Why mountain bike crank length matters

play article
Subscribers can listen to this article
E-bikes have been a driver of getting mountain bikers onto shorter cranks (Photo: Roo Fowler
E-bikes have been a driver of getting mountain bikers onto shorter cranks (Photo: Roo Fowler
  • Mountain bikes mostly come with three standard crank lengths, but which one is best for you?
  • Longer cranks have leverage, but how much does that matter?
  • Depending on the terrain you ride, it might be time to be a bit more unconventional in your approach to crank lengths

Cycling can often be a world of very small measurements, that make a big difference. And nowhere is this more evident, thanks crank length.

Those aluminium or carbon-fibre crank arms that your pedals are threaded into might only vary very slightly in length, but they can dramatically influence your ride comfort. Especially off-road.

In what way should you be thinking about crank length? The first issue to consider is your gearing and leverage.

mountain bike cranks
More brands are offering shorter mountain bike cranks, like Rotor's Kapic, with 165mm arms (Photo: Rotor)

Thinking about big and small circles

The shorter a crank arm is in length, the better your pedal-to-power response will be when cranking away. A shorter crank rotates in a smaller circle and allows you to ride at a higher cadence.

By the same logic, a longer crank applies more leverage force, which could theoretically aid your climbing on steep gradients. But the truth is, that these power and speed gains are quite marginal.

The most important thing you should be thinking about when evaluating crank length, is your knee health and terrain contact – especially when mountain biking.

Bicycles tend to follow a familiar trend and correlation of crank lengths. On a size small bike, you’ll find 170mm crank arms, size medium ups that to 172.5mm and on a large, you’ll be spinning 175mm cranks.

You might be wondering how the 2.5mm difference between those component sizes makes a difference, but it does.

mountain biking tech
Cranks might primarily convert your leg power to chain rotation, but they also need to clear obstacles (Photo: RaceFace)

Crank clearance is important - on challenging trails

Every experienced mountain biker has endured the annoyance of a pedal strike. That moment when your awareness of terrain does not translate to pedal discipline, and there is the inevitable ‘clunk’ sound, as your pedal hits a rock or root.

Ordinarily, it is only a matter of annoyance. But if you are aggressively climbing on a steep bit of technical terrain, a pedal strike can trigger momentary imbalance and a nasty low-speed crash.

The shorter your cranks are, the less likely they are to strike terrain. Many years ago, when most mountain bikes were transitioning to 1x10 gearing, riders struggled to compromise between having short enough cranks to mitigate pedal strikes, but with enough leverage to turn those cranks up steep climbs.

A succession of breakthroughs in mountain bike gearing has delivered 1x12 drivetrains that have enormous climbing gears. As such, the need for a longer crank, to apply that climbing leverage, is no longer a necessity.

Mountain bike cranks that are too long, can also trigger knee discomfort, at the top of your pedal stroke. But if you have good knees and think there is the opportunity to go shorter, it is a worthwhile option, reducing your risk of pedal strikes.

We live in a world where facts and fiction get blurred
In times of uncertainty you need journalism you can trust. For 14 free days, you can have access to a world of in-depth analyses, investigative journalism, top opinions and a range of features. Journalism strengthens democracy. Invest in the future today. Thereafter you will be billed R75 per month. You can cancel anytime and if you cancel within 14 days you won't be billed. 
Subscribe to News24
Editorial feedback and complaints

Contact the public editor with feedback for our journalists, complaints, queries or suggestions about articles on News24.