Primitive Archer
Main Discussion Area => Bows => Topic started by: carpholeo on March 01, 2015, 07:47:13 pm
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So i got this IPE board cut out to 3/8 at the tips, 1-1/8 at the fades, 3/4 thick and 64" long... and the thing will hardly bend. I want to back it with bamboo but im wondering how thin will i need to make this for it to bend?
(http://img.photobucket.com/albums/v603/r33s33/stuff/ipeblank_zpsklbvomtx.jpg)
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I'd pre-tiller the ipe so both limbs bend evenly and together until you can bend it into the reflex you want.
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just did a boo backed ipe, with 1/8 inch of boo. it got to good floor tiller at about 3/8" of ipe plus the boo
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Wow! i should slice it in half and make 2 bows then
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Yes. And make it no wider then 7/8" out of the handle.
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Yes... you could easily get 2 bows out of it, especially if you're going to back it.
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7/8 at the handle with a straight taper to the tips at 3/8, or should i go 1/4 at the tips?
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Ipe is about twice as stiff as most common bow woods (maples, oaks, elms, osage,… modulus of elasticity: c. 22 vs 10-12 for these other woods). So for the same total limb thickness you can make the limb nearly twice as narrow throughout its length as you would for these other woods (and end up with the same poundage). So if you normally make limbs 1 3/4” (~4.5 cm) wide, make them 7/8” (~2.2 cm) wide for ipe. (you could also make it a bit thinner if you still prefer a 1 1/8” wide bow; but the backing should be accordingly thinner as well).
I’d make the tips also narrower than usual (but not narrower than its thickness), as Ipe is very heavy (dense) wood.
Don’t be fooled by thickness either: suppose your stave at ¾” (19 mm) pulls 100#, removing 1 mm (about 3/64”) reduces it to 85#, 2 mm to 71#, 3 mm to 60# and 4 mm to 49#.
How to calculate this: take ratio of thickness, raise it to the third power, and multiply by draw weight at initial thickness; e.g.: weight at 19 mm = 100#. Expected draw weight at 15 mm =100# * (15/19)3
Force draw curves on straight (long) bows are fairly linear. If I take my own FD curves for straight bows, weight at 28” is about double of the weight at 15-16”. So if you carefully tiller the bare ipe belly to 15”, you can estimate what it would be at full draw.
Suppose you have 3/8” belly of ipe drawing 15# at 15”, and want to add 1/8 of bamboo, the expected draw weight at 28” would be c. 65-70# (if the MOE of bamboo is the same as of the ipe).
Or you could just do it like most people do it, by experience, gut feeling and fingerspitzengefühl. If you lack these (like I do) you can use such arithmetics to compensate for it.
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You lost me the calculus stuff. Lol. I guess I'm old school. But that was interesting
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Forgive the question but what is ipe?
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I beleive brazillian walnut
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Yep, braziian walnut is another common name, though it's not in the walnut family. Tropical hardwood. Commonly used as flooring on outdoor wood decks ... expensive outdoor wood decks.
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I'd pre-tiller the ipe so both limbs bend evenly and together until you can bend it into the reflex you want.
Rule #1
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joachimM, my head just exploded
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Yah... me too. I think you're putting way more into bow making design than there is. And, making wood bows by 'feel' is the best way IMHO. Wood is wood, and each piece is very different, even of the same species, or even from the same tree for that matter. I don't think blueprints and mathematical designs work for wood bows. Just my opinion.
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Is the bow going to bend through the handle?
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semi bend through handle, but im open to suggestions at this point
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Yah... me too. I think you're putting way more into bow making design than there is. And, making wood bows by 'feel' is the best way IMHO. Wood is wood, and each piece is very different, even of the same species, or even from the same tree for that matter. I don't think blueprints and mathematical designs work for wood bows. Just my opinion.
I think you are right.....except I think if you start at a given size (any size) weigh it, you might get close if both pieces weigh close to the same with the same MC. The mathmatics is a good base line, although a bit over my head....math is not my strong suite.
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It's not that mathematics or engineering and design principles somehow 'don't apply' to bow building with organic materials, as they most certainly do. Rather, the issue is in the inability to effectively measure. Applied Mathematics is the slave of measurements. Without effective means to measure ALL the variables, a near impossibility, the mathematics can get no more effectively close than an educated guess. When we can either eliminate the variables (i.e. straight grained wood with no run-out), or improve our ability to measure the factors involved (i.e. by using a moisture meter or digital calipers), we make the results of our mathematics work MUCH more reliable.
