Author Topic: Testing some recurves  (Read 3903 times)

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Offline DC

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Re: Testing some recurves
« Reply #30 on: September 21, 2020, 01:07:49 pm »
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,,, (-S
My wife had me throw away all the rocks. I do have a bucket of lead. I hate to even think of lifting it. My back started to hurt just mentioning it.
I just weighed it 99.7#
« Last Edit: September 21, 2020, 01:19:11 pm by DC »

Offline DC

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Re: Testing some recurves
« Reply #31 on: September 21, 2020, 01:45:14 pm »
I hung the lead on it and the bucket hit the floor. Nothing broke. No pic.

I reduced the yew thickness to just over 1/4"(approx) First pic.

I trapped the back so the back is 5/16" wide and the belly is 1/2" wide Yew thickness is still just over 1/4" Boo is about 1/16" thick. Bucket hit the floor. I have to modify my rig. Still everything is sound, no chrysals. Second pic

Edit changed to just over 1/4"

Offline DC

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Re: Testing some recurves
« Reply #32 on: September 21, 2020, 02:40:00 pm »
I partially emptied the lead bucket to get it down to 50#. Then I hung it on the recurve. Then I started filling it back up. It's tire weights mostly. I got it up to 67# and called it quits. It was about halfway straight. Way, way more than I would ever want on a bow I was pulling. The recurve was also leaning a tad so it was seeing torsion. I lifted the weight off and it went back to normal. The picture is comparing one straight off the caul and the test subject. No sense wrecking them, I may use them on a bow. I figure that I can take a bunch off my bows. There's probably 5 fps in there.

Offline mmattockx

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Re: Testing some recurves
« Reply #33 on: September 21, 2020, 03:59:59 pm »
torsional stability just might be the controlling factor. what cross section shape is best for that?

Round is the best in terms of carrying torsional loads and for torsional stability. What bowyer's call a torsional failure mostly looks like lateral buckling to me, which is a different thing.


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.

Square will be better than flat. The closer you get to a circular section, the better for torsional properties. Solid versus hollow makes no difference, it is the section shape of the outer surface that matters.


Triangular is best from a weight versus stability point of view.

That would be for lateral stability, which I think is the real issue.


I partially emptied the lead bucket to get it down to 50#. Then I hung it on the recurve. Then I started filling it back up. It's tire weights mostly. I got it up to 67# and called it quits.

In that case you could probably still take more material off, but it does get sketchy when you are down to those dimensions. My last bow was 5/16" wide at the nocks and looks impossibly delicate and thin to my eye, but works fine.


Mark

Offline PatM

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Re: Testing some recurves
« Reply #34 on: September 21, 2020, 04:20:36 pm »
Best to decide if you want a belly groove style or a  longer  loop straddling a teardrop cross section.

Offline willie

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Re: Testing some recurves
« Reply #35 on: September 23, 2020, 04:14:48 pm »


Triangular is best from a weight versus stability point of view.

That would be for lateral stability, which I think is the real issue.

Mark
Mark,
Yes, after some more reading, I believe lateral stability seems to be the controlling factor. thanks.  My google-fu must not be very good, as finding some resource that confirmed triangular as the best cross section was not to be.  Can you link to a reference useful for doing some calcs?
 

Offline mmattockx

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Re: Testing some recurves
« Reply #36 on: September 23, 2020, 10:03:40 pm »
My google-fu must not be very good, as finding some resource that confirmed triangular as the best cross section was not to be. 

I don't know that you would find anything on that sort of reference, it is pretty specific to our discussion on bow making. The best cross sections for bending strength and lateral stability are wide flange beams and if you wanted to maximize the structural efficiency would design the tension flange to be near yield at max load (tension is inherently stable, so there are no buckling considerations) and the compression flange to be near buckling failure at max load. That is impractical in wood, so we usually make a trapezoidal cross section that balances the tension and compression sides in a similar manner.


Can you link to a reference useful for doing some calcs?

I would have to do some looking for something like that. When I am worried about lateral buckling it is usually in regards to a steel structure I am designing and I use the steel design codes as specified in the applicable building code or industry regulations, I seldom do it from first principles. I suspect any mechanics of materials or structural design text would have a section on lateral buckling of columns or beams. All of that should be online these days. Maybe a search on 'lateral buckling of beams in bending' or similar would find some class notes from one of the many universities that put all their resources online.


Mark

bownarra

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Re: Testing some recurves
« Reply #37 on: September 24, 2020, 12:59:17 am »
As I stated earlier stability is the issue. Just check out Turkish hornbows :) Nothing new under the sun :)

Offline willie

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Re: Testing some recurves
« Reply #38 on: September 24, 2020, 09:58:40 am »
Quote
if you wanted to maximize the structural efficiency would design the tension flange to be near yield at max load (tension is inherently stable, so there are no buckling considerations) and the compression flange to be near buckling failure at max load. That is impractical in wood
thanks Mark, the sinew  build up in sections 9 and 10 of the horn bow illustration above, seems to verify your analysis.

Quote
Nothing new under the sun
mike,  I believe if one of the designers of this bridge had been a turkish hornbow shooter, things might have turned out differently.

https://www.youtube.com/watch?v=lXyG68_caV4

Offline bradsmith2010

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Re: Testing some recurves
« Reply #39 on: September 24, 2020, 10:25:00 am »
very great info guys, thanks for posting,, :)