Joe, It seems my thinking on this is wrong. I feel sure that I read the ideas I set out above, and the reasons given were to do with the bow hand, pivot point, being below the centre of the system, rather than string hand being above centre.

Originally Posted by geoffretired

Interesting ideas Job,
What I have been picking up from Border Bows is that the bottom limb, if left as a perfect copy of the top one,would be bent more during the draw.Having further to travel to get back, it would arrive later than the top one....

Nope. If Usain Bolt and I take part in a 100m 3 legged race we cross the line together (even though I've been dragged along by the faster/stronger runner for 98m). The limbs are tied together like the 3 legged scenario so at any instant one limb is dragging the other. The end result in practice is that the limbs oscillate (each keeps overtaking the other) - it's a self compensating system. The practical effect on the arrow is a high frequency nock oscillation in the vertical plane.

If you bolted the riser to a post, so the riser couldn't rotate, then all these "tiller problems" would disappear. What tiller is really about is that if the riser rotates during the power stroke then the limbs cannot energy balance, at the end of the power stroke the limbs have different remnant energy and you get post shot limb oscillation.

Know nuffink about barebow but with the Olympic recurve bow tiller is controlled with the stabiliser system not by playing about with tiller bolts.

Originally Posted by geoffretired

Joe, It seems my thinking on this is wrong. I feel sure that I read the ideas I set out above, and the reasons given were to do with the bow hand, pivot point, being below the centre of the system, rather than string hand being above centre.

Since the Gold Medallist, risers have been vertically symmetrical around the throat of the grip (always a little manufacturer give and take of course).

The pivot point is therefore below centre and how much depends on the size of the archers hand and the high/low grip set-up. The nocking point is always above centre. By straddling the the bow centre line you minimise the limb energy imbalance when drawing the bow. Three under or string walking changes this limb energy balance and hence the different approaches to tiller. With conventional recurve and Mediterranean grip then more top limb bent more than lower limb with full draw geometry.

The distance between the bow pivot point and the pressure button has for years been known to be a "tuning parameter" as regards bow design. Lots of comments historically by me, Win & Win, Vittorio Frangilli and probably Uncle Tom Cobbley.

Originally Posted by geoffretired

Interesting ideas Job,
What I have been picking up from Border Bows is that the bottom limb, if left as a perfect copy of the top one,would be bent more during the draw.Having further to travel to get back, it would arrive later than the top one. So, the bottom limb can be made a little stiffer than the top one. Now, it still bends more than the top one, but not so much, so it has slightly further to travel, but being stronger/faster, it will catch up if it is given the right amount to distance to catch up in. If that is correct, I guess the bottom limb gives a greater output than the top one. I can only guess that it might be about what energy is left in each limb by the time the string goes tight.( as Joe said earlier)
How that happens, and/or what the positions of the limbs is like at that point in time, I need help understanding.

Where I've got to now, having measrured my bow, is that the arrow rest is about 2" higher than centre. This means in effect that the top limb is shorter, and therefore stiffer than the bottom limb. To equalise the power of each it is necessary to weaken the top, and stiffen the bottom, so that when they arrive at the end of the shot joined by the string, one won't be pulling against the other. Ideally they would be pulling together? what I've been doing now is watching the vibrations at the end of the limb tips. The top now is almost still at the end of the shot, the bottom one vibrating slightly. I'm assuming that if I'd got it right they would both be still(ish)? Or not?

Originally Posted by joetapley

You've got it. For a quick summary of tiller see https://sites.google.com/site/archer...llerbasics.pdf

Just follow Chrispy's advice above and talk to the experts. Believe normally zero or negative tiller depending on the bow or style.

Don't know about mathematical models of barebow tiller - still at the "janet & john" stage of technical archery development here.

Just read that article about dynamic tiller. I get this back roll on my bow, and I've stuck a 1 lbs lump of lead on the front so that I now get a vertical neutral alignment after the shot. What I don't get is the design of the riser. The top of my riser is 10 3/4" from the rest, the bottom is 14 1/4" from the rest. This should mean that the bottom part of the riser must be heavier than the top, and should therefore counteract the backroll? If this backroll is caused by excess power then can it be countered by further weakening of the top limb, or simply designing a bow where the arrow is shot from the vertical centre? Like some of the compass longbows?

