Have you ever wondered why bicycles come with a relatively narrow range of crankarm lengths?
Most cranks commercially available range from 165 to 175 mm. This 1 cm span equals about 1/3 inch yet is supposed to fit riders from under 5 feet tall to well over 6 feet. Very tall cyclists can buy 180-mm cranks from a few companies, but that’s generally the upper limit without an investment in custom production.
So while widely available cranks differ in length by less than 1/2 inch, the riders on any weekend club ride can vary in height by almost 2 feet. That’s a huge discrepancy and seems completely out of proportion. Shouldn’t a rider who is 25% taller than another ride on cranks 25% longer?
Of course there are good reasons to avoid really long cranks. At a certain point they’d require changes in bicycle design. Long crankarms necessitate a high bottom bracket so you don’t hit a pedal on the ground when the bike is leaned over even slightly in corners.
Crank lengths are standardized over a narrow range more to fit existing bicycle design parameters than for any theorized physiological efficiency. There are traditional and aesthetic reasons too — extremely long or short cranks simply don’t look right on a bike. Over the years, guidelines for crank length have been generally accepted. Riders with crotch-to-floor measurements of less than 30 inches (76 cm) are usually sold 165- or 170-mm cranks, riders who measure 30-33 inches (76-84 cm) are guided toward 172.5-mm cranks, and taller riders end up on 175s. A few studies have attempted to find the ideal crank length and decide if rider inseam or height is the determining factor, but the results have been confusing and contradictory.
Martin’s Revolutionary Study
Jim Martin, Ph.D., from the University of Utah, has studies crank length. Using special adjustable crankarms, subjects rode with lengths ranging from 120 to 220 mm — a far wider range than is available commercially. Riders sprinted all-out for about 4 seconds on cranks of various lengths and their average power was recorded.
Most of us would assume large differences between the power output a rider is able to generate on crankarms only 120-mm long compared to 220-mm long. But amazingly, max power varied only 4% from the shortest to the longest cranks. And even more surprising, in a more normal crank range of 145 to 170 mm, the difference in power was a miniscule 1.6%.
Some riders agonize over crank length differences of 5 mm. Should I get a bike or a new crankset with 170-mm arms or 175-mm arms? If power in Martin’s study was affected only 1.6% with a 25-mm change in crank length, we can safely assume that it would change less than 1% when comparing the normal range of available crank lengths.
Even more surprising, there was no correlation to rider height and leg length. Commonly used 170-mm cranks compromised the power of the shortest and tallest riders by at most 0.5%. That’s only 6 watts out of the 1,200 watts generated by a powerful sprint. How about tests longer than 4 seconds? In a 30-second maximum effort, using cranks from 120 mm to 220 mm produced no differences in power output or rate of fatigue per crank revolution. Crank length simply didn’t matter.
So what’s the bottom line? According to Martin, “Cyclists can ride the crank length they prefer, without limiting power.” So the crank length that came on your bike is just fine. And you don’t need special cranks for time trials, sprinting or climbing.
Attention: Time Trialists
Martin’s study has a specific application for time trialists. Traditionally, TT specialists switch to 2.5- or 5-mm longer cranks for races against the clock, believing that greater length provides more leverage to push big gears during a time trial’s steady application of force.
However, Martin argues that because crank length doesn’t affect power production, time trialists should use the shortest commercially available crankarms — 165 mm. This will let them have a lower and, therefore, more aerodynamic position that’s likely to result in a faster ride.
After all, long cranks are longer not only on the downstroke but also at the top of the stroke where, in a time trial position on aerobars, the rider’s thighs tend to hit the chest. The longer the crank, the more the chest must be elevated, thus increasing frontal area and wind drag.
So with short cranks, a rider can get the chest lower without leg interference. The importance of a low frontal area in time trials is unquestioned. Pro teams spend thousands of dollars for wind tunnel research so they can determine the lowest and fastest position for their riders without compromising power. But they do it on bikes with long crankarms — often even longer than normal. Thanks to Martin’s study, we know that short crankarms allow a lower aero position without compromising wattage output.