A Correction

In a previous post, “A Little Bow Geometry,” I made a mistake that needs correction. (I include that earlier post below so you don’t have to go fishing for it. SR) The error involves the diagram:

In this diagram you can see that I put the centerline of a recurve bow between the pivot point and the arrow rest hole. This might be a good place to put it but almost all recurve bows are designed with the center line right at the pivot point, not where I put it in the diagram. That deign places the center of pressure on the bow’s grip about as far below the centerline as the arrow is above.

The rest of the post is about right.

A Little Bow Geometry

Everything about archery consists of tradeoffs, starting with the design of bows.

Consider the recurve bow at brace in the figure. The horizontal centerline of the bow is midway between the arrow rest hole and the pivot point of the grip. The nocking point is about one half inch above the level of the arrow rest, so that arrow (depending on size) is centered about 3/8˝ above level and an additional inch or so above the previously mentioned centerline. All of these are examples of many of the various tradeoffs necessary to design a bow.

Basically this results in the archer’s fingers (plus tab), being 2-2.5˝ high, being practically centered on the bowstring (see fingers in diagram in relation to bows centerline (CL) which is also the string’s centerline). The bow hand is on the bottom half of the bow, creating what is called a “tiller” problem. (The word tiller means the same as the word tiller associated with sail boats; it means a thing “to steer.” By holding the bow asymetrically, that is on the bottom half, we in effect have made the top limb longer (and therefore weaker than the bottom limb). Many people adjust for this by turning the limb screws to create a slightly stronger limb on top than bottom. (In the old days, they actually sanded one limb more than the other to make it weaker.) Others address this issue by adjusting the nocking point location, leaving the limb bolts alone. If you move the nocking point up, you are decreasing the leverage you have on the top limb, making it effectively stronger, etc. and apparently only small adjustments in nocking point location are necessary to adjust for the problem that comes from holding the bow on its bottom half.

The same issues come up whether you shoot a longbow, recurve bow, or compound bow. The simplest approach is to set the tiller at “even” and then adjust the nocking point location while tuning (bare shaft test). Tiller is determined by measuring from the top or bottom of the riser to the bowstring (at a right angle), then “tiller = top tiller measurement – bottom tiller measurement.” Typical recurve settings are +1/8˝ to +1/4˝ (string closer to bottom limb than top). (Barebow is lower, sometimes a 0˝ or even negative tiller settings.)

This situation has benefits, though, in that as the bow is drawn, a torque is created hinging on the bow shoulder that helps to raise the bow. Basically the draw is part “back” and part “up” (using the bow arm as a boom.

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2 responses to “A Correction”

1. Hi Steve Do you ever balance a recurve bow? With everything attached for Olympic style or just barebow?

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• Yep, Barebow, too. We want our hows to “hang” from our bow hands straight up and down. To get that we want the center of gravity (COG) to be in the central plane of the bow. There are jigs, believe it or not, that allow you to do this but they are a bit expensive. Instead, you can simply hang your bow from a bit of thread to see if it hangs straight up and down. You can shoot a photo of the hanging bow (from the side) and then draw a line on it from the thread, straight down. The COG must be on that line. Hand the bow for a different point (any point), take another photo (again side on) and draw another line. down from the thread. Now comes the tricky part. If you printed the two photos onto paper, hold the two up to the light and arrange the pages so that the bows overlap. The COG of your bow is where the two lines you drew intersect (The COG must be on both lines, right?)

Of course you can do the same thing with a photo editing program, making one of the two photos somewhat transparent (to be able to “see” through it.

OR archers want the COG of the fully configured bow to be slightly ahead of the bow hand and slightly below it. This causes a roll over forward to the target after the loose. Barebow archers can’t do this as they don’t have the benefit of longrods, etc. Barebow archers tend to have COGs slightly behind the bow hand (and below if enough weights are used). The bow reaction from a naked recurve bow or a well-setup barebow, is top limb tip rocks back toward the archer.

Modern compound bows tend to be heavily reflexed (speed, speed, speed!) and so are very front heavy. (As the handle was moved so close to the bow string–I find them unshootable.) These bows have to have weights added to amke them less front heavy, otherwise they roll over too quickly, adding motion to the bow when we want it to be still.

If you are interested in the “bow balancing jig” Larry Wise put out a DVD with the manufacturer showing how it works. [image: image.png]