ELB Bamboo backing thickness

[Tommy D]

I am thinking of laying out my first ELB .... it will be bamboo backed Ipe.

I ve see a few pictures where the bamboo backing looks quite thick. I was always under the impression with bamboo backing that thinner was better so it doesn’t over power the belly. Having said that, bamboo is lighter than Ipe and if the compression stresses are on the belly and the tension on the back then surely having more lighter bamboo in the “neutral plane” makes sense.

I am happy to do a tri-lam if this had distinct advantages? Was thinking of trying to get some maple boards - if the timber yard is doing “click and collect”...

Would love some advise from some of the fine ELB builders on this forum   

[Del the cat]

It only needs to be about 3-mm thick at the crown, so if you prep it down to almost nothing at the edges it will be fine when the bow ids worked down to finished dimensions.
https://bowyersdiary.blogspot.com/2014/11/bamboo-backing-build-along.html
Del

[Tommy D]

Thanks Del - yup that’s what I always thought - but I saw somewhere an ELB where the bowyer had purposefully left the bamboo quite thick. Was wondering if there was a big advantage to this?

[bownarra]

No not really.
With a boo/ipe elb @ 73" long. 7/8ths thick at the handle tapering to 7/16ths at the tips is thick enough for 50#@28.
Width taper centre 12" 1 inch wide, taper to 3/4" wide 8 inches form the nocks, then into 3/8ths at the nocks. Leave the tips 1/2" wide at rough out.
It is nice to have a core to make your ipe go further. Taper your maple to 0.006" per running inch. A simple lap splice 1" long for the core lams is fine.
Remember elliptical tiller if you don't want your teeth shaking out! From no bend to only a minuscule bend in the handle that can only be just felt at full draw. The bend should increase as you move along the limb right out to a couple of inches from the tip. Elbs can shoot beautifully or definitely not if the tiller is wrong

[Del the cat]

Thanks Del - yup that’s what I always thought - but I saw somewhere an ELB where the bowyer had purposefully left the bamboo quite thick. Was wondering if there was a big advantage to this?

It's an advantage if your belly lam is thinner than you ideally wanted   
Del

[Hamish]

Hey Tommy, +1 for thin boo, and great advice from Bownarra on layout, etc for a boo backed ipe.

[Tommy D]

Alrighty then ... it’s on the “to do list” ... right behind everything the wife has put on it!

[PatM]

Remember that the thinner you make boo the relatively heavier it becomes.   Many people caution against starting too thick  because you'll run out of belly material.   Conversely  the thinner boo backing and thicker Ipe belly may weigh as much or possibly even more.

  Some bamboo is actually heavier than Ipe if you just go by the extreme outer layer.

[Tommy D]

Remember that the thinner you make boo the relatively heavier it becomes.   Many people caution against starting too thick  because you'll run out of belly material.   Conversely  the thinner boo backing and thicker Ipe belly may weigh as much or possibly even more.

  Some bamboo is actually heavier than Ipe if you just go by the extreme outer layer.

Are you suggesting there is no harm/ might be a benefit in leaving the bamboo thicker so long  as I have enough belly wood to tiller still?

[PatM]

Sure.  I have seen many examples of Ipe bows that had no harmful effects from slightly thicker backing.  You won't actually see too many with the bamboo meeting the Ipe with a knife edge joint.