Relatively new recurve archers are often confused by tiller. (Heck I still get confused as to what positive and negative tiller are!) So what is being referred to when people talk about tiller?
The reference is to the same thing a tiller does on a sailboat, it “steers.” The “steering” referred to in archery, though, is confined to an up-down plane, not left-right. It stems from the basic structural paradox of bows. (No, not the Archer’s Paradox.) The structural paradox of bows is this: if you grasp the bow dead center, then the arrow cannot rest on the bow’s centerline, it must be above center. If the arrow is placed at the bow’s center, then the archer’s grip cannot be there, so a compromise is in order. Typically with regard to recurve bows, the center of the bow corresponds to the pivot point of the grip. The center of pressure of the archer’s bowhand is thus a couple of inches lower and the arrow is a couple of inches higher. Basically, the difference is split 50:50.
Note In compound bow design, the arrow is often placed on the bow’s centerline. This results in a very low bow hand, but the bow’s letoff, which results in a much more gradual acceleration of arrows, and the bow’s high inertial mass allows this strategy to be successful.
A consequence of this recurve bow design compromise is that since the center of pressure (COP) of the archer’s hand is on the bottom half of the bow, the bow is longer above that point than below that point. Since the archer’s fingers are closer to the middle of the bowstring than the COP is to the center of the riser, the pull on the two limbs is unbalanced. The limbs bend different amounts and, even though the two limbs aren’t exactly equal in characteristics, they aren’t that different. This has the consequence of one limb moving faster than the other and the bowstring traveling in such a manner to move the nock up or down as it travels (depending on the situation). This is not conducive to high levels of accuracy.
There is a test to see if tiller is a “problem” you need to address.
The Recurve Bow Tiller Test The technique I was taught was to remove stabilizers (to allow the bow freer movement), then raise your bow into shooting position and line your aperture up with a spot on the wall (or in the distance), then draw straight to anchor slowly (not drawing low and raising up, it has to be straight). If the aperture moves down the bottom limb is too stiff (or top limb too weak). If the aperture moves up, the reverse is true. When the bow is “balanced” (tiller means to “steer,” remember) the aperture stays level during such a draw. This procedure takes the test out of the usual draw and shoot sequence, isolating the tiller per se. Trying to observe tiller effects while shooting is very difficult. Of course, you need relaxed hands for a successful test.
Fixing Your Tiller There are a couple of approaches to fixing this. One is to fiddle with the limb bolts. Typically the bottom limb is screwed in a bit to make the bottom limb “stiffer,” to compensate for the lack of leverage on the “shorter” limb. This results in tiller measurements (from top or bottom of the riser squarely to the bowstring) in which the bottom tiller measurement is usually somewhere between 1/8˝ and 1/4˝ smaller than at the top (the bowstring is closer to the bottom of the riser than it is to the top).
The other approach is to leave the two tiller measurements the same (if they were set that way initially) and tune out this difference with a slight nocking point adjustment. Moving the nocking point changes the angle the bow sits in your two hands and a very small change in nocking point location can re-angle the bow to bring it into “balance.”
In any case, the ultimate arbiter of your tune is arrow group size. Basically anything that makes your groups smaller is good, anything that makes them larger is bad. The de facto ultimate group size at any distance is the size of a group in which all arrows fit into the highest scoring ring. (Typically this is the diameter of the ring plus twice the diameter of your arrows (allowing for outside-in “touches” that score that ring’s value).