Tiller vs. Front profile TBB4

[bigcountry]

So, I am confused.  Not new.  But anywho.  I have read thru the TBB4 a few times, mass and performance, and read the part about how tiller should not always be circular, but match the front bow profile.  They showed my most loved profile of up half the limb, being parallel in width and pyramiding to a the tips.  They recommend very little bend in the parallel area, and most bend last half of the limbs.

But I was taught that you want the most work from the widest part for the least set. 

But doesn't this contradict a molly profile and tiller?  With a molly or homie, don't you want all your work in the widest part?

Maybe I am reading it wrong. 

[bubby]

ya gotta remember on a molly the narrow part of the limb is stiff with no bend, so the wide part of the limb basically does all the bending, Bub

[Badger]

  Bubby, the bend should start at the fade like any other bow but with the paralell limbs the bend should slightly increase through the paralell area until it starts to pyramid to the tips, optional to leave the last few inches stiff or not. The elyptical tiller is a littlle less shocky and tends to be a bit more efficient. For a more circular bend just start the side taper up closer to the fades.  A pyramid bow bends evenly with equal thickness limbs and also distributes the stress equally. A paralell bow has to be thinner to bend equally and if it is thinner it is under less stress instead of equal stress if it doesn't bend a bit more.

[bigcountry]

ya gotta remember on a molly the narrow part of the limb is stiff with no bend, so the wide part of the limb basically does all the bending, Bub

Thats what I am saying.  TBB4 says the widest part should bend the least, and the pyramid part bends more.  Contradicts the molly or homie principle

[zenmonkeyman]

Took me a long time to cotton on to the reasoning for this.  I hope I finally have it right!  Here is how I interpret it:  It says in a few places that wood will compress or stretch about 4% without taking too much set or breaking.  Now with the parallel limbs, the thickness has to taper, or all the bend would be at the fades.  Another thing is, the thicker the wood, the more the back has to stretch and the belly compress to match a radius of curvature.  So; the upshot is, the thicker wood near the fades, bends less to use up its 4% than the thinner wood of the same width closer the tip.  If the limbs are parallel, and you have an arc of the circle tiller, or more bend at the fades, even, then you're either overworking the near-fade wood, or you're underworking the near-tip wood, or both!  So the worst case, if the wood doesn't break, is a high-set bow with heavy outer limbs, which would give you a sluggish bow.

Contrast this with a well-designed pyramid bow that has no thickness taper.  That's why the arc of the circle tiller works for it: all the wood is doing it's proper 4% part.

Hope I'm making sense here!

[Badger]

  Zen, that makes perfect sense to me. I like to think that thickness determines how far something can bend and width determines how far it will bend. Add that to the concept of trying to keep equal strain on a bows limbs and it will pretty much explain itself.

[zenmonkeyman]

Thanks Badger!

[bigcountry]

Interesting Zen. 

You might have it.  Too bad, they didn't articulate it as well you did.  Thanks a bunch

[bubby]

badger, all I was getting at was that on a molly the wide part of the limb, the working portion is where all the work happens, and that as on this bow the last 14" on each limb of a 66" bow, dont bend, Bub

[mikekeswick]

You are thinking only about the width. You must think about the thickness of the limb. The tips on that bow don't bend because they are thicker than the working limb.
Think in more dimensions!

[bubby]

mike the tips are thicker than the working limb, but they are also only 3/4" wide at the widest point compared to 2" on the working limb, the levers dont bend 'cause they are designed not to, believe me, if ya build bows ya think in dimensions, but I just take wood off till I got what I want, circular, elliptical, whatever, Bub

[mikekeswick]

I've made loads of bows with stiff tips and understand the design.
These bows are normally made wider and thinner than an equivalent flatbow (length,poundage,etc), so that the now shorter bending section can accomodate the extra bend nessacary.
The tips don't bend because they are thicker! If there was no increase in thickness they would bend too much. The thickness has more effect on the amount the wood can/will bend than the width does. To quote the TBB - to halve the draw weight of a bow you can either halve the width or reduce thickness by 1/8th.

A pyramid bow should have an almost arc of circle tiller because the thickness will be the same along the limb - therefore the bend the wood can handle before excessive set also be the same.

A parallel width limb that then tapers to the nocks should have an elliptical tiller because the wood will be thickest near the handle and reduce in thickness almost all the way to the tips. So the thicker wood near the handle cannot bend so far as the thinner near tip wood can.

A 4 x 4 will not bend far without breaking

A shaving will bend in a full circle with no damage.

[bubby]

mike, I was only addressing big country's statement on the molly bows, and I know why the levers don't bend, that's all, peace out, Bub