If you are custom building a bike you will have a choice of crank length. Choosing the right crank arm length, like the rest of this, comes down to your size and riding style. Longer cranks give you more leverage, helpful if you like to push big gears at a low cadence (hill climbing, time trailing, etc.) For time-trials and mountain stages, Miguel Indurian, for example used 180 mm cranks in time trials instead of his usual 175 mm. On the other hand, if you like to spin at a high cadence, you’ll do better with standard cranks. As a starting point, we often recommend 170mm cranks on 54cm (21") C-T (centre to top) road frames and smaller or heights below 5’9", 170–172.5mm cranks on 55–61cm (21" – 24") frames for people from 5’9′ to 6’1", and 172.5–175mm cranks on 62cm 24" and over frames or over 6’2" tall. Mountain bikes generally get cranks 2.5–5mm longer than road bikes; that is, you might want a 175mm crank on your mountain bike if you’re set up with a 172.5 on your road bike. Velodrome (track) riders can reduce this estimate by 2.5 mm if the races are short or the track is steep.
Longer cranks tend to cause a sharp bend in the knees at top dead centre, right when power begins to be applied. Cranks that are too short won’t be as efficient because riding at higher pedal speed involves more movement of the leg mass covering a larger rotation compared to a shorter crank.
While a larger rider, in particular one with longer femurs, will be better suited to a longer crank, an increase in crank length on road bikes also increases the total movement of the knee to chest on a rider with an absolutely smaller relative frame. This might increase the tendency to "ankle" (heeling down on the down stroke of the pedal)
Since the seat height is not limited by the high but by the low stroke (leg extension, upper/lower leg angle) a longer crank requires a lower seat position. On the other hand, to decrease the range of femur motion one wants to raise the saddle. One is constrained here by geometry.
In Article 49, the paragraph on Technical Specifications, of the UCI regulations, the height of the "bottom bracket" is limited to the range of 24 cm minimum and 30 cm maximum.
Since this article does not allow for morphological differences it represents absolute limits in design not function. Using ultra-long cranks and the maximum allowed 30 cm BB height the bicycle would have insufficient pedal clearance to do much other than navigate a straight smooth course. Even if the UCI where to allow for a morphological clause and permit higher bottom brackets, the bicycle would provide inferior handling with such a high bottom bracket.
Gearing
It is advisable to run a gear that results 60 rotations per minute (1 rotation per second) or faster to avoid overloading muscle tissue and knees. A faster cadence will help circulation and reduce lactic acid build-up. This is especially so for longer rides.
