limb thickness vs draw weight?

[shawnkfl]

do you guys have a chart to figure the draw weight for building laminated limbs? i saw the one on binghams, just wondering if that was the end all be all, or if there is a formula to figure out the lamination thickness. how about for all wood bows? is that a thickness calculation based on wood type?

[jsweka]

The Binghams weight chart is specific to their bow design.  Deviate from that and you'll get different results.  You really can't have one formula to figure out appropriate lamination thickness for a desired draw weight because there are just too many variables - bow design, bow length, bow width, riser length, width taper, and thickness taper.  Professional bowyers take a lot of notes on each bow they build and once they have settled on a design and made many of the same design with different lamination thicknesses, they can figure out what they need to meet a customer's request. Yes, wood type can have some influence on final draw weight in a given design too.

[IdahoCurt]

The chart is only good for the bow profiles he sells.Those numbers will get you in the ball park but will take some fine tuning on your part(multiple bows) to dial in your design and lam stack thickness.
Write everything down and keep consistant in your design and you will have your own formula.Lots of trial and error.

[shawnkfl]

being a mechanical engineer, i knew there would be a bunch of factors involved. i guess i was hoping to see a chart or figures that are "close" to the finished draw weight. i mean, i would hate to build bow thinking i was building for a 60# draw and end up with a 30# draw. thats a lot of time and material. i guess i could start from the binghams chart and that would at least get me in the range. then i could tweak from there.

[Pennsyltuckey pete]

I have experienced just that on the first bow that I designed from  scratch.  Way light on the first try, then way way heavy.  Third and fourth were  close to controlled.  Part of the learning curve.  I now have a 26# practice bow that has gotten a lot of use by me and my friends and their kids!

Not a bad outcome and definitely not a waste.

pete

[Tucker]

What JSWEKA said!

What I do is build several bows on the same form with the same thickness of glass, but different thickness's of laminations, keeping everything else equal - like the length and the width of the finished limbs.

Then I plot on graph paper the relationship between limb thickness to draw weight.  Draw a line through the dots. After doing this a few times the graph yields predictable results to plan for a bow of a known draw weight off of a specific form.

Yes it does take a lot of materials and expense, but it pays off in the end.

For the beginner - find a bow very similar to what you want to build or exactly like you want to build (like a Bingham Projects bow). Take note of the thickness of the glass on it and the total limb thickness. (note- the limbs may have tapered laminations. Pay attention!) Note the draw weight of the bow similar to what you want to make.

Bingham's predictions of .001" thickness change per pound of draw weight for recurves and .oo3" thickness change per pound of draw weight for longbows is pretty close. For hybrid bows, or varying R/D bows, extrapolate between these two numbers as an educated guess to get you started.

Building bows is like eating Lays potato chips...nobody can build just one!

[Swissbow]

I got a chart from Jason for a longbow with a mild D/R-profile. I used it on several bows and it worked perfect so far. I always choose the stack 5 - 10 lbs heavier, thus giving me some more room for tillering and I can shape the tips very slim. If you want to add more reflex in your design you can take the chart as a starting point and choose the stack like 5 - 10 lbs lighter than on the chart ( depending on the amount of reflex you want to add ).

You can download it on my site ( in the download section )...

http://swissbow.com/

The more bows you build the better you can adapt the chart to a new design.

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Andy

[Bradford]

I kind of used a little of everything and actually came close.

I  used Binghams chart and swiss bow chart.. then found some common factors about riser size, limb size and such.  I actually hit my marks using these factors.

If you look at the two charts you will see some commonalities that you can use.

Always shoot high..  you can sand the sides of the limb to lower weight.

Good luck

[SEMO_HUNTER]

Originally posted by Swissbow:
I got a chart from Jason for a longbow with a mild D/R-profile. I used it on several bows and it worked perfect so far. I always choose the stack 5 - 10 lbs heavier, thus giving me some more room for tillering and I can shape the tips very slim. If you want to add more reflex in your design you can take the chart as a starting point and choose the stack like 5 - 10 lbs lighter than on the chart ( depending on the amount of reflex you want to add ).

You can download it on my site ( in the download section )...

 http://swissbow.com/  

The more bows you build the better you can adapt the chart to a new design.

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Andy

Andy, would the longbow R/D chart also work for a take down longbow?
I'm building a short longbow, which was originally intended for my nephew.

The riser is only 10" and I'm looking for a total length not to exceed 60" and a poundage between 40-50lbs.
I'm having a hard time figuring out what size lams I would need.
Thinking about an Osage core lam, and Bo-tuff clear glass outside lam. Or possibly Zebra wood inside lam? Don't know if that would make any difference or not?

Sorry to bust the post, but I thought this would tie right in with the topic.

Thanks,
SEMO

[IdahoCurt]

A Take down LB will need less lam thickness due to the shorter working limb length.A 10" riser? that is really small for a take down,cool!

[SEMO_HUNTER]

It was supposed to be a Christmas present for my nephew, but I was running out of time so I put the take down aside and started on a regular 1 piece Osage longbow. I knew that I could make one of those from start to finish in just a matter of a week or so.

Here's the riser:

 

[IdahoCurt]

Nice,that looks very solid and I bet it won't have any hand shock.Post pics when you're done.

[SEMO_HUNTER]

That's what I'm hoping for, just not real sure where I want to go with it at this point?
Laminated limbs, or natural Osage limbs?
Probably start with all natural, the I could always get the materials for a laminated limb project later on. I'm not getting much helpful info on this subject, so I'm trying to figure it out on my own.

It would be nice to build a set of 30# limbs, and make heavier ones as my nephew grows into them. Work up to 40# and 50# or 55# but at some point the limbs would probably out grow the riser?

That's ok though, cause I've already got some cool ideas for a different riser using walnut. Our place here in Missouri is full of walnut everywhere.