Hi,

Does anyone know of an accurate way to measure the amount a bow's back stretches when drawn? I'm thinking of a device/method which would give 0,1mm accuracy or better. According to my calculations my steel bows' backs stretch ~0,6% (0,48cm if bow is 80cm long). I've considered a few simple options which don't seem adequate:

A good tape measure would be accurate enough for measuring the total (or average) stretch of the back, but not good enough for measuring shorter sections. A pair of (vernier) calipers would be accurate, but the results would have to "calibrated" somehow, because the surface being measured is not flat. They might be useful for certain relative measurements, though.

Any ideas?

Sounds like some good methods. Unfortunatly the calculations might be the most accurate method, unless you could get access to a CMM
Sounds like an intersting challenge, what is this for?

I'm just trying to verify that my calculations are correct, which is hard to do with calculations . Anyways, "The Practical Guide to Man-Powered Weapons and Ammunition" gives another variant of the tape measure method:

• Tape a piece of paper the length of the bow limb to the center
  
• Cock the bow
  
• Measure the distance between the edge of the paper and the end of the limb
  

This method has the advantage that one can be sure that one end of the "tape measure" (=paper) stays in one place thus increasing accuracy.

I'm thinking that if I measure the stretch on each half of the limbs (inner and outer) separately, I could calculate the stretch in any part by interpolating the results... meaning that if the outer half of the limb stretches 0,5% (on average) and the inner 0,6% (on average), I could deduce the that the tips stretches less than 0,5%, whereas the center stretches more than 0,6%. More exact values could also be calculated easily. This should work ok as long as the stretch increases roughly linearly.

I hope I'm making any sense .

What a simple and easy idea I like it.

I believe the stretch should be pretty uniform across the surface. I guess that would depend on if this is a straight limbed bow or one with a bit of curve to it. but i would think the change should still be rather linear. of course that is assuming a proper limb with no joints or stress concentrators in it but that really goes with out saying