Crank length. It might not be something you’ve considered before; surely everyone just sticks with what comes on their bike, right? Well yes, that’s probably true, but depending on how and where you ride there’s evidence that switching to a shorter crank length could make a difference to your riding. In this article we’ll outline why it could be worth making the switch.
The industry standard
Go into any mountain bike shop and chances are every bike in there will come with 175mm or 170mm crank arms, with 175mm usually found on M–XL sizes, and 170mm found on XS–S.
It’s the industry tradition and has served riders well for many years – taller riders need longer cranks for their long levers and smaller riders need something shorter.
However, delve a little deeper and there’s evidence to suggest that smaller cranks could be better for most riders, regardless of height.
It doesn’t really affect your overall maximal power
Back in the day it was thought that longer cranks were better, because they increased the pedalling leverage, thus enabling you to create more power.
However, several studies have shown that even dramatic crank length differences have only a small effect on how much power you can produce.
One of the most famous studies was performed by Jim Martin of the University of Utah. In his experiment he found that even when riders switched from huge extremes, such as 120–220mm cranks, the difference in maximal power would differ by as little as 4 percent. When you consider that the difference between commercially available cranks is much narrower e.g. 165–180mm, it’s safe to assume the difference would be even smaller.
So, if you’re worried about losing any of your sprinting power, it seems like shorter cranks will have almost no effect.
Shorter cranks can help with acceleration
A study in 2010 by Macdermid aimed to look into the differences between 170–172.5mm and 175mm for female cross country riders. The tests comprised of a supra maximal test, an isokinetic test at 50rpm and a maximal aerobic capacity (v02 max) test. What this basically means is they measured the spectrum of the rider’s overall performance across a wide range of metrics.
The results showed that there was no difference in the rider’s peak power or maximal aerobic capacity across the different crank lengths. However, the shorter (170mm) cranks were quicker to accelerate up to maximal power.
This could have benefits for all the racers out there, be it Enduro, Downhill or XC, as being able to accelerate those pedals faster could have you sprinting out of corners, closing gaps, or starting quicker than before. It could also help with those micro accelerations that riders make to clear obstacles or technical features.
Shorter crank can help with bike fit
Shortening your cranks can also help with opening up your hip angle whilst you’re pedalling. This makes it easier for you to pedal through the dead spot at the top of your pedalling stroke. Once again if you’re a keen XC or Enduro racer, this could help you to pedal more efficiently.
This article, albeit focussed on road riders, has some great animations which demonstrate how differing crank lengths affect your pedalling style.
Mountain biking isn’t like the smooth, uniform tarmacked world of road cycling. Trails are full of rocks, roots and ledges, which can all cause you to come unstuck with a nasty pedal strike.
Switching to shorter cranks obviously lessens the chances of this happening as your pedals are a little bit further away from the ground, and sometimes a little bit is all you need to get through that technical section cleanly.
Shorter cranks also put less force through your joints, so if you have been injured before, or are particularly prone to knee injuries then shorter cranks could lessen the chances of this happening.
From the evidence above, it seems that crank length has little effect on your maximal power or your maximal aerobic capacity. However, it may actually help you to make small accelerations quicker and open up your hip angle for a more efficient pedalling style. Finally, it can help prevent rock strikes and that may in turn lessen the chance of a knee injury.
So, if you’re currently on 175mm cranks, it could be worth looking into something shorter, like 170mm. If you’re a shorter rider, then something as low as 165mm could be a decent investment.