Bamboo backed selfbow

[mmattockx]

I have been gluing bamboo backing on osage for a very long time, and the sum of the 2 is way more than a single piece of the same thickness.

Have you measured the difference? Much of the data I have seen indicates that bamboo and osage are similar in terms of the modulus of elasticity (MOE), so the combination shouldn't be much different than either on its own at the same thickness as the lam bow. If one is much stiffer than the other (higher MOE) then the combination would feel much stiffer.

I'm thinking that all we care about in a backing is tensile strength. Correct me if I'm wrong. I was looking at the Wood Database and it doesn't give tensile strength. Is one of those things listed there equivalent? Oh, if we only had an engineer

If only, huh?

The wood database calls the tensile strength the Modulus of Rupture. It shows an MOR of 11,020-24,450psi for bamboo and an MOR of 18,650psi for osage. That number is a fair bit lower than other data I have seen for osage, though. MOE for bamboo is 2,610,000-2,900,000psi and 1,689,000psi for osage. Again, that is low compared to other data I have seen on osage. If it is accurate, then bamboo would really stiffen up osage in a lam bow.

could well be the boo but if I glue two wood lams together with a little reflex, they become several times stronger than the original wood it seems...?

That could be the reflex. I was searching for info on gluelam vs wood beams and there was a few comments that it was because the gluelams are arched.

My experience with reflex is limited, but so far I would agree that reflex seems to really stiffen up the limbs out of proportion to the change made/added.


Mark

[Selfbowman]

Ok got y’all thinking.

[willie]

So it’s bad to have the Osage less than one third the thickness of the limb. More set or what?  Arvin

More set if the bamboo and osage differ significantly in their stiffness. I do not see any reason not to feel comfortable with 1/2 and 1/2 in thickness before thinking ahead to different tillering options.  Side tillering may be an option if it's not in an area of the limb that might want to twist on account of the tips being further back.

How thick is the bamboo just before the hook compared to the thickness of your self bows with the shorter hooks (measured in the same place)

[Selfbowman]

When you get it figured out . See how this trap stands with your thinking. 1-1/2 at fades is the width=
1-1/16 at the top of the trap. Mid limb 1/1/16 wide top of trap7/8. Tips 3/8 wide 1/4” top of trap. Does thickness matter? This is as perfect diminishing mass as I can think of. If you pros do . Do we get the tips bending first on the tiller? Arvin   

[Selfbowman]

Pic

[Badger]

DC, You are allowed to build up the handle on a simple composite bow, this would include a power lamb as long as it doesn't go past the fades or exceed 12" length I think. I need to double check on the length.

[Selfbowman]

Ok trapped for now. It will bend like a hundred pounder now. the pic is pretty good representation of the limb and the bamboo thickness. So I could take the Osage to about 3-1/6 and that would put the two materials pretty close on compression and tension. Arvin

[Selfbowman]

Pic

[mmattockx]

When you get it figured out . See how this trap stands with your thinking. 1-1/2 at fades is the width=
1-1/16 at the top of the trap. Mid limb 1/1/16 wide top of trap7/8. Tips 3/8 wide 1/4” top of trap. Does thickness matter? This is as perfect diminishing mass as I can think of. If you pros do . Do we get the tips bending first on the tiller? Arvin

You should be trapping as a percentage of width and thickness, not random amounts. Say, reduce the back width to 80% of the full width and then taper down maybe 70% of the side depth. Don't trap to a sharp edge on the belly, make sure you leave at least a bit of a flat on the side. To get a consistent back width, measure every 2" along the limb and mark the reduced width you want on the back  and mark how far down the side you are going to go. Then connect all those dots with a line and rasp up to that line.

This keeps the trapped proportions the same full length of the limb and doesn't result in stronger and weaker areas. Being consistent with the trapped cross section will make your life easier when tillering.


Mark

[Selfbowman]

Thanks Marc!!! I will read this probably 10 times before I make another stroke. Tip overlays going on.

[Selfbowman]

Pic at fades thickness.

[Selfbowman]

Ok Steve what should this  bow s draw weight be. 67”,9” handle. 1-1/2 at fades 3/8 tips you know the design. Thanks. Arvin

[sleek]

Boo is glued to the SELFBOW! 
Arvin

Hey, we gadda move this thread now, he is CLEARLY breaking all the rules!!!   

[Selfbowman]

No power lam😁

[Eric Krewson]

Narrow bamboo, I don't use it, I cut out the exact bow pattern out of my bamboo then thin and flatten the bamboo until the edges are 1/16", I use a belt sander with a 36 grit belt for the process.

As you can see my bamboo backing is thin, I cut the slats from 4-5" diameter culms that I harvest from a patch across town. It is mandrake, the rind always looks green even when it is completely dry. The bamboo is tan under the rind.