....The way you calculate the TOTAL mechanical advantage of the cam, is by measuring the ratio of string/fed-to-cable/reeled.
All cams transmit load through the axles.
The load is always applied on the plane of the cable/string. So, in order to find the starting and stopping points, you need to "draw a line" from the string, to the axle at 90 degrees to the plane of the string.
Mark the location at that point on the string, and draw the bow back with a draw board. Repeat the same measurement at full draw and measure the distance between the two marks. The difference in ATA from brace to full draw, tells you how much cable was reeled in. That will give you the total ratio of mechanical advantage.
That ratio, will be the inverse of the peak draw weight-to-peak limb load. For most modern bows, the cams feed out 3-4 inches of string, for every inch of cable reeled in. That means a 70# bow, has about 245# of total limb load at full draw.
Now, that seems like a lot, but it doesn't even factor in
let-off!
We all want let-off right? Well, everything you take from the string, goes right to the cables. If you have 80% let-off, that means that while the string is under 15# of load, the cables are under 230!
At brace, they were all at about the same amount of load. At full draw, the situation changed a bit didn't it?
In order to provide a 30" draw length, a short ATA bow needs to feed out more string, than a long ATA bow does. However, since the total limb compression is limited by the amount of deflection the limbs can withstand, the cams can't be made to reel in 3 extra inches of cable.
That is why short ATA bows, are harder to tune. The popularity of them has increased over the last 10 years, and with it, the cam lean "phenomenon", has been realized.
The
only way to solve the problem, is to use cams with an odd number of tracks, or attachment points.
3, or 5 track cams, can maintain lateral balance during the entire draw cycle, including let-off. 2, or 4 track cams, cannot. It is impossible.
When you add the fact that the cable guard itself exacerbates the problem, and adds to the issue by not allowing the cables to pull the limb tips directly at each other, you might possibly realize why these bows are so hard to tune!
There is only one real
solution.
You
have to shoot the arrow, through the cables.
You
have to shoot the arrow, through the cables.
You have to shoot the arrow, through the %$@#$# cables!