Does zero set mean "overbuilt" ?

[Richard B]

For the sake of a theoretical comparison a simple spreadsheet model of a bow assuming some basic published data for yew (Modulus of proportionality of 6.48 e7 MPA and elastic modulus of 9.1 e9 MPA) suggests that a bow that is 70" NTN 1.44" wide at the fades and 0.5" at the tip should take about 0.75" set if perfectly tillered at 52 lb at 28 inches.

To keep the strain in the yew below the proportional limit and achieve zero set, at the the same draw  and weight, the bow would need to be 81" NTN 2.25" wide at the fades and 0.5 " wide at the tip, which seems a bit over the top.

Clearly all staves are different (even around the same trunk). I quite like the approach in TBB vol 1 where bend tests are done on wood close to (or part of) the stave being used so that a judgement can be made about achievable draw weight and profile before its too late.

[Danzn Bar]

For the sake of a theoretical comparison a simple spreadsheet model of a bow assuming some basic published data for yew (Modulus of proportionality of 6.48 e7 MPA and elastic modulus of 9.1 e9 MPA) suggests that a bow that is 70" NTN 1.44" wide at the fades and 0.5" at the tip should take about 0.75" set if perfectly tillered at 52 lb at 28 inches.

To keep the strain in the yew below the proportional limit and achieve zero set, at the the same draw  and weight, the bow would need to be 81" NTN 2.25" wide at the fades and 0.5 " wide at the tip, which seems a bit over the top.

Clearly all staves are different (even around the same trunk). I quite like the approach in TBB vol 1 where bend tests are done on wood close to (or part of) the stave being used so that a judgement can be made about achievable draw weight and profile before its too late.

Wow............
DBar

[Badger]

 Richard, that sound pretty accurate, I like to go about 1 5/8 for a 68" yew.

[scp]

For the sake of a theoretical comparison a simple spreadsheet model of a bow assuming some basic published data ....

Where can I get this model?

[Richard B]

Badger,
The precise numbers I gave in my last post might be a bit misleading....
My spreadsheet model can never be that accurate, because of the uncertain nature of wood. Even if you have measured the properties of a sample of wood close to the stave you are using, that actual stave will be different (wavy grain, knots, ring density etc. etc.) so any theoretical model will only be an approximation at best. You will still need to feel your way with you particular bit of wood as you shape it and tiller it.
What I use it for is establishing a first guess at what draw weight I can achieve with my stave and producing a target tillered shape that should give me approximately the target weight at the target draw weight, with even distribution of stress down the limbs (so should, in theory, give me even set). It helped me achieve my target draw weight and draw length on my first (elm) bow and I have posted screen shots of the target tiller curves superimposed on the tillered shape of my current bow.

http://www.primitivearcher.com/smf/index.php/topic,45456.msg684371.html#msg68437.

My spreadsheet is based on one by David Dewey, which you can find on google and download. I produced my own because I wanted to understand what is going on and would like to improve the way it is modelling set (which adds a lot of complexity and uncertainty), as well as extending it to look at asymmetric bows, and make it a bit more user friendly.