Mechanical Properties of Wood for Building Bows and Arrows

[willie]

If you don't believe that thick bamboo or Hickory lets weaker woods crush then present an alternative explanation.

Pat, an alternative has already been suggested by Dave.

I suppose you could take a sample that, say, was found to be strong in tension, and trap the back through various iterations.

Mark,
 bamboo and cherry would not be an easy combination work with, because of the highly differing strengths, but that does not mean bamboo is not a good backer, nor does it mean cherry is not good in compression.

[gfugal]

Tim Baker showed this with his flax backed pine right? Flax is very stiff (high MoE) pine is not so much. The result was a over stressed belly with tons of chrysals. Anytime you bend a bow the back undergoes tension and the belly compression. If there is tension that means the back is trying to stretch, if the bow actually bends, then it does stretch. But depending on the belly material it may stretch more or less. If the belly is less stiff the flax makes it compress more than it normally would thus reducing the amount it stretches. Vise versa, If the belly was stiffer it would make the flax stretch more. If there was no belly material then I guess you could say it doesn't stretch. But that's only cause flax isn't rigid, and super flexible in compression with practically no compressive stiffness, thus you can loop it or flex it however you want. But if you glue it onto something rigid, then it must stretch if you bend it like a bow, unless the belly material undergoes so much damage that it isn't rigid anymore. Styrofoam might do that, but even the most chrysaled wood still maintains some rigidity.

[gfugal]

Also I'll add that when we say stretch we don't mean visible stretch, we're talking about microscopic amounts less than a 1% change.

[Marc St Louis]

Mark,
 bamboo and cherry would not be an easy combination work with, because of the highly differing strengths, but that does not mean bamboo is not a good backer, nor does it mean cherry is not good in compression.

Bamboo is an excellent backing, for certain types of wood.  Good in compression is an ambiguous statement.  Good for what?

I bought a Bubinga board many years ago, a very strong and fairly heavy wood.  It was fine when backed with Ash or Maple but as soon as I backed it with Bamboo it chrysalled all over the place.

[willie]

Good for what?

Good for a lighter draw cherry bow I suppose, at least that what was reported in TBB.

Mark, do you think the Bubinga would have done better with a thinner or narrower piece of bamboo?

[willie]

Greg,
When Tim put flax on pine, he purportedly proved the combo is mismatched, but was it because he chose High MOE flax to put on low MOE pine or did he just put too much on? What about flax on ipe? or sisal on pine?
Interestingly, Joachim said in an earlier thread that flax "stacks". That might bear further investigation as it indicates some sort of work hardening might be occurring.

we're talking about microscopic amounts less than a 1% change.

I actually think that smaller amounts of strain means less hysteresis.   .5%  stretch on the back would worth looking into if it could be matched with an elastic wood of comparable strength on the belly.

[gfugal]

Less amount would help, but will it be enough? maybe. That's something to look into. I've wondered the same thing about bone. Supposedly bone has a similar, if just slightly less, flexiblity as wood. But it's really brittle making it poor in tension. Yet concrete is poor in tension too, but they use it in engineering all the time for it's compressive qualities, even in horizontal load bearing floors multiple stories up. Bridges are another example. If you optimize two materials for the tension and compressive qualities like rebar and concrete (in the right position and ratio mind you) you can manage to construct things that otherwise seem unlikely to work. Bone has a a high MoE and is excellent in compression. Reason tells me if done right it would make a good belly for a bow, but we haven't seen it done very successfully yet. I wonder if that has to do with the ratio's you mention. Maybe there been too much of it? Or maybe it was simply a design flaw. Bone is not nearly as flexible as horn, and maybe people used it as a horn substitute not minding to change the design.

[PatM]

Bone bellied bows have been done. For all intents and purposes antler is bone.

