Probably an old question

[PatM]

Most wood can only stretch about half a percent before failing.   The old test of cutting apart a bow along  the middle after some use tells the story.  The back goes back to straight and the belly may even show more set than it did as a whole bow.

[DC]

Most wood can only stretch about half a percent before failing.   The old test of cutting apart a bow along  the middle after some use tells the story.  The back goes back to straight and the belly may even show more set than it did as a whole bow.

I thought about that and that tells you how much the belly has collapsed but it doesn't tell you what the back is doing at full draw.

i can do it when i get home and tell you what i find. im going to do this to the belly to! this is interesting!!

I thought I would try it too but it may be a two person job to avoid leaving the bow at FD.

[PatM]

 It tells you that if the bow doesn't break it still can't be stretching much at all.   It HAS to be compressing  more

 I guess the argument would be does wood store more energy by stretching .5% or compressing    however much it does at full draw.

[Deerhunter21]

i did the experiment with my dad. my bow is 65" ntn has about 1.5 inches of set. around 45# at 26". the backs lenght before it streached was 65". after it was streached it was 66"ntn. if the bow was perfectly tillered each inch along the bow would be streching 0.01538461538 per inch along the length of the bow. please tell me if i missed to say anyting. i know I said this before but seeing how much a bow bends i would say it definentially compresses more than it streches. i think there is a line where the back will stretch and the belly compresses and it can be uneven with the back stretching less than the bow compresses. but then again what sleek said is that they both take the same amount of psi but what if it takes more compression to take the same amount of psi as the back?

[DC]

Thanks. So that's a 66" bow pulled to 26". Not overly stressed I would think but it still stretched 1.5%. Interesting.

[Deerhunter21]

now how to find how much it will compress

[sleek]

Thanks. So that's a 66" bow pulled to 26". Not overly stressed I would think but it still stretched 1.5%. Interesting.

Does that make us professional board stretchers?

[Deerhunter21]

Thanks. So that's a 66" bow pulled to 26". Not overly stressed I would think but it still stretched 1.5%. Interesting.

Does that make us professional board stretchers?

I guess that does!


also DC i dont want to be a stickler but when we do stuff like this the tinyest thing can change the whole thing. its 67 tip to tip and 65 nock to nock

[sleek]

Laws of math apply here, just dont ask me to do it lol...

If you know the circumference of the back of a drawn bow, and its thickness, you can calculate the circumference of the belly of the bow. That will tell you how much the belly is compressing vs its straight length. Of course, being as the back and the belly is separated by the distance set by the thickness of the limb, there is a ratio between the back and belly that can be applied.

Now if I could only remember how to math....

[Deerhunter21]

dont worry about it sleek, ive just been taught like 3 years ago what we need and still using it . only problem, my bow i think is eliptical tiller and i need to tweak one side a couple of scrapes. i should really post it. well ill post it right now.

[Selfbowman]

That’s spot on sleek to the common sense  guy. The thickness will be a portion of the equation. Correct me if I’m wrong. .050 of glass on the belly and .050 on the back of a glass bow the part in the middle just keeps them apart. Yes or no? So Marc’s comment on them doing the same amount of work will give you a perfect tiller for that piece of wood. I think. Arvin

[sleek]

dont worry about it sleek, ive just been taught like 3 years ago what we need and still using it . only problem, my bow i think is eliptical tiller and i need to tweak one side a couple of scrapes. i should really post it. well ill post it right now.

Well, as I am quite used to playing with a short deck, I figured another way. Put the image on the computer and use an arc rendering tool to trace the back and belly, then measure the length of each arc drawn. A CAD program would be great.

[sleek]

Further musings....Red and Green style
Full draw the bow and while at full draw, lay a strip of duct tape on the back. No need to trim it, just lay it flat on the back. Let down on the bow and trim the tape excess that hangs over the tips. Pull the tape and measure it. Its length is the length of the strung bow.

Do the same on the belly, law tape at full draw, and mark the tips with sharpie. Then yank it off before letting the bow down or it will stretch the tape out as when you let down.

[Selfbowman]

Sleek duck tape will stretch. Arvin

[sleek]

Sleek duck tape will stretch. Arvin

yeah, That's why I was saying to lay it on the bow AT full draw, but I suppose pulling it off the bow will stretch the tape also....

I'm certain there is a computer program that can do this with an FD image.

Could just trace the bow at full draw on a chalkboard... back and belly, then  measure those lines.