I started cycling over twenty years ago, for the first few years I had no idea what crank length I used but as I continued in the sport and purchased my first mountain bike I began to know a little more about the machine I was riding.
I have the dubious honour of having virtually the shortest legs in the peloton but have for most of my cycling life used standard 170mm cranks on my road bike and 175mm on my mountain bike.
I have been fitting riders to bikes for nigh on fifteen years and wrongly assumed that bike and component manufacturers knowledge is superior to mine, after all they have been doing it longer. At Cyclefit we design custom frames to be millimetre perfect but generally use cranks that vary in length by only 0.5cm for customers whose inseams range from 75cm to 93cm.
What we are aware of at Cyclefit is that bike manufacturing is an industry and for a company to survive it must make a profit therefore reducing the number of products it produces and has to warehouse the better.
Most bike manufacturers now only produce five frame sizes to cater for the population as opposed to eight or nine ranging from 48cm to 62cm. We are now categorised as either extra small or large or medium/large etc. Across this range of frame sizes cranks were specced usually in three sizes 170mm, 172.5mm and 175mm, much easier for component manufacturers then to only make three crank sizes and why not stop making triple chainsets as well so inventory is reduced and we have to struggle up mountains in a 34/28 gearing.
After a season of trying to ride up mountains and hours of both physical and mental self-analysis in September I decided to reduce my crank size in proportion to my leg length. 155mm in fact.
The main reason for the change was that at the point of maximum torque in the pedal stroke a short leg will be bent more and the sheering stresses at the knee greater. This became very apparent as I struggled up the Tournmalet in the summer at about 3rpm.
Would a more open knee angle through the dead spot at the top of the stroke and maximum torque further down the stroke reduce the sheering stress and the amount of force required to move the pedal?
We based our calculations on the average saddle height of average sized riders who would normally use 170mm or 172.5mm cranks, worked out the ratio of leg to crank (not including feet) and used this to decide which size cranks to use. It is not fool proof because femur length and foot size also get involved but the actual difference of 1.5cm is a good place to start because the body will need plenty of time to adapt to such a major change.
See table below: