How thick are your limbs for common wood bows 40-50#?

[gfugal]

There is another thread talking about whether 1/2" of limb thickness on an Osage bow 66" long would be enough to get a 80+ lbs bow. After reading through the comments I was surprised by how thick people were saying their Osage bows are. How thick are your bows that are around 40-50#? How do you measure thickness: from top of crown to belly crown, or more of an average? I guess people have a much rounder tiller than I do where the crowning affects the thickness much more, (whereas the average thickness may not be so drastic as 1/2"). My bows tend to be basically flat on the belly.

This is why I ask. I have had two bows break recently around 40-50 lbs at full draw of 28-29". They were both over 60", nearly 2" wide, and a tad over 1/4" thick. 1/4" is much less thick than the 1/2" people were talking about with their Osage bows. Granted my bows weren't 80# nor made of Osage, but still, you would think they should be able to handle a stress much less than that since they were half as thick. After all, thickness and bend radius determines the stress on the wood. I keep asking but is there a point where too thin limbs actually increases the likelihood of a bow breaking? If my limbs are getting too thin should I try to reduce width more rather than keep scraping the belly?

[Pat B]

The thickness is determined by tillering to your intended draw weight at your draw length.  I usually start at about 3/4" for staves just like boards.

[PEARL DRUMS]

Every bow, every piece of wood, and MOST importantly every bowyer is different. What doesn't work today will work fine tomorrow, or for the next bowyer.

[willie]

I keep asking but is there a point where too thin limbs actually increases the likelihood of a bow breaking?

generally not. I would suspect something had changed with the bows that broke. went out of tiller? got too dry?

woods like osage and yew can be thicker because they are more elastic. longer bows need thicker limbs, all else being equal.

[Badger]

  I am surprised yours broke at 2" wide and 1/4" thick. I very often use short working limb areas in which case I will be thinner than 1/2" but I don't remember anything going down to a 1/4"?  Did you have any grain run outs or violations. Osage will break in a heartbeat if the grain runs off the edge.

[Badger]

 I just measured a 70" 50# osage that is .450. Pretty thin for a long bow like that. I never measure thickness as a rule, I just watch for set and keep tillering to whatever it ends up.

[Jim Davis]

I make my bows to a set of dimensions. They are all pyramid.

I make them all 66" ntn, 1-1/5" wide or a little more, and I CUT them ALL to about 5/8" thick on the band saw, then thin the limbs from fade to tip to about 9/16", then start tillering for an even bend. They end up around 40# depending on width, wood and how careful I am.

I too am surprised that your bow broke, being only 1/4" thick, but 60" is kind of short for a full draw...

[George Tsoukalas]

Kind of short at 60" for 28-29" draw.
I'd go 67" ntn minimum.
Other than I do what Pat does.
Jawge

[BowEd]

Just got through tillering a 65" NTN osage and @ 28" it's 50.5#'s and @ 1 and 5/16"" wide midlimb it's 13/32" thick.Thickness @ fades is 18/32".It's eliptical tillered @ 1 and 3/8" wide at the fades.So a 1/8" narrowing to midlimb too.Measuring midlimb from center of handle.

[PatM]

  The bows were OVER 60 inches.

[gfugal]

Here's the two bows I was talking about.

The most recent was a Canyon Maple bow about 67" NTN but with Recurves


You can see in this next two pictures how thin it is


In this picture, my ring is about 1/4" thick and the limb is about that thick past where I'm holding (it fluffed up quite a bit where it blew up so it's a little thicker there so it's not a good example). I know I measured it with calipers and it was almost 1/4" but I can't remember exactly. Probably 5/16th. Unfortunately I threw both away so I can't remeasure them.


The other bow was a Red Oak Board bow from Lowe's. It was about 64". It was a little thicker than the maple, closer to 3/8.




Both were inferior woods with more radical designs (recurves for at a shorter length). They also may have experienced damage before the final tillering that caused the breaks (serious torque while trying to get out twist on the maple; on the Red Oak it raised splinter while attempting to brace). I backed both with linen due to these issues. However, both still got more than an inch of set, which was telling that it wasn't just a freak internal weakness but was stressed considerably for the design as well.

[PEARL DRUMS]

Your oak board needed to bend a lot more from the fades out 10", that's why it broke and why it took a lot of set.

[gfugal]

Your oak board needed to bend a lot more from the fades out 10", that's why it broke and why it took a lot of set.

Yeah, you can tell that my tillering improved from the oak board to the maple. The oak board was also a board with recurves and not the longest at 64". So really I'm not too surprised that it broke thinking back retrospectively.

[PatM]

How dry is it where you are?  Your moisture content might be dangerously low too.

[gfugal]

How dry is it where you are?  Your moisture content might be dangerously low too.

This winter inside my house it's been pretty close to 40% humidity at 70°, and I tiller inside and store them inside while I'm tillering. There are times where it will drop below 40% maybe 38% but rarely 35%. More often it increases to 50% or so but never getting much higher than that.