Help on stack calculation

[BCWV]

As some of you know, I just built my first glass bow with help from John Sweka and I'm very happy with how it turned out.

 I'm planning another but want to do it with more narrow limbs. I used John's build along and had followed his lead. My limbs are 1 1/4" Wide from center for 18" then had marked out to 1/2" wide at the tips. I was checking weight as I profiled and stopped at 5/8" at the tips to end up at my desired weight.

 I would like to be 1 1/8" at the fades for 18" and taper to 1/2" at the tips on this one so should be able to run with the same tapers as the last one. I've searched for a calculation for width reduction verses lam stack but didn't find anything. There may not be such a critter. I thought of going with 6 more pounds using Brad Jansens calculator but am just throwing out a number.

 What would be an educated guess for an increased lam stack for an 1/8 " decrease in width? Is there a rule of thumb?

 Thanks
 Brad

[BCWV]

I'm sorry, should have mentioned it's a Hill style straight longbow build.
Thanks
Brad

[bjansen]

For a simple decrease in limb width, the calculation is easy.  Every percentage change in limb width, should equal a percentage change in draw weight....thus decrease the limb 10% from your original, I would expect a 10% draw weight reduction.  


Thus once you determine an estimate of the limb reduction (lets say 10%), reduce your draw weight on the original bow by 10%. Then plug that reduced draw weight in the lam stack calculator with the original lam stack thickness you used and you can adjust your desired draw weight to caculate the new butt thickness of the new lam stack.

[BCWV]

Well crap! I completely missed it. I had about worn my calculator out trying to come up with a number. Can't see the tree for the forest.
Thanks Brad!

[monterey]

Be careful as you make your limbs narrower.  You may need to add more thickness taper to avoid too much bend close to the fades.

[BCWV]

What do you think Mike? I want to go 1/8 less limb width over all. I was thinking that I could tweek tiller with more width adjustment if I needed to. I could go more narrow at the tips if it was bending too much at the fades. Or would another .001 per inch work better?

[MoeM]

You can keep the limbs parallel over few inches after the fades to prevent some bend.
I even got some nice and almost perfect full compass tiller on a Hill style with a "progressive widness taper".
First I draw the outlines from the fades aiming to very wide tips and then, starting from the middle of limblenght on the first drawn outlines, I marked a second pair getting closer to the centerline, aiming for desired tip width.
After sanding to this line I carefully flattened the break.
Other option could be some kind of powerlam or wedge over the fades to reinforce this area.

[7 Lakes]

Count on dropping about 12 lbs when going from 1.25" to 1.125" width. Just to be confusing I need to add the following:  When you narrow the bow you are getting rid of some of the limb taper.  You will need to add overall taper (from butt to tip) to the limb lamination to keep everything the same.

[monterey]

5/8ths at the tips was pretty wide and I think by going to 3/8ths or even a bit less you would probably be ok.

What is your rate of thickness taper?????

[BCWV]

Mike L,
 I went with John's .002 taper with his profile also and it worked out great. I did stop at 5/8 at the tips as I was watching my weight and tiller as I profiled the limbs.

 Mike B,
 I didn't realize that an 1/8" width decrease would make that kind of difference. This must be why some use more taper on the Hill style bows. How much more taper would you suggest?

 Thanks fellas.
 Brad

[bjansen]

I use .006 taper (per inch) on my hill bows.  Using a straight line taper from limb widths of 1.1" (at the fades) to about .450 (at the tips).  I also put a 7ish degree trap from back to belly.

Might be a better way, but that is just how I do it.

[BCWV]

Thanks guys.
 I have too many irons in the fire at the moment so I'm going to have to put this one on the back burner for a bit.
 I appreciate the help.

[jsweka]

There's some good food for thought here.  I've been using basically the same design (1.25" limbs and 0.002 taper) for every straight limbed bow I've built and got it down to where I can come real close to a desired draw weight.  Now you guys got me wanting to experiment with narrower widths and more taper.  Hmmmm......

[7 Lakes]

.005/" to .006/" depending on the drawlength you're building for.  You can also go less than 1/2" tips if needed. Particularly if you are using tip wedges.

[BCWV]

Thanks Mike B. I appreciate it.