Hollow Limb Design and the Mass Principle

[half eye]

My thoughts run along with Doc's. I just look at it from a different angle, that being: for any given width the two surfaces (one concave and one convex) with greatly increase surface area over the same width limb with flat surfaces. Increased surface area means increased distribution of load....either compression or tension.

I have no interest in "maximum Performance", meaning comparing one thing with another, but I really like this design because it's a new way of looking at a problem......and that's always good. I hope Simon's design kicks some serious butt.
rich

[PatM]

Some FG  lam bows have been made in that design. I can't remember which. It will come to me...or not. Jawge

Just the ACS design  Jawge.

[IdahoMatt]

Well put Rich. 

[George Tsoukalas]

Thanks, Pat. Jawge

[Badger]

  Jawge, it was Ol Adcock and the ACS. He had such a minor amount of concave affect I don't think it would come close to qualify as a comparison. It turned out quite a few bows at that time were actually faster than his. I think he used it as a sales tool more than a real tecnique.

[PatM]

Steve, I never actually saw definitively which direction the concave limb was facing. Was it cupped to the back or belly?

[Badger]

Pat, it was concave and convex on the back, he used a press to slightly shape the limb, it really was more hype than anything as the concave was almost imperceptable if you weren't looking for it.

[Cameroo]

I had a whole novel typed up, and then realized it was mostly speculation, so I decided to just sit back and watch the show instead 

Ah heck, who am I kidding! I tend to agree that increased surface area does not necessarily mean the load is distributed any better.  A lot of that added surface area is just making a larger neutral plane. I would think all of the compression forces would be concentrated on the outside edges, and that any forces applied that would be enough to "flatten out" the arch must cause set-inducing damage to the wood along those edges.

Just a hypothosis 

[Badger]

  The most obvious simple test would be to simply saw out a bamboo pole into a pretillered shape then check out what it does. I realy don't know id anything is gained or not. Seems like the compression on the ridges would be excessive but he has successfully completed several of these now. I wouldn't be surprised if some primitive tribe hadn't been doing this for centuries allready.

[Mohawk13]

Compression force less on the edges and increased in the middle when drawn. Really messing with a slide rule and a piece of bamboo. As it flattens the pressure in the middle increase by a coefficient of about 10, until full draw where it drops to about 3. Initial force creates intense force and then the load is distributed to the flatter surface, thus evening tension across the flattened width of the limb......or something of that nature

[Del the cat]

  The most obvious simple test would be to simply saw out a bamboo pole into a pretillered shape then check out what it does. I realy don't know id anything is gained or not. Seems like the compression on the ridges would be excessive but he has successfully completed several of these now. I wouldn't be surprised if some primitive tribe hadn't been doing this for centuries allready.

That's waaaay too sensible
BTW, regarding arrow mass. If you calculate the g force as an arrow is first loosed it's pretty huge, so there is plenty of inertia for the bow to work against at the start of the loose (or at the maximum accelleration point, which is presumably somewhere near the start of the travel).
Presumably the problems all occur after the arrow leaves the string, and the limb tips are having to stop?
So obviously if the arrow has gone, it can't be the thing that's causing the problem
Del

[mikekeswick]

First off i'll start by saying I am off to get one of these roughed out. I've got a perfect maple sapling that is just about dry now.

So to my mind the flattening of the limb is only going to be minimal , it has to be or else you would get a split sapling! I really don't think that the bow is trying to 'flatten out' during the draw. The poisson effect which goes on in every bow that is bent is actually trying to increase the belly concavity and make the back concave too. So maybe it's actually more about opposing forces resisting each other. The best cross section to minimise the poisson effect in a bow limb is a rounded belly and a flat back.

I can't wait to get this bow finished and shoot some throught the chrono. It will reveal the truth  I aim to make my HLD bow the same draw weight and length of my fastest straight stave bow and see how they compare.

[Badger]

  Mike, with the poisen effect I believe the developed ridges are facing away from the bend.

  Del, the idea is to stop the limb tips before the arrow leaves the bow or at least slow them as much as possible.

[simson]

I have posted the measurements of the HLD Elder bow on the other thread!
For comparison here are some other elders of mine:
http://www.primitivearcher.com/smf/index.php/topic,35631.0.html?PHPSESSID=ojscmbeir9rl5rm6ccho63ej64
http://www.primitivearcher.com/smf/index.php/topic,37923.0.html
http://www.primitivearcher.com/smf/index.php/topic,38689.0.html
http://www.primitivearcher.com/smf/index.php/topic,38720.0.html



First sketch is the bow unbraced with depth d1 and width w1 and groove g1

Second sketch shows the bow drawn with d2, w2, g2

What did happen?

d1>d2: you get a bow with a thinner limb, only a few millimeters make a great affair!
w1<w2: you get a bow with a wider flatter limb, the increase isn the last inches is lower than 'normal'.
g1>g2: you get a belly surface with compression force vertically (same as on every 'normal' bow) and additional tension force horizontally. Perhaps this effects preventing chrysals and lower stress. The wood cells have more room caused from zhe tension force for the vertically compression.

My thoughts for designing HLD:
- the walls must be thin enough to flatten out a bit when bow gets drawn
- the groove must be pronounced enough to get an decreasing limb thickness
- the Cross section should show a horizontally taper to prevent splitting. that means the wall is thickest in the middle and tapers  out to the ridges
- there should be a vertically taper (of course!) to get the bow even bending. But this also must prevent the sudden 'snap-effect' as can be seen on metal ruler tapes
- the bow must be tillered in the right proportions to get all that things working. But when done alright, you have a bow with high early drawweight, lower increase in the last inches ( of course no 'let-off'), the graph of the fd - curve is in the first inches steeper (more energy is stored) and there will be less stress on the belly     -   always compared to a 'normal' bow.

[dwardo]

Fascinating Simson.

Somebody get that man a set of digital scales and a chronograph.