[willie]

like rebar and concrete (in the right position and ratio mind you)

My thought exactly. If we can learn more about tension qualities of the materiel's we work with, I think there is room for improvement. So often the limitations we work to, are on the compression side, and most of what we learn about bow wood is about compression. If we minimize compression damage and consequently, compression hysteresis, by design, I think we can begin to see more about what makes for a better back. I have been watching Badger make better bows with his "no set" tillering method, and think that it may be a practical way to observe back quality differences, and help answer the question, "What natural materiel's are the "Yew and Osage" of the tension side?"

[PatM]

Don't we already know that?  It's tempting to think something magical  or a combination of some materials will provide radical improvement but I think we are just putting numbers and trying to explain scientifically  what we already know.

[PatM]

Don't we already know that?  It's tempting to think something magical  or a combination of some materials will provide radical improvement but I think we are just putting numbers and trying to explain scientifically  what we already know.

 This material might be such an example but practically it's not an option.
https://en.wikipedia.org/wiki/Resilin

[DC]

There has to be more than just tension and compression stats. A steel backed, concrete bellied  bow would have great numbers but it wouldn't last. I think that for bellies we need a wood that will compress substantially and return without damage. How much it will compress we can control by how much wood we use.

[gfugal]

There has to be more than just tension and compression stats. A steel backed, concrete bellied  bow would have great numbers but it wouldn't last. I think that for bellies we need a wood that will compress substantially and return without damage. How much it will compress we can control by how much wood we use.

Right. Weight is a huge factor. Steel and concrete may be great for stationary construction projects but bows have to propel an arrow. The bone bows that have been done, like the inuit design of sinew over the antler, work for sure but both sinew and antler are heavy materials. They didn't really have wood to work with up in the tundra, but something tells me you could optimize things better if you reduced the amount of the material taking most of the stress (sinew and bone) and have most of the bow be wood still. That way you still get the benefit of the bone and sinew, while reducing weight considerably. But that brings up the point that if you build a wood bow well within it's stress limits then it should be lighter for it's draw weight than a composite bow of sinew, wood, and bone. Therefore a matchup utilizing bone would be most useful if it was incorporating stresses wood normally cannot take. Therefore a heavy war bow would best suited for this material, something with a draw weight over a 100 lbs. Likewise you see that with steel too. It doesn't make a very good bow material for two reasons, it's heavy and repeated bending ultimately leads to the weakening of the metal and it breaks. The few times it has been done successfully is in very heavy weight crossbows where the material can handle that extreems poundage where would couldn't and the draw length is short as to minimize damage done from over bending it.

[willie]

something magical................ will provide radical improvement

most would be happy with incremental improvements, I think
 
recombinent resilin, hmmmm. sorta natural, but definitely not primitive. OK for around the campfire, I guess. Be sure and post pics.

[PatM]

There has to be more than just tension and compression stats. A steel backed, concrete bellied  bow would have great numbers but it wouldn't last. I think that for bellies we need a wood that will compress substantially and return without damage. How much it will compress we can control by how much wood we use.

Right. Weight is a huge factor. Steel and concrete may be great for stationary construction projects but bows have to propel an arrow. The bone bows that have been done, like the inuit design of sinew over the antler, work for sure but both sinew and antler are heavy materials. They didn't really have wood to work with up in the tundra, but something tells me you could optimize things better if you reduced the amount of the material taking most of the stress (sinew and bone) and have most of the bow be wood still. That way you still get the benefit of the bone and sinew, while reducing weight considerably. But that brings up the point that if you build a wood bow well within it's stress limits then it should be lighter for it's draw weight than a composite bow of sinew, wood, and bone. Therefore a matchup utilizing bone would be most useful if it was incorporating stresses wood normally cannot take. Therefore a heavy war bow would best suited for this material, something with a draw weight over a 100 lbs. Likewise you see that with steel too. It doesn't make a very good bow material for two reasons, it's heavy and repeated bending ultimately leads to the weakening of the metal and it breaks. The few times it has been done successfully is in very heavy weight crossbows where the material can handle that extreems poundage where would couldn't and the draw length is short as to minimize damage done from over bending it.

  Bone bows have been built in conventional composite style.  Also steel bows have successfully been done in ancient and modern times.. Not just as crossbow prods.