Recurve shape when strung

[whittler01]

Curious.  I measured the distance from string to top and to bottom of riser on my Winterwind recurve.  There is about an inch difference in distance between the two measurements.  I did this because I can see that the top limb does not bow as far back as the lower limb when strung and kind of looks offset to my eye.  Is this normal or OK?  I don't know much about this stuff.  Is it something I should send back to get fixed?

[Bill Carlsen]

The lower limb is often shorter/stiffer than the top limb...it's all about tillering and if you shoot split finger or 3 under. If you  have a question this may not be the best place to ask...call the guy that made the bow.

[burnie]

Its called positive tiller. You want a positive tiller due to the difference in dynamic and static fulcrums.  You brace lower than center, while the arrow shelf is cut at center, so to  compensate you tiller the upper limb with a bit of positive tiller to equal out the difference in the two levers (lower and upper limb).  As mentioned, different shooting styles call for different amounts of positive tiller on the upper limb.  There is a little difference in a bow tillered for three under or split finger.  Most bows are tillered for split finger and for those of us that shoot indian style (three under) we compensate by just moving the knock up a bit from where it is on a split finger shooter.

[Orion]

Most builders locate the arrow shelf from 1 1/4 to two inches above the center of the bow/grip.  If the shelf is 1 1/4 above center, the top limb will be longer than the bottom limb.  If it's two inches above center, the limbs will be about equal length.  The lower limb is usually made a little stiffer to compensate for the fact that the shooter places two or three fingers below the arrow when drawing and shooting, placing more stress on the lower limb.  What you're looking at is probably normal, unless the bow's tiller is way out of whack. Should be about 1/8 to 1/4 inch more space (positive tiller) between the tip of the top fadeout and the string than there is from the tip of the bottom fade out to the string.  Measure those distances with a bow square or a t-square.

[hickstick]

you sure you got the limbs on right?   :-)

[whittler01]

Hey thanks, I get it I think.

[Dan Bonner]

The top limb should bend more than the bottom limb. Take an arrow with a field tip, put the point of the tip at the fadeout (where the limb wedge ends on a 3pc or where the riser ends on a 1 pc)on the belly (shooter's) side of the top limb. put the point at the middle of the limb from left to right, hold it so that the arrow crosses the string at a 90 degree angle. Now use your finger and mark the distance from the limb to the string. Hold your finger in that spot on the arrow and move the arrow to the bottom limb. The distance should be from 1/8 to 1/4" shorter on the bottom limb. The way you described it sounds like you have the limbs reversed on the riser or the bow is out of tiller. if the bottom limb is weaker than the top send it back to the bowyer and let him retiller it. When you say thelimbs are offset do you mean from left to right as in poor limb alignment or from top to bottom as poor tiller?

[DCM]

"Most builders locate the arrow shelf from 1 1/4 to two inches above the center of the bow/grip. If the shelf is 1 1/4 above center, the top limb will be longer than the bottom limb."

If the fulcrum of the grip is at the dimensional center, how could the upper limb be longer than the lower w/ the arrow pass 1 1/4" closer to it's string groove?  Isn't this backwards, or have I misunderstood what you've written?  Because what you've written pretty much says the upper limb is, by definition, 2 1/2" SHORTER than the lower (taking 1 1/4" away from the lower and giving it to the upper).

"If it's two inches above center, the limbs will be about equal length."

Again, if the arrow pass is 2" closer to the string groove of the upper limb, how on earth could they now be of equal lenght, and not the most obvious conclusion that the upper is now 4" SHORTER than lower.

"The lower limb is usually made a little stiffer to compensate for the fact that the shooter places two or three fingers below the arrow when drawing and shooting, placing more stress on the lower limb."

