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Cam Lean - Does it Really Matter? By Steve Johnson
Aug 13, 2007 - 6:07:04 AM
About 10 years ago we called a bow manufacturer about the lean of the cams on one of their bows. At rest, the top cam was very noticeably leaning (not parallel with the string). At full draw, the cam leaned a lot more. We were told that as long as the string and cable stayed on, the cam lean didn’t matter. The bow would shoot accurate and consistent, if we were having a problem it certainly was not the bow’s fault. We were told the problem was in us, not the equipment.
So, we made a tool to check the axle hole squareness of the limb. The axle hole was over 7 degrees out of square with the length of the limb. We started checking all the limbs we could get our hands on. None were as bad as that first one. In fact, the most perfectly square axle holes were on a set of limbs made by the same manufacturer that we had called in the beginning.
We decided that it would be easier to fill the axles with JB WELD (a filled epoxy glue) and redrill the holes rather than deal with the manufacturer. So we did (both limbs). The improved shootablity of the bow was so dramatic that we now check axle hole squareness of every bow we work on.
We had discovered that cam lean definitely had an affect on the shootablity (forgiveness) of a compound bow. So began a very long journey to find perfect cam lean. Using ourselves as the guinea pigs, we tried all sorts of different setups. But the going was slow. Often, a great deal of time was spent shooting a new setup with a different cam lean, without any clear results. Shootablity and forgiveness are very hard to measure. Especially when we are in such desperate need of both.
We soon found that measuring the cam lean by “eyeball” produced different results depending upon whose eyeballs were used. So we made a tool to measure the cam lean by projecting a laser dot to the opposite cam that would show if the cams were parallel to each other. This also gave us an indicator for how far they were out of line with each other.
So with our trusty laser tool, we started working on finding the perfect cam lean. We found that even if the axle holes were square in the limbs that cam lean still existed, and varied from one bow to another. There wasn’t much we could do about it until we discovered the split-harness cabling.
Split-harness cabling is not to be confused with the split cable setup. The split-harness hooks on the outside of the limb and ties together about 6 ½ inches down the cable. The split cable setup requires shooting between the split and it seems to aggravate cam lean. But the split-harness setup (standard on most bows today) makes it possible to manipulate and minimize cam lean simply by shortening (twisting) one side of the harness and not the other. This gives such good control that we converted all our bows to this and won’t work with anything else. (Unless something new comes along that might work better. We are always looking.)
We started lining the cams up with each other (using the laser tool and the split-harness) so that at rest the cams would be in perfect alignment with each other and parallel with the bowstring. We noticed that the best shooting bows also had the least amount of change in cam lean as the bow was brought to full draw. Few bows were able to maintain a parallel alignment of the cams as the bow was drawn to full draw. But, the ones that had the least amount of change in cam lean gave better arrow flight, more consistent accuracy, and didn’t seem to always be changing how it shot when compared to those with a lot of change in the cam lean.
To find out what was going on, we started comparing how the arrow was being delivered by the different bows. Using a shooting machine and a high-speed digital camera, we discovered that the bows with the least amount of change in cam lean always had less left and right nock movement as the arrow was being delivered (less string oscillations). We found that if we balanced the cam lean, so at rest the cams would lean one way and then at full draw the cams leaned the other way, then the change in cam lean was less and the string oscillations were less. The arrow was delivered straighter.
Hot on the trail and smelling blood, we started repositioning the cams on the axles, shimming limbs, playing with the cable guard, anything we could think of to reduce the change in cam lean. As the arrow delivery got better so did our shooting. One hunting bow went from a state of the art piece of garbage, to the sweetest most forgiving hunting bow (it killed an elk this year).
We could not seem to do much about up and down nock travel (seems mostly dictated by cam design). But, minimizing the change in cam lean certainly reduces the string oscillations from side to side.
The arrows are delivered straighter (equals better arrow flight and better broadhead delivery). The launcher prongs will wear more evenly (equals more forgiveness). The cam bushings will not wear out as fast (equals longer consistency). The limbs are less twisted (equals less limb fatigue).
Most of the discoveries were made on dual cam bows. But we found that the top idler wheel of a solo-cam bow had the same effect on string oscillations as the top cam did. The solo cam on the bottom had the same effect as the cam did on the bottom. The only difference is, there is no way to control the change in cam lean on the cam of a solo-cam. The idler wheel generally has a split-harness and is not a problem. But, to align the bottom solo-cam does not and cannot have a split-harness. What you buy is pretty much what you get. We have found that some are better than others, but even two of the same bow (that are suppose to be identical) will not always have the same change in cam lean.
When we use the laser dot to measure cam lean and change in cam lean, we have found that different bows need to be judged differently. High-energy storing bows always have the most change in cam lean and are the least forgiving. But any bow (dual-cam or solo-cam) where the laser dot moves more than an inch (top or bottom) when drawn, we fix or abandon until we can figure out how to fix it. (We have quite a few gathering dust).
Our best shooting bows have less than ½ an inch of laser dot movement (top and bottom) when drawn. We need all the forgiveness we can get!