Upping bow weight, thin backing strips or belly lams

[LittleBen]

I just finished up a laminated bow and its ~30# @25". It's hickory backing, maple core, jatoba belly.

Want to increase the draw weight to 45# @25". I calculated that I need to add .060" thickness

I'm debating two possible options:

1) add a second backing strip of hickory or white oak at ~.060" thickness.

2) add another belly lam of jatoba at ~.060 as well.

Any thoughts on the better approach?

I'm leaning towards a thin white oak backing. .060" is about 1/16".

[Pat B]

Leave it as it is and make a new one. Same amount of work but you end up with 2 bows. I'm sure you can find someone to give the light weight bow to.

[Roy from Pa]

Give it too some young kid and make another...
And pay attention the next bow build:)

[LittleBen]

Haha you crack me up Roy. I labeled wrong again! I didn't tiller them way below the draw weight, they are  trilams and they came off the form already at 30# ... not sure if thats better or worse ...

Maybe I should just give it away ... It's a takedown and I love takedowns but they are such a pain in the rear to get everything glued drilled shaped aligned etc. The thought of doing this again anytime soon is not particularly appealing.

Maybe I can pawn it off on my Dad, he wants a bow, it'll be his first bow and at 60, maybe 30# is not too bad.

[canopyboy]

I learned a lot adding a belly lam to a bow after the fact once, and was quite pleased with how it came out.  Given what I know about your contraints at home, I would say fix it.  Worst case is you build another one anyway.  More than likely you'll learn a few things and be a better bowyer for it.  I think you can't beat a hickory backing, leave that alone and add some more jatoba.  Don't forget to go a little more than .060, you'll be losing some grinding the limb down to a good, flat gluing surface.  

BTW, how'd you get to 0.060?  Sectional moment of inertia had to increase proportional to poundage?

[LittleBen]

yeah basically Dave. just modeling the limb as a cantilevered beam, I came up with a 15% increase in thickness for a 50% increase in draw weight ... that worked out to .060".

The back and belly are still essentially perfectly flat because the limbs came off the form already tapered so I just strung it up, it was immediately obvious that they were way too light and I also had 1/4" negative tiller. I actually need .070 on the bottom limb and .060 on the top limb. That should bring me up in weight and even the tiller. At least most of the way.