Heavy weight bows...

[upstatenybowyer]

I've been practicing shooting a lot and feel pretty comfortable at around 60# of draw weight. So, I'm wondering, how does one go about making a heavy weight bow without getting a whole bunch of set?

It seems like as soon as I go for anything above 55, the potential for set starts increasing exponentially. This has been my experience even with slow careful tillering using Steve's "no set tiller" method.

My guess is that a heavy weight bow needs to be longer and have D section limbs (ELB) for the wood to handle the tremendous compression force exerted on it.  Is it possible to make let's say a 70-75# recurved flatbow, maybe with shallow D section limbs that takes very little or no set?

[Badger]

 Just as you said, wider or longer. A 20# bow should be under the same stresses a 150# bow will be under. You can only go so thick given a radius before it starts to take set and then you have to start going wider or longer. 60# will be about 20% wider than a 50# or longer.

[upstatenybowyer]

Thanks Steve. I never really thought that a 20# could be under the same stress as a 150# bow. That seems crazy at first, but makes complete sense. I know I've read about this a million times, but there's something about hearing from a person in "real time" here on PA that helps me understand better.

20% wider for 10 extra pounds of weight seems like a lot. Has anyone ever come up with an equation to determine % longer or wider for x amount of added weight?

[bradsmith2010]

you have to use more wood ,,its relative,,  if you increase the draw weight (50%??),, you will have to use more wood to offset, the bow breaking or taking set,,
there is not really an equation, cause each piece of wood is different,,
but if you tell us what weight you want, what kind of wood,, and how long your draw is,,,, we can get you in the ball park,, then you will have to listen to the wood as you tiller,,
better to start wider than you think, and then side tiller if you are overbuilt,,if the bow starts to take set ,, then thats all the wood will do,,
what I see at times is guys want a bow that is 1 1/4 wide but 70#,, and then dont want it long, and wonder why it fails,,,,,, you have to make what the wood will stand,,,,, not what you think it should look like,,or not worry if it seems wide,, you dont have to go 20% wider if you go longer, Badger is just getting you width adjustment if you dont want to go longer,,
you could go a combination of wider and longer etc etc etc,,, so its easy but complicated,,
better to be on the safe side as you go heavy,, longer and wider,, you cant really go wrong,,
if the bow does not take any set,, and you feel it does not have good cast,,
you can shorten it and side tiller to hit your weight and reduce the mass,,
in bow making sometimes things are not as they seem, but if Badger says 20% you can bet thats bout right,,

[loon]

there's the mass principle in TBB 4
slightly rounded/mostly flat is still likely easier to make a heavier bow with/more efficient. Warbows were apparently more oval sectioned and a D section was used on Victorian English target bows.

[Stick Bender]

The osage stave I'm working on now is short for my draw it's 57 in I'm attempting a self bow so the same principals sorta apply but only going 48 lb but the simple way I'm doing it is go as wide as I can in this case 1 7/8 in. And after getting it bending and good tiller at 20 in. Or so just start side tillering mass tell I get to proper mass,draw & draw weight I like using the mass principle because of the simplicity of it takes a lot of the variables away when working with a new design !
 

[Del the cat]

For heavy bows, length (relative to draw length) is your friend... spread the stress over good long working limbs.
Del

[upstatenybowyer]

Thank you all for the input.

Brad, I'm following your logic for sure. I haven't got a particular wood in mind yet, but I'll definitely let you all know when I do. 

[SLIMBOB]

If you have a 1 inch wide bow, at a given length, that draws 40 pounds at 27 inches.  If you double the width, you double the draw weight, so 2 inches would be 80 pounds.  There is your equation.  The rub is that you cant use it with precision because 2 different pieces of wood will behave differently.  It will get you in the ball park however.  Going longer is your friend as Del said.  I don't know if I agree with D section on Osage.  If by that you leave the belly crowned, I don't do that with osage.  Others may disagree, but but I want it flat.

[Del the cat]

As Slimbob says... I'm not a big fan of D section as it gets interpreted as a high arched D and I had a part finished bow to finish for some one that was more like a V  . Rectangular with the corners taken off then slightly rounded, is more like it IMO
Del