Tiller shape vs front profile

[Don W]

We seem to interprete speed with efficiency, and speed can be gaged with each situation. So how do I constantly make my heavy hunting arrows go faster? What exact profile do I need to start with and expand from.

Interesting question. With heavy arrows, a bow becomes more efficient, so efficiency gains are harder to tease out of designs.  So maybe just a heavier draw weight? BTW, for me, a bow that is at my upper draw weight limit "feels" better if it is longer.

Everything is a trade off. It's written over and over in TBB. Obviously a short bow is more convenient for hunting, but I agree a longer bow seems more comfortable to shoot. I don't typically sit in tree stands, and don't use commercial blinds much, so I need to get past the need for a shorter bow I think. I'm not even sure where that desire came from.

[mmattockx]

I'm not trying to strain the wood evenly i'm trying to strain it optimally for a bow!  Two different things

Not necessarily. By straining all the wood equally you are getting the most work/energy storage possible without overloading any particular part of the limb. That sounds pretty optimal to me.

+1. It has never made sense to me that people think the limbs are strained/stressed evenly along their whole length.

They certainly can be and this would be the optimal way to store energy in the limbs. A pyramid profile with a true circular arc tiller will see equal strain along the whole limb. Why do you think that isn't possible?

It's round.  A typical circle is round.

Answer of the week! A better definition might be that circular tiller has a constant bending radius. Elliptical tiller has a larger radius at the base of the limb with the bend getting progressively tighter towards the tip.

I think it’s helpful to think of these topics in terms of examples and thought experiments. Say you have a pyramid bow and a typical parallel limb taper (American longbow) bow and both have a circular tiller with the same amount of set. Which one would shoot faster and why? (Same poundage/draw length, length, etc. )

Probably the pyramid bow because the outer limbs are lighter, but that is a SWAG at best.


Mark

[scp]

I'm not trying to strain the wood evenly i'm trying to strain it optimally for a bow!  Two different things

Not necessarily. By straining all the wood equally you are getting the most work/energy storage possible without overloading any particular part of the limb. That sounds pretty optimal to me.

+1. It has never made sense to me that people think the limbs are strained/stressed evenly along their whole length.

They certainly can be and this would be the optimal way to store energy in the limbs. A pyramid profile with a true circular arc tiller will see equal strain along the whole limb. Why do you think that isn't possible?

.... .... ....

Mark

Semantics. Bow limbs are not, and should not be, strained evenly. The total stress should reflect the relative position of the measurement. It's much more stressed near the fades and much less near the tips. But if we consider the amount of "wood" in each positions,  each units of "wood" are, or should be, strained "equally".

[SLIMBOB]

I agree it is semantics, but for a different reason. When we say “equal strain” along the limb, I think we some how conflate “equal force”, “equal strain” and “mass placement”. The force applied along the limb is equal. The mass placement along the limb is what determines the tiller. Having thought about this after Pats question, clearly, a bow limb is not ,”strained” equally along the length, depending on your interpretation of exactly what that means. A narrow, thick, working handle should not “feel” the strain that the working limb feels. At least in my opinion. The inner limb should not feel the same strain as the mid limb, and the outer limb should not feel the same strain as the mid limb. Mass placement determines how much stress is felt along the entire length of, I suppose, any bow. And that mass placement seems to me to be, the very essence of what we do. We strive for proper mass placement, not equal strain.
Edit. Mass placement will determine our tiller shape, elliptical, circular, or some variant between the two. This brings us full circle back to, what is the proper tiller shape for a given front profile?  But we can at least dispense with the “equal strain” tangent.

[Don W]

This is why a thread with examples and "here's what right and here's what's wrong would help. Some of the  discrepancies is interpretation.

[willie]

Some of the  discrepancies is interpretation.

agreed.  Looking up the word strain in the dictionary gives many definitions. When strain is used in engineering, it is a percentage length measurement.
a bow back surface segment that measures 10" when the bow is unbraced, and is stretched out to 10.1" at full draw is said to be strained 1%.

Also, as defined in the engineering  world,  stress is a pressure measurement. For example, if the above bows back is sinew, then the stress/pressure at the sinew back surface is not very much compared to an otherwise similar back, bent just as far, but made from much from a stiffer materiel such as hickory.

For folks coming from a non-engineering background, different definitions of the same words are frequently in use.
Our language is like that.   Are there definitions from TBB that differ?

