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effect of sijahs on draw weight

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mikekeswick:
No!
A bow is different because there is no fulcrum so the term leverage isn't correct. You don't get leverage as such you get a mechanical advantage through the string over the limb. Think about when first bracing a highly reflexed bow...you started with long string tillering now you need to increase the brace height...so you shorten the string and try to brace the bow, chances are you didn't shorten the string enough first go and the  bow stretches the string that it goes rom a couple of inches brace height to non with the string tight along the belly.....that is mechanical advantage.
What matters is the angle between tip and limb(and consequently tip and string). Less angle = less advantage, more = more!
To find out the difference between one bow and another they would have to be identical apart from the tips. Failing that the maths is very complicated!

stuckinthemud:
Cheers Mike!   The not quite short enough string on a reflexed bow is very familiar - the string did stretch, quite a lot!!

You can tell when its a quiet day in the office when the worm in my head says 'yes, but why??' ;D  I know that the bows we learn from are the very best, that the proportions have evolved over centuries of trial and error until they reached the perfect form, its just that that little worm (when I'm bored and there's nothing to do except stare at the screen) says 'Yes that's the best, but why?' >:D If you understand the principles you are able to design your own but if not, you are stuck with copying or making many un-intentional errors, like I wonder what will happen if.............  but me and complicated maths don't get on, not even a little bit, shame there is no rule of thumb in this instance! :D

gfugal:
I was doing math for determining actual limb movement for each inch of draw length, and I stumbled across an interesting occurrence. No matter the length of the limb, the limb movement will be the same, with a constant draw length and brace height. For example, a 50 inch bow's limbs are going to bend 16.6 inches at a 32 inch draw, when it's braced at 6 inches. Similarly a 100 inch long bow's limbs will also only move 16.6 inches at a 32 inch draw, with a 6 inch brace height, despite having twice as long limbs. The difference is the comparative stress on the limbs. While they are bending the same distance, the second is much less stressed because the bend is at a much lower percentage of the total length.

I'm still trying to wrap my head around what this means for levers but I believe by adding them it does twofold. 1) as already mentioned, it aids with string angle, which I'm still trying to wrap my head around. 2) It increases the length of the bow slightly (only with sijahs, recurves actually decrease the length of the working limb). What this does is move the point the whole limb has to travel further out past the working limb. So while the whole limb, with the sijah  will travel that same fixed distance per given draw length, the working part of the limb doesn't have to move as far. Thus they aren't being as stressed.

DuBois:
Like an atlatl is an extension of the arm. Still throwing it with the same effort either way but...

Throw a ball for your dog with your hand and then try it with one of those fancy dog ball throwing levers and see the difference in distance.

BowEd:
That's my interpretation or enhancement of distance on it but don't know about draw weight of it.

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