"Trapping" a Backed Bow

[tom sawyer]

Trapping as a means of getting more belly wood working against a backing, sounds simple but isn't so easy to accomplish in practice.  We all probably round or bevel the edges of our boo backings, this seems like a prudent practice and I suppose it helps a little as far as reducing the strength of the backing.  However, an actual trapezoidal cross-section is going to present some problems both in execution and in other practical aspects.  Trying to bevel the sides of a bow limb to a specific angle over the whole length (or even just the middle third) is hard to do and keep it even.  This is one drawback.  Plus this results in a very sharp edge on the belly side which I think would be prone to splintering.

I guess if you can execute a really oval cross-section where you bevel your backing so it ends prior to the widest part of the oval, that might be a decent cross-section as far as reducing the backing/belly ratio.  But something like a 30 degree bevel on the sides of a trapezoidal cross-section, while accomplishing something in theory, isn't really the best idea from my perspective.

I think the better idea is to really get the backing super-thin so you no longer have a maximally-strong backing that puts the neutral plane right at the glue line.  You are flirting with being too close to the nodes this way, but its seems to me to be the approach that is working best for people.

What do you all think about that?

[Dano]

Getting the backing the right thickness is prolly the best way to go, but a trapped back looks so cool.

[Gordon]

I read somewhere that the power fibers lie just under the surface in bamboo. "If" that is true (and I don't know that it is), then would it make any difference how thick the bamboo backing is?

[Dano]

If that's the case, the extra thickness in the boo is waisted, might as well make up the difference with belly wood, and make the boo as thin as ya can. Course all this depends on the choice in belly wood.

[Slivershooter]

Dean Torges, in his video Hunting the Bamboo Backed Bow, does a good job of explaining where the power fibers are and how to distinguish them from those that are not as powerful.  According to him the power fibers are most often close to the outer part of the bamboo but the amount and thickness of them do vary.  I usually trapezoid the back of my BBO’s to avoid overpowering my belly wood.  The second BBO I made overpowered the Osage belly (I actually got lucky and saved it with a patch) so I started thinning the boo more and trapazoiding (new word) the back and haven’t had any problems since.

[kra]

To bevel the belly in a way, that you get a wider belly than the backing is, will be, from my experience, the savest design.Rhen you can take a backing from a smaller bamboo pole (8").

One exception, possible for backings from thick poles,  may be to cut down the backing so that there is no visible edge of the bamboo when the bow is finished. From the forces point of view, this can be seen as the same as describes before.
And if you have a strong wood for the belly as it is with good osage and a backing from a thick bamboo pole (10"), you can also go with a normal design (no beveled sides) and a backing approx or a little more than  1/20" thick at the side.

The oval cross section I do not like so much for this bows from performance point of view, as it implies is some "dead mass" in the limbs side which may slow down the bow.

Thats just my impression from sonme, but not tooo much bows.

[Rich Saffold]

It's not that hard to do Lennie. It just takes a good scraper and an eye to get the edges rounded enough   There's a couple reasons I and others do it.

The first is to keep a thick backing from being too strong relative to the belly..And with bamboo, a thicker piece has a lighter section inside the power fibers and this can act very similar to a middle lam. 

The second is you can build limbs with lower limb mass using this type of cross section, and this translates in to a faster, yet thinner bow.

 Do I use this on all my limbs? No. mostly on bows where the primary emphasis is speed, and when using a lighter belly wood..

Rich

[bobnewboy]

Hi Guys,  this all sounds very interesting, but do you have any pictures of a "trapped" back?  I might be a dummy, but I'm having a few problems getting a clear picture of exactly what you mean in my head ....If one of you could perhaps draw a quick diagram of a cross section of a "trapped back" that would help a great deal.

//Bob

[Dustybaer]

       back
   ___________
  /__bamboo __\
 /                      \
/____osage_____\   it looks kinda like this

      belly

[bobnewboy]

Hi Dusty,  thanks for that. 

I can see where the trapezoidal name comes from now. I could see that having the BB thinnner at the edges is a good thing, and in my case this happens anyway when I round the backing.  While rounding the backing I also round the belly edges, and I s'pose this brings my limbs into a slightly more lenticular section (but not by much).  However I am always rather dubious about doing too much to a BB-backing, because those splinters would be really nasty, and not just for the bow 

//Bob

[Pat B]

Lennie, On a few BB bows I've made I left the boo too thick and it overpowered the bellies. I trapped the boo and left the belly proud along the sides. The cross section of these bows was a rounded trapezoid rather than a "squared off" trapezoid.    Pat

[mullet]

  I do the same as Pat and round the belly. When I backed the last Ipe bow I thinned the boo as thin as possible because of the advice on hear about thow hard it was to drop the full draw weight.

[Jesse]

I re did a hickory selfbow and put ipe on the belly. I trapped the ipe on the belly side drastically and it turned out real good.