a high brace height an had splinters on most sets I shot a lot sooner or later. now running low brace heights on all our bows an no problems so far. Better for my bows I think for limb stress JMO I,ve never found the sweet spot I here so much about yet on any of my bows they seem to shoot great at any brace height
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Re: Brace height vs limb stress

Post by Shoebuck »

How far is your string off of the bumpers? I just replaced the limbs on one of my Micro 335's and I am very diligent at keeping my brace at 1/4" off of the bumpers. The bow was cocked and sitting in a ground blind when the limb split length wise. No damage done to anything else that I can see.
My question to you is, do you think that my brace height is too high?
I want to give a big ata boy to Dan Miller at this time. The limb broke Wed. of this week and I called him on my way home. The new limbs are on my bow as I write this. That is amazing service.
Very interested in your theory, and will be waiting for your thoughts. Dave
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Masboy
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Re: Brace height vs limb stress

Post by Masboy »

My bows are all older models with matrix limbs with s5 on them that gives them a high brace height if string is 1/8- 1/4 off rubbers. so I cut about a 1/2 inch off the ends of the post an now my ends of the stop rubbers sit on my low brace line that,s 5/8 inch from the riser joint.don,t know anything about the matrix riser or the stops on those models . so I,m using a brace height of 11/16 -3/4 mostly on my bows.

I see at most shops I go to the strings are buried in the rubbers on the matrix line.i think if it was any harm to shoot a bow like that excalibur would take care of that. don,t see any harm from that JMO . good for rubber sales :lol:
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wildcatter
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Re: Brace height vs limb stress

Post by wildcatter »

I don't think brace height can be to high,,, but I am not sure??? Like I said, the higher you go with brace height, the more preload you have on limbs, and the more you shorten your power stroke, I can't see how this could possibly hurt the limbs. I also don't know how high you would have to go to loose speed, but you will not increase draw weight, you will simple shorten the power stroke. I am about 1/4" off my bumpers,,, at 1" Brace Height set from the riser rail seam.

I have thought about using a couple spacers about 1/8 or 3/16" to shim the REDS bumpers off the riser farther to make a closer gap between them and the string. I realize this would be harder on the rubbers, but think it might quite the bow a little and cushion the shot more???? Haven't tried it yet but after season, I am thinking I will.
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wildcatter
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Re: Brace height vs limb stress

Post by wildcatter »

Masboy wrote: I see at most shops I go to the strings are buried in the rubbers on the matrix line.i think if it was any harm to shoot a bow like that excalibur would take care of that. don,t see any harm from that JMO . good for rubber sales :lol:
That is bad and the problem I see is these sellers don't know any better. It is bad,, and will give you way to low a brace height. The biggest reason it happens is these bows are sent out with proper brace height, but as they sit with string streatch and nobody keeping them adjusted, it is just lack of maintenance. Sitting this way isn't what hurts the limb tips, shooting it this way is!
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Masboy
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Re: Brace height vs limb stress

Post by Masboy »

How can you not put more draw weight on your limbs if you raise the brace height an that makes your limb width narrowed when cocked ?can we get a third opinion as I have screwed up more than once with a tape measure with my brain :lol:
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Re: Brace height vs limb stress

Post by wildcatter »

Masboy wrote:How can you not put more draw weight on your limbs if you raise the brace height an that makes your limb width narrowed when cocked ?can we get a third opinion as I have screwed up more than once with a tape measure with my brain :lol:
You are right, the more you raise brace height the narrower your limbs get, and the closer your string gets to your trigger claws, just like cocking the bow, if you make the limbs narrower,,, they fold back farther and farther to the trigger claws. If you set your brace height at 2" you longer have a 13.1" power stroke, you will now only draw the string back to 12.1" the shorter the string the higher the brace, the higher the brace, the narrower the limbs, the narrower the limbs and shorter the power stroke, means you lose more power stroke,,, I don't know how else to explain it. You do not lose draw weight, you simply gain preload.

1" is what both my 380's have been shot at for close to a 1000 shots,,, over 600 on one and over 400 on the other,,, they shoot lights out!
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