Trapping the Back on Hickory...

[Onebowonder]

What ratio do you recommend for trapping the back of a Hickory bow stave?  I've heard 2/3 to as much 1/2.  what variables need to be considered in the decision???

ALSO - is there a resource available that would give suggestions for how to trap different species of wood?

OneBow

[RyanY]

My last four bows have been trapped to 1/2 the width of the belly. I'm not sure how I came to that decision other than just going for it. David Dewey over on paleoplanet was able to calculate the degree of trapping he could do on a red oak bow he made that, I believe, was pictured in TBBV4 in the section on trapping. He'd be the one to ask how to determine the degree of trapping.

[Jim Davis]

Onebo,

Hickory is about 3.5 times stronger in tension than compression, so you can get away with having the back half as wide  as the  belly with strength to spare. There is little point, however in narrowing the back near the fades, since that part of the limb moves so little. The outer two thirds of the limbs are the best area for narrowing, but stop the reduction well before reaching the tips to make  sure there is plenty of wood left for the nocks.

I narrow the backs of all my bows, but have had two fail because I did too much where the limb was very narrow near the tips.

This happens to be white ash, but I have hickories made the same way.

Jim Davis

[mikekeswick]

This is a really under-used trick on tension strong woods. Certainly there is a weight saving to making the limbs like this but the fact that you are evening out the stresses the wood has to take just plain makes sense and when done correctly the results speak for themselves.
That picture of the ash bow is exactly how my ash bows backs end up looking. Ash is a great candidate for trapping and if also heat treated can make awesome bows. A lot of people don't rate ash and other tension strong/c ompression ok-ish woods but if made like the above example I think those people might change their minds about them!

[Will H]

What do you guys think about trapping the back on Osage?

[Jim Davis]

What do you guys think about trapping the back on Osage?

I'm interested in this answer too. The good old Forest Products Laboratories neglected to report the mechanical properties of dry Osage orange. The numbers for green Osage are pretty useless, since we don't use it that way.

Some new tests would be wonderful.

Jim

[RyanY]

My guess with osage is that it can probably be trapped but not to as great a degree as white woods because of its density. A good show of if osage can take trapping is looking at if we can make bows with a high crown and flat belly.

[Will H]

Ryoon, I'm confused as to how destiny relates to trapping?

[Onebowonder]

Ryoon, I'm confused as to how destiny relates to trapping?

I'm not the Ryoon, ...and don't play one on the Internet, ...but I'd like to take a stab at your question.

I'm thinking that density itself is not a direct relationship to trapping, but a tangential one. 

It is more related to the relative strength of the particluar species of wood in tension vs. in compression.  Is this wood better working in the back or belly of a bow?  If the wood is more of a tension strong back wood it would be trapped narrower on the back and wider on the belly.  If the wood is stronger in compression or a good belly wood, it would be trapped wider across the back and narrower in the belly.

As far as the Osage question, it is good in both tension and compression.  I don't know what the ratio of one to the other is, ...but that is the number that would determine which design for trapping would be most servicible.

What I'm still hunting for is a species by species reccomended range for typical and less typical bow woods.

OneBow

[Jim Davis]

Nearly all wood is stronger in tension than in compression. Notable exceptions being yew, cherry and red cedar.

And, wood has very little dimensional yield in tension before breaking like glass. Most of the bending elasticity is in compression. So, the best ratio of back to belly for a given wood only changes the weight of the limb, not whether the back does more or less of the work.

Narrowing the back by a third is a good safe and fairly efficient standard from which to work.

Jim