I have been involved in an ongoing debate for the past year or so about how the tiller shape on an archery bow affects arrow flight. I realize most of you have never given much of a thought to bows and arrows so I will try to pose the problem as simply as possible.
A bow has an upper and lower limb, each limb is a spring with a string attached to the ends. We place an arrow on the string at approx center of the bow. We are connected at 3 points, each limb tip and the arrow knocking point. If one limb is weaker than the other it will display itself with one limb bending more than the other casuing the brace height to be unequal. It will also cause the arrow knocking point to rise or fall as the string is drawn backward favoring the direction of the stronger limb.
My contention is that if I adjust the tiller shape of the bow (relative strength of each limb in relation to one another) So that the arrow pulls sraight back and forth if pulled by the arrow nocking point I will have a good tiller.
Everyone elses contention is that the tiller shape of the bow should show even at full draw relative to the placement of the shooters fingers on the string. They contend that the string and arrow will return in an equal and opposite path to how it was drawn. I contend it forgets how it was drawn and will instantly responds to only the arrows mass under accleration which is about 1 1/2#.
From an experience standpoint I have probably built more bows than nearly anyone on the planet.
While it is theoretically possible for a material to “remember” the path along which it was flexed, I would consider it extremely unlikely in this case, since it usually only happens if you have extreme twists, or if you’re taking some measure to deliberately ensure it.
I wonder if they’re just assuming that loosing the arrow is the exact reverse of drawing the bow, only quicker?
There’s no particular reason why it must be. I don’t think the behaviour of released springs is necessarily linearly proportional to their behaviour when being stretched.
What’s “tiller shape” and “brace height”? I know what bow limbs are and what the nocking point is, but if you could elaborate a little it’d be great. I love materials science so I’m eager to hear. And just out of curiousity, how straight do bows of unequal limb strength shoot?
You have a contemporary with the game of Billiards called “Dr. Dave”. He has proven his theories (along similar lines in detail as to yours) with “high speed videos”. These videos clearly show what is going on at a basic level.
Here is a link to his non-billiards videos, quite interesting…
http://high_speed_video.colostate.edu/
Note: You will probably receive quite a bit of flack as to your ideas. I have a rule that the more correct someone is with their new ideas, the more opposed all the idiots out there will be to it!
You are following in the footsteps of some great people - like Louis Pasteur (doctors fought his ideas of bacteria causing disease as being ridiculous), Galileo, and Joseph Lister. These intelligent people were fought tooth and nail as to their ideas which were later found to be quite correct!
Anyway because so many people are opposed to your idea, in my opinion, it must be correct!
Brace height is a measurement of a bow with the string attached and under tension. The limbs are bent and under tension at this point. Tiller shape refers to the shape of a draw bow. We look for specific things like ballance and wether or not the limb is bending prgressively or al at one time.
Careful, there. Yes, they laughed at Galileo, but they also laughed at Bozo the Clown. The real test of correctness is not that an idea is attacked: It’s that the idea stands up against all the attacks.
I have introduced 3 major theories into primitive archery over the past 15 years or so that have held up very well inspite of going against the thoughts of some of the phd’s out there whose word was pretty much thought of as fact. I know I am not smarter but I also know I have spent a lot more time than them thinking about it and testing.
A few years ago I intoduced a theory related to histerisis or lost energy in the limbs not attributed to vibration. Strictly reffering to losses within the wood itself. No one has ever been able to quantify it so they simply attributed a predemined amount of histerisis to various bow woods. They based this on a faulty test they use to measure histerisis.
I contended that histerisis was virtually non existent until the limbs started taking on compression damage while being bent. My contention was that if you never allowed the bending bow to enter into the plastic range and stayed within the elastic range histerisis could be largely avoided. This would require changes in design as well as tecniques used to get the bow bending.
I was able to prove this and actually measure the histeris by simply making comparisons using widely accepted tests to factors not previously compared to. This was the moment I started loosing my passion for archery because I considered it my goal all along to simply find that lost energy, once I found it I lost interest.
Isn’t the hysteresis loss just heat? That is to say, after any vibrations are damped out it all ends up as heat. And just how for to you have to get a bow to bend to get it undergo plastic deformation? Is that close to failure?
Thats a great question. The placstic range on different woods varies greatly. Some woods will take on 4" or 5" of plastic deformation before failure while other woods will take only an inch. A common thought in archery is that you have to teach the bow to nbend properly. This is actually a horrible way to think about it. A properly built bow will have no memory of ever being bent. Almost impossible to achieve perfection here but using certain tenniques you can caarefully track your progress and the condition of the wood as you progress.
we refer to plastic deformation of the wood as " set or permanent set" for future reference.
I know set pretty well. Set is the jaw of the crane operator when you ask him, “Weren’t these rings on the lift straps round when when started?”
It’s actually not surprising that archery should have some folk wisdoms in place of hard science. Like other sports it seems to crop up in the fight between anecdote and tradition when faced with some hard evidence. I’ve seen it cycling, baseball and running, certainly. It sounds like you’re on the side of the angels here.
You could set up a rig to do a low speed, controlled experiment to see if the knock point moves up and down, during pull and release.
Lock the bow. Use a device to mechanically push or pull the knock point directly back. A screw or lever system. The device has a free turning roller at the knock point. The knock point marked. If the bending limbs are indeed that uneven. You should see the marked point move up and down in relation to the roller. If that does happen. Then it would seem that the arrow could get some vertical distortion forces when released. Arrows do distort when fired. Flexing quite a bit during release and flight.
This is a standard procedure used durring the building of the bow. Many tiller the bow shape according to how the bow is held. I contend it should be tillered according to how the arrow is shot. I tiller mine to move straight back and forth if pulled from the point wher the arrow nocks on the string.
It is.
But the limbs can be imperfect in ways. They can twist in or out. Be a bit unequal in bending force. A lot of variables, that manufacturers try to eliminate. As they age, the differences can vary over time. It is usually such small amounts, that a good bow is very accurate and repeatable.
A lot of us still do it that way except we use one over and two under, 3 fingers because the bows are heavier than kid bows.
Another popular way that is actually considered a superior method for aiming is to shoot with three fingers under the arrow. This is where the tiller shape gets thrown off.
Using a mechanical release. Some use a loop that is attached to the string. Attached above and below the knock point. So the draw point is exactly in line with the center of the arrow.
Basically a bow is a pair of springs. Presuming it follows Hooke’s law, at least to the first order, assuming the arms of the bow are balanced, result should be the same. You get into second and third order problems if, as you say, you do not bend the arms identically because the flat part of the string includes the three fingers below the knock point… But how far off symmetrical will that be?
I suppose if you wanted to prove a point, you could taper the bow arms different from each other such that the arms bend differently at different tensions until at one point, they have bent the same distance - but then you wouldn’t get a symmetrical tiller shape, would you?
