Testing some recurves

[DC]

I thought I would make up some Bamboo/Yew recurves(just the curved part) out of scraps and test them to destruction to see how small I can make them. The questions are how to test them and how much weight should they take.

  I think the most strain would be at FD when the recurve has "opened up". I thought I would grab the recurve in the vice and hang a 5 gal bucket from it. Then slowly fill the bucket with water until the recurve failed. I considering grabbing the test piece at a bit of an angle to simulate a bit of twisting to the mix. I'll make them as thick as I do now and then shave them thinner as I test them.

  I'm assuming that the tip of each recurve on a 50# bow takes 25#(I'm about 87.5% sure of that ). I'm wondering how much of a safety factor I should add. I've heard everything from double to six times the load for stuff but I don't know what is reasonable in this case. I pretty much make sure I have primo wood for bending the tips. So for a 50# bow how much should I hang off the tip and expect it to survive.

[PatM]

You could probably hang a  small vehicle from a piece of laminated wood of that shape.

[DC]

That had occurred to me but that would eliminate Simple Composite from the works. In this quest for speed I find myself drifting further away from the stickbows that got me into this. It's starting to weigh on me a bit. But then so is having 75 bows hanging on the wall.

[bradsmith2010]

I have tested tips to destructioin,, I put it in vise, and pulled with string that had a scale attached,,they usually didnt explode,, just started to fail,,and I knew about the weight they started to fail,,
   also I remember a thread,where the brace weight of a bow was more than I thought,,on the string and tips,,test it to make sure,,

[DC]

I was thinking about using a scale but the shrapnel was scaring me off. I thought the water bucket also preserved the weight at the moment of breakage so I could look at the weight at my leisure rather than have to catch it right at the moment.

Did you find that your tips were way stronger than they had to be? I've always suspected that they were but I was too chicken to test my theory.

[bradsmith2010]

yes they had to get incredibly small to fail,,

[willie]

I'm assuming that the tip of each recurve on a 50# bow takes 25#(I'm about 87.5% sure of that

if you put a bow in the vice at the handle and pull on one limb with a scale, the readings will be very different as the angle changes. Finding a way to pull the same deflection with 1/2 the draw weight is not as easy as is seems.

[DC]

I'm assuming that the tip of each recurve on a 50# bow takes 25#(I'm about 87.5% sure of that

if you put a bow in the vice at the handle and pull on one limb with a scale, the readings will be very different as the angle changes. Finding a way to pull the same deflection with 1/2 the draw weight is not as easy as is seems.

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

[Tommy D]

I was thinking about using a scale but the shrapnel was scaring me off. I thought the water bucket also preserved the weight at the moment of breakage so I could look at the weight at my leisure rather than have to catch it right at the moment.

Did you find that your tips were way stronger than they had to be? I've always suspected that they were but I was too chicken to test my theory.

You need something like this DC ... http://www.billfishtacklesupply.com/Scales/scales.htm. They use them to test drag on heavy fishing reels. There is a slide that remains at whatever point the drag got to so you can read the weight. These only go to 55lbs... but maybe 2 side by side would be enough - I am assuming that gives you 110lbs...

[DC]

This is a one off test, I don't want to spend any money. And I'm cheap, you were thinking that anyway

[Tommy D]

A smart man like you could fit a “one off” sliding bit of scrap yew to a cheap scale that I am sure you already have! 

[mmattockx]

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

You are correct that each half of the string on either side of the arrow nock carries half the draw weight. But the string tension is not at all equal to 1/2 the draw weight because it is puling at an angle to the arrow shaft. Simple geometry can be used to calculate the string tension if you know what the angle is at the arrow nock. Even just measuring the angle from a full draw picture will get you in the ballpark.

For instance, on a recent R/D recurve design I have been working on the string angle from the arrow shaft to the string is 65 degrees at full draw. Using trig you can calculate what the string tension will be.

1/(cos 65) = 2.366

So the string tension in my case is 2.366 x half the draw weight.

I would say figure on the string tension being at least twice 1/2 the draw weight, maybe closer to 2.5x.

Also realize that the string tension is lowest at full draw, so you need to test your recurves with several string angles and higher loads to simulate the loads the recurve sees as the bow is drawn. It is very possible the worst loading condition is somewhere closer to where the string lifts off the recurve than at full draw.


Mark

[bradsmith2010]

I just pulled a scrap tip,,cut from a bow,,at about 90 degress,, not scientific at all,, but it gave me a general idea,, I just kept reducing the size of it, till I could pull it and make it fail,, I had the vise on the floor and put my foot on it and pulled up till it would start to break,,,,

[bradsmith2010]

I thought I read it was a little more like that,,

[DC]

I never even thought about string tension. I've measured it on some of my RD's and it was at least double DW at brace but it dropped steadily until FD. Now, at brace, if I'm right, when the string is laying on the recurve there is no, or minimal, bending force on the recurve. It's only when the string lifts off that bending starts to happen. 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