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A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion. Bows generally have two concentric cams at the end of each limb.

Figure: Round cam, eccentric cam, oval cam, and elliptical cam geometries. 

A diagram of different cams


There are four popular cam systems used in modern compound bows. 

Figure: Compound bows utilizing single, hybrid, binary, and twin cam systems. 
Diagram of different cam systems


The ratio between the radius of the outer cam and the inner cam measured at the point where the cable makes contact with the cam.

Figure: Example of a gear ratio between the outer cam (yellow) that has a radius of 1.8 in and the inner cam (gray) that has a radius of .75 in. The gear ratio in this example if equal to 2.4. That means that for each full revolution of the outer cam, the inner cam completes 2.4 revolutions. 

A regular cam diagram.
Gear Ratio = 1.8 in .75 in  = 2.4

On a compound bow, the outer cam typically has an oval shape, which allows the radius to change as the cam rotates. This means that the gear ratio also changes.

Figure: As the cam rotates, the gear ratio is reduced. This helps the archer pull the cable to full draw without having to overcome an excessive amount of draw weight. 

A diagram of a cam at rest

At rest

Gear Ratio =3.6 in.8 in = 4.5
A diagram of a cam at full draw

At full draw

Gear Ratio =2.25 in.8 in = 2.8


A system of two or more pulleys with a rope or cable threaded between them that is used to lift heavy loads.

On a bow, the inner cams act as a block and tackle that pull the limbs of the bow together as the cable is drawn. The pulleys create mechanical advantage, multiplying the force applied by the archer. 

The shape of the cams further enhance the mechanical advantage by altering the gear ratio and providing "let off" as the cable reached full draw.

Figure: As the cable is drawn, the cams rotate and the limbs of the bow flex inward towards one another. The elasticity of the bow limbs combined with the mechanical advantage of the cams amplifies the force that propels the arrow. 

A diagram of a block and tackle in a bow