I've been getting a lot of emails asking me to post more here, so I thought I'd share a few thoughts about tiller. When people ask about tiller they are usually told to set it somewhere and leave it, or that it simply doesn't matter. For some people that may be the best, but for people looking for the last measure from their bow, a working knowledge of tiller is just as important as knowing how to adjust brace height or plunger tension. I've read several books on tuning and not one of them gave an adequate explaination, so here you go:
Tiller is proabaly the least understood and most confusing part of tuning, but it is actually very smple. The tiller adjustment simply controls the balance between the force that the top and bottom limbs exert on the arrow at release. That's it. The measured tiller that a bow needs to shoot correctly depends on whatever mismatch exists in the limbs, the design of the riser, and the input from the shooter. Let's take those one at a time.
Most ILF limbs are designed at zero tiller. One exception to that is Border limbs and there may be others. Zero tiller means that an attempt was made to match the top and bottom. In reality that can vary a bit. That is why limbs are always marked "top" and "bottom". It's important that you keep the limbs in the same position that the bow is tuned in, unless you know for sure that they are EXACTLY matched. Being slightly mismatched is not a problem, but just requires that understanding. In fact I have deliberatly put the bottom limb on top in a few cases where I needed the stronger or weaker limb in that position.
The riser design is in the hands of the engineer who designs it, so I won't say too much about that. I will say that any well designed riser should require minimal tiller. Any bow or riser that demands high tiller to shoot right is not properly engineered or perhaps constructed poorly.
Shooter input is where a knowledge of tiller can pay dividends. The tiller that a bow needs depends on the grip pressure you exert, your finger placement on the string, and the distribution of pressure between your fingers. For this reason, any tiller adjustment is best done by people who have fairly repeatable form. If you are changing these inputs on every shot, then adjusting tiller will be an exercise in frustration. The fundamental building block of any tuning attempt is the ability to shoot decent bareshaft groups. OK, now to the crux of the bisquit.
The most fundamental adjustment of tiller is actually made by adjusting the nock point. Since most trad bows do not have adjustable tiller, the only way to achieve balance is to adjust where you contact the string. On a well designed bow, you should be able to reach the balance point fairly close to having the arrow level. The farther out of tiller the bow is, the higher your nocking point may need to be. Your nocking point may also have to be higher if you shoot 3 under with a bow designed for split finger. Raising the nocking point is fine to a degree, but if extreme nocking points are needed, then you will set up an imbalance in the bow that will create vibration and noise. This is why so many people think shooting 3 under is noisy. There is NO truth to that. Shooting a bow that is out of tiller or imbalanced by excessively high nock points IS noisy. My idea of excessive nock height on my bows is anything over 3/8". All of my personal bows run 3/16" to 1/4" and no more. Any adjustment past that is made by using the tiller bolts, or by swapping the limbs around, or both.
Here's how the process works:
First I set up my bow with a fairly low nock point, say 1/8" to 1/4" and shoot a group of bare shafts and fletched arrows into a bale. If the bare shafts group just slightly low, then I start creeping the nock point down. If the BS group is high, then creep the nock point up. If the group spacing is over 5", let's say, then I would leave the nock point alone and adjust the tiller slightly.
ADJUSTING THE TILLER TO A MORE NEGATIVE STATE WILL RAISE THE BARESHFT GROUP, going more positive will lower it. Since you are adjusting the difference between the limbs, you can either adjust one bolt or both (in opposite directions). Here are some rules of the road:
1. Positive tiller means that there is more preload on the bottom limb, so that tiller measurement will be smaller. Generally split fingers requires 1/16 to 1/8 of positive tiller, meaning that the top measurement will be 1/16 to 1/8
larger. Generally 3 under requires close to zero. The term positive tiller comes from "top limb positive" meaning that the top measurement is larger. Sorry, but it makes about as much sense as "back" and "belly".
2. Tightening a tiller bolt will increase preload, loosening will decrease it. To get more positive tiller, you would tighten the bottom bolt and loosen the top bolt, or you could do one or the other alone for a smaller adjustment. One trick that I use is to put the strongest limb on bottom for positive tiller, or on top for negative tiller. You can find the strong limb by adjusting the tiller to zero and looking at the space under each limb. The limb with the biggest space is the strong one.
3. Always draw the bow a few times or shoot a couple shots to settle the adjustment before measuring.
4. Since you may have changed the draw weight slightly, the last step is to check the draw weight and readjust if needed. Always turn the tiller bolts in the same direction and EXACTLY the same amount to keep the tiller adjustment good during the weight adjustment.
5. Make sure you have properly spined arrows before adjusting tiller. Really stiff arrows can bounce off the shelf or rest and give you some screwy results that are not tiller related.
Make very small adjustments, like maybe 5 minutes on the hand of a clock. Some bows will be much more sensitive than others. You will learn through experience how far to adjust your bow. My go-to bow is a 58" DAS shooting 295gr arrows. The short length and light arrow weight make it very sensitive. I can change the point of impact of my bareshafts by 6" with a change of 3/16" of tiller (turning the bolts from about 12:00 to 4:00). Longer bows will be less sensitive. Make small adjustments and shoot a test group after each one. Remember that you are looking at where the bareshafts impact relative to the fletched arrows. When you get close, lock the bolts and use the nock point for that last tweak. Like I said, I like to end up with about 3/16" to 1/4" of nockpoint rise.
Remember that the goal is to get all the settings as close to center as possible. In other words, 1/2" of tiller compensated by a 3/4" high nock point is not good even if the bareshafts look reasonable. What you have done is compensated one big error with another big error. Try to work it down so that a small tiller setting is offset or complimented by a small nock point setting. That way your bow is not fighting itself and it will be quieter and vibration free. The better designed your riser is, the closer you can get to zero on everything.
OR If none of that makes sense, just set it at zero and leave it 
Here's a little bedtime tiller story. Many of you know I got my index finger caught in a machine over a year ago and my shooting has been a challenge ever since. I've pretty much overcome the handicapped finger with a somewhat onorthodox string pickup. I shoot 3 under, but I can't curl my index finger anymore so I get a good hook with my middle and ring finger, and let my index finger sort of stick out as an anchor point. The result is that it shifts the string pressure down on the string farther than a normal 3 under. For that reason I actually shoot a slightly negative tiller on my bows (about -1mm). Nothing at all wrong with negative tiller, it's just unusual for most people to need it. It could also be used to some advantage by stringwalkers.
David
A few words about tiller
Re: A few words about tiller
The people here are awesome in that nobody can fly BS or even a simple mistake under the radar. The tiller thing has been frustrating because finding good clear info is tough. Most of the sources I've read talk about the bow pulling unevenly, but I gotta tell you: By the time the tiller is so far out that the bow pulls evenly, you've got a bow that is almost unshootable. That explaination is in my mind a copout. There is a whole world between unshootable and perfection.
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