The Case for Short Cranks

By Lennard Zinn

It’s probably clear in my previous “Why Proportional-Length Cranks?” blog post that I generally see performance benefit for tall riders with long legs in using long cranks. And by long, I mean longer than the standard 170-175mm (or even 165-180mm) offerings from the big component brands. In fact, I have found that riders will generally climb best (and often feel best) on cranks that are in the range of 20-21.6% of their inner leg length.

 

That said, there are some applications that offer clear performance reasons for long-legged riders to use standard crank lengths (and for shorter riders to use extra-short cranks). Those applications are on bikes with aero handlebars: individual events against the clock (road time trials and timed velodrome events) on flat and rolling terrain, as well as in triathlons in which drafting is outlawed. This same advantage applies for the same reason on aero-bar racing for riders for whom standard crank lengths work well for climbing to use extra short (shorter than 170mm or even 165mm).

 

The reason for this is that, beyond 12mph, aerodynamic drag is the biggest resistance force the rider must overcome. Since the bulk of the aerodynamic drag of the bike and rider is created by the rider’s body, the higher the speed, the more important is the body shape. The fastest body shape is low, flat, and narrow, and crank length affects how low the rider can get. 

 

Crank Length with Aero Bars 

Some impediments to getting the rider’s back into a horizontal position are: the knees coming up and hitting the belly and/or the chest, the hip joint becoming impinged from being folded into such a tight angle, and the low back and hamstrings being overly stretched. Triathlon bikes traditionally counteract this with dramatically forward saddle positions so that the feet are further back relative to the hips, thus opening up the hip angle. This, however, is not allowed on road time trial bikes and track bikes; the International Cycling Union restricts how far forward the saddle can be ahead of the cranks.

 

While tall riders could produce incrementally more power by using longer cranks that are better matched to their legs from the perspectives of leverage, muscle extension/contraction, and knee angles, this power output increase can be more than offset by increased aerodynamic drag. If the handlebars have to come up higher to prevent the knees from hitting the chest, the hips from impingement, and the low back and hamstrings from strain, then the upper body is going to be pushing more air ahead of it. It often takes more power to overcome this than was gained by the longer cranks. The solution is to use shorter cranks to allow the upper body to get closer to horizontal. 

 

Rather than using a crank that is 20-21% as long as the inner leg length as Tour de France results have consistently shown works well overall, for time trials that proportion could be more like 18-19% to maximize speed. This results in a reduction in crank length of around 15-20mm from climbing crank length to time trial crank length. The rider’s knees thus come up 30-40mm less high at the top, so the handlebars can come down that same 30-40mm, which can make a big difference aerodynamically.

 

There is some application of this reasoning in mass-start professional road racing, where the tallest riders often have the job of being lead-out men for their team’s sprinters. While not using aero bars in this situation, the lead riders have to get as low as possible on the bike, because reducing aerodynamic drag is super critical when leading out a sprinter at the front of the peloton in the last few kilometers, when the sustained speed can approach 80kph (50mph). A good example of this is Mario Scirea, whose 194cm (over 6’4”) height was rare in the pro peloton. His job for much of his career was to lead out super sprinter Mario Cipollini. His handlebars were far below the height of his saddle to reduce his aero drag, and even though his legs were long enough to ride 200mm or longer cranks effectively, the 175mm cranks he used allowed him to be very low in the wind during those wild, high-speed windups at the front as the peloton hurtled toward the finish. 

 

Note that how high the knees come up is also dependent on how long the thighs are relative to the lower legs. Great swimmers often have long lower legs (and feet) and short thighs, whereas great cyclists often have long thighs and short lower legs. The great Tour de France riders of the 1980s, Bernard Hinault, Laurent Fignon, and Greg Lemond all were able to ride with very low handlebars and a nearly horizontal back with 175mm cranks that were about 20.5% of their leg lengths simply because they all had extremely long thighs relative to their lower legs.

 

When aerodynamics is all-important, shorter cranks can often be the fastest solution.