About Bamboo backing

[arachnid]

Hi Guys.

When you back a bow with bamboo you get the bamboo to a 1/8" thickness.
But, since the bamboo is crowned, you actually get 1/8" thick crown and a very thin edge (maybe even 0 thickness).
Is this a problem? Does the tension spreads equally along the the entire bamboo back?

[Pat B]

That is exactly what you want 1/8" at the crown at the handle and a knife edge along the sides.  You can taper that from 1/8" at the crown and a bit thinner at the tip, but that part is not necessary. In most cases a boo backing will limit the bows width to about 1 3/8".

[bushboy]

I think most folk use moso boo which is larger diameter @ 4" so the crown will be a lot less.i have only used what I think is Tonkin that grows to under 3" but is a stronger species I think? A 2.5" pole should make for a 1-1/4" with a knife edge.i think if you backed a 20# bow it will be a 60# coming out of the gates and very compression wood is a must for such a narrow design

[Springbuck]

  Not a problem.  It's kind of like having a crowned stave of strong wood like elm or hickory: the crown will take it.   Bamboo is very, very tension strong, unless the fibers are cut into much at all.  The edges can peter out, and this is a good reason to use larger diameter bamboo, because super thin edges then have to be radiused more, and splinters can lift, etc...

But, just like trapping a limb can help balance forces between tension and compression, so can a crown.  But, not too high a crown, right?  If you have a very high crown, then the thickness at the middle is grossly and abruptly much thicker than the rest of the limb, and concentrates the tension/compression forces more than is good at the highest part of the crown AND the middle of the belly opposite the crown. The rest of the limb, the sides, are doing very little.  So get the widest bamboo possible.

Also, Badger, back in the day started leaving his bamboo thicker than 1/8"  Like 3/16" or almost 1/4" depending on the belly material and bow length.  I started doing it, and found benefits.  You need less belly material, for one.  It's less work, for another.  The total weight of the limb can be less.  And I have an easier time dealing with the nodes if they are raised like in most moso bamboo slats.  If the backing is 1/8", the nodes might rise 1/8" above that, creating a big difference in thickness, which then has to be dealt with while tillering, like a big knot on a self bow, and makes it harder to get the bamboo closer to even thickness.  If you leave the boo a little thicker, it evens out easier.

All my first dozen or more laminated bows were WAY to thick to start with.  I didn't know what I was doing, and was paranoid of not having enough.  After thinning a 1" thick total glue-up of bamboo backed ipe or bulletwood with hand tools down to a snidge over 3/8" thick on a couple 55 lb R/D bows, I figured out that a 3/16" backing and 1/4-3/8" slat on the belly will make about any weight flat bow or R/D bow you want, esp with a good power lam and or tip wedges (that's advanced clas, I know.)

[bushboy]

That being said with the right materials and design, nothing shoots faster than a bamboo backed bow(mother nature fiberglass)save a sinew backed composite of sorts.

[Badger]

   Something that I have never really understould but always wanted to know is how much the back of the bow is stretching compared to how much the belly is compressing. We always think of the nuetral plane being in the center of the bow. If the back is not stretching then the nuetral plane would be right under the surface of the back.

   A real simple test that I have always wanted to do requies a simple jig and some accurate measurements. If you took a 1/2 circle and wrapped a strip of wood around it and then measure how much longer it got on the outside and how much shorter it got on the inside you would know exactly what part of the bow was doing what. It would be cool to compare bamboo, hickory, maple, ash for instance against various belly materials we use such as maple, osage, yew, ipe etc. I might make this one of my retirement projects. I will add it to my bucket list.

[arachnid]

That being said with the right materials and design, nothing shoots faster than a bamboo backed bow(mother nature fiberglass)save a sinew backed composite of sorts.

I always heard preises on BBI so I really want to try it. That and the fact that I had a real hard time
finding straight grain white oak for backing strips.

I have a friend that want to make a long flatbow, BBI 70" long and glue on siyahs on the tips.
I told him that it`ll make the tips heavy so the bow will be faster if he glue the backing on with reflex.

Which of the designs will be faster?

[Badger]

  A 70"bow with siyhas glued on wouldn't really give the recurves a chance to open up much if he has a normal draw length. I would go more for a little reflex and thin tips.

[arachnid]

About the glue, can a bamboo backing be glied up with a regular wood glue (I use Elmer's for all of my bows with good results) or do I have to use epoxy?

[Pat B]

I've used all three of the TB glues and Elmer's Glue-All with hickory and boo backing, all with good results.

[Stixnstones]

TBII n TBIII are the glue's for me