Isn't positive tiller required because the upper limb is almost always shorter than the lower?  Having the upper bend more readily allows it to span the distance to full draw in unison with the lower, despite it's disadvantage of being shorter.  Which leads me to wonder how the lower could be under more stress, if it doesn't have to bend as far as the upper.

  http://www.goarchers.org.uk/mechanics/#

[Dan Bonner]

DCM
The limbs are exactly the same length on practically all glass bows. Generally the throat of the grip is at the center or fulcrum of the bow. Most bowyers place the shelf between 3/4 to 1 1/4" inches above the throat (mine are 1" over so the bow behaves as though you are shooting off your hand). So functionally, as you stated, the top limb is shorter. The bottom limb is under more stress. Ask any bowyer who has built carbon skinned limbs which limb always breaks when the carbon blows... always the bottom. The reason is that most shooters bend the bottom limb more because they "palm" the grip putting most of the draw force below the fulcrum and on the bottom limb disproportionatly. This is the reason for positive tiller. The idea is to spread the draw force evenly as you stated and to get the bending and recovery of the limbs in time with each other.

[DCM]

Dan,

"The bottom limb is under more stress.  Ask any bowyer who has built carbon skinned limbs which limb always breaks when the carbon blows... always the bottom."

In the dozens of bows I've built in the last ten years, admittedly not from carbon lams, I have not found this to be the case.  And I would think bows of natural materials (both self and backed) run much closer to thier elastic limits than those composed of modern, even carbon, materials.  In fact, the opposite is the case.  I tend to blow out the upper limb at the first boo node above center, in a highly stressed design.  But more to the point, the upper limb on selfbows and natural bows, again cases where we accelerate the degredration and run closer to the materials load capacity, tend to pull more positive over time, indicating acclerated fatigue on that limb.  It's no wonder, since by placing the arrow pass above dimensional center we put it at the disadvantage of being both shorter and weaker in relation to the lower.

"The reason is that most shooters bend the bottom limb more because they "palm" the grip putting most of the draw force below the fulcrum and on the bottom limb disproportionatly. This is the reason for positive tiller."

An interesting idea, but I prefer to think of it another way.  Every archer "heals" the bow to some degree yes, but only because there is a practical limit to just how close one can position the fulcrum relative to the arrow pass, and more importantly the nock point.  I tend to prefer to regard this as inherent in the geometry, versus the behavoir of individual archers, which are actually all at the mercy of this effect to some degree.  This inherent asymetry is the reason we are forced to use positive tiller, but we can then adjust it to suit each archer by moving nock point for each individual's unique style.  So, I would argue nock point is a better way to describe how we compensate for healing, not tiller.

My purpose for posting was to make a simple point, the farther we put the arrow pass (but more importantly the nock point) above the fulcrum, the more positive tiller is required to compensate.  And the shorter the upper limb is, the more positive tiller we need, and these both contribute to exaggerating the stres on the upper limb.  This topic is widely misunderstood, in my view, and I think we do a real disservice to the craft by repeating the misunderstanding or leaving it unchallenged.  Also because I invested a great deal of time and brain power to understand it myself, and would like to get some return on my investment.  LOL

Hope I have not offended, and would love to entertain discussion to the contrary.  But I would hope rebuttal could be more informed than heresay and antecdote, as these while entertaining have exhausted my interest.  To that end I posted a link in my first post, which I think explores these topics well from a scientific perspective and is substantially the foundation of my understand and thus my position.  I would not be so bold as to just make this crap up myself, and then confront others.  LOL.  But it does follow my experiences in bowyery and in a very wide variety bows from 76" to 36", 80# to 8#, reflexed, deflexed, recurved and straight bows, in a very wide variety of natural materials and in glass.

 http://www.goarchers.org.uk/mechanics/#

[Orion]

Dan:  On most glass laminated bows, the throat of the grip is not in the center of the bow.  It is above it.  Depending on the shape of the grip, the shelf is usually about 3/4 inch above the throat, at least on an indexed or recurve type grip.


DCM:  I know it sounds counterintuitive, but measure a few of your bows.  Here's another way of looking at it.Two things are going on here.  First, if the center point of the bow moves from 1 1/4 below the shelf to 2 inches below the shelf, the top limb must get shorter and the bottom limb must get longer to move the center down.  Second, an arrow shelf two inches above center is cut higher into the upper limb, and  when you measure limb length from the arrow shelf, rather than the center of the bow, the upper limb gets shorter.  

I measure all of my bows, and it's surprising how much variability there is from one builder to another.

I think you may be right about the upper limb receiving more stress as the arrow shelf is raised, effectively shortening the working part of the limb and thus requiring more positive tiller to compensate.

