Testing some recurves

[mmattockx]

So by the time enough of the recurve is under bending pressure the string tension has dropped way off, probably to near DW. Am I close

It is around draw weight at full draw, but that is a very crude approximation as it changes a lot through small changes in string angle.

I realized I should have put the string angle numbers up for the string tension range I mentioned above. A string angle (as measured from the arrow shaft up to the string at full draw) of 60 degrees results in string tension equal to draw weight. A string angle of 66 degrees results in string tension of 1.25x draw weight. This means you have a 25% change in tension for a 10% change in angle.


Mark

[willie]

So you're saying it's not 1/2 the DW? What does science say? My gut says it's half

Marks got you covered with the science.

I put a bow in the fixture and drew it to full draw and marked where the limb was drawn back to. then I unbraced and pulled only one tip to the same mark with the scale attached to the string. I am still confused by my results. try it.

[Tommy D]

DC seems to me a less scientific but more practical way to achieve what you are trying to figure out is to sacrifice one of the bows you have built and basically just keep reducing the tips/ recurves until they fail. This will also show you the string angle that they fail at as well.

[DC]

I've already glued up a pair. They have to cure til tomorrow. I didn't realise it was going to get so complicated. I'll read back over this tonight and see if I can come up with something that will satisfy me. I have a feeling that these things are strong enough that string alignment and having they break off sideways is more of a concern. That and slamming them in car doors.

[willie]

are strong enough that string alignment and having they break off sideways

torsional stability just might be the controlling factor. what cross section shape is best for that?

[bradsmith2010]

yes breaking off sideways is a concern,,

[DC]

are strong enough that string alignment and having they break off sideways

torsional stability just might be the controlling factor. what cross section shape is best for that?

Well, if it's steel tubing round is best but solids may be different. I think I've heard that wide is best. They always say to keep the limb wide up to the base of the recurve but there may be a difference between torsion which could involve the whole limb and just the recurve bending sideways.

[bradsmith2010]

I just worked on a recurve,, and the tips wanted to bend off sideways, I couldnt get to line up,,I cut them a bit shorter and they were stable at the shorter length,,the bow still holds 6 1/4 reflex,, so I think I was just being overly optimistic bout the longer curves,,the bow is 46 inches long, I have been putting off narrowing the tips, ,Im afraid they wont be stable again,,

[DC]

I like that. Is it backed?

[bradsmith2010]

yes about 1000 grains of sinew,,the tips literally tried to twist off when I pulled it on the tiller tree,, I thought I ruined it,, but was able to get it to line up and shoot,, it was tricky to tiller,,

[willie]

Well, if it's steel tubing round is best but solids may be different. I think I've heard that wide is best.

I did some reading , and round and smooth seems to make sense for most torsion bar applications, as they are expected to flex millions of times, and cracks tend to start at corners. found nothing on tubes versus solids. I suspect more squareish might be better than flatish for rectangular.

here is a illustration of a horn bow that may be informative. I read often the fg guys issues with stability. their limbs are typically flatish retangular, and they often are trying for more quite radical reflexes though.

[bownarra]

I was just about to say......Turkish hornbow tips Triangular is best from a weight versus stability point of view. Flat back, slightly rounded sides to the tip. Nock cut into the back.
Their tips were 10mm wide at the base of the tip, 17mm thick (average) but these dimensions were for 100# + bows with radical reflex.
More than half the problem with reflex/recurves/stability is laying out your centre line and width profile absolutely perfectly. Get this dead right and you can safely push the limits.

[DC]

My precision test rig.

0 pounds

40(approx)pounds. I had a 4.5 litre bucket(9.9 lbs) that I filled from the pond so there was a little spillage Edit-- it's 47#< I weighed it.

[bradsmith2010]

rocks work good too, i think you gonna need to get to 90# to see any failure,, maybe more,, I love the test rig,, looks very effecient and safe,,,

[DC]

So you're saying it's not 1/2 the DW? What does science say? My gut says it's half

Marks got you covered with the science.

I put a bow in the fixture and drew it to full draw and marked where the limb was drawn back to. then I unbraced and pulled only one tip to the same mark with the scale attached to the string. I am still confused by my results. try it.

I did roughly the same thing. I put a 50# bow in the vice, put a scale on one tip and pulled it in the right general direction to the FD mark. I got around 40#. My vice rotated a bit so this is a little low. I didn't start off in the braced position because there was too much junk in the way. But to me it's looking like a 50# is 50# for one limb at FD. I know the braced string tension is way up but because of the angle it's pulling at I don't think that's an issue. Is that the surprise you got Willie?