Hollow Back experiments

[Jan de Bogenman]

Spent a few weeks on making selfbows with a serious concave or hollow back. To see if it is possible to reduce mass and to gain speed. Given a concave tape measure, one would think so.
But wooden bows rarely work as you might think, so first let's see if it's possible with a serious cavity at all.
So far I have made three Hollow Back Designs (?) And they are not broken....yet.
The first has a cavity of about 3 mm, the third 6 mm.
One outcome is clear: Tillering such a bow is a thrill! With this thin lines on the back, you expect to hear that small "tsjick" or big "krack" every moment.
I will post some specifications later on.

[Jan de Bogenman]

First attempt: a hickory flight selfbow.
Lines are taken from the example described by Dan Perry in his article in, yes, Primitive Archer Magazine. (Thanks!)
60"ntn, 35#@24". Cavity 3 mm (0.12"). Width 1.5"@ fades 0.24"@ noks. Set 1.5".

[Jan de Bogenman]

Second try. A more conservative design. And in terms of dimensions and draw weight a copy of another, fairly new bow.
Too little material for conclusions yet. But compared to the twin brother, the hollow bow weighs about 10% less and is about 10 fps faster. But as said, I think it's not time for conclusions yet. Much is in the details, especially in speedtests.
Cavity 4 mm (.16") Both ash piramid selfbows, 70"ntn. 40lb@28". Width at fades 48mm(1.9"), at nocks 9 mm(0.35").
Out of curiosity about the results, and if she would hold anyway, I was somewhat hasty during tillering . Set is therefore somewhat larger than expected, but with about 1.5 inches it is still within the limits, I think.

[Philipp A]

Hi Jan, this is a very interesting design. I never thought you could violate the back to this extent and get away with it! The tiller is looking good as well. How does the bow draw? Is there any kind of let off when it flattens out? I am interested in trying this design as well once you have gathered some more results from it. Awesome that you are trying it for sure.

Cheers,

Phil

[Philipp A]

another question, is the belly flat or convex?

[Jan de Bogenman]

Work in progress: A selfbow made from the inner side of split elm.
Cavity about 6 mm.

A relatively short bow, length is 61". Widest point 5 cm (2"), nocks 6 mm (0.24").
Current state: about 40lb @ 25 ". I am aiming for 26"to fit my flight arrows.
Maybe it's time to draw one conclusion: the design is stronger than I expected!
And I think a deeper cavity and sharper edges are possible.

Sorry for the last picture, I was shaking a little.

[joachimM]

The grain of the back isn't violated, it's much like a board bow. But I gather it's very challenging to tiller.
But the principle, I think, is very sound and worth while investigating.
Jan refers to a metal tape measure: if you hold it concave, it takes more force to buckle than when you hold it upside down with the back convex.

The easiest way to make such a bow (and test the merits of the design) is from split bamboo poles, which are naturally concave/convex depending on whether you take the inside or the outside as the back.

I'll let Jan do the hard work first 

[Jan de Bogenman]

Hello Phillip,

I did not take any measurements for a draw curve yet. But I do not feel a "sweeter" draw. Given some experiments with bamboo and pvc I'd rather expect the draw to be rather "sturdy" and will not flatten out.......untill very sudden collaps.
I suppose the flattening out is thwarted by the concaving Poisson effect.

First build some more, than one for overdraw!!

The belly is a little convex with rounded edges. I am not sure yet this is the best design. I just follow my intuition and experience on this part. I need an engineer!

[willie]

Quite an investigation, and some nice bows,too.

I suppose the flattening out is thwarted by the concaving Poisson effect.

That would be interesting to measure, just how much shape changing is going on. One would hesitate to ask a guy to leave a bow at full draw long enough to take measurements, though.
Just curious what you might be asking an engineer? You might get more questions than answers from such a
conversation.

[Jan de Bogenman]

@Joachim. Actually the Ash and Hickory bows are board bows and the grain was violated moderatily in this staves. I have enough confidence in this woods for such a challange. The elm bow is made of a split log. And there are two little flattened knots in the high ridge on the back!!

[Philipp A]

Coming to think of it the design again it is very similar of that of a turkey feather or any bird feather for that matter. Two main spars on the back and the belly slightly rounded. That design is very efficient for being light weight and carrying high cyclical loads. In a feather there is a central groove but the middle is not as widely hollowed out as in your design. You are carrying all the tension on the two side spars. I think if properly executed you can very nicely balance the load on the back and the belly and avoid set. I have made a "feather bow" with a central groove on the back hollowed out into the spars but did not carry it all the way to the tip like you have, but that bow stayed perfectly flat and took no set. I had it thinned out a lot into the tip though. I think the way you do it is correct where you gently run out the hollow part into the tip just like in a feather.

I am looking forward to see the finished products!

Cheers,

Phil

[Jan de Bogenman]

Wow, made arrows for years and never saw feathers that way! Very interesting and a good example.

And Wille, thanks. Maybe an engineer and a nice 3D modelling program could come up with some quick answers on a optimum shape. And how many load the material takes on what place, compaired to a more flat design. And yes, very great chance of a windy discussion! But experimenting and guessing is nice too.

I have tried to see a change in the cross section while moving and holding the ash bow. Little to see. But thats the same with pvc tube and bamboo slats. They even tend to tighten, or close, it seems.
Will try to do some measurements.
Jan

[leonwood]

Jan, nice to see you here as well!
Great experiment if you ask me! The design scares the $#*^ out of me but somehow you make it work! Keep us posted on the elm version, I think you are on to something here.

[willie]

3d modeling sounds like fun, but I guess optimization would be a matter for working through the inevitable "trade-offs" that seem to be a part of every design process.

[joachimM]

when looking at the pictures of the drawn bow from above on the tillering tree, it rather reminds me of a crowned back. But instead, the high portions are on the sides, not the center. Theory suggests that crowned or trapped-back designs take less set (at the expense of risking tension failure) because they move the neutral plane towards the belly. So maybe the advantage isn't any different from a trapped back, unless the concave back flattens out during the draw.

As for grain violations in European ash: I've had a bow violently explode on me two days ago during tillering of a crowned back, on a knot that was violated ever so slightly. scared the hell out of me.