[DCM]

It occurred to me I haven't actually said directly what the purpose of tiller is.  While all of our discussion relates to forces working against it, the purpose of tiller is to have the arrow come off the bow neither pushing into the arrow shelf or up off of the shelf, with the preference to err on the side of the latter obviously.  In Tapley's words:

"The aim is to have the total accumulated angular momentum equal to zero when the arrow leaves the string."

I've actually written a couple of replys in the last two days I did not post.  I don't enter this discussion lightly, and sometimes forget what part I've thought and written, versus what part I'd actually posted.  LOL

I have a hard time with the concept that a limb in equilibrium with another but which is both shortened, and weakened, can be under less stress (strain I think it actually the correct technical term).  It think of it as follows, when I hinge a limb, it doesn't set in the area that bends LESS.  LOL

Orion,

I'd hoped we were saying the same thing.  If you'll re-read your first reply, you can see I think the root of our misunderstanding.  I've measured quite a few bows actually, and even experimented with different measures while building with focused purpose on this precise question.  But I'd agree, there's a good bit of variability, with no apparent, or at least dramatic, differences amoung the various scenarios.  I disagree about the center of the bow, it does not move, by definition.  Only it's relationship to the arrow pass changes, just as you've described your last post.  The consequence is ALWAYS that the upper limb is shortened, and in the examples we've discussed the case of 2" above moreso.  I find that when things are counter-intuitive, there's a good reason.  LOL  But in these discussions, and I've had my fair share, there is more frequently opportunity for simple miscommunication, versus real differences in our understanding and application of these concepts.  What works, works, after all, and there ain't no cheating that reality.

[Dan Bonner]

Orion and DCM
I design all my bows so the pivot is at the throat of the grip. The highest point of the radius of the shelf is 1" above the throat which is also the pivot. Most of the bowyers I have talked to also Put the throat in the center infact all of them that I know.

I saw Mike Palmer do a demo once where he used a mock up bow that had single lam ilmbs that were all fiberglass and drew about 5#. When drawn you could easily see the effect of throat placement and grip on the bend of the limbs. As long as he kept only the web of his thumb and forefinger at the throat the limbs bent eavenly. When he heeled or palmed the grip the bottom limb bent more.He was making a case for a high wrist grip actually.

I believe the limbs should be of equal length in relation to the pivot point or fulcrum of the grip not measuered from the arrow shelf. Most of the contact the archer has with the bow is below the pivot on the handle and below the dead center of the string (depending on knock hight and split vs 3 under style)so dont you think this would put more stress on the bottom ilmb? I do... and my experience bares this out.

Now DCM heresy and antecdote?... Give me a friggin break.   All wood bows are different, very different. I have built more than 10 bbos and selfbows and they simply dont compare to glass let alone carbon bows. I too have invested lots of brain power in bow design. I have designed and built two  forms for  glass / carbon LBs and 3 carbon recurves and have put "dozens" of bows through them and have hit lots of bumps in the road along the way. I have consulted with the some of the best bowyers in the world and learned from their experiences as well. I have also invested a lot of money in materials and tools along the way so it is very important to me to be employing the best design principles possible.I read the thesis paper just like you did. I think you e wrong abot the top limb taking more stress.

[DCM]

Dan,

I hope you can accept my apology for your having been insulted by my choice of language.  It was not my purpose, and my language was accurate.  You and I can wax poetical all day about what we've done, or what other bowyers have done.  But these are all antectdote and hearsay, my having cited wooden bows tendancy to pull positive tiller over time is an example.  And we can cite what other expert bowyers have told us, and who agrees or disagrees and why.  Yes, I have well recognized names in my pocket too, but I won't play that card.  Because appeal to authority is logical fallacy and none of it advances the ball in either direction.  Our ideas, our argument, needs to stand on it's own merit.

I looked over your posts on this board, and admired your work.  It is impressive, and does inspire hope that you can articulate an argument which supports your obvious experience and achievement.  I sincerely hope you can tame the emotion, which we have all experienced, and continue with the discussion.  But I think ultimately we will find we know the same things, and simply express our understanding in different terms.  