The limits of materials science are amplified the closer you approach to the limits of the performance of the material. Being as the best performing bows are made from the least possible amount of material, logically, higher performance bow building is at or near the limits of the material.
The intuitive method of bow making does not ignore the mathematics of materials science involved, ...it just uses a process of perceptions that are more difficult to measure, define, or quantify. After a while we say things like, "I just knew that I could get a few more pounds out of that limb" or "I had a feeling it was going to give way at that point". Hardly a predictable science, but the guys that do it are far too consistent to all just be lucky!
OneBow
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Sorry guys for the math...
I tried to give an answer to your question: how thin does the board need to be for it to bend (and make a bow). Since I don't know how much it draws at the given thickness, I tried to provide a way how to calculate the required thickness yourself.
I know that not all pieces of wood are the same, but to a large degree, there are constants on which to build. Otherwise we wouldn't all want to make osage bows instead of willow, and you could as well take a belly of douglas fir instead of ipe. Onebowonder gave a good account of the limits of maths in bow design. Within a single board, odds are high this math approach will yield useful results.
Let me try to make it simpler through a few well-know facts in archery:
As bow width doubles, the draw weight doubles. Very logic and intuitive: you doubled the amount of bow, so the draw weight follows that ratio.
as the bow thickness doubles, the draw weight is multiplied by eight (TBB vol 1: tillering, by Jim Hamm). This is much less intuitive.
So if you have a bow of draw weight 20#, a similar bow of twice the thickness would draw 160# at the same draw length (if the wood could stand the compression and tension at that thickness and draw length). This is because thickness and draw weight follow a cubic (power 3) relation.
doubling thickness: original thickness times 2. Draw weight is then 2*2*2=23=two to the power of three = 8 times more
Adding or removing a tiny bit of thickness has a large influence, and this is what makes tillering so critical: one pass too many with the rasp can make the difference between a 40# and a 60# bow.
Suppose you increase or decrease thickness not by double or half, but by some smaller degree, what is the expected draw weight?
You take the relative amount you increase or decrease in thickness, multiply by itself two times and multiply by the former draw weight.
To go from 1 cm thick at 30#, to 1.1 cm, you multiply by 1.1. Take this to the power 3 (1.1*1.1*1.1 = 1.33), and this number tells you how much the draw weight is expected to increase. So at 1.1 cm thick, all other things being equal, you end up with a draw weight of 30#*1.33=39.90#
You can also do the opposite: if you know the draw weight at some thickness but want another draw weight, how thick should the limb be? (this was the initial question here, more or less).
Suppose the thickness is some value A and gives a draw weight of say 80#, how thick should I make it to get to my target draw weight of, say, 60#? Or for your ipe board: how thick should the belly plus backing be to get to your intended draw weight?
I added a very simple excel sheet that allows you to calculate this by giving current draw weight, current thickness, intended draw weight. The output is the required total thickness (ipe plus bamboo) for the intended draw weight.
For you imperialists, I even added a inch-decimal inch-cm conversion chart ::)
So you can forget all i wrote above, and just enter the numbers in the excel sheet and see how the output (in red) changes. Since you will need to pre-tiller the ipe belly for best results, you might as well have an idea what a certain thickness of bamboo lam will do to the draw weight.
Hope it helps.
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(http://img.photobucket.com/albums/v603/r33s33/stuff/headache_zpsc0v7vysu.jpg)
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https://www.youtube.com/watch?v=8Y0yolPv7tU
As long as you realize that shaving off wood will make the bow easier to draw, you can just goad ahead on that board and shave wood off till you can bend it. That will give you the answer to your question how thick the board should be in order to bend.
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https://www.youtube.com/watch?v=8Y0yolPv7tU
As long as you realize that shaving off wood will make the bow easier to draw, you can just goad ahead on that board and shave wood off till you can bend it. That will give you the answer to your question how thick the board should be in order to bend.
Glad you posted that.....I feel better now.....about those beans..... :laugh:
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:-\...I'm gonna need that slower and dumber please. .. lol. .. :laugh:
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Ipe is very striong.. It took me at least 5 bows early on, doing boo/ipe to realize that I was scraping off more ipe than I was leaving...ending up in hunting weights, 1.5" wide 68" R/D bows that were barely 3/8" thick. Ipe is STIFF.
I must be the only guy here who has never done the ELB with it.
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joachimM - You are dead on.
Onbowwonder - You too.
People say oh this stuff isn't applicable to wooden bows blah blah....true you don't need to use maths but boy oh boy can it take the guess work out. Rulers aren't much use calipers allow you to measure accurately enough for the forumals to be useful.