Originally Posted by Job

Just read that article about dynamic tiller. I get this back roll on my bow, and I've stuck a 1 lbs lump of lead on the front so that I now get a vertical neutral alignment after the shot. What I don't get is the design of the riser. The top of my riser is 10 3/4" from the rest, the bottom is 14 1/4" from the rest. This should mean that the bottom part of the riser must be heavier than the top, and should therefore counteract the backroll? If this backroll is caused by excess power then can it be countered by further weakening of the top limb, or simply designing a bow where the arrow is shot from the vertical centre? Like some of the compass longbows?

You generally have to add weight to a bow to get a good (aiming) balance anyway, normally done with stabilisers. With a custom bare bows, risers are often heavier and have the facility to add weights below the grip. Lowering the bow centre of mass below the grip (bottom heavy) acts to rotationally stabilise a bow. Also remember that as a bow is tilted to shoot any distance the mass below the grip moves forward of the grip, so having the centre of mass below the pivot point means the centre of mass moves forward of the pivot point improving the bow stabilisation.

Hi Joe and Job,
I am getting too far from what I have " understood" over the years; I need to back track and see where I go wrong, in order to go forward.
Shooting a bow, like a crossbow, with the bolt on centre etc, would not create any need to tiller the system, assuming the prods were matched.
With a normal bow, the hand gets in the way; hand and arrow cannot share centre spot.
1) if the arrow and draw hand are placed above centre, riser vertical to start and allowed to pivot, the riser pivots as the draw force levels, horizontally behind the grip pivot point.The riser tips back at the top towards the archer.If that pivoting is not allowed, the top limb bends as the riser at the top is pushed back to upright.

Originally Posted by geoffretired

With a normal bow, the hand gets in the way; hand and arrow cannot share centre spot.
1) if the arrow and draw hand are placed above centre, riser vertical to start and allowed to pivot, the riser pivots as the draw force levels, horizontally behind the grip pivot point.The riser tips back at the top towards the archer.If that pivoting is not allowed, the top limb bends as the riser at the top is pushed back to upright.

Our resident 'expert' demonstrated this to me. As he drew my bow, he pointed out the shelf pivoting backwards. He said this can be corrected by tiller. Now I don't want to totally disregard all he says, because he is a very good national recurve shot. However, I now know that his advice does not apply to me, because he is talking about freestyle or Olympic recurve, with all the stabs, etc. I am shooting Barebow, which is totally different. The advice I am going with, is when 3under to shoot zero tiller. As I understand it, shooting 3under effectively lowers the finger release by about 1". This will in effect weaken the top limb. The zero tiller option means that with 3under the energy from each limb is now the same? I don't get how this can be, because there is still a 3" difference in string length (4" on my bow with Mediterranean release). But I take the experts word for it, because they are far better shots than me. I would really like to understand though this discrepancy between effective limb length. Someone said to me today that top manufacturers make their bottom limbs stiffer than their top? Which is why pairs are marked top and bottom. If this is so it would go a long way to help in my understanding. I am sure there must be some formula for this. Say add a pound in stiffness for every extra required inch in effective length? The Japanese must understand this since their traditional bows are extremely out of proportion?

Hi Job, I understand your situation. Experienced archers, I find, are worthy of our respect. They have learnt much in their time. However, not all experienced archers are good shots and not all good shots have a great understanding of the problems you want answers for.Much in archery is old wives tales that have stood the test of time. Mainly because they went unchallenged. Challenge them and many are being shown to be flawed.
Barebow is much like recurve bow, not so much weight out on sticks, but that is a small detail in this issue.
Let me address the point about the tilting arrow shelf.( a longrod magnifies the tilt and makes it easier to see.)
Hold your bow up as if to start the draw, with hand on the string. IF you have the bow upright, the draw hand is slightly higher than the bow hand.With one finger above, this is even more obvious, and makes my demo slightly clearer. Next, start to draw the bow, just a little.
There is a pretty good chance that the draw hand will drop as the draw progresses and it will drop until level with the bow hand.If the bow hand remained in its original position, the bow will now be tilted back a little.The limbs have hardly bent at all so tillering would not get rid of a tilt that was caused by the dropping path of the draw hand.
The bottom limb is stiffer on many bows.That is done because the bow hand is below centre and the string hand isn't usually level with it.
When I exaggerate this by putting my bow hand lower than normal and the string hand higher,I pull the string and see the bow tilts back at the top.I also see the draw appearing to be pulling up and away from the bottom limb, bending it more than the top one.I understand that it is made stiffer to resist some of that extra bending.( my understanding may be wrong)
Japanese bows are shot well below centre; but I think the hands are almost level.My guess is that IF they were made from a natural shaped branch, the bottom of the bow would be the thick end of that branch, making it stiffer, like recurve bows built in one piece.