From Webster's dictionary:

"strain:

d: deformation of a material body under the action of applied forces"

"stress:
a: a force exerted when one body or body part presses on, pulls on, pushes against, or tends to compress or twist another body or body part; especially : the intensity of this mutual force commonly expressed in pounds per square inch

b: the deformation caused in a body by such a force"

As far as I'm concerned in this application deformation and bend is the same thing.  Whichever limb bends more is the one under more strain.  If the lower limb bends more at full draw, the bow does not demonstrate positive tiller.  I'm not suggesting this is the case, neither am I criticizing you or your work but I don't think you are advocating negative tiller.  Rather, I'm simply observing that this is the argument you have promoted.  If not, and the upper bends more (and moreover is shorter), which is the model of positive tiller we all follow and the one illustrated by Tapley I referenced, the only model which allows us to achive our objective ("The aim is to have the total accumulated angular momentum equal to zero when the arrow leaves the string.") then the upper is under more stressed (or more specifically strain) by the definitions cited.  

"All wood bows are different, very different. I have built more than 10 bbos and selfbows and they simply dont compare to glass let alone carbon bows."

Absolutely.  They are very different in the one respect that these bows work very much more closely to elastic limit of the materials employed.  But they differ in no other regard physically.  If you think so, you'll have to demonstate this in some way other than bald assertion, because geometry and physics doesn't change depending upon what material it is applied to.  In short, our glass bows can be overbuilt in one aspect or another, and we'll never know the difference.  But a 58" selfbow to make 60# @ 28" will put the test to our design choices in a way a glass bow never will.

Again, I apologize if I insulted you.  It was not my purpose and I sincerely hope you can see past my choice of language and address the underlying argument.

Aren't bending and deformation the same thing?

Is the limb that bends more the upper or the lower?

If the lower, doesn't that contradict the  definition of positive tiller?

If the upper, how can a limb the bends more, deforms more, not be under more stress, given the definition of stress as:  "deformation of a material body under the action of applied forces"?

I honestly don't know why some bowyers' lower limb blew out while experimenting with carbon, but the physics and geometry of bowyery do not support the hypothesis that the lower is the more strained limb (in the conventional positive tiller model).

I've been thru the crucible too, and you won't scare me off with loud voices and hand waving.  Make your argument.

[DCM]

"...and below the dead center of the string (depending on knock hight and split vs 3 under style)so dont you think this would put more stress on the bottom ilmb?"

Fold the string off your favorite bow where the arrow nock lays, and the report back the difference, upper segment versus lower.  2" to 3" shorter on the upper is typical in my experience.  That means, not surprisingly, arrow lays well ABOVE dead center in nearly every case.  This is what the arrow sees once we loose.  It's the reason for positive tiller.  But I agree completely with the rest of your post.

[Dan Bonner]

DCM
There is no doubt that the arrow is above the center of the string and the bow's center. On that we agree. But where are at least 2 of the fingers on the string and the weight of the bow hand at full draw and release? At or below center on the string and bow respectively. My argument is that the forces applied to the string and bow are below center making the lower limb effectively shorter during draw. After researching it there are bowers moving the throat in relation to the true center of the bow in an effort to solve this very conundrum. But I still believe that more draw force is put on the lower limb.

[Orion]

Dan:  Of course the throat of the grip is the pivot point, but it is not necessarily in the center of the bow.  On bows that have the arrow shelf cut 1 1/4 inch above center, the center of throat of the grip below it would be close to center, probably within a half to a quarter inch above center.  However, if the arrow shelf is two inches above center, the resulting pivot point is about an inch to 1 1/4 inch above center.  

This isn't my opinion.  This is fact, the result of measuring a lot of longbows, including Robertsons, Morrisons, Great Northerns, Dwyers, Hills and a few others.  Where the arrow shelf and thus the pivot point below it are located are a partial reflection of how bowyers intend their bows to be shot.  For example, let's say the center of the pivot point, the smallest part of the throat of the riser is 3/4 inches below the arrow shelf.  Then, an arrow shelf 1 1/4 above the center of the bow results in the center of the throat being only 1/2 inch above the center of the bow.  This throat position is probably best for high wrist shooting because it almost centers the web of the bow hand in the center of the bow.  On the other hand, a low wrist grip, which requires placing more palm into the grip, would do better with an arrow shelf cut 2 inches above the center of the bow. This would place the thinnest part of the throat, the pivot point, about 1 1/4 above the center of the bow, causing the shooter's bow hand palm to exert more pressure on the center of the bow/riser/grip area.

