Hollow Back experiments

[ohma2]

Ive got no business in such a post like this cause i like a very simple self bow.that said i really like where you guys are going with this.if it werent for tinkering and technology we wouldnt be typing and posting about what we like to do.

[DC]

I think it's a neat idea but I have no idea how it is staying together Are the edges of the back one ring?

[Jan de Bogenman]

Joachim, its about mass in the first place. I suppose that if a trapped back is more economicaly in mass per strenght, all tape measures would have had that shape.

[Jan de Bogenman]

Ive got no business in such a post like this cause i like a very simple self bow.that said i really like where you guys are going with this.if it werent for tinkering and technology we wouldnt be typing and posting about what we like to do.

Haha thats exactly how I feel about bowmaking myself. For 15 years now I am making bows and a simple selfbow of some local wood, made with simple handtools, gives me most satisfaction. But this hollow back design was on my mind for some time somehow and was triggered bij the hollow belly bows. Dont know yet if it's worth all the effort. But I think it can be, and it is big fun already. And like all wooden bows, big chance it has been done already, sometime, somewhere!

[Jan de Bogenman]

I think it's a neat idea but I have no idea how it is staying together Are the edges of the back one ring?

It is hard to understand what you see happening on the tillerboard. I expected it would explode long before half of the draw length.
For the ash bow, the cross section is about 15% higher than in a flat limb with the same width. Due to the shape there is less material, but this material is further away from the neutral plane. So maybe tension is a little less.....I guess!
And other forces are working too. If you bend a half bamboo pipe this way, it will suddenly flatten out and split, not break. Now, bamboo is a wonderfull material with different characteristics than wood, but maybe something alike happens in this bows.

In the ash and hickory bows, the back does not follow the grain for 100%. In the elm, the ridges follow one ring. But has two little flattened knots on the ridges. The staves were originally selected for 'normal' flatbows.

[Jim Davis]

I like the looks of this design.  I also like the fact that you have removed mass from the back. Wood is 3 to 4 times as strong in tension as compression, so the back is the right place to remove wood.

I am sure the woods that have interlocking grain are better suited for your design. A limb that tapers has the grain running off the edges of the back at a small angle. The edges are bearing almost all the tension strain in  your design, and with hickory or elm, can probably survive.

The tiller looks very good to me.

Here is an extreme experiment I did with a miniature bow to leave the middle of the back doing the work. The strip down the middle has no grain runout until near the tips. It worked very well, but I have not scaled up the experiment.

[Pat B]

DC, I think it like a decrowned bow. as long as the grain lines run parallel you should be OK.

[Jan de Bogenman]

Jim and Pat, thanks for your input.
I agree that the grain is probably more important than in a 'normal' limb. But I did not want to try this with material that differs from my other bows,  to be able to compare.

Nice experiment too Jim. Not everything can be scaled proportionally, but this should work I think. Bamboo slats can do great work here too?
Thinking of it, maybe worth trying two narrow slats off bamboo along the sides.

Jan

[willie]

Jim
am looking forward to seeing the up-sized model when it happens. getting just the "right" amount of backing might be telling.

all tape measures would have had that shape.

Jan, tape measures are designed to be bent over backwards and rolled up inside the case, too.

[Jan de Bogenman]

Willie, very true indeedindeed!
Maybe try to store my bow that way.

[Pappy]

I once made an Osage that had that concave naturally in the back, the only problem is that the rails on either side do all the work unless you leave the belly thicker to off set the concave. It worked fine for a while but after several hundred shots the rails couldn't take the strain and lifted a splinter, since then if I have one with the concave back [Hollow] I leave it thicker on the belly so the limb is the same thickness all the way across. Guess that would kind of negate what you are trying to do by reducing mass.    Looks cool , hope it last 
 Pappy

[wildcat hunter]

  Just thinking about your bow design. In theory ( mine ) it should be a durable design. The "area" of the back has more square CM's than a standard  tillered back. Like a standard 1 1/2" back to a 2" back, more area in a wider back.  If you could measure it somehow following the curves - up and down and flat. Would be interesting to compare the area to a standard bow back of equal pull.  Then like you are doing compare the durability.
I like it.

[gianluca100]

Years ago i had the exact same experience as pappy, it was an black locust flatbow. It shot very good unless the edges gave in and splintered.
I also feel that the reduction of weight in the reduced width of the back works as in a trapped design. Nevertheless perhaps the "tape measure effect" could play a role. Seems quite difficult to find out. though.
Maybe you should start to make samples of crossections (a short length), bend and measure  how they behave (weight, bend strength, poissoning etc.). So you could test multiple cross sections without having to build alot of bows...

Very intersting topic, thanks for sharing an doing the work 
gian-luca

[Selfbowman]

Interesting for sure. But the tabe measure bends to a point then brakes over when to much tape is is let out. I think it will work unless over strained. I expected it to split down the center. Arvin

[simson]

I dunno think this is a HLD bow, because there are not the advantages like on a hollow belly. But that groove in the back is mass saving for sure.
I believe it is no good idea  to groove out the back and violate the tension fibers, better search for a natural concave back stave.
The work you invested in that concave back would be much more useful in an concave belly (and you get the same amount of mass saving).