| Compound Crossbows
Crossbows exist in different variants. One way to classify them is the acceleration system ie;compound or recurve crossbow. A compound crossbow is a modern crossbow and is similar to a compound bow. The limbs of a compound crossbow 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 crossbows use either (one or both) cam shaped or excentrically mounted pulleys in order to provide a "let off", such that the trigger sears are not holding against the maximum draw weight of the bow while cocked. 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. When cams are used, this unique feature in design allows more arrow velocity in a given draw distance or (powerstroke) as opposed to a recurve crossbow of the same weight pull. Two factors determine the amount of energy a crossbow can hold. Its draw weight is the amount of force required to draw the bow. A crossbow's draw weight increases the farther back you pull the string. Its draw length is the distance between the crossbowstring's position at rest and its position when drawn. The total amount of energy that a crossbow can hold is approximately equal to its draw weight times its draw length, divided by two. In other words, a bow's overall strength depends on how hard it is for you to pull the string and how far back you are able to pull it.
Crossbow manufacturers express this strength in terms of: The crossbow's energy, measured in foot-pounds, The arrow's velocity, measured in feet per second. | 
