A crossbow is, in its simplest form, a handbow with a braced mechanical release. Had a crossbow no more draw weight (force required to pull back the String) than a typical handbow, then this release mechanism would be required to do nothing more than mimic the sliding release of an archer's fingers. This can be seen in notchlock crossbows of low draw weight, where the string slides out of a notch from behind the bolt.

However, this action tends to wear out bowstrings when used in conjunction with the higher draw weights of more powerful crossbows. At draw weights that were typical for military crossbows, the abrasion of the string against such a notch would quickly destroy the string. Further, the friction between the string and notch would strongly resist the crossbowman's efforts to trigger the bow.

What is needed therefore is a release mechanism that holds the string against draw weights of half a ton or more, releases the string without abrasion, and can be triggered with reasonable effort by the crossbowman. The following diagram is of such a mechanism, which was typical of crossbows of moderate to high draw weight, and in widespread use for centuries.

The Stave would be to the left of this illustration, which is also the direction of firing. The String is held back in the notch of the Roller Nut. The Nut is held from rotating and releasing the String by the Tickler. When the Tickler is moved and the Nut is suddenly free to rotate, the release of the String is essentially instantaneous. Whereas the String is stationary until the very moment of release, it is then suddenly free to rush forward, resisted only by the inertia of the Nut. There is no heavily loaded sliding of the String as seen in our other examples.

A thumbnail analysis reveals how the load applied by the drawn string is distributed and controlled by this release mechanism to allow the crossbowman to easily trigger the most powerful crossbows. The distance from the center of the Nut to the point at which it is supported by the Tickler is three times the distance from the center of the Nut to where the String rests against it. This 3 : 1 ratio means that the load applied to the Tickler will be only one third the load applied by the String. Given a draw weight of 1000 lb. for example, then the load on the tip of the Tickler would be 333 pounds. Given the coefficient of friction for steel on steel of .10, and assuming that the surface with which the Nut bears against the Tickler is properly radial, then the resistance to the motion of the Tickler at that point is only 33 pounds. The 2.5 : 1 ratio suggested by the limbs of the Tickler would then allow for a load of only 13 pounds to trigger the mechanism and release the String.

Of equal importance is how this mechanism distributes the loads imparted by the drawn String. The greatest resultant load in this system is the sum of the loads of the String and Tickler against the forward facing surfaces of the Nut, or 1,333 pounds as in the above example. This load is well distributed by the large bearing area of the front of the Nut against the Mortise Reinforcement. Note further that this load does not contribute to wear between the Nut and Mortise, as it exists only when the Nut is stationary, and dissipates the moment the Nut is free to rotate.

All in all, it is an impressive design; simple, sophisticated, yet over 850 years old! It brings true meaning to the expression, "period science."