The owner of a new bicycle often underestimates the length of the crank, neglecting an element that could be decisive in the result of the man-bicycle system.
The incapability of understanding the advantages and the disadvantages of a crank of different lenghts lets wrongly think that the choice of this component could be hurried.
If we control the numbers derived from different choices, we can immediately understand how much is important an adequate length of the crank.
First of all it is good to remember that the choice of the crank’s length is influenced by anthropometrical, physiological and situational factors.
The anthropometrical factors are easy to understand, because they are related to the length of the segments which form the lower limb: thigh, leg and foot. This last one, often underestimated, is the element which proportionally influences the most the length of the crank.
If two people have a similar limbs lengths but different foot lengths, can adopt cranks of different lengths. This is mainly due to the fact that the foot, changing its declivity, can extend or reduce the “lower limb” system by many millimetres and, consequently, can change the dynamics of all the elements, particularly of the knee.
The physiological factors mainly concern the contractible attitude of the proper muscles districts, and, consequently, the capability of developing the best performance at different pedaling rhythms. It is clear that a crank of a shorter size facilitates a higher pedaling rhythm compared to a longer sized crank.
The situational factors refer to the type of the performance requested. For example, it is clear that, in a cicle race in a circuit where there are a lot of 180° buoy turns, it is necessary to slow down and relaunch the bicycle in quick times. In those cases, a crank of a lower size facilitates the reinstatement of the speed.
In some competitions it is necessary to adopt really short cranks for practical needs, such as, for example, in fixed-gear bicycles, where the pedals are obliged to rotate even in curves.
A shorter crank allows the execution of a “banked” curve, avoiding the pedal from touching the ground.
Passing over all these “accessory” considerations, we can move on the choice of the crank’s length, taking into consideration only the anthropometrical factors.
This choice should mainly take into account the arch of the circle marked by “the centre of the knee” during a complete rotation.
The more we keep this arch in a vertical position (in relation to the pedal’s vertical axis) with the 90° crank, the most efficient will be the push on the pedal.
This does not entirely depend on the length of the crank. The largeness and the orientation of the arch of the circle marked by the knee is strongly influenced also by the position of the saddle in terms of its heigth and its retreat.
Always remember that pedaling has a circular movement, which has, as only reference point, the rotation centre. Therefore, shorter or longer cranks do not influence the distance between the saddle and the central movement.
The alteration of the distance between the saddle and the central movement according to the variation of the crank’s length, can considerably modify the largeness and the orientation of the arch of the circle marked by the knee.