Predicting Draw Weight?

[jeb532]

New to this forum and bow making...but have been a mechanical engineer for 40 years.  

Starting my 5th simple self-bow.  The first 3 small ones (made for grandkids) (40", 40" and 48" all about 12# max)came out about as I expected draw-weight wise.  They were made from 1x2 red oak, cut "edge-wise"...i.e the limbs were 3/4" wide.

The fourth (64" long)was cut with the limbs on the 1-1/2" wide dimension...and came out about 25% stronger than expected.  

I've been using red oak from home depot..simply because of cost and availability.  

I've been working on an excel spreadsheet for large deflection cantilever beam bending.  I have also made some preliminary mechanics models in Pro Engineer....but I don't get answers that match results on the first three bows.  I have to use E value about 40% of published values for 12% moisture content (published is ~2.3 Mpsi) to get the proper magnitude of deflection in the ProE model.

Any comments about predicting draw-weight would be appreciated.

   

[Holm-Made]

When I used to build self bows I was amazed at the different poundages a bow would turn out for a given amount of wood.  One osage self bow with given demensions would be way higher or lower in poundage then another of the same densions.  Some wood is denser then others.

With self bows you have to read the wood as your working the tiller so that you hit the poundage when the tillering process is complete.  With your history as a mechanical engineer it is going to be difficult for you to put the computer aside.  The computer program is only going to give you a rough estimate when it comes to wood as every piece is different.  Chad

[Pat B]

K.I.S.S.!!!  Try not to put so much thought into it. We aren't building Swiss watches.

[jeb532]

Not building swiss watches..granted..but...some rules of thumb are usually helpful.

And yep, every piece of wood is different...the home depot guys groan when they see me coming...I usually go thru their whole stack to get one or two good pieces.

[Ray]

Throw your calculator away,like Holmmade said every piece of wood is differant.
 Never draw your bow over its intended drawweight.Take your time removing wood,work it slowly to avoid set.
 Remember though it is wood and not a glass bow and you will succeed.Enjoy the primitiveness of it.--Ray/mamba

[jeb532]

Humm..."never draw your bow over it's intended draw-weight"...sounds like a conundrum to me...since I started this thread asking about predicting what the intended draw weight might be...

[Dano]

I guess this is why engineers like to work with steel and engineered lumber, they can't deal with all the variables. Seriously, how big of a program would it take to do what your asking? You'd have to plug in density, moisture content, weight, temp, you name it, and you'd still come up wrong. The fun is in the challenge.

[Steve Kendrot]

You don't predict your draw weight as much as determine what you want it to be, then remove wood until you have what you want. At least with organic wood bows. With laminated glass bows you can follow a "recipe" to come close to your intended draw weight, but if you are making self bows, board bows or backed board bows, almost anything 1/2" thick and 1.5 inches wide will start off as a 100 pound bow. After you've got it floor tillered,  get it bending evenly on both limbs with a long string until you've got it bending enough to brace it with a short string. Pick your draw weight and pull it until you hit that weight. Remove wood, exercise the bow and draw it to your desired weight again. Repeat this process until you are able to pull it back to your draw length. That's it in a nutshell. Follow that process and you won't need to worry about the variability between boards and computer models and the like. If the technology and engineering turns your crank, try a laminated bow. Its a bit more formulaic.

[bjhaug]

Yep,

I have a degree in Civil Engineering, Math, and Accounting (couldn't decide what I wanted to do I guess), and have found that it doesn't matter one bit all the math skills and physics that could be involved.

Just start out with what is an incredibly heavy bow and remove material until you get down to the weight you want.  Steve hit the nail right on the head with his answer - great reply.

[AKmud]

Originally posted by Steve Kendrot:
You don't predict your draw weight as much as determine what you want it to be, then remove wood until you have what you want.  

+1

When I start a bow, I have a target weight in mind.  The trick is then working the wood to achieve that weight.  It seems to me getting the maximum weight out of any given piece of wood would be a lesson in futility since you would always be riding the fence on what it will tolerate.  One wrong move and it will be kindling.

[ChristopherO]

As stated above, "Wood Density" is very important even within species.  I made a sinew backed static recurve in Pignut Hickory this past winter from a tree I harvested expecting it to turn out as tough as the other pignut I've been using.  Not even in the same league.  The strength wasn't there and the sinew is overpowering the belly.  They are all different.  I should have known better but had a "Formula" in mind and ignored what the wood was telling me.  
As for your condundrum, we wood whittler's usually pick up a stave or board with a target weight in mind and work to get that from the wood, if it will let us.  In that process we should never pull past the target weight while tillering.  Having been a home builder in the past and using static wood creating static structures building bows is a completely different animal.