effect of sijahs on draw weight

[stuckinthemud]

Ok, so the purpose of levers on lever-arm bows is to reduce the effort but by how much? Is a Turkish or Magyar drawing at 57# apparent weight actually pulling at 75#? How can we calculate the effect of sijahs on draw weight?

[loon]

it'd be a lot more if you put the strings on the "knees" (where bending limbs and levers meet)
I think it's to have a fatter draw/force curve, make the short bow less stacky..

[mikekeswick]

Too many factors to consider to know 'how much' for various different designs.
The whole idea is simply to reduce string angle. Cut the tips off any recurved bow and see how it feels without them

[loon]

If you had a Molle and a pyramid, both with even bend on the bending limbs, same max width, same length ntn, same brace height, both straight bows, same draw weight at 28", I think the molle would have a "fatter" draw force curve. Stores more energy.. would it not?

[stuckinthemud]

I kinda want to keep this really simple and really basic, as Loon says, if the strings were on the knees of the limbs then they would be much harder to draw - for the purpose of this question, I would say that was the 'true' draw weight, the weight of the bow if the string were at the end of the working section of the limbs.  If you put a lever on the end of the limb, that reduces the effort required - like putting a torque lever on a jam-jar lid when its stuck.  Is one of the reasons that horn bows are so effective is that the use of levers moves impossibly powerful bows within the capability of the archer to draw them?

[loon]

Well, if you put levers on them, then the inner limbs won't be bending as much for the same draw length, but the same amount of movement by the limbs moves the arrow more than without the levers, so it might give the bow a mechanical.. advantage? or disadvantage? Not the best at physics.

[stuckinthemud]

I think that is true, a huge mechanical advantage - short, fat, strong working section to the limbs can be bent by a lever more easily than without, with the bonus that the lever also maintains a decent string angle; just wondered if anyone knew how much of a mechanical advantage levers offered - if it can be calculated then the ideal length for the levers could be found for a given design?

[loon]

I think the ideal length of levers could change given materials, arrow weight and draw length..

[mikekeswick]

A recurved tip (of any kind) simply reduces the angle between the string and the tip thus giving greater 'advantage' over the increased angle you would have without them.
This allows you to bend a thicker limb for the same effort on your fingers. Storing more energy during the draw as a result.
Horn and sinew have the capacity to store more energy for a given weight compared to wood alone - that is why the composites can be better bows if designed correctly.

[stuckinthemud]

Thanks for clarifying that Mike, I was visualising it more as a lever/fulcrum thing, but is there any way of quantifying the advantage - longer recurves, greater advantage. A two inch recurve would not reduce the effort as much as an eight inch one, but is there a rule of thumb here? You know like, every additional inch of recurve reduces effort by so-much %?

[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??'   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?' 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!

[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.