Trad Gang
Main Boards => The Bowyer's Bench => Topic started by: longrifle on November 01, 2014, 09:28:00 AM
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How do you guys deceide on which thickness glass you should be using? To date all I've used is .050 for all my bows both longbow and recurve in the 55# range. I'm going to be making a few 35 # bows and was thinking of thinner glass.
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you lose about 5# for every .010 of glass you lose, so going from .050 to .040 on both you back and belly should drop you about 10#.
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I would have to go back and look up in some of my bow building books but i thought that there was a formula for a percentage of glass to wood. For some reason I settled in on using .040 glass a few years back for all my one piece r/d longbows.
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Yeah I thought there was a formula also.
Does thicker glass make a bow more sturdy or slower?
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You want the glass to take up 20-25% of the stack is the general rule of thumb.
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I wonder who wrote the rule? Would be interesting to see a 45 or 50 lb recurve made with only 25% glass.
I will typically go from .043 to .050 glass (uls is .043 and not.040) when building heavy longbows or longer longbows.
Going from .043 to .050 will make up the difference in adding two inches in length to most longbows.
Some bowyers never change to keep thing simple...or to stock less supplies.
BigJim
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Yeah I was wondering who made that rule up as well??
And why not build an all glass lam bow?
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Just some thoughts on an all glass bow. Experience has taught me that the more glass or ratio of glass to wood/grass/ will tend to make the bow harsh in the hand, handshocky, etc. The performance is good, but you need to preserve the elbow too. I mainly use .050 glass, except on the lighter weight bows, 40 lbs on down. Good luck, if you have the resources, by all means experiment!
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My limited experience also shows that thinner glass (given good core wood) makes a smoother bow that is soft in the hand. But there starts to be a trade-off in performance.
As for why not build an all glass bow? In a bending limb, the outer fibers take the most stress and strain. You have tension on the back and compression on the belly. The middle (or neutral axis) is where the tension and compression meet and the net strain and axial sterss is zero. What you need in the middle of an ideal composite laminate is a core material whose only job is to keep the outer fibers separated. If you make an all glass bow, you're using very heavy glass with good axial strain characteristics where it has no benefit. That weight will hurt your limb performance.
An ideal laminate (which you can see on ILF olympic limbs, high performance sailing, etc) would be outer fibers as thin as possible with maximum axial strain carrying characteristics -- i.e. carbon fiber. The core will be light and have no axial properties to speak of but will hold up very well in shear (the force of the two outers skins trying to pull together as you bend the limb) -- i.e. foam or end grain balsa.
For those of us making wood cored limbs, the wood laminates do carry some of the strain energy, even though the glass carries more. As a result, there is a point at which too much glass makes the limb heavy and slow and too much wood makes the limb less snappy. For a given limb design, this optimum balance could be calculated using Young's modulus and a bunch of dynamic finite element calculations for all materials and masses involved. More than likely people wanted to avoid that kind of migrain and just went with trial and error to find that 20-25% was generally close to the optimum point.
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Thanks for all the input guys
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Canopyboy, your second paragraph sounds straight from Jack Harrison's book "more unnecessary fun". I agree with both of you completely about tension/compression of the limb. 20-25% in a longbow and 35% in a recurve seems to work for me as far as the ration goes.