Short Cranks
Upright bike cranks are mostly available in lengths of 170mm or 175mm. With a few offerings of 145mm and 150mm used on BMX bicycles. We are talking here about the use of cranks from 145mm on down. The shortest experimental set has been 75mm.
With any crank there is a lower limit to bottom bracket height where the riders heel strikes the ground, or the underside bodywork of the trike. By using the shortest cranks possible the BB can be as low as possible, and any streamlining bodywork also can be lower without the riders toes striking the top side bodywork.
It turns out that there are unexpected benefits for some riders.
The shorter crank results in a smaller angle of motion for the riders knees. When power is applied with the knee bent less there is reduced stress in the joint yielding greater comfort.
Because the pedaling circle is smaller the rider can pedal at a much higher cadence. For some riders this results in better performance (higher and more sustainable speeds). Some report sustainable cadence increases from 60 rpm to 120 rpm and above.
Because the shorter cranks equate to a smaller mechanical advantage, the rider must use lower gears for the same ground speed. It may be that some compensation by installing lower gear ratios is necessary to maintain the same climbing ability in the lowest gears. Also the highest gears may become unusably high as well.
