Can I convert acalculated energy figure into approximate draw weight?

[stuckinthemud]

I have a lever bow. The levers reduce measured draw weight but I cannot get a string to stay tied to the end of the working section in order to measure the actual bow-weight.  I can find arrow weight and velocity which in turn gives me an energy figure.  How do I convert energy into draw weight?

Thanks in advance,
Andrew

[bradsmith2010]

can you send pic,,and why wont string stay on,,

[Badger]

 I f you tell us how much an arrow weighed and how fast it shot at what draw length we can give you a rough idea of your draw weight. You can look up KE conversion calculators on line. If you assume about 75% efficient you should be pretty close.

[Badger]

 I am a bit confused as to why you can't measure the draw weight, can you elaborate on that a bit?

[GlisGlis]

"The levers reduce measured draw weight"
The force on the string comes from working sections and levers combined
When you draw the string you are acting on the tips of the bow not where the working sections end

[Badger]

 There is no difference between draw weight and measured draw weight they are one and the same.  The levers may produce a bow with a higher force draw curve meaning they store more energy than is typical for a specific bow weight. The bows stored energy is simply another measurement. Typically, a bow will store a bit over 90% of its peak draw weight in foot-pounds of energy, A very extreme bow may store 110% but that is very extreme. 

[stuckinthemud]

The performance of the bow is terrible, far below what I had expected and I would like to know how hard the limbs are working.  I think the centre is too weak, possibly the tips are too heavy, and the string is too thick.  I even wondered if the arrow was too heavy - its not, I went to the post office and weighed it properly........I will lighten the tips and make a new string and increase brace height a little.  For fun, I sinewed it to see what difference that might make but I didn't find a way of stopping the string sliding down the limb before a really hot day gave perfect conditions for applying sinew.  When everything has dried, I'll put on a leather wrap and draw the bow at the end of the working limbs and at the levers and see what the difference is.

[bradsmith2010]

what is the perfomance
length of bow
arrow weight
draw length
any pictures,,

[superdav95]

Agreed.  Would be nice to see a few pics of the bow too. 

[Badger]

I get more confused each time you answer a question. If it is a regular bow why can't you just put a string on it and check the draw weight? Levers and working limbs don't need to be accounted for when measuring draw weight.

[bradsmith2010]

thanks for saying that Badger,,, I was confused too,,,

[stuckinthemud]

sorry, I tend to have muddy thought processes and  I very often go off at a tangent.  the limbs have very little taper and the string slips an inch or two or three every time I try and tie on, even using a timber hitch.

These are work in progress not finished tillering - I stopped working on them as they were clearly going to end up underpowered



finished one


So far I have roughed out 4 of these things, 9 or 10mm at the end of the bending section, the limb is constant taper on 1mm per 3" for the elm, and 1mm per 2" for the R/D apple.  The bow is 36" ntn, with a width at the end of the bending section of 25mm and at centre 0f 30mm.  Draw weight comes out at between 55 and 65 lb at 10.5" draw.  The dimensions of the tip are set by an archaeological fragment, the width at centre also.  Length is unknown.  The short elm was a lot stronger but overstressed and the one limb collapsed on a row of pin knots I dealt with badly. 

Kinetic energy calculations come out with a figure of around 15 foot pounds, I would expect about 23 +- 3lb.