Pyramid bow limb thickness?

[Paintmonkey]

How do you determine limb thickness when laying out a pyramid bow?  I'm planing my first bow, red oak board 2.5 inches wide.  6 foot overall length, 7 inch stiff handle. 3 inch stiff tips. Trying for 35# draw @ 28 inches.   I think that leaves 21 inches of working limb assuming 2 inches of fade at tip and handle. 
Any advice would be appreciated.

[bubbles]

Just under 1/2" is usually where mine end up, and I rough them out about 1/16 over 1/2".

[Pat B]

I tiller pyramids just like any other bows...remove wood until the limbs are bending evenly and together. With a pyramid profile the thickness will automatically be pretty constant.

[bubbles]

I think bubby had a maple pyramid board build along where he took the limbs down to around 1/2" with a table saw and then tillered from there.

[bubby]

2-1/2" is too wide you shouldn't have to go over 2" wide

[bubby]

http://www.primitivearcher.com/smf/index.php/topic,35312.0.html

[paco664]

http://www.primitivearcher.com/smf/index.php/topic,35312.0.html

+1 thanks for that very informative link!

[Paintmonkey]

Had actually already bookmarked your build along bubby, just thought I might have missed a method of estimating limb thickness/draw weight.  Also is there a down side to wider limbs? Thought the bowyers bible suggested more width=less stress, set, and maybe breakage.

[dbb]

  Also is there a down side to wider limbs? Thought the bowyers bible suggested more width=less stress, set, and maybe breakage.

There is no downside to wide limbs in it self, but with the lenght and targetweight you have it might be hard to get the weight down without getting the limbs very thin.

[joachimM]

I have a 70" red oak bow which draws a bit under 70# at 27". It's 18 mm thick throughout most of its length, 5 cm wide (2") but trapped on the back.
To get a 72" bow of the same materials to 35# you'd need to make it just under 14 mm thick according to my calculations.

[Paintmonkey]

Joachim, how do you do the calculation?  That is exactly the info I was hoping for.

[joachimM]

I made a spreadsheet for a previous question, see there
http://www.primitivearcher.com/smf/index.php/topic,51846.msg702489.html#msg702489

[Springbuck]

I think bubby had a maple pyramid board build along where he took the limbs down to around 1/2" with a table saw and then tillered from there.

This is where I usually start, but you aren 't going to get out of tillering, just because it's a board bow.  If you can thickness it first with power tools, before you cut side profile and length, do it in stages if you can, and quit when a little floor tillering push, just about 3" bend or something, feels about right.  Then quit there, and do the side profile.

Otherwise, just make sure you are at least down enough to leave that handle area stiff.  1/2" is a good guideline, but on a very wide/long bow even that might land you too light. So, you CAN then shorten it or narrow the taper,  ut only if you've left yourself that little fudge factor.

[Springbuck]

Oh, and normally, I would disagree  (sweetly) with the above post by Bubby about the limb width, and say get all the width you can on oak.   But, at your length and targeted draw weight, that bow is very low strain anyway, even with marginal wood (and oak runs from marginally to pretty darn good).  The strain is low, so you don't have a major stress problem that needs solving with width.

Determining the this, and determining the thickness are basically the same problem, and sometimes you have to creep up on it.  If you had a 2" wide bow 70" long, and another one 66" long, both pyramids, and started with 1/2" thickness on the limbs, one bow might be perfect on a 40 lb draw weight, but the shorter one might be 15 lbs heavier or more draw weight.  So, if you WANTED that 66" bow at 60 lbs, you got it, but it would take more set than the 40 lb/70" bow.  BUT, if you had left the shorter one 2-1/2" wide, and a HAIR under 1/2" thick on the limbs, it would take less set.

Get it?

[Paintmonkey]

Thanks for the replies everybody.  I think I have a good starts worth of info here.  I'll probably trim it to 2 inches wide as it seems like the consensus is there won't be too much stress at the length and weight. 
If it doesn't break right away I'll let you know how it works out.