Crank length is an often overlooked aspect of bike fit. A majority of production road-oriented bikes come outfitted with either 170mm or 175mm cranks. You do occasionally see 165mm cranks spec’d on the very smallest frame sizes, and some manufacturers offer 172.5mm cranks on their mid-sized frames, but it’s a rare occasion to encounter a crank under 165mm or over 175mm on a bike primarily intended for road riding.
There are a number of methods for determining proper crank length. Peter White recommends 18.5% of the distance from the top of the femur to the floor in bare feet. Leonard Zinn recommends 21% of inseam measured in bare feet. Bill Boston recommends measuring femur length from the center of the hip joint to the end of the bone in inches, then using this measurement as crank length in centimeters (for example, if your femur measures 17 inches, you would use a 17cm [170mm] crank).
Let’s use Peter White’s method as an example. For simplicity, let’s assume a rider with a top of the femur to the floor measurement of 919mm (36 1/16″). Multiply by Peter’s 18.5% (919mm x .185 = 170mm) and you end up with a recommendation for 170mm cranks. That’s convenient for someone with a 36″ floor to femur measurement, but it gets more complicated for individuals outside the “normal” range. Shorten the floor to femur measurement by three inches (33″) and the recommendation quickly drops down to 155mm (838.2mm x .185 = 155mm). Lengthen the floor to femur measurement by three inches (39″) and the recommendation jumps up to 183mm (990.6mm x .185 = 183.25mm). Both fall outside the normal range of sizes typically offered in road cranks.
I believe there’s quite a bit of room for personal preference in these formulas. I also think the human body is amazingly adaptable and we can probably get used to just about any crank length currently being manufactured, regardless of our leg length. That said, it’s only reasonable to assume that crank length should vary based upon an individual’s physiology. As Lennard Zinn said, “If you accept that muscles and joints work most effectively when operating in a certain range of motion, then it only makes sense that muscles, bones and tendons work that way for everyone. Short riders should not be required to force their muscles through a greater range of motion than the person with an 80cm inseam riding a 172.5mm crank. And on the other end, 7-foot basketball players do not bend their legs any less when they jump than shorter players. So why should they use minimal knee bend and operate their muscles only through a tiny part of their range when they ride a bike?”
Assuming Mr. Zinn is correct, taking a closer look at crank length should be particularly advantageous for those who are significantly taller or shorter than average. On the outer edges of the bell curve, it’s very unlikely people are riding cranks that are close to their ideal length.
While it’s not easy to find unusually short or long cranks, they’re out there. The Specialites TA Carmina is a beautiful, if expensive, production crank that has interchangeable arms available from 155mm up to 185mm. Lennard Zinn offers oversized cranks, and da Vinci Designs offers custom cranks in almost any size within reason.
Those who aren’t involved in the recumbent community probably don’t realize that recumbent riders have been experimenting with ultra-short cranks for some time now (see the links below). Some of their findings are quite interesting and may possibly be applicable to upright bikes. Short crank advocates claim reduced knee strain and higher crank RPMs among their advantages.
Ultra-long cranks may introduce more knee strain, and they will definitely reduce cornering clearance, so both of these facts should be taken into account if you’re considering retrofitting a longer crank on your existing bike.
And finally, whether you’re going up or down in length, keep in mind that a major change in crank length will very likely require a change in saddle height, which, of course, will affect overall bike fit.