No2: Crank length

Your cranks are essential for transferring pedalling force into forward motion, so why are most of us using the wrong length?

It’s possible a good number of people reading this don’t know what length cranks are fitted to their bike. Many might not even be aware that they come in different lengths. Yet cranks affect how effectively we generate pedalling force, as well as our overall comfort on the bike, so shouldn’t we be paying more attention?

Let’s start with the basics. Cranks are measured from the centre of the pedal axle to the centre of the bottom bracket spindle. Lengths most often range from 160mm up to 185mm, in 2.5mm increments, and typically bigger bikes will come with longer cranks.

This might seem obvious, so why consider changing? It’s time to defer to the experts.

‘There are a number of reasons, but hip range is the big one,’ says Phil Cavell of London’s Cyclefit, bikefitter to the pros. ‘It’s especially important to be able to ride more effectively in an aero position.’ He’s unequivocal that many of us are riding cranks that are too long for our bodies to cope with and bike companies need to change their ways.

‘They’re stuck in the past,’ Cavell says. ‘A 54cm bike coming with a 172.5mm crank, when that could be ridden by someone who’s 5ft 6in, is just nuts. We’re fitting 165mm cranks all the time now. I literally can’t remember the last time we fitted a 175mm crank. It’s an obsolete item for us.

‘Shorter cranks will almost certainly help most riders be more comfortable on a bike,’ he adds. ‘They help soften the impact of cycling on the body. Think about it: the equation is 2πr, so crank length changes that circle significantly, and going shorter appreciably reduces the range of joint movement. We didn’t evolve around producing power with a flexed knee and a flexed hip. If you can do anything to open out the hip angle it’s most often a good thing.’

Faster, stronger

If we use shorter cranks won’t we be forfeiting leverage, and therefore losing power? Not according to Jim Martin, associate professor at the University of Utah in the United States.

‘Our tests revealed that extending the range a long way from the standard [170-175mm] has no substantial impact on power or efficiency,’ says Martin. ‘We tested right down to 120mm and up to 220mm. There was a substantial fall off [in maximal power] below 145mm, but we’re talking about cranks more than an inch shorter than most of us ride, and even then it was just a 4% drop.

‘You do have to take into account pedalling rate,’ he adds. ‘With a shorter crank you need a higher cadence, but that’s a small adaptation that happens very naturally for most. As far as maximal sprint power and metabolic cost are concerned, crank length can be anywhere from 145mm to 195mm and it really doesn’t matter.

‘A longer crank is basically a lower gear ratio. It might allow you to climb better, but its effect is tiny compared to shifting up two sprockets on your cassette. What is more important is the influence it has on the relationship between your thigh and your torso. This is about comfort, the basic feeling of your thigh coming up into your chest or stretching your muscles until they are like guitar strings, just to get over the top of the pedal stroke.’

Time for change

‘I would say at least half of your readers aren’t as aero as they could be because their cranks are too long,’ says Martin. ‘Anyone in the range from 5ft 8in to 5ft 10in won’t be able to get a horizontal body position with standard length [170-175mm] cranks. It will typically be worse for women, who are shorter on average, not to mention anyone a bit older, who will almost certainly have reduced range of movement in their hips.

‘Think about it like this: can you squat more weight from a deep squat or a shallow squat? Shallow, right? That’s a bit like using a shorter crank. Plus, if you shorten your crank by 20mm, then you then need to raise your seat height by 20mm too, so that means your leg is now 40mm more extended at the top of the pedal stroke and your hip angle is much more open.’

Martin’s findings debunk the myth that long cranks produce more power, his conclusion being the individual rider is essentially free to choose. However, shorter cranks would bring a lot of positives.

‘I didn’t set out to discover “it doesn’t matter”,’ says Martin. ‘I wanted to discover the optimal, but as it turns out it just really doesn’t matter when it comes to power. Shorter cranks certainly matter for riding aerodynamically, though, not to mention aiding mobility or joint pain issues and even ground clearance for pedals. I’d say in most cases, your cranks are probably too long.’