Steam bending bamboo recurve?

[DC]

Strange thing about Bamboo. If you bend it with the outside as the back it will take set. If you bend it with the inside as the back it doesn't take set. I tried it with an offcut.

[nabiul]

The inside of the bamboo is soft like white woods while the outside is like true hardwood. All of the useful fibers are within like an 1/8th of an inch deep from the outside and they overpower the chalky soft inside fibers causing set.

The thing is the curved cross section makes for a really ugly bow with the inside of the bamboo as the back, unless you have really thick bamboo that can be worked flat without losing poundage.

[DC]

Would you rather have an ugly bow that shoots well or a pretty bow with a bunch of set?

[nabiul]

As the craftsman I'd take it as a challenge to find a way to have both. But set doesn't mean a bad shooting bow, all olympic recurves have a ton of deflex which is 'hidden' by the recurved tips and they shoot just fine. If you think about it, deflex and lower initial string tension should make for a smoother drawing bow so I don't totally see it as being a negative thing unless the set straightens out when the bow is left unstrung.

You also have the difficulty of attaching a handle and doing proper fades when using the bamboo in reverse; you'd probably have to make a handle that sticks out in front like on an assyrian style bow. Lots of things to try that could keep one busy for years to come.

[nabiul]

Well I took the bow out to the range and it shoots like crap; like a 20-25lb bow even though it was near 40lbs at my draw. The set just kept increasing as I was shooting until there was very little difference when it was unstrung, but it straightened out again when left that way. The cast is terrible, all of the work was being done after 28" when it stacks rapidly so small differences in draw on each shot caused wild variations in speed. The handle was awful too, the curve kept rocking in my palm leaving no consistency on placement between shots, which I thought might happen as the handle felt more comfortable to hold backwards while I was making it. I think the temperature difference between my cool basement and full summer weather also played a part in how wimpy it felt.

Lots of lessons learned and I've already started on my next bow; I'm going to try what I said in my last post with the bamboo bending in reverse with the handle glued onto the front of the bow. Basically the same as my first bow but held in reverse and no pyramid taper this time to try and get more poundage out of the bamboo.

[nabiul]

Ok so I've got bow#2 in glue up and have some reservations about bending the bamboo in reverse. The handle is on the front of the bow so when drawn there will be a huge peeling force along the glue joint and I don't think my hemp wraps are up to the task of stopping it from splitting the joint or the wood itself. Also Tim Baker's article mentions to tiller on the back when using bamboo in reverse, but how do you deal with cutting across the layers of fiber?

I think I have a good heat treat on the limbs this time; I did them in the oven and haven't glued on the siyahs yet so it's not too late to make it a conventional horse bow. What do you guys say, glue it up in reverse and try to make it work just for the sake of acquiring experience?

[nabiul]

Welp I guess I'm seeing this through to the end. I'll be sure to take pictures when the handle rips off and takes my fingers along with it.

[nabiul]

So far so good, pulling 28-29# at the position in the picture, the bamboo feels very light and 'weak'; it's not like wood at all. No signs so far of taking any set. I screwed up and misjudged the thickness of the left piece, a hinge has formed where I flattened it to glue on the handle; it bends right into the joint as I expected. I wonder if wrapping that part would help or would it just disintegrate over time from flexing?

[DC]

If you are at 30# where you are just don't do any more "wood" removal in the fades where it's thin. As you remove wood from further out it will take the strain off the thin part and it may(should) even out.

[nabiul]

I was doing that, but then I got greedy and tried to see if I could get more than 30# out of this bow. The back fibers let go right at the hinge at around 33-34lbs would be my guess. It's a shame because I think it could've made an efficient lightweight shooter if I tillered it to bend evenly all through, but I have no use for such a bow anymore; I need more weight. The good thing about this type of construction is that it's easy as piss to make a new limb, just a pain to disassemble and glue up again.



I removed the wrappings and cleaned up a bit of the glue, I was sure the joint would start peeling at least a little but no signs of it at all.

[nabiul]

At what point do you stop reducing the thickness of limbs and start looking at reducing width? I'm back where I started; 30# at pictured which looks to me like a low brace. The new limb is on the left which is stiffer... and straighter than the right... which I still have to do more work on to get it bending evenly which means I have to take more material off the left limb to compensate. I also want the tips to be curving more to take stress off the new hinge which is right at the handle on the right limb.

I mean I get that the thinner the material gets, the closer the back and belly become to the neutral axis and the overall stress/strain on the wood reduces, but there must be a point where it's too thin for a safely functioning bow?

Yes I am tillering with a belt sander.

[DC]

I know this is going to sound like a wise assed answer but I really think that when you start to wonder if it's getting too thin, that's the time too start narrowing it. I think you have to build a few bows with a particular wood to get a feel for the width/thickness thing. I believe that you get the best bow from the thickest wood you can that doesn't try to bend sideways. I also think that a thicker limb puts more stress on the back and belly so, again, it depends on the wood.

[nabiul]

I give up; the new limb exploded while I was trying to string it. It was stronger than the other limb but there was a node in the working section and the fibers split horizontally and lifted up right at the node while the power fibers on the belly peeled away. The entire limb also split lengthwise, I suspect the natural crown of the bamboo being on the belly caused this. I don't ever see this design working without a serious backing like sinew, but then how would you tiller it? Maybe a short draw long bow would work, but not the horsebow I'm trying to make.

I'm just going to have to live with the string follow from using bamboo conventionally.

[nabiul]

This is the latest iteration of the horse bow with the outside of the bamboo as the back, didn't even bother tillering much; it bent pretty evenly from glue up and was weak enough to brace and draw. I didn't think it was possible but it shoots worse than my first bow; initial string tension and stacking is not an issue, but it's so sluggish to cast and there is a ton of energy left in the limbs after the shot which goes right into your hand once the string hits the siyah. My plan was to use wide limbs to get poundage out of the bamboo, but I think now there's too much mass at the limb tips. Also lifted a 1/4 in section of the back near the handle I guess since no real tiller caused too much stress there, but that can be fixed.

Actually I'm liking my first bow now, it isn't a bad shooter at all if you pull a little bit into the stacking and use lighter arrows.

[scp]

This is the latest iteration of the horse bow with the outside of the bamboo as the back....

I would make the front profile look like a pyramid or even Eiffel tower. You might have to back the belly with bamboo or wood.