It might not seem a major concern to many riders, but to some, and myself included, the length of the bike crank arms is one of the most important factors to fit and performance when riding. Yet compared to other geometry factors, it seems to be overlooked and stuck in the past.

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So why is crank length so important, and how do you know what length is best? We speak to a bike manufacturer Mason, an MTB expert from SRAM and bike fit expert Phil Burt to dive into it in more detail.

The length of the crank arm is measured from the centre of the bottom bracket to the centre of the pedal axis. The most common sizes across all disciplines are 170mm, 172.5mm and 175mm. However, there are other sizes either end of these distances used.

The size of the crank arm is often dependent on the size of your bike, which relates approximately to the rider's height. The bigger the bike, the longer the crank arms – usually.

Height is not the only factor as there are other variables to consider, including the type of riding – MTB, gravel, road and even within each of those, there are differences. On the road, a crit racer might choose a shorter crank arm length for extra cornering clearance, whereas mountain bikes, and more downhill orientated bikes, will usually have a shorter crank arm length than cross-country to avoid pedal strikes.

Your crank is the lever for pedalling, so you might simply think that the longer the lever, the more force you can transfer into the pedals and to the wheels, but it isn't quite that simple, and many biomimetic factors come into play. A few studies have been done in the past, with Inbar et al. (1983) and Martin and Spirduso (2001) both showing only a very minimal power difference, up to a few percentages even at extremes of 120mm and 220mm lengths. If power isn't a factor, why are we so set on the three major sizes currently available? For me, the answer is simply tradition and how things have always been.
 

If power isn't the biggest factor, perhaps the fit on the bike is the reason why mountain bikes will generally have longer crank arms than road bikes? We spoke to renowned bike fit professional Phil Burt, who has worked with some of the top athletes and riders across all disciplines, from triathlon the mountain bike.

“Crank length is an interesting and important factor, although for power production it is only a factor in maximal cycling, such as the start of a team sprint on the track where the first few revolutions where the influence of torque has an effect, but in normal circumstance, you have to go to extremes, such as 80mm or 330mm for it to have any effect.

Some of the best road cyclists previously raced as mountain bikers, including Peter Sagan, Tom Pidcock, Cadel Evans and many others, and the cycling community will usually agree they are the most efficient pedallers, and likely due to regularly climbing steep, loose climbs.

In those situations, the crank is moving very slowly and a longer crank may be a factor, but this would only be on the very steepest climbs. If you want to go up really, really steep stuff all the time there is a potential argument for a longer length crank. For most people, mountain biking is a whole mix of circumstances, uphill, downhill and flats and then other factors, such as clearance will play a factor.

There are lots of things in cycling that are culturally accepted and don't move quickly with the industry stuck on these arbitrary numbers. For many people their crank lengths are too long and they will notice some positive differences when dropping down. The advantages of holding a long crank length for the extremes, where torque is a factor are far outweighed by the benefits of reducing the crank length. These include better clearance, smaller circles to pedal, greater stability, the opening of the hip angle and perhaps less important for mountain biking is the ability to obtain a more aerodynamic position.

At some point in the future, one of the big manufacturers will jump and the rest will follow and I would not be surprised if in 10 years we see riders on 155 to 165mm crank lengths.”

Within Mountain Biking specifically, one of the biggest factors is pedal clearance. It is the reason why so many cranks will come supplied with protectors for the ends and I will bet that if you check your own mountain bike, there is a high chance of the end having some form of scuff or mark from where it has caught the ground when pedalling. For me, as a predominately cross-country rider, it is a common occurrence, especially riding up more technical rocky tracks that we have around South and Mid Wales.

There are some signs that the industry is starting to make a switch, with more trail bikes coming with shorter crank arms fitted