Bow and wood testing

[Badger]

  That article by Dick Baugh is what inspired me to test out the no set tillering theory. Aside from going to designs such as Mark St Louis in known for which really have to be disected on their own, My findings are similar to Dicks but I feel I tested it out in a more applicable manner.
  Using a shooting machine and fast flight strings for uniform testing purposes I find with rare exceptions a self bow will exceed about 176 fps. These are bows with some reflex, either flipped tips or slight d/r. Backed bows will rarely exceed 186 fps. I consider anything around 170 fps for a self bow or 180 fps for a backed bow to be excelent performance.
     As for the Hysterisis tests done with a working bow. I can't attribute 100% of the gains on a no set bow to removing hysterisis but I know it is a large portion of it. I can seldom get a normal sized bow past about 23 or 24" draw without at least some breakdown of the wood. On one occassion I was able to get one out to 26 3/4" before the first hint of weight loss took place, this particular bow was my personnal fastest at 192 fps, after a slight overdraw the speed was reduced to a respectable 184 at 28" draw 10 grains per pound. This particular bow was backed.
  As for various designs, I can't really see a lot of difference. Some designs will slightly favor heavier arrows while others might favor lighter arrows but when properly executed and backset being about the same most designs are very close. Always a bonus for skinny tips.
   My testing shows a dramatic pattern where the tiller shape of the bow should match the front view of the bow. And both of these factors need to be decided on based on a reasonable mass figure. In other words, a red cedar deep section elb 66" long would not have the same tiller shape as a 66" long bamboo backed ipe bow. When the density of wood is so great that a bow cannot be made any narrower then we have to start pushing the bend on the tiller out away from the fades to justify the mass.

[rossfactor]

Steve - "When the density of wood is so great that a bow cannot be made any narrower then we have to start pushing the bend on the tiller out away from the fades to justify the mass."

That right there is a helluva statement. Really interesting. 

So, is the 'ideal' amount of bending limb a function of wood density? I mean, taking an idea to its extreme:

If a bow wood was extremely light, but also had extremely high elasticity, would  3 inches of bending limb and 23 inch narrow static levers be faster than a bow with 13 inches of bending limb and 13 inch static levers?  I know there are other things going on here than mass placement.... just thinking on e-paper 

Gabe

[hurlbri1]

Very cool conversation here!  I love that I can totally roll this into my physics classes

On the ellipse versus parabola: My guess is that there is a quantifiable difference...but that is what I am trying to determine in my little spreadsheet tool!  I'll keep ya posted.

On hysteresis...cross-sectional area would definitely have an impact on hysteresis.  If energy loss is due to heat, and heat is caused by molecules moving against each other causing friction, then more molecules would equal more friction, right?  Maybe...if I were a guessing man, I'd guess that histeresis is closely correlated with bow mass...

This is an excellent discussion, by geek brain is loving it!

[rossfactor]

I think the development of hysteresis is more related to elasticity than mass.

Steve - "Wood has almost no hysterisis or at least very small until it starts to develop some memory of being bent."

And memory of being bent is a function of elasticity.

I could be wrong about this.

Gabe

[Badger]

  Gabe, I agree with you. If a wood has more elasticity it can be built at lower mass. Most woods are pretty close in elasticity. I don't know the numbers by heart but I do know that both yew and osage excel in being more elastic. Plum and ocean spray are also good examples of exceptional woods. Mass is proably the easiest thing for us to measure so working with averages it can give us at least a guideline until we get to know a specific type of wood.
   If we took a piece of ipe at .9sg and built a bow with the same design using say maple with a .6 sg. we would have to make the maple bow about 50% wider and the same thickness if the elasticity were the same. If we tested the wood and found one to be more elastic than the other we could make that bow narrower and deeper with lower mass. I based the mass theory on the results I got from avrage woods. Primarily I was using hickory, osage, maple, oak, ipe and yew for the bulk of the bows tested with a good smattering of other types of wood I was able to get ahold of.
     As for the example you gave earlier. about one with long stiff limbs and another with more working limb would be hard to call because it wouldn't be possible with wood unless it was extremely wide but. Shorter working areas tend to be more efficient because they don't vibrate. If the limbs were very light and bending right next to the fade you would have the best of both worlds so it probably would be faster.

[Jim Davis]

 Hi Steve! I have done some thinking and reading since I posted about this sort of thing on the other site.

First, you were right about raw strength not illuminating how the back of the bow behaves during the draw and loose. Turns out, a tension break in wood is in the category of a "brittle" rupture like glass and ice--there is no stretch before failure.

What this means is that I was off the mark in thinking that narrowing the back would make the back work more. You were right that it seems only to reduce unneeded weight in the limbs--a useful outcome.

It also means, that the belly is always doing all the work of being a spring. That may figure on that other subject of whether a belly should be flat.

On the subject of an end grain core, the Forest Service Wood Products Laboratory data shows that hickory, hornbeam and locust (the only woods I took a quick look at) are about 5 times stronger in compression parallel to  the grain as 90 degrees to the grain. (Yew is about 4 times stronger parallel to the grain.

Another strength factor I have pretty much ignored is the shear strength parallel to the grain. Don't know if there is some yield there before rupture. We don't find bows failing in shear very often, but if  there is some distortion short of rupture, that could also be part of the energy storage equation.

Nice thread.

Jim Davis

[Badger]

  Thanks Jim, good topic on the shear forces. I am with you on that. I feel they are more important than we give them credit for. I have had several yew bows fail in shear over the years. I think shear forces help to explain why laminated wood bows will usually outshoot self bows. Lately I have been rethinking the concept of cores being just a seperator and the lighter the better. They may have more effect on sheer and the way the belly compresses than we give it credit for.

[TBod]

About why narrow tips/nocks are fast..

Was just thinking that the aerodynamics is also better on narrow nocks. Don't really know how fast the nocks move when you shoot.

I actually think that contributes more why narrow nocks are faster on a bow than the lowweight explanation!

[Badger]

  TBod, the air resisitance doesn't really account for much loss. The tips are going less than 1/2 the speed of the arrow and most of that speed is gained toward the end of the shot. But no doubt it does acount for some.

[TBod]

Ok, but the air resistance during acceleration increases exponentially. Maybe I'll do some tests not totally convinced. Thanks!

[Buckeye Guy]

Badger
Thank you for sharing your knowledge !
I am glad you have the words to explain these things to others  !
Your knowledge confirms many of the things I feel and believe to be true but don't have the words to express or any means to prove!
Your work will fuel many generations to come !
Thank you !!
Guy
When you get bored with what is going on in the bow consider the arrow ,just a stick ,or is it ?