Ipe is about twice as stiff as most common bow woods (maples, oaks, elms, osage,… modulus of elasticity: c. 22 vs 10-12 for these other woods). So for the same total limb thickness you can make the limb nearly twice as narrow throughout its length as you would for these other woods (and end up with the same poundage). So if you normally make limbs 1 3/4” (~4.5 cm) wide, make them 7/8” (~2.2 cm) wide for ipe. (you could also make it a bit thinner if you still prefer a 1 1/8” wide bow; but the backing should be accordingly thinner as well).
I’d make the tips also narrower than usual (but not narrower than its thickness), as Ipe is very heavy (dense) wood.
Don’t be fooled by thickness either: suppose your stave at ¾” (19 mm) pulls 100#, removing 1 mm (about 3/64”) reduces it to 85#, 2 mm to 71#, 3 mm to 60# and 4 mm to 49#.
How to calculate this: take ratio of thickness, raise it to the third power, and multiply by draw weight at initial thickness; e.g.: weight at 19 mm = 100#. Expected draw weight at 15 mm =100# * (15/19)3
Force draw curves on straight (long) bows are fairly linear. If I take my own FD curves for straight bows, weight at 28” is about double of the weight at 15-16”. So if you carefully tiller the bare ipe belly to 15”, you can estimate what it would be at full draw.
Suppose you have 3/8” belly of ipe drawing 15# at 15”, and want to add 1/8 of bamboo, the expected draw weight at 28” would be c. 65-70# (if the MOE of bamboo is the same as of the ipe).
Or you could just do it like most people do it, by experience, gut feeling and fingerspitzengefühl. If you lack these (like I do) you can use such arithmetics to compensate for it.
