Board Bow Schematics

[BryBow]

What is the mathematical formula for determining the thickness of a board to attain a desired pull weight if ripping on a table or band saw before gluing on the riser and tillering?

[bornofmud]

There isn't one.  Depends on the wood you're using, the width, the width taper, what it's backed with,the length of the bow, length of the riser, etc etc.  The best you can do is find someone that's made something similar to what you want, and ask them what their thickness was for a bow around the draw weight you want.

[Roy from Pa]

A board a half inch thick will get you well over 55 pounds. A lot depends on wood choice.

[John Scifres]

Never heard of a formula for thickness.  Tell us what you are after and we'll give you a starting point.  But you go from there and tiller to weight most of the time.  That's due to the variability of wood, even among species, even among a single tree, sometimes even between the two limbs.

[BryBow]

I think I'll stick with the gang. Seems like some higher ups at another popular site thought my question was a bit stupid. So much for thinkers in some of the other forums.

I've tillered a plenty since my new found addiction and I find it a comforting and peaceful process.

I'm not much of a mathematician and there are some folks on here that are allot smarter than me in that area.

My thoughts were that most kiln dried red oak boards bought at say H.D. or Lowe's would all be about the same density so if you were building a 62" long by 2" wide with a taper to 1/2" tip then you should be able to come up with a mathematical formula that would get you pretty close to the correct thickness.

[LittleBen]

Density in ring porous woods like red oak can vary quite a bit, mainly because of the varying ratio of latewood to earlywood.

However, since the elasticity of the wood should be the same, regardless of that ratio, you can make certain predictions.

Lets say that you build a red oak bow, of 64", pyramid bow and you're buddy wants one just like it. Your first bow was a fairly low density piece of red oak and was 2.5" wide, and ~7/16" thick.

You go to the store and get another board for your buddies bow, but you find a much more dense board! Great! assuming you want the bow to have the same specs, you should make the bow the same thickness but slightly more narrow in order to reach the same draw weight.

Hope that makes sense.

Good luck with your calculating ... my experience with tri-lams and calculations is that it can be tricky and can be hit or miss.

Try out supertiller if you wanna try calculating.

[Grey Taylor]

LittleBen... no, no, no... if you find a better piece of wood for your buddy's bow then you give him your old bow and make your new bow from the new wood.

Guy

[LittleBen]

HAHAHAHA! I knew that was coming Guy! I think you're right on ... I just didn't have the patience to go back and reword my example.

[PEARL DRUMS]

[Mike Most]

4est trekkers, board bow build on this site, uses 15/32 as a start on a 2.5 inch wide board bow tapered to 1/2 inch at the tips, but it is a 68" bow, I took a lot off that belly by hand and it still came in at 61 lbs at 28. Use the Gizmo by Eric Krewson works great

[Pat B]

You remove wood from the belly until the limbs bend evenly and together and you achieve your desired draw weight at your draw length.

[PEARL DRUMS]

Originally posted by BryBow:
[QB] I think I'll stick with the gang. Seems like some higher ups at another popular site thought my question was a bit stupid. So much for thinkers in some of the other forums.

/QB]

That same popular site with no thinkers got the answer you wanted didn't it?

[Pat B]

I think you are putting too much FG thinking into building wood bows. One of the things I like about building selfbows(board or stave)is the lack of precision needed to build one. Its pretty simple...remove wood from the belly until both limbs bend evenly and together and you achieve the draw weight you desire at the draw length you are shooting for.

[LittleBen]

Well said as usual Pat.

I think Pat's comment in combination with what Mike Most said is the big picture here. You can get a rough idea of thickness (i.e. 15/32 in th eexample mike presented) but at the end of the day, every piece of wood is a little different and you're going to have to tiller it the old fashioned way.

Even when I've made tri-lams where I carefully calculate stack thickness, carefully grind lams, and use all bookmatched wood (i.e. everything should theoretically be the same, I still have to fine tune everything ... it's never really dientical and you're gonna need to use some elbow grease.