Trapping the back, how and why?

[dane lund]

I'm currently working on an Oregon Ash stave that I got from our buddy Carson.  He also turned me on to a posting of an Ash bow with side nocks that he did. In his explanation, he mentioned that he  "trapped" The back of the bow.
I'd like to know what that means, why and how it's done?.

Any help?  :embarassed:

[Newindian]

Trapping refers to a trapezoid crosction meaning the back of the bow Is narrower then the belly , it is done with materials that do well in tension but not in compression so that the bow doesn't fail in compression.

[bow101]

Trapping the back also reduces weight.  Apparently makes the bow quicker from what I understand,  have yet to try it.

[dane lund]

Carson had mentioned that it had a trapezoidal shape, but I wasn't sure if the back or belly was the narrower side of the trapezoid.

[Crogacht]

I understand it the same as Newindian.

You can trap the belly or the back depending on the properties of the wood and where you want to relieve stress.

I don't know how accurate my understanding is, but in my head, it seems like trapping the back or belly would move the neutral plane slightly, which would change how the stress is distributed.

[Jim Davis]

Here's the most important thing to understand about the stresses wood withstands in a bow.

Almost all wood is 3 to 4 times stronger in tension than compression. The exceptions are cherry, red cedar and probably some others.

The next most important thing to understand is that most wood has almost no elasticity in tension. Yew is an exception. In a tension failure, there is almost no stretching before rupture. Nearly all the elasticity in bending is in the compression wood. So, narrowing the back of the bow leaves it plenty strong enough for the work of tension and does lighten the limbs, allowing a faster return.

On a related concept, backing a bow with hickory or bamboo helps prevent problems with grain runout, but the weakest part, before and after backing the bow, is the belly.

Jim Davis

[DavidV]

This is hickory I did a while back for perspective. It's only mildly trapped and the edges could be rounded more.

[Badger]

The slight crown we naturaly have on a good stave is a perfect method of trapping just naturally. Trapping kind of mimics that.