Trapping

[DC]

I'm new to trapping. Do I trap the bow as soon as possible, like along with shaping the front profile? Or is it OK to do it later? Do you lose much DW when you trap, like is it too late to trap if you've only got 5# to go?

[Selfbowman]

All good questions DC. If m in your boat an need info. Arvin

[Pat B]

I like to trap as I'm building. Trapping later will reduce draw weight some but can also change tiller. The purpose for trapping is to give advantage to a weakness, ie. to decrease tension in a weaker compression wood like with hickory. Hickory is very strong in tension but a little weak in compression so by narrowing the tension strong back you relieve the stress on the compression side. I think with wood bows you only reduce the back width while trapping and it is for practical reason. In glass bows it can go both ways but only for aesthetic reasons.

[DC]

It does look kinda cool. I've done it now and I'm just about to brace.

[Morgan]

I’ve wondered the same thing. I have read a lot about people trapping backs on bows to lessen the tension of the back. If this works, then how is trapping a good thing and high crown a bad thing? Haven’t been able to wrap my head around that.

[DC]

I think the high crown thing depends on the wood. As near as I know you only trap a strong back. I have made a bunch of Ocean Spray(very tension strong)with very high crowns and had very few problems and I can usually write off the problems to something other than the high crown. So you may want to decrown a wood that isn't so tension strong.

[Pat B]

On a high crown bow most of the tension is down the center of the crown. Of a trapped backed bow the stresses are(should be)evenly distributed along the working limb.
 DC, be sure to round over those edges for your safety and the bows.    Very little of the sharp edges do any work, just in it for the ride.

[DC]

Yup

[mmattockx]

I'm new to trapping. Do I trap the bow as soon as possible, like along with shaping the front profile? Or is it OK to do it later? Do you lose much DW when you trap, like is it too late to trap if you've only got 5# to go?

I would do it as soon as possible, since the goal is to balance back and belly forces. Waiting until well into tillering just risks having a bunch of set occur before you finalize the limb cross section shape.


Mark

[DC]

Mark, is there a way of telling how much trapping is too much? Is there a point where the slope is too shallow for the belly width to be effective. If the back was 1/2" wide and the belly was 1" wide and the limb was 3/8" thick would the outside 1/8" of the limb be doing anything?

[bownarra]

People always bandy these terms like 'strong in tension'. What does that mean? What we are actually talking is the woods resistance to being stretched or compressed. Elasticity comes into this as well of course. Eg where is the woods elastic limit.
DC make some mini test bows with the wood you are using. Overdraw tehm to induce a good amount of set. Then turn the bow on its side and cut it in half on the bandsaw, Now see what the back piece does and what the belly piece does.
Trap until you get it soi that both pieces are even when cut apart. Then you have a balanced limb.

[willie]

Then turn the bow on its side and cut it in half on the bandsaw, Now see what the back piece does and what the belly piece does.
Trap until you get it soi that both pieces are even when cut apart. Then you have a balanced limb.

Equal deformation back and belly does seem like a good way to balance the materiel properties.

Don, are you considering to trap a lam bow or a self bow?

[willie]

Morgan,

you can't actually lessen the over all tension on the back, but with trapping, you can change the "unit" tension, as in pounds per square inch.

For two bows of the same weight pull, one rectangular and the other trapezoidal in cross section, The one with the narrower back will actually have a higher unit tension.

As bowanarra points out, the terms  "tension strong" and "compression strong" (or weak) are a bit nebulous, as the properties are relative to the other side of the bow. Tim baker makes use of such terms in the bowyers bible, and they serve well enough to get the bowyer considering the merits or shortcomings of their materiel's, but usage of such terminology without stating quantities, does get confusing.

[mmattockx]

Mark, is there a way of telling how much trapping is too much? Is there a point where the slope is too shallow for the belly width to be effective. If the back was 1/2" wide and the belly was 1" wide and the limb was 3/8" thick would the outside 1/8" of the limb be doing anything?

I saw one post from years ago where David Dewey made an offhand comment about not trapping too much or you can run into stability problems. He did not elaborate on that, though. My experience with structures says stability should not be an issue when removing wood from the tension side of the limb, but I haven't tried it to see. I can see that it would be possible to get such an odd shape section that it loses much of its torsional stability, but I think you would end up with failures in the wood well before reaching that point.

From a mechanics of materials perspective the slope doesn't get too shallow for the belly to work properly until you start having buckling failures due to the thin edges failing in an elastic fashion. I wouldn't expect to ever get close to that with wood, but it is always wise to leave some thickness at the widest part of the belly and not go to sharp edges.

People always bandy these terms like 'strong in tension'. What does that mean?

To me it means that a piece of wood when bent will fail on the compression side before it fails on the tension side, meaning the maximum allowable stress is lower in compression than tension. Trapping allows you to shift the neutral axis towards the belly, lowering stresses on the compression side and raising them on the tension side. This lets you bend the limb farther (or bend a thicker limb the same amount) before you get compression failures. If you were to perfectly balance it then you would have chrysals just starting to form as the back blew up.

Then turn the bow on its side and cut it in half on the bandsaw, Now see what the back piece does and what the belly piece does.

The only thing to note with this experiment is that you should cut the limb apart on the neutral axis and not on the center of the thickness. I'm not sure if you could ever get equal deflection on the two sides because tension doesn't seem to produce any set while compression always seems to. I would say that a better measure would be to trap test pieces and then bend them to failure. Trap more and more until you see compression failures occur just before the back fails. I would always keep it a touch on the safe side and stick with compression failures occurring first simply to avoid blowing up a bow while holding it.

Don, are you considering to trap a lam bow or a self bow?

That's a very important and relevant question.


Mark

[DC]

I'm trapping a Boo backed Yew. I've done it before and it seemed to help the speed. I kind of attributed it to less mass in the limb. The Boo will easily handle the strain so why not get rid of some weight. I'm just curious as to how much to trap it. Bownarra's idea sounds good except I'm too cheap to sacrifice the amount of wood/boo that it would take I'd rather make a bow out of it.
As far as the words "tension strong", what other words would you suggest? We do know that some woods are stronger in tension than others. I wouldn't know how else to say that.