Knee Angle and Crank Length
After the saddle has been adjusted to the correct height the knee angle is tested. The angle between the upper and lower leg is smallest when the pedal reaches the top of the up stroke. The top pedal position is the beginning of the down stroke, in which the most force is used for propelling the bicycle through the extension of the leg. If, in this moment, the knee angle is smaller than 90°, then the kneecap is being pressed against the surface of the joint with excessive force. In this case knee problems are bound to occur. As soon as the angle is greater than 90° this is no longer a problem. (See Fig. 2)
The knee angle is affected by the length of the cranks. The crank length should be no more than one fifth of the leg length. Otherwise the knee angle will be too small. The standard crank length is 170 mm. This length is suitable for most riders with a leg length of at least 80 cm (31½”), which corresponds to a minimum height of approx. 170 cm (5’7”). We recommend shorter cranks for riders of smaller stature.
Most riders less than 170 cm (5’7”) in height tend to ride with a relatively high saddle position in order to achieve a healthy knee angle at the top of the pedal stroke. This is actually a compromise because these riders must extend their feet to reach the pedals at the bottom of the pedal stroke. Althout this added foot extension is generally not bothersome, a shorter crank should be used if possible. Shorter cranks are now available with gear systems that have a single chainring in front.