Hollow Back experiments

[PatM]

Check out the old bows on this page.
http://www.bow-international.com/features/traditional/bows-over-the-borders/

[sleek]

It seems to me, that you wouldnt want a bow with a curved belly or back to flatten out. It seems to me that would cause it to split down the middle and try to become 2 bows rather than one. It would be likely to tear in two unxer tension, sending cracks up the spine of the limb.

[willie]

It seems to me, that you wouldnt want a bow with a curved belly or back to flatten out.

if it bends a couple of different ways, can store more energy?

I ripped a piece of plastic pipe and tested the ripping in a spine tester. crown up bent more than crown down, (with the same weight). Doesn't this mean energy storage can be improved with a better or different crossection shape?

[Philipp A]

@sleek, I think you are correct with your assumption, except that you would never bend the bow so much that it would split lengthwise in the centre. I have watched a 4 part series on how to make a River Cane quiver on Youtube and it was very interesting when the presenter bent the river cane halves (crown out) that make up the spines of the quiver for the bottom part. They split lengthwise in the centre after having been bent so both ends were up and formed the rounded bottom. It would represent an extreme draw on a HLD bow and you can nicely see how the river cane half flattens out when bent just before it splits.

[sleek]

Here is the knowlege i am going on to form my opinion. With heat straightening osage, when i am taking out twist, if i concetrate on to small a spot, i have opened up cracks simply by twisting the wood to correct it by simply clamping it to a board flat.  These are usually where the twist is in one spot on the limb.  I dont know how it will play out when spread out ober the entire limb length, but with a curve flattening on the entire limb, it seems it would still be likely to split and all at once and at full draw. As a safety measure ud wrap sinew in a few place up the limb to hold it together shoukd that happen. And it may not, but there us a point where it will, whether the curve of the hollow must be deaper, bow must be shorter, draw longer and weight hevuer, idk, this is all new, but there is a limit. Id reccomend finding it to learn more if anyone had a mind to.

[willie]

I took some more measurements on the half pipe. when bent in the hollowed belly direction, I can see the flattening effect. But when bent in the hollow back direction, the cross section shape does not change.

[sleek]

I took some more measurements on the half pipe. when bent in the hollowed belly direction, I can see the flattening effect. But when bent in the hollow back direction, the cross section shape does not change.

That is interesting. Especially because id expect the hollow back to want to curl up rather than flatten out.

[Jan de Bogenman]

A little test. Two hollow back bows. With a thin thread around the limb. 3 persons where asked to feel the tension in the threads. 2 of them do not care about bowmaking and that is good for once!
Unonimously conlusion: the tension decreases when the limbs bend.
This seems to indicate that the limbs try to curle up. Probably forced by the Poisson effect.
When bend severely, the limbs will be forced to flatten out (Poisson is loosing the battle) but that point is beyond the draw lengths of these bows. Anyway the first reaction is to curl up.

[Jan de Bogenman]

Check out the old bows on this page.
http://www.bow-international.com/features/traditional/bows-over-the-borders/

GREAT!! Love the Scotties! And there really is nothing new under this sun.

[willie]

 

Anyway the first reaction is to curl up.

I expected the same and with an easy bend, it seemed it was curling up some, but with a severe bend, it seemed to be right back where it was when straight. Not enough movement to be conclusive, and my test was less than optimal, measuring a half round of plastic pipe with not so precise calipers. One thing that that was apparent was lots more change in width when the hollow was to the belly.
It will be interesting to see how these bows shoot. And how much they weigh. Have you read about the mass principle? Badger wrote it in TBB4, and it can be a practical indicator of efficiency.