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Training points: Are your cranks the right length?

Stu Bowers
24 Nov 2020

Your cranks are essential for transferring power into forward motion, so should we be paying more attention to getting their length right?

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.

The problem is, the industry settled long ago upon the ‘right length’ crank for a given size of bike – for example, try finding a size 56cm without 172.5mm cranks attached.

Yet, thinks Phil Burt, we should consider challenging this status quo. And he should know, having spent 12 years as head of physiotherapy at British Cycling and five years as lead physio and consultant to Team Sky.

‘When I first met Bradley Wiggins he was riding 177.5mm length cranks, but for the Rio Olympics team pursuit he rode 165mm [Wiggins won gold]. He was on similar for his Hour record. That wasn’t by chance – it was planned – and if it can work for someone who has won the Tour de France, it might be worth you considering.

‘Crank length can influence a number of things. In a low, aero position the hip joint angle becomes very closed, which makes breathing harder; hip flexors become tighter and the hip extensors [glutes] spend longer waiting to engage.

'Shorter cranks help open up the hip at top dead centre of the pedal stroke so help alleviate these things, thereby making an aero position more efficient and sustainable.

'Shorter cranks also lessen the total kinematic loading of your knee joint – think how much easier it is jumping onto a 20cm high box versus a 1m high box. That’s crank length.’

Bikefitter Phil Cavell of London's Cyclefit agrees. ‘There are a number of reasons to change crank length, but hip range is the big one,’ Cavell says. ‘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.’

Burt makes a similar point.

‘The research evidence is clear: crank length makes no difference to power on the road – track is slightly different – unless you go as short as 80mm or as long as 320mm. And as a bike fitter and physiotherapist, I’ve never had a reason to go bigger.

'So if anything raised here resonates with you, try dropping your crank length by 5mm, but also remember, if you haven’t got any issues, leave well alone!’

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