50" Red oak board. Need rough out suggestions for 16" draw.

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I have a 50" red oak board that is 1.25" wide top to bottom and 3/4" thick. I'm looking for 16" draw length. No target draw weight. Heavier the better. I'm thinking to pyramid style design with taper going from 1.25" @ middle to 1/2" at the tips, also thinking to taper thickness from 3/4" middle to 3/8" at the tips. Not concerned with draw weight. It will be backed. My main concern is knowing how thick is too thick? If it is too thick in the middle, will it take set or crack? Why 16" draw length? Primitive crossbow prod. I've made 2 prods, both have broken but I've learned from both. You can't just take a 26" prod, make it 3/8" end to end and expect it to work, for long. Now I want this next one to work so I think I need to taper both the width and thickness to get what I need. I'm just worried about it taking set. Don't know how thick is too thick in center.

[Badger]

  I have more questions than answers possibly. Red oak is not your best choice but may work if it has absolutley no grain run offs. Is the board 1/4 sawn with straight grain? It sounds like you are trying to build a bow by numbers and not many of us do that here. If you are cutting the limbs pyramid you won't need much taper. You do need to tiller your bow out just like any bow and not try to just cut it to predetermined dimensions. If I were doing it I might start at something like 5/8 in the middle to 1/2 at the tips. The way you find your maximum weight is to approach it slowly. Put a long string on the bow, one that hangs down not less than 4" and not more than 6". The very first thing you do is get the bow bending evenly. Once you have it bend evenly you start the approach to find out what your max weight can be. Read your yardstick on your tiller tree just as if your bow was braced and don't worry about tip movement. Pull it down about 2" (6' mark on tape with 4" loose string)  and carefully read your weight and make a note of it. Now you start progressively adding weight to your pull and after each pull going back and see if the weight at 6" has changed. If you have to remove wood to even up your bend then go back and recheck the weight at 6". Lets say you try it at 30# 40# 50# 60# and at 70# you notice a little loss of weight at your 6" mark. That would mean 60# is probably going to be your max.

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It is straight grain but It has some run off. I'm hoping backing it will keep it from lifting.

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So I'll start with tapering the board, back it then put it on tillering tree and get it even and work it from there. Let the numbers be what they may.

[willie]

48" nock to nock @ 16" draw with a little number crunching gives 9/16" as a final thickness for a pyramid style limb, so I would say Badger was really close with his thickness estimate. Keep in  mind that the wood will only bend so much for a given thickness, so the best way to design for poundage is by adding width.

I would expect about 20 lbs per inch of width for a moderately strained limb, (which would be 1/2" thick),
and about 33 lbs per inch of width for a 9/16" thick limb, which is pushing the oak fairly hard.

Of course, rough out depends to a large degree on how much extra you need for the tools you use 

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Sorry willie, Im not following the 20 lbs per inch of width, if it were 1/2" thick. I understand poundage should come frm limb width and not thickness. What does that mean for a 48" ntn?

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I've been reading posts that state pyramid bow charts and math but can't find the charts or formulas. Where can I find these things?

[Badger]

 Willies estimate goes right along with what you heard. Thickness controls how much it can bend and width controls how much it will bend which is the same as hitting target weight.

   Willie, did you figure that based on 16" draw".

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Roger, I guess I can start by making the board 9/16" thick to begin then taper limb width. I'm not sure what 33 lbs per inch of width correlates to in a 1.25" wide limb? 41 lbs? 50" bow though. If its 1.25" wide limb then it should be 41 lbs at 16"?

[willie]

Steve, both calcs are based on a 16" draw.

Blank,
sorry for the confusion,  missed the design width you stated earlier.

given a max width of 1.25", you could tiller your roughed out board (48" ntn) to 40 lbs, and expect to finish the thickness at about 9/16. this is asking a quite a bit  from a red oak board.

for a lesser strained plan, if you tiller with a 25 lb goal, you should finish out at about 1/2".

I presume that you will tiller with the "never pull it harder than the finish weight" method.

I would cut my width taper to pretty much finish dimension, and tiller by reducing thickness. Otherwise you risk over straining the center third of the prod during tillering
 

opinions vary about using math and computers to design with, 

http://www.primitivearcher.com/smf/index.php?topic=39779.0


this thread should help you find the calculator I use.

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http://paleoplanet69529.yuku.com/topic/40108/Woodbears-Selfbow-design-sheet?page=1
and supertiller can be found here.

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http://www.buildyourownbow.com/build-alongs/how-to-use-supertiller-build-along/

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Willie it is very clear now. I appreciate your patience and explaining. Id rather have a bow work well at 25 then to snap at 40. I appreciate your help!

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One last question. How does draw length affect poundage? What if I wanted a 14" draw or  a 20" draw?

[willie]

the basic premise is that thickness determines how far the wood will bend, so for the same poundage, given a 1.25" width, a 14" draw would need to be thicker, and a 20" draw would be thinner.
Poundage is basically a function of width, all other things being equal. More draw length does increase arrow energy, though. Play with one of the spreadsheets a bit, for a good understanding of the interrelationships of the various parameters.

Id rather have a bow work well at 25 then to snap at 40

I will bet your second one breaks... ;)
Have fun and post some pics.

[Blank]

Hmm measured the width again and it's 1" 11/16. Not 1 1/4.

[willie]

Poundage is basically a function of width

to be a bit more precise, it is directly proportional.

so

1-11/16" divided by 1-1/4 is 1.35 or 35% more

therefore 25# becomes 33#    and 41# becomes 55