Originally Posted by geoffretired

1) if the arrow and draw hand are placed above centre, riser vertical to start and allowed to pivot, the riser pivots as the draw force levels, horizontally behind the grip pivot point.The riser tips back at the top towards the archer.If that pivoting is not allowed, the top limb bends as the riser at the top is pushed back to upright.

Almost have to remember that with modern recurve the handle is not gripped but is free to pivot so need an explanation of why the bow doesn't tip backwards significantly when you draw it.

What hypothetically would happen if say you replaced your string fingers with a pulley. As you start to draw the riser would tip backwards, as you state above. The pulley would then run up the bow string and the bow would continue to tip backwards until pretty much horizontal. The reason the bow stays vertical during the draw is the result of friction between the string and the tab/fingers. This friction holds the bottom limb tip in place i.e. the string tension is higher in the bottom string section then the top section. Why the top limb bends more than the bottom limb on the draw is more to do with the string geometry than the bow geometry.

Someone said to me today that top manufacturers make their bottom limbs stiffer than their top? Which is why pairs are marked top and bottom.

Nearly all limbs made and sold today are "zero tiller" limbs i.e. top and bottom limbs are identical. Limbs are marked top and bottom a) historically relating to rules regarding logos on limb surfaces and b) you need to be consistent about which limb is put where. You can quite happily put the bottom marked limb into the top pocket but you need to be consistent about it - even my top end (2000) Samick Masters limbs are not exactly identical - though near as dammit. There are bowyers still making limbs for the "Traditional" market, i.e. no stabilisers , and these have a built in tiller of up to around 5mm positive.

The basic reason the top limb bends more than the bottom limb is because the top string section is shorter than the bottom section. This results in the string generating a larger torque on the top limb (basically because the string to limb angle is larger). With a stabilised bow with the centre of mass forward of the grip you get a little additional top limb bending from the gravity torque.

As already mentioned as long as the riser is rotationally stable having a different amount of limb bend/stored energy between top and bottom limbs, within reason, doesn't matter as the limbs, being connected, sort themselves out during the power stroke. On a practical level all handled by playing about with the nocking point which determines the relative string length. Can now see why string walking is all about making compromises.

When I exaggerate this by putting my bow hand lower than normal and the string hand higher,I pull the string and see the bow tilts back at the top.I also see the draw appearing to be pulling up and away from the bottom limb, bending it more than the top one.I understand that it is made stiffer to resist some of that extra bending.( my understanding may be wrong)

Hi Joe, just posted this as part of a reply to Job.

I mentioned about using the camera set for long exposure time in Kinovea SW thread:
"...maybe it was somewhere in this site - somebody was using the camera, taking a photo on the the long time shutter as this has caught the path of reflexive fletching and helped him to diagnose the up/down arrow disturbance or something..."

As I´ve red this thread, this kind of diagnostic tool seems to be fitting

Originally Posted by geoffretired

Japanese bows are shot well below centre; but I think the hands are almost level.My guess is that IF they were made from a natural shaped branch, the bottom of the bow would be the thick end of that branch, making it stiffer, like recurve bows built in one piece.

I hadn't thought of yumis being made from a branch. That would make sense of how they arrived at those proportions, and would give a natural stiffness to the shorted thicker end. Perhaps they simply worked out the proportions by trial and error by observation over the centuries, and passed their conclusions down? All empirical, no magic formula afterall?

[QUOTE=joetapley;533685]Almost
Nearly all limbs made and sold today are "zero tiller" limbs i.e. top and bottom limbs are identical. Limbs are marked top and bottom a) historically relating to rules regarding logos on limb surfaces and b) you need to be consistent about which limb is put where. You can quite happily put the bottom marked limb into the top pocket but you need to be consistent about it - even my top end (2000) Samick Masters limbs are not exactly identical - though near as dammit. There are bowyers still making limbs for the "Traditional" market, i.e. no stabilisers , and these have a built in tiller of up to around 5mm positive.
QUOTE]

My grasp of this is slipping away again. I cannot see how such a small relative difference in NP or release position can counter the big differerence in string shortness without some other factor coming into play? Do recurve risers have a different angle in their limb pockets top and bottom?