Crank length: what the science actually says and why shorter is probably better for you.

One of the most common questions I get in the bike fit studio is about crank length. It usually starts with something like "I ride 172.5s because that is what came on my bike" or "I have always been on 175s and I do not want to lose power." Recently, with Vingegaard and Pogacar dropping down to 150 and 165 mm cranks, the conversation has picked up even more.

So I want to lay out what the research actually shows, why the fears around going shorter are mostly unfounded, and how I think about crank length when I am fitting an athlete at Revo.

The power argument is basically a myth

The concern most cyclists have with shorter cranks is that they will lose power. It feels intuitive. A longer lever should give you more torque. But that is only true if the force at the end of the lever stays the same when you change lever length, and it does not.

The best data on this comes out of Jim Martin's lab at the University of Utah. He and his group tested trained cyclists across a wide range of crank lengths, from 120 mm all the way up to 220 mm, and measured maximum power output at each length. What they found is that across a 50 mm range from 145 to 195 mm, there was no significant difference in maximum power. None.

And when they zoomed in on the practical comparison most of us actually care about, 145 vs 170, the difference in peak power was less than 1%.

The reason this works is that when you shorten the crank and keep the saddle height set for the same leg extension, you also change the position your hip and knee are in at the top of the pedal stroke. Shorter cranks mean less hip and knee flexion. Think about the difference between a partial squat and a deep squat. You can lift a lot more weight from a partial squat because the joints are in a more mechanically advantageous position. Same principle applies here. Shorter cranks let your muscles produce force in a stronger part of their range, which offsets the shorter lever arm.

Torque also does not scale linearly with crank length. When Martin's group looked at torque production across lengths, going down 25 mm from 170 to 145 only cost about 7% torque, not the 15% you would predict if it were purely a lever arm effect. That 7% loss in torque, at the peak of the power curve, translates to essentially no change in power.

Efficiency does not care about crank length either

If you are an endurance athlete, what you probably care about more than peak power is metabolic cost. If shorter cranks make you burn more oxygen at the same power, that matters more than any theoretical peak power number.

Same lab, different study. They tested trained cyclists at submax intensities across crank lengths from 145 to 195 and measured expired gases. What they found is that metabolic cost is determined by two things: how much power you are producing and how fast the pedal is moving in space. Crank length itself was not a factor.

Even more interesting: when cyclists were allowed to freely choose their cadence at each crank length, they self selected a cadence that kept the pedal speed roughly constant. On shorter cranks they spun faster. On longer cranks they spun slower. And when you do the math on that, the shorter crank athletes may actually get slightly more power for the same metabolic cost. The physiology does not punish you for going shorter.

What actually matters for choosing crank length

If crank length is not really a performance variable in the range most of us are using, what is it? It is a fitting variable. And this is where my background as both a PT and a fitter matters, because I care about what the crank length is doing to your position and to your hips.

Here is what I look at:

Hip flexion capacity. In the last blog post, I wrote about how limited hip flexion drives low back pain in athletes. Long cranks force you into deeper hip flexion at the top of the pedal stroke. If your hip cannot access that range cleanly, you are going to compensate somewhere, usually at the lumbar spine or by rocking the pelvis. Shorter cranks reduce the demand on hip flexion and often clean up the low back symptoms I see in cyclists.

Anterior hip impingement. Athletes with any structural or functional hip impingement will feel it every single revolution on long cranks. Dropping crank length is one of the fastest ways to make those athletes comfortable on the bike without changing anything else.

Aerodynamics. Shorter cranks let you get lower at the front end without your knee hitting your chest at the top of the stroke. This is why we are seeing the pros drop down. When you can close the hip angle by 5 or 10 degrees without losing power, that is a real aerodynamic advantage.

Ground and pedal clearance. For mountain bikers, cyclocross racers, and gravel riders, shorter cranks mean fewer pedal strikes. That is not a small thing when you are picking your way through a rock garden.

The adaptation is quicker than you think

The other pushback I hear is "I will not adapt to a different crank length." In Martin's original study, subjects were doing max effort tests across a 100 mm range of crank lengths after only a brief warm up on each. The riders reported that after the test, when they got back on their own bikes with their normal cranks, those felt weird for the first mile or so. Then it was gone.

Your nervous system is good at this. Give it a week or two of riding on the new length and it will feel normal.