Worms: Now in cans! -- Front-Profile and Overall tiller shapes

[BL]

I'm surely not the only one trying to follow another recent thread with some interesting contradictions about tiller shape as it relates to front profile, so I thought I would try to clarify for my own sake as well as others'.  Also, this thread gives those interested a place to collect their thoughts and share opinions on the subject without hijacking a bow post.  I use the word opinions due to the somewhat disparate examples and experiences being offered, and encourage all to come with an open mind and keep things as lighthearted as one can be while having a disagreement.  Hopefully this will give us newbies some food for thought on all sides.

I believe that a major source of the argument was a discrepancy between people talking about profile-matched bend versus what portions of a limb should in general bend more for optimal performance.  These are two separate concepts and were being discussed as one.   Let's separate them!


Part 1! 
Here's what I "know" so far, in as concise a few bullet points as I can muster about how front profile and tiller shape relate.  Please weigh in and correct me where I've misunderstood, as I'm still pretty new to all of this.

1. Thicker wood stresses the functional surfaces more quickly when bent.  Thus, a thicker piece of wood reaches the same amount of surface strain as a thinner piece while bent less.  Simply put, you can only bend a certain thickness of wood so far before it will break.  The thicker, the quicker.  Here's a link for you engineer types.  http://paleoplanet69529.yuku.com/topic/28701/minumum-bend-radius?page=1#.UPDz82czTrJ

2. By making a portion of the limb wider than the rest, you can keep up weight while reducing thickness to safely and effectively concentrate more bend into that part of the working limb (a mollegabet is an extreme example) while still having equal strain throughout.  So the widest parts of a limb "can/should" bend more when drawn, which pulls your tiller away from an even circle shape.

Conclusion on front profile: When people talk about tiller matching front profile, they mean that wider portions of a limb have the ability to carry more curve and narrower portions will be less curved when drawn while still having equal strain along the limb.  This same principle is in effect when people discuss how wide overall to make a bow to achieve a certain poundage at draw length based on the overall length of bow, but can be applied to smaller portions of the limb as well if you'd like to move more of the bend to one portion of the limb or another.  For example, in the difference between a molly and a parallel limb, to achieve equal strain, the molly bends more at the widest and less at the narrowest, while a parallel limb will bend closer to the same throughout.  This all makes wonderful sense.


Part 2!
The second topic of discussion, which is what it seems like Ryoon was trying to call to the fore with his provocative comments about gizmos and a parallel limb still not demanding circular tiller, is of overall bend placement as a general rule for performance.  Correct me if I'm wrong, but here's where I think the actual discussion lies.  Once you have your personal ideal for overall bend placement, then you can adjust that per your front profile.

If we take a hypothetical bow that is 1 inch wide straight from tip to tip with no tapers, and want those tips to slam home to the "greatest effect" regardless of draw weight or wood used, what will the bend look like?  Would it be a perfect circle?  Would it be whip-ended?  How about a double helix!  What trade-offs are there for hand shock, accuracy, durability, etc. versus increased speed?  This is a question both of personal experience (art) and of engineering (science) with validity in both sides. 

Let the opinions fly like so many arrows from a well-tillered stick!

[Del the cat]

... For example, in the difference between a molly and a parallel limb, to achieve equal strain, the molly bends more at the widest and less at the narrowest, while a parallel limb will bend closer to the same throughout...

The above is quoted from your conclusion to part one.
This is IMO wrong an several counts.
1. A parallel limb will not bend 'closer to the same throughout', as there is much more leverage at one end, thus a paralle llimb is generally tapered in thickness to give an even bend.
2. OK a Molly could be said to have equal strain throughout if built perfectly.
You have put your interpretation on what the 'front profile ' exponents mean. I don't believe your interpretation is right.
I think they mean deflection at any point (lets call this 'point deflection') is inversely (ammended for clarity. e.g narrower bends more which is fair enough) proportional to width, thus pyramid shapes and pyramids with extra narrow tips have point deflection increasing with distance from the grip. (which is of course untrue of the Molly, as point deflection in the narrow levers is virtually nil).

This leads to the confusing term 'elliptical' I've heard it used to indicate ellipses in both directions! (e.g Bows with more bend at the grip (e.g a non tapered bow) and bows with more bend at the tip (e.g tending toward whip tiller)). This interchangeability allows proponents of the 'front profile' to flip their argument to fit any case, as it can be applied to almost any tiller that isn't circular.
If you want sensible logical argument you need to first agree your terms, which requires careful and consistent use of language (which is very tricky in any technical discussion).
I have tried to nail down some of these points before, but it's like trying to pick up an eel. People are unwilling to give yes/no answers to specific questions.
Del
PS
To be honest I'm rather tired of the 'front profile' discussion, but I feel duty bound to try and quash it, for the benefit of newbies, who could get thoroughly confused by it.
I feel it is contradictory, counter productive and actually wrong.
Just look at some of the real extreme paddle bows ... the tiller shape bears no relationship whatsoever to the front profile.

[steve b.]

You did a good job of articulating the issue and I'm sure this thread will be around for awhile now as many opinions will come.  I'm not smart enough to understand the physics, other than to correct your statement in part 1 about the widest part of the limb bending more.  On a pyramid type bow the fades are widest and they bend the least.  I probably misunderstood the context.

