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Bows by the numbers - modelling a bow before cutting timber
Aussie Yeoman:
This week a friend and I made a pair of bows on the basis of some mathematical modelling.
I modelled a pyramid bow using non-published, aggregated/averaged data from some white oak. I used a working strain that should theoretically have induced about an inch and a half of set. Between a friend and I we made two - one 50 lb at 26 inches and the other 40 lb at 26 inches.
I haven't made exhaustive measurements, but the early indicators are that the final dimensions (thickness) are within 1 mm of the projected data generated by VirtualBow. You can see from the photos that we didn't get the finished bows to match the projections exactly, but they're pretty close. And the average set of two limbs is 40 mm - just slightly over 1.5 inches. As predicted.
You can see we didn't quite match the projected tiller shape exactly, but it's pretty close.
sleek:
Fascinating work. What are the limbs dimensions?
Aussie Yeoman:
Here they are!
sleek:
Thanks that's perfect. If you had made them 11 to 15 % less in draw weight what does your calculations say you should predict for set? Per mine, 11 to 15% less the ideal range for no set for white oak.
willie:
--- Quote from: Aussie Yeoman on September 07, 2024, 02:14:35 am ---
I modelled a pyramid bow using non-published, aggregated/averaged data from some white oak. I used a working strain that should theoretically have induced about an inch and a half of set.
--- End quote ---
Aussie,
deliberately designing to 1-1/2" of set and coming in at 40mm is great. I wished some of mine came in that close. Guess it is all in the test data. Have you considered or tried a simple bend test to obtain a working strain?
@sleek
you should give it a try
https://www.virtualbow.org/
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