Crank length

Velocity of shortening(cm/sec)

Load(g)

If the crank length increases and the pedaling frequency remains the same, the muscles will contract over a longer distance as a consequence of a larger circular movement of the legs. However, this should take place within the same time span, which means that the contraction speed of the muscles will increase. At a higher contraction speed, the extent in which power can be exercised in the muscles will decrease (Hill, 1938).

The second variable is the crank length. This is the distance from the heart of the bottom bracket axle to the heart of the pedal axle. Contrary to the pedaling frequency, the crank length cannot be altered during the cycling process. It is of the utmost importance that the crank length is adjusted very precisely to the physical characteristics of the cyclist. In addition, a given crank length is only optimal for one specific pedaling frequency. In other words, it is of very little use to determine an optimal crank length if the pedaling frequency is not taken into consideration. Hull and Gonzales (1988) have been looking for an optimal combination between these two variables. In order to indicate that both the pedaling frequency and the crank length have an influence on the power exercised on the bottom bracket axle, they suggest the following formula:

P = Fn.Lc.ȏ

P: power exercised on the bottom bracket axle
Fn: effective component of the power exercised on the pedal
Lc : crank length
ȏ: angle velocity of the crank arm

When the pedaling frequency increases during a continuous and effective power exercised on the pedal and with a fixed crank length, the power exercised on the crank spindle will increase. The same thing happens when the pedaling frequency is kept constant but the effective power increases. On the basis of this comparison it can be concluded that the pedaling frequency should be as high as possible and the crank length as long as possible; however, as stated earlier, the pedaling frequency must stay within certain limits in order to reach its optimum. Because pedaling frequency and crank length are interrelated, the crank length must also stay within certain limits.
Because pedaling frequency and crank length are interrelated, the crank length must also stay within certain limits. Earlier research showed that the longer the crank, the lower the optimal pedaling frequency. On the basis of this fact, each individual cyclist can make a choice. But there is a third variable that must be taken into account, and those are the physical dimensions in general, and the length of the legs in particular. Hull and Gonzales (1988) came to the conclusion that the optimal pedaling frequency becomes lower and the optimal crank length gets longer when the length of the legs increases. This can be explained by pointing at some of the principles of the way the muscles work..

Why does somebody with longer legs need longer cranks?
Muscles have a certain optimal reach within which they can exercise the most power. This reach gets wider when muscles get longer. In order to use this optimal reach to its full capacity, the length of the cranks must get longer, so that the angles of the joints get larger and, as a result, the length of the muscles can vary over a larger distance. Hull and Gonzales (1988) found an ideal combination for a test person with a length of 1.77 meters: a pedaling frequency of 110 rpm and a crank length of 145 mm. This crank length deviates quite substantially from the standard crank length of 170 mm. When calculating the cost function for both crank lengths, a difference was noted of 2.4%. However, it should be noted that Hull and Gonzales derived their results from models rather than from experiments.
The diagram shown below presents a more pragmatic approach and gives an indication of how crank length relates to body length (Burke 1996).