Is the early wood the weak link to cause set?

[sleek]

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

Mark, I agree with you ALMOST all the way. I disagree that a belly and back stretch and compress the same on a rectangle cross section bow. I disagree because if that were the case, then a bows wood would be equal in tension and compression strength and we know that not to be the case because all woods we are familiar with are either stronger in compression or tension. Which leads to Arvins question of trapping. We trap to compensate for the difference in strength.

[PatM]

 Plastic strips do not mimic  the wood exactly, so even if you locked it in place as you should have it only tells you how much that material might stretch.

[Selfbowman]

No Pat the strips are a constant length in the radius . If you don’t trust the test do your on if you have a bow without 3 inches of set.

[mmattockx]

Mark, I agree with you ALMOST all the way. I disagree that a belly and back stretch and compress the same on a rectangle cross section bow. I disagree because if that were the case, then a bows wood would be equal in tension and compression strength and we know that not to be the case because all woods we are familiar with are either stronger in compression or tension. Which leads to Arvins question of trapping. We trap to compensate for the difference in strength.

You are confusing strength and stiffness. Assuming the wood is the same stiffness in both compression and tension then it will deflect the same amount on both belly and back. If it is not the same stiffness both ways (there was a thread on this a while ago with an indeterminate conclusion on this) then it will not deflect the same amount on both sides. The ultimate strength has nothing to do with the amount of deflection until you reach the limit and a failure occurs.

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?

All flatbows are very similar but not the same. Assuming you have a typical bow hardwood the compression limit on set will be around 0.55-0.6% strain. The tension limit will often be around 0.9% strain. If your bow has a flat unbraced profile and you use a 6.5" or so brace height that will get you around 0.4% strain like Arvins bow above. Max strain at a 28" draw will be around 0.7-0.75% strain. You would need to trap enough to shift the neutral axis more than 10% to completely eliminate set, which is a lot.

The amount of trapping required can be calculated more accurately if you know the maximum strain your design will see and the strain level your wood can take before set occurs. You have to balance the reduction in belly strain with the increase in back strain. For maximum performance the goal is to have both belly and back reach their limit at the same moment, but that is not always possible depending on the two limits. You can always get closer to balanced than you get with a basic rectangular cross section, though.


Mark

[Deerhunter21]

Plastic strips do not mimic  the wood exactly, so even if you locked it in place as you should have it only tells you how much that material might stretch.

the strip isnt and really shouldnt be locked on. the strip isnt bending with the bow, its just a consistent length thats used to measure before vs after its strung.

im pretty sure hes just using the clamp to make sure the strip doesnt fall when he takes the picture.

[Selfbowman]

Same results with my world record bow.

[Badger]

     I just did the same test on a finished bow and got exactly the same result Arvin got.

[Deerhunter21]

Ok so i have a question, do you get a different result with bend in the handle bows?

[Selfbowman]

Try it deerhunter. It’s going to be close to the same in any bow. Yes a stiff handle should make slightly different variables .

[Deerhunter21]

i would if i could, i havent made a bend in the handle bow... im working on a stave that i can turn into a bend in the handle bow though  .

[Allyn T]

I don't understand why you even need a strip just draw two pencil marks on the back of a bow and measure them. and if you want to know whether the back moves more or the belly just draw perpendicular lines on the edge of the limbs and see whether the lines on the back open more or the lines on the belly side closed more.It will probably vary depending on which part of the limb you're measuring to because some limb bends more than the rest

[sleek]

The shortest distance between two points is a straight line.


Idk, just wanted to sound smart.

[gutpile]

to me set is caused by over stressing the bow during tillering.. even a heavy early wood bow can be made with no virtually no set provided it is tilled properly and slowly.. using a cable system and never pulling within 5 lbs of projected target weight range is a good way to reduce and or eliminate set provided bow wood is properly dried also and the moisture content correct..... gut

[Selfbowman]

Gut do your bows never take set? If so I’ll send you a bow blank and give you the needs I have for a finished bow expecting to get a bow back with out set. And what would that charge be.

[avcase]

Arvin,
I am just catching up on your thread. This would be an excellent example for an engineering strength of materials course to help explain the relationship between stress, stiffness (elastic modulus), neutral plan, and strain. Without detailed measurements to go by, the amount of stretch (strain) you are measuring on the bow back seems reasonable.  If you can measure it, you will probably find a similar amount of compression (shortening) on the belly side, then the neutral plane is right between the back and belly. 

You might find some difference depending on how much of the belly-side thickness is horn.  More horn = less strain on bow back and more strain on the bow belly.

I don’t have any experience with gemsbok horn. I have a small sample, but it is not in good enough condition for a 3-point bend test. It seems a little stiffer than my water buffalo horn, but half to a third as stiff as a decent piece of hickory of same thickness.

Alan