[Don W]

One of my biggest issues with TBB when I first started was it's lack of definitions. I spent a lot of time internet searching words. Even then, if you don't put it in the correct context, it becomes unclear, and it's hard to know the context, until you "know the context". TBB is one of those books you need to read, build a few bows, read again, build a few more, read again, rinse and repeat. I don't know how many times before you've soaked it all in. I'm not close yet.

[SLIMBOB]

I would agree with that completely. I read TBB 1 as I built my first bow. Then the others as I built the next few. Over the next 25 years I have reread them countless times. Not front to back, just chapters relevant to what I was working on. I’m a reader to begin with, so I have enjoyed them all, but there is only so much that can be conveyed in a book. Building them teaches you things that contradicts some of what was written, and some things were absent from the books entirely. Still, they are a very good resource.

[Don W]

I agree they are a "must have" resource.

[RyanY]

Mark, it’s not that I think it isn’t possible but that it isn’t optimal for bows to be strained equally along their whole length. This is what Tim Baker’s Mantra suggests and that seems to make sense to me depending on the bow. I would say that it’s not terribly common for that to be the case in the real world anyways given that bows rarely show the same amount of set along the whole limb. Certainly the last 6-8” of a bow limb doesn’t take set in almost all bows unless quite whip-tillered.

[scp]

If we divide the bow limbs into many sections, the total stress for each section should be quite different. It should be much higher in the fade section than in the tip section, etc. That's the mantra.

But if we look at the unit stress of each section, by dividing the total stress in each section with the mass units in the section, it should be same for all sections. All "wood" in the limb should be evenly and equally strained. This is the underlying principle for the mantra and no set tillering.

Just my SWAG, scientific wild-ass guess.

[RyanY]

If we divide the bow limbs into many sections, the total stress for each section should be quite different. It should be much higher in the fade section than in the tip section, etc. That's the mantra.

But if we look at the unit stress of each section, by dividing the total stress in each section with the mass units in the section, it should be same for all sections. All "wood" in the limb should be evenly and equally strained. This is the underlying principle for the mantra and no set tillering.

Just my SWAG, scientific wild-ass guess.

Doesn’t really make sense that it would be stressed the same if there are areas of more mass to do less work. That’s actually not the mantra. If we define stress/strain as how close the wood is to deformation (set) then the inner limbs and tips should be less stressed so they don’t take set and the mid limb takes the brunt of it.

[BowEd]

I think the fact that a limb needs to taper from fade to tip is easy to comprehend. I think once you start to study, learn and experience you get that to much stress causes set, and set is not your friend. But there has to be a better way to explain the correlation between profile and tiller and how to get to an efficient (or more efficient) bow. Even if that means creating categories like hunting, target, general, it's for heavy arrows, for light arrows, etc. We seem to interprete speed with efficiency, and speed can be gaged with each situation. So how do I constantly make my heavy hunting arrows go faster? What exact profile do I need to start with and expand from. I think the mass theory started down that road, but never got to completion. But there is probably more to it as well. Sorry for the ramble, but I am really trying to wrap my head around this.

Don...For heavy hunting arrows your power needs to come from your inner limbs.

[Don W]

I think the fact that a limb needs to taper from fade to tip is easy to comprehend. I think once you start to study, learn and experience you get that to much stress causes set, and set is not your friend. But there has to be a better way to explain the correlation between profile and tiller and how to get to an efficient (or more efficient) bow. Even if that means creating categories like hunting, target, general, it's for heavy arrows, for light arrows, etc. We seem to interprete speed with efficiency, and speed can be gaged with each situation. So how do I constantly make my heavy hunting arrows go faster? What exact profile do I need to start with and expand from. I think the mass theory started down that road, but never got to completion. But there is probably more to it as well. Sorry for the ramble, but I am really trying to wrap my head around this.

Don...For heavy hunting arrows your power needs to come from your inner limbs.

Which means what? How do you make the power come from the inner limbs?

[BowEd]

It means to get the bow to work close to into the fades but not take much set.
http://www.primitivearcher.com/smf/index.php/topic,67543.0.html
PS....A mid section working bow makes a good hunting bow too.
You pull a heavier bow than me nowadays and should be able to get a 550 grain to 650 grain arrow to go plenty fast enough for any big game kill.