Tiller Question?????

[hook]

I have this little (43.5") osage bow started for my grandson. I want to end up about 25 LBS at 20" or so. its roughed out and rawhide backed. It was bending OK on the floor tiller-on the strong side, so I put a tiller string on it and pulled it a little just to see where the limbs were working. Any way long story short I needed to remove more wood from the limbs. I got this picture in my head that showed the growth rings kinda diminishing down the length of the limb with nice pretty V's pointing towards the tips. I started tracing this image on to the belly of the bow with my scraper when (maybe too late) I discovered that in doing this I have some concavity in the belly. It might not be too late to take it out but....IS THAT BAD?
By the way, after approximatley 12 attempts I have not made a bow that didn't break
Does the belly have to be flat or should it follow the grain like the back?

[turtle]

I wood think that concave would make the outside edges of the belly do most of the work. If it was me i would try to get the belly flat.

[Pat B]

How are the limbs bending? If they are bending evenly and together that's all that matters!

[Jude]

I remember the first self bows I had ever seen.  They were made out of beech and the makers had misunderstood the meaning of chasing a ring.  They had scraped the belly concave, down to the inside of a single growth ring.  The bows were pyramid profile, about 2" at the fades, and shot really nice.  They were strung, so I don't know how much set they had, but it was likely quite a bit, since the outer edges of the belly were doing most of the work.  It's probably no different than having a rounded belly and making the center do most of the work.  Both should cause the bow to take some set and relieve some of the strain from the back.

Julian

[randman]

I have been wondering the same thing because I have a couple of staves that have a concave shape on the belly as a result of being wide section character staves. My thinking is that the concavity would actually provide more strength while maintaining consistent thickness across the width of the widest part of a pyramid bow with a crown in the middle of the width. The same way an arch would provide more strength than a flat span over an opening in a wall (or a crown on the top side of a floor joist). The bow is not finished yet so I have yet to test my hypothesis. I am also basing my hypothesis on the paragraph on the Poisson effect in TBB Vol4 where the back of a rectangular piece of wood achieves a slight concavity when bent while the belly opposite achieves achieves convexity to match the back's concave cupping. Tim Baker postulates that this is an argument for creating a slight concavity on the belly side so that at full draw the belly would be flat again to achieve maximum compression resistance. I'll let y'all know whether it works or not. I'm considering it part of my apprentice training

At any rate on a narrow 25lb bow I wouldn't be too worried.

[hook]

I'm gonna leave it concave for now and work the other limb down. Then i'll put it on the pully and like Pat B said if its bending ok I'll leave it alone.
sound like a plan?

[George Tsoukalas]

It is ok to leave the belly on osage rounded a bit. For future little tyke bows it is ok to leave them long so they can grow into the bow. You are skirting length limits now and that maybe why you've had some failures. Double your draw and add 20%. That's 48 in for your grandson's bow. Let the backing be insurance and not absolutely needed to keep it together. Later on you can test the limits. Jawge