Get a copy of Stephen Graf's "The American longbow: How to make one and its place in a good life." It will answer most of your ASL questions, and it's just a good read.
"When I look at my ASL I see that the centerpoint is above the midpoint of the handle." Not sure I know what you mean here. Do you mean the arrow shelf is above the center of the bow? It should be. On most bows, it's about 1 1/4-inch above center. This makes the upper limb the longer one. You have 1 1/4 inches of grip above the center of the bow and 2 3/4 of grip below the center of the bow, resulting in a shorter lower limb. So, yes, the center point of the bow is above the center of the 4-inch riser grip. If the arrow shelf is 2 inches above the center of the bow (and some bowyers do build this way) then the limbs are of equal length.
Tiller has to do with how the limbs bend in relation to each other. It's determined by measuring on a strung bow from the string to the end of the upper and lower riser fade outs. Positive tiller will yield a measurement that's a slightly greater distance, usually 1/8 to 1/4-inch, to the upper riser fade out than the distance to the lower riser fade out. If both measurements are the same, it's called neutral tiler. If the lower measurement is greater, it's called negative tiller.
Since most bowyers place the arrow shelf 1 1/4-inch or so above the center of the bow, the top limb is longer, but because the shooter's top fingers are above the center of the bow, whether shooting split or 3-under, the finger position in effect reduces the string length to the end of the upper limb, flexing the limb more. The 1/4-inch positive tiller built into the upper limb weakens the limb just a bit to accommodate the greater flexing to balance it to the lower limb.
By comparison, the bottom limb (and string) is shorter to begin with than the top limb and thus is built a bit stiffer because it will flex less on a bow with with a standard 1/4-inch positive tiller. In this case, moving the nock point and fingers up creates a longer string below the fingers to the lower bow tip, which flexes the lower limb less, requiring more stiffness. That's why some bowyers tiller bows neutral or negative tiller for 3-under shooters.
All this being said, many 3-under shooters shoot positive tiller bows and just set the nock point a little higher to balance out the limbs.
My three shooting fingers span about 2 1/8 inches. So on a bow with the arrow shelf 1 1/4 above the center of the bow, plus my 1/2-inch nock height minus the thickness of the nock, say 1/4 inch, my lower finger will be on the string about 5/8 inch below the center of the bow, placing my middle finger on the string just about in the center of the bow.
Now, if I switch to split, my two lower fingers are about 1 1/2 inches across so on a bow on which the riser is 1 1/4 inches above the center of the bow, the bottom of my lower finger will be on the string right at the center of the bow.
The 3 under hold on a bow cut 1 1/4 inches above center, centers the three fingers on the string at the center of the bow, but when the limbs are of unequal length, that might not be the best location. The fact that 3 under is noisier than split finger shooting is an indication that the limbs are somewhat out of tune/balance.
Topic: Centrpoint,limb length, tiller (Read 563 times)
