'glass thickness vs draw weight

[critman]

If you take the same dimensions on two identical bows BUT, one has .040 'glass and the other .050 'glass what would the weight difference be?  From what charts I have looked at it looks like almost 10 pounds which is a lot.

[heartlandbowyer]

It's gonna be close to ten, is on a bow off my form anyways. A lot of strength in the glass.

Cory

[jsweka]

I agree with Cory.
You will need to increase your total stack thickness proportionately more when dropping from 0.050 to 0.040 glass.

[NYArrow]

It seems draw weight relies heavily on the glass thickness. More so than one would think. We spend a lot of time discussing stack thickness and lam combinations on this forum... and it seems that on a straight limbed longbow .01 of glass = pretty close to 10#. Of course varies with bow length and limb dimensions. I know there are no hard, fast rules but from what I have read this seems to be pretty close.

[critman]

I have had a few request for some younger folk's bows. I need to make my 10 year old daughter a new one also. I was wondering about this and what stack thickness I would use for a 60" mild r/d bow around 30#. Thinking around a .250 using .040 thickness glass should get me close at 1 1/4 wide limbs. What say ye?

[Swissbow]

The glass thickness itself is not so important for the draw weight. The total stack is what's important. In fact I have I have built the same bow with different glass thickness but the same total stack and I haven't noticed any significant difference in draw weight. But since glass is much heavier than wood/boo lams there are differences in performance. The limbs get heavier when using thicker glass and therefore the bow will cast slower and hand shock will likely increase.
__________
Andy

[critman]

Thanks Andy. Good info!

[KenH]

About 88% of the strength of a wood-glass composite comes from the glass; only 12% from the wood.  As mentioned, it's not glass thickness, per se, but total stack.

If you have a for for a bow and have made weight and length X in that style, you can use the Lam Stack Calculator (that can be found here at Trad Gang)to calculate the lam stack and length for a new bow of the same design.  Worked just fine for me recently on both recurve and flatbow designs that I'm finishing and will show when they're done.

[critman]

KenH, i'm not real computer savvy, when I go to the link it says "upload file" I don't know what file name to put in to open. I know this is stupid but can you talk me thru this? I really would like to play with some stack thicknesses but don't even know where to start. I have downloaded openoffice although i do have MS office as well.

[Wolftrail]

Originally posted by Swissbow:
The glass thickness itself is not so important for the draw weight. The total stack is what's important. In fact I have I have built the same bow with different glass thickness but the same total stack and I haven't noticed any significant difference in draw weight. But since glass is much heavier than wood/boo lams there are differences in performance. The limbs get heavier when using thicker glass and therefore the bow will cast slower and hand shock will likely increase.
__________
Andy

Thats good to know, like they say learn something everyday........

[Mad Max]

Originally posted by critman:
KenH, i'm not real computer savvy, when I go to the link it says "upload file" I don't know what file name to put in to open. I know this is stupid but can you talk me thru this? I really would like to play with some stack thicknesses but don't even know where to start. I have downloaded openoffice although i do have MS office as well.

X 2

[KenH]

Critman and Mark -- check yer PMs...

[quail]

Critman,

Using a "Kenny M" R/D design form for a 58" bow, this is what I have come up with after experimentation: .030 Glass, .130-.002 Taper Back Lam, .070-.001 Taper belly lams. Bow scales right around 26-27 lbs @ 28".

[critman]

Thanks quail. Is that the mild R/D or more reflexed design of kenny's?

[quail]

More R/D design. Bow shoots really nice!