Though the actual measurements varied, that's the relationship I found on all of the bows I measured above.  My relatively straight gripped Robertsons and Dwyers have their arrow shelf's 1 3/4 to 2 inches above center.  The Morrisons, which are higher wrist, about 1 1/4 inch above center.  The one anomalie is the Hills, which are definitely low wrist bows.  Their arrow shelfs are cut 1 1/4 inch above center.  Go figure.

[DCM]

Well put Dan.  I think we are having what is substantially a symantic argument.  

"But where are at least 2 of the fingers on the string and the weight of the bow hand at full draw and release?"

Two fingers are still above dead center of the string, and the force on the bow hand is below dead center of the bow.  This is the inherent asymetry positive tiller combats.

"At or below center on the string and bow respectively."

No, above in the case of the string, although granted not much above, and in the case of the bow, yes, below dimensional center, in the typical scenario.

"My argument is that the forces applied to the string and bow are below center making the lower limb effectively shorter during draw."

The lower limb, and the lower string segment, cannot change their lenght during the draw.  The fulcrum can change, by healing the bow, and because the bow (if heavily positive tiller) tends to rotate upper forward during the draw.  While admittedly difficult to mitigate entirely, this is not actually desirable, imho.  And it doesn't really address the issue of stress, or more correctly strain, with regard to the relative strain on the upper versus lower, unless you are suggesting the lower should bend more than the upper at full draw.

I think we strive to bave the forces balance, but tend to err toward the side of overworking the upper limb, by having it be both shorter and bend farther.  The differences in practice are actually quite small, getting down into the less than 1" area (lay your three draw fingers on a ruler, nock at 4" above with 3" of finger width, or 3" - 2 1/2" are typical), but again typically erring in the area of 1" or less ABOVE dead center.

As regards draw force on the lower limb, it introduces a very interesting aspect of the equation, bow rotation.  If by "more draw force" you mean stiffer lower limb, the bow MUST rotate in the hand.  It's simple teeter-totter physics and does happen in practice.  Don't know how you suspend your bows on the tiller tree, but I use a single point of contact, and usually not a very pronounced throat, to capture the bow.  Bow rotation is easy to see when I work a bow on the tree.  I would argue we want the bow to rotate upper limb forward, if at all, versus the opposite, in order to "steer" the arrow away from the shelf on the loose.

More to the point, don't we want to minimize rotation where practical, minimize the inherent imbalance a shortened, weakened upper limb represents?  This is the area that I find most interesting, where the potential benefits hide.  When I investigate folk's understanding and application what I find is not much consideration of the alternatives, and a good deal of discontent with having been confronted with the question.  As a mind experiment, what would the effect of having the arrow pass, simply as a proxy for positioning the nock point, be at or nearer the dimensional center?  It's essentially just a little step forward from 1 1/4" above versus 2" above.  What about 1/2" above, or dead center?  What's the downside?  What's the upside?

I just think the topic merits more exhaustive treatment than it typically gets, and that it sure as hell enjoys a well earned reputation for repetition of misinformation at the most callous characterization, or miscommunication at the more generous.

I sincerely appretation your participation, and again, I apologize if I've stepped on your toes.

[DCM]

Interesting observations regarding throat of grip versus dimensional center Orion.  Had not seen your post when I posted.  Talk about counter-intuitive, for the life of me I can't imagine the benefit of having the fulcrum always fall above the dimensional center.  I'd be curious to know the relationship of degree of positive tiller, and nock point, to the various scenarios of arrow pass and fulcrum.  Whether for example 2" above tend to be more positive, and higher nock point, for example, or vica-versa.

[Dan Bonner]

Now we may be getting somewhere in terms of benifitting bow design. If you put the arrow pass at the demensional center it would place the bow hand farther below center and it sure seams intuitive that it would stress the lower limb more. What do you think?