Is the early wood the weak link to cause set?

[Selfbowman]

I take the no set back. It probably took 3/4” set

[sleek]

Hey, for fun, try shoot an arrow at 26 inches if you haven't tillered it out yet...

[RyanY]

I understand that the horn likely won’t take set in compression but will that be off set by the extra mass on the belly? This design doesn’t seem to suit horn without any reflex. Seems like too many variables to know what’s going on. I have nothing to base this on but I had the same sense as Pat that the horn won’t necessarily over-stress the back. Board bows seem to break often enough without horn...   

[willie]

Here is the bow we are working on and a Osage with Horn on the back. The one with horn on the back took no set. That one is same length but 1-1/2 at fades. 48@28

Arvin, I seem to recall you posted about the bow with the horn back, but could not find much. Can you link to a previous thread?

[Selfbowman]

No Willie but I will open another can of worms. Some say the back does not stretch. I put some painters tape on on he back once and it did not brake after taking the bow to full draw so the tape either did not stretch and that would make the back the  neutral  plane or the tape stretched. I will do another test today by clamping a steel tape or strap to the back and mark both ends and take it to full draw and see if back stretch’s. The tape should move some guessing 1/8 to 1/4 inch if the back is not the neutral plane. If not the back does not stretch. Leaving the belly to take it all. Then  I would say horn would be good for the belly. Arvin

[Selfbowman]

Ok smart guys I need math on a given length. Let’s go with 60” of tape or strap 1/2 thick limb. I’ll try to find the straightest limb bow I have to find out about the neutral plane being at the back or not. In 20” of bend if the back is not the neutral plane how much should the tape move? Arvin

[PatM]

You cant generalize backings and say they stretch or don't stretch.   Tape  and sinew will stretch,  bamboo and steel much less so.

  When people say wood backs don't stretch they mean that most of the movement in a bending bow is on the belly side.   Wood simply can't elongate much before breaking.

 So a bow is more about the belly grudgingly compressing like a hard rubber ball than the back being like an elastic

[Deerhunter21]

No Willie but I will open another can of worms. Some say the back does not stretch. I put some painters tape on on he back once and it did not brake after taking the bow to full draw so the tape either did not stretch and that would make the back the  neutral  plane or the tape stretched. I will do another test today by clamping a steel tape or strap to the back and mark both ends and take it to full draw and see if back stretch’s. The tape should move some guessing 1/8 to 1/4 inch if the back is not the neutral plane. If not the back does not stretch. Leaving the belly to take it all. Then  I would say horn would be good for the belly. Arvin

I would think tape would be just like a brown paper bag backing. i did a measurement of how much my bow back stretched when drawn a while back.... i cant remember how much it stretched, it wasnt a lot though, but it did stretch. i think i tested it by measuring the whole bow nock to nock undrawn by putting a string up against the bow and marking the string where the nocks were (and then measuring it ofc). then i had my dad draw it and i took the same string, put one mark on one nock and put it up against the back and marked where the other nock was now. if i were to try to guess how much space was between the markings for the same nock, drawn vs undrawn... i would say around a quarter inch.... but dont quote me on that. it was about a year ago. all i can remember for sure was that the nock lined up on the string (when drawn) further than when undrawn.

[Selfbowman]

Ok I went Lowe’s and got some plastic lumber strap I found center on the 60” strap marking each end and clamped it on the unstrung bow. I then braced it . This was the result. Better explanation please Pat.

[Selfbowman]

Results

[Badger]

 Way more than I would have expected, I was thinking you might see about 1/10 of an inch. Are you pretty confident that was a good test?? If that is the case it implies that wood stretches about the same as it compresses.

[Pat B]

How about if the banding is permanently attached.

[Deerhunter21]

intresting... now im curious to see how much a bow compresses compared to how much it stretches... that would mean we would have to measure only one limb and only the amount that bends... i would but my bow is kinda out of commission... i gotta fix my bow now so i can test some!! 

also i should point out, that this is two bending limbs plus the handle which is not bending, so the amount were seeing is twice the amount of one bending limb.

i also feel like including the handle in the measurements might change the end result... it might not but thats just my thought... another thing i wanna test...

Also, is the band as taught as when it measured the unstrung bow? i

Also, what wood is it? (it looks like osage but just making sure!) different woods differ in tension and compression strength so it would affect how much the belly compresses and the back stretches

m sorry for the questions? this is just all super interesting to me!!

[mmattockx]

Ok smart guys I need math on a given length. Let’s go with 60” of tape or strap 1/2 thick limb. I’ll try to find the straightest limb bow I have to find out about the neutral plane being at the back or not. In 20” of bend if the back is not the neutral plane how much should the tape move? Arvin

Strain is measured in inches of deflection per inch of length. So 1% strain would equal 1" of stretch over a 100" length. I guarantee that the neutral plane is not on the back, it is not physically possible for that to be the case on an all wood bow.

Results

It is a pretty crude measurement but you have about 1/4" of change over 60". That equates to ~0.4% strain getting to brace. That seems about right, as the limbs typically see close to half their maximum stresses getting to brace on a typical flat bow. On reflexed bows they see more than half getting to brace.

Way more than I would have expected, I was thinking you might see about 1/10 of an inch. Are you pretty confident that was a good test?? If that is the case it implies that wood stretches about the same as it compresses.

That is a reasonable result based on the numbers I calculated above. Yes, the back will stretch as much as the belly compresses with a rectangular limb section on a self bow. Why is there so much skepticism about this? It is basic mechanics of materials and has been well proven for well over a century now, maybe much longer than that.

How about if the banding is permanently attached.

One end has to remain free indicate the length change. If you solidly anchored both ends of the band it would just change length along with the bow.


Mark

[Selfbowman]

Thanks Mark . Now can you give me the approximate length change at brace so I can decide weather to trap belly or back to work prevent set?