Is there a way to calculate bow stack?

[arachnid]

Hi guys.

Say I've built a certain bow design and it came out at a certain weight.
Is the a way to calculate  the bow stack to achieve a different weight for that same design? Or do I just have to build more and more and just guess?

And how about using the same design at different lengths? Say a 60", 62" and 64"? How much weight is added/subtracted in general

[BMorv]

I started using 0.003 - 0.004"/ lb for my wood tri lams  and it gets me close enough.  Not sure if that crosses over to glass bows though.

[KenH]

There is a Lam Stack Calculator for wood/glass composites -- I can send you a copy.  

But you have to make a prototype bow first, and finish it enough to get a draw weight at desired draw length,  to get a baseline for the calculations.  

I make the cheapest prototype possible -- junk pine lams and riser and cheap black glass.  You're not gonna use it for anything, probably, no sense in spending much time and money.

You gain/lose about 5# per inch of length difference, if I remember correctly

[monterey]

There are several stack calculators out there.  Google bjansen stack calculator.

On my ASL builds, .003 per pound is dead on.  Most calculations are based on the formula that says "double the width, double the weight. Double the thickness, octuple the weight."

[BMorv]

Yep.  It's been a while, but the bending strength is proportional to the moment of inertia.  In the case of bows with roughly rectangular cross section the moment of inertia formula is I= (width)X(thickness^3)/3.  So you can plug in a few numbers while keeping the width constant and see what you come up with.  I show that 0.003" = 1.81% increase in strength.  0.004"=2.4%, 0.005"=3.03%, up to 0.010"=6.12%
These calculations are based off of one material with known properties, so taking off thickness of one lam will change the overall properties of composite.  But it's close enough for approximations.

[jsweka]

Here ya go. Brad Jansen originally made this, but Ice Mike posted it on a website others could download it from.  There are some notes on it, but you can ignore those for your purposes. Just enter your info in the green cells.  Works great and I use it all the time.

  http://www.facilityautomation.com/hosting/Lam%20Stack%20Calculator.xls

[Krasus]

Sucks but I figured mine out by simply building lots of bows and playing around.  

[arachnid]

Thank you guys so much. That helped a lot!

[kennym]

Up for Miles

[mlsthmpsn]

Thanks, Kenny.      

Looking forward to building my first laminated bow on your form.

[wood carver 2]

Originally posted by Krasus:
Sucks but I figured mine out by simply building lots of bows and playing around.    

LOL! Same with me, but then I'm thick headed.      
I've built a couple that could easily shoot 3/4"rebar arrows, if I could draw them!
Dave.

[TradBowyer]

build two and then you got it. my recurves change 1# for every .001" in stack thickness. my mild D/R longbows change 1# for every 0.004" in thickness. only way to really get that information is to build 2 bows off the same form. everyone's form will be a little different so taking someone else's numbers won't exactly work. taper rate of lams also affects it