Thickness vs width questions

[Woody roberts]

I’m not sure how to ask the question but here goes. All woods being individual notwithstanding is it better to make a bow wider and thinner or vice versa for a target draw weight. I realize soft woods will be thicker than harder woods at a given draw weight.
For instance would a hickory pyramid bow 2” x 1/2” out of the fades be better than the same bow 1-3/4 x 9/16” out of the fades.

My suspicion is the thicker bow may shoot faster but the thinner bow may last longer. I could be wrong, I often am.

[Pat B]

I think it depends on the characteristics of the wood. Dense woods like osage can be narrower and thinner but woods like hickory and maple work better a bit wider and thinner.

[mmattockx]

My suspicion is the thicker bow may shoot faster but the thinner bow may last longer.

That is pretty accurate.

Limb thickness determines the strain/stress in the limb wood. The thicker it is the more stress the wood sees for any given amount of bend forced on it. Width determines the total draw weight because it determines the volume/mass of wood working.

The way I look at it is each piece of wood has a maximum stress level it can withstand before failing (typically by taking set in most hardwoods). This maximum stress determines the maximum thickness the limb can be for any particular bow design. At that maximum thickness every inch of width will develop a certain amount of draw weight. Width is then determined by what weight you want the bow to have.

Maximum efficiency for any given design is achieved when you take the wood right up to its stress limits, but not beyond. This equates to a thicker/narrower bow overall but if you go too far it will take set and lose performance.

Since wood is not predictably consistent in its properties like steel and other structural materials you either have to test and measure each piece individually or use experience and rules of thumb to guide how wide you make your bow and then refine it during tillering.


Mark

[Woody roberts]

This question may need a separate thread. I like to work my limbs very close to finished thickness before doing much if any bending. For hickory I start with 1/2” . Hackberry,walnut etc I start at 9/16 to 5/8”

I’ve only made 3 Osage bows and haven’t worked out a starting thickness yet.

Do these starting measurements sound about right or should I adjust?

[Pat B]

I basically don't measure thickness. I do start out about 5/8" to 3/4" and go down from there as I get the bow bending.

[PatM]

Thicker is quicker.

[mmattockx]

Do these starting measurements sound about right or should I adjust?

That depends on the bow design details (length, draw length, weight, width, style, etc.). A recurve will use different thicknesses than a flat bow, which will be different than an R/D design. Stiff handle, stiff tips, etc. will also affect it.

FWIW (given my small number of bows), I have never had one end up thicker than about 0.425"-0.435" thick. These are mostly pyramid bows with little reflex, ~67" long and around 40#@28" made from hard maple or red oak.


Mark

[bownarra]

Chapter 4  TBB vol4 (?) chapter on design and performance.
And yes thickers quicker

[Selfbowman]

Mark is right if I understand this correctly. So if you have a 50# Osage that is 7/8 wide and 7/8 thick will it be quicker than 1-1/2 wide by 3/8 thick? Of course the ends will taper from mid limb. . Arvin

[RyanY]

The assumption that soft woods will be thicker is wrong as density does not necessarily correlate with elasticity.

The general assumption is that a thicker narrower limb will have less mass and therefore be quicker. Badger often chimes in about a bow(s?) he made years ago that we’re wider thinner but lighter (less mass) than narrow limbed bows. I don’t think he measured the performance of those bows.

The issue with going too narrow is that you risk needing increased thickness for a specific draw weight but that increased thickness will be under a higher strain and more likely to take set. There is probably a sweet spot for low set/strain and low mass determined by a bows cross section. If your bow isn’t taking set late in the tillering process you have room to remove width for more mass reduction.

[Selfbowman]

I think the density will determine the size in general. The mass weight will be close no matter the material if it’s all the same material. For example bamboo cores in a composite bow will maybe mass weight lighter if the glue does not add to mush weight. So my simple composite bamboo backed  bow came out lighter because of thicker bamboo than normal. I also trapped the rind because it’s more dense. The mass weight came out 3-4 oz lighter than a normal Osage selfbow. That one is fast! Arvin

[BowEd]

Correcto Arvin.That's my observations also.It'll happen putting horn on the belly too,but then it is'nt a self bow any more.

[Allyn T]

Ryan, if it isn't taking set would it be necessary to narrow or would it be like the wider bows you just mentioned? Do you have to go extremely wide and thin for the benefits?

[RyanY]

Allyn, to be honest I’m not sure. I think we’d have to build a number of bows with the same or zero set with differing thicknesses. I am wondering if there are two advantages to wide thin bows. The first being that thin limbs will be under less strain and are less likely to take set. The second being that a higher percentage of working wood is closer to the surface with less non working wood in the middle/neutral plane of the limb possibly leading to lower mass.  We often talk about the outer surface of the limb doing a majority of the work but there is some gradient of stretch between the surface and neutral plane. My thinking is that thin limbs make that gradient very “steep” if that makes sense.

I should add Steve’s thinking (if I remember correctly) that even in low set bows there is the possibility of damaged wood that isn’t contributing to work but adds mass. Making a bow extra wide may help avoid this due to very low strain. The big issue is no one wants to make 3” wide bows. 

[Allyn T]

Lol yes, my first bow attempt was 3" wide at the fades and I didn't realize at the time how much harder wood removal is at that width. It made me never want to try it again.