I just think that the bottom line here in figuring out the efficiency of a bow design comes down to arrow speed, in the end.  The design that casts the fastest arrow without overstraining is the winner.  And I know fanatics have been running these tests for 100 years  now and there has not been that much improvement in design over that time. 
So as I mentioned to Ryoon, this recent quest for efficiency will only yield a couple fps at best for any given bow.  I personally would rather have a slight inefficiency in exchange for durability.

[blackhawk]

I don't have enough knowledge or experience to speak an opinion,but I just try to make it bend even n that's good enough for me 

[spyder1958]

I'm very new here and have only built a few bows all different, and several types of wood as well as design. without the help here I'd still be "lost in space" well I'm still lost but have a direction. My take is to try and build the same bow using different designs and see which works over all the best, Then I will know for sure, what works best for "me".
But this sure is entertaining and helpful in a lot of ways. I got the popcorn on. 
Keep up the great work.
        Graylan

[BL]

Del - I totally agree with you.  It sounds like you've been told by some that front profile makes a bow bend a certain way regardless of thickness.  That doesn't make any sense.  As you said, it's the thickness taper that decides what will bend how much a lot more than width.  However, as best I can understand it, it's not that the front profile CAUSES the limb bend a certain way... it ALLOWS you to tiller it a certain way safely if you want, as in the case of the molly having more bend in a shorter working limb but making it wider to hold the load.  To use your statement of point deflection being proportional to width, that would follow exactly, as the greatest point deflection is in the widest part.  I may be misunderstand you again though!  To achieve that equal strain with parallel limbs you taper thickness.  If your front profile tapers closer to a pyramid, your thickness will tend more constant right?

Everything I said above is assuming that you are controlling the thickness taper to make the bend, and that one's choices in front profile just allow you to safely move the bend around if desired to allow for levers, stiff handles, or otherwise.  Sorry if I didn't state that directly.  I agree entirely with what you said about thickness.

As far as the term elliptical, I always took it to mean a smooth arc, but a little less bend in center, and a little more bend the farther you go out the limb.  I haven't been around here long enough to be familiar with the slippery devils you've discussed this issue with before, but I can see how that would get aggravating!

Are most paddle bows pretty short?  Otherwise are the wider parts just overbuilt? 


Steve - Yeah, I wasn't saying that the widest part of the limb has to bend more, but that you can make a wider/thinner limb bend farther than a narrow/thick limb at the same draw weight if that's your goal.


Chris - Whether you can spell it out or not, you've got some awesome bows to back up your opinions on tiller.  I've been getting antsy waiting for number 4 by the way!


Thanks for taking the time to post thoughts everyone.  These contentious topics are always enlightening.  I hope it doesn't get too caught up with the first half of the post.  The second part is what confuses me the most, as a purely circular tiller is so appealing to my eye, yet I read how some bows, especially longer ones, "should" have a little whip to the ends.  Is all of that based on the length of the bow? If someone goes for a more elliptical tiller instead of circular, is there a general thought as to how much more the tips should bend?  How does that compare in comfort and speed to the exact opposite design... a stiff-tipped bow which also is touted as an enhancement?  I made two bows for my wife's younger cousins for Christmas, and I felt as I was working on them that I was mostly making these choices at random.  Both had stiff tips, one bent clear through the handle and the other was mostly stiff, I made both as close to circular as I could.  I'm all about aesthetics and making the full draw an eye-pleasing line, but I also appreciate the value of informed design decisions and would like to know more.  Perhaps it also time to read through my stack of TBB's again.

[BL]

Scott - sometimes wording really is everything.  That's a really good explanation.  Thanks!

[koan]

Wow! This is interesting.... But I have to admit I just let the wood tell me what to do... Geuss im just not that worried bout max performance.. Glad some of my Brothers on here are doin the research for posterity but I aint gonna sweat it, lol. I am worried tho that we might be confusing newbes and makin it more difficult for them. JMO... Brian

[Del the cat]

BL...
My Bad, I should have said, I believe the proponents of front profile determines tiller are saying that Point deflection is inversely proportional to width.
My bad grammar .
However I don't expect any proponent will step forward and explicitly state exactly how thety think front profile relates to tiller.
I've ammended my original post.
Del

[SLIMBOB]

Well, I'll be your huckleberry.  It's really just good ole horse sense.  You can make any front profile bend to any shape you wish, and if it suits you, so be it.  If however you wish to improve the performance of the bows you build, lots of things can be done that will incrementally improve the cast and longevity.  One of those is moving heavier wood a shorter distance, again, just common sense.  Parallel limbs are going to be wider mid limb an so, be heavier mid limb.  Reduce the bend at the fades and move it out towards the center, and that heavier wood does not have to move as far.  Better use of the stored energy.  Elliptical tiller for parallel limbs 101.

[Del the cat]

Bravo Slimbob!
if you was over here I'd buy you a pint!
Del

[SLIMBOB]

Do they sell Shiner Bock on that side of the pond?  Blue Moon?  Sam Adams?  Might just take you up on that Del.