Page "Crossbow" Paragraph 23

A compound crossbow is a modern crossbow and is similar to a compound bow.

The limbs are usually much stiffer than those of a recurve crossbow.

This limb stiffness makes the compound bow more energy efficient than other bows, but the limbs are too stiff to be drawn comfortably with a string attached directly to them.

The compound bow has the string attached to the pulleys, one or both of which has one or more cables attached to the opposite limb.

When the string is drawn back, the string causes the pulleys to turn.

This causes the pulleys to pull the cables, which in turn causes the limbs to bend and thus store energy.

Other types of compound bows use either ( one or both ) cam shaped or eccentrically mounted pulleys in order to provide a " let off ", such that the archer is not holding against the maximum draw weight of the bow while trying to aim.

But in a crossbow the string is held back mechanically, so there is no advantage in providing a let off.

Therefore, compound crossbows generally use only pulleys that are both round and concentrically mounted, in order to capture the maximum available energy from the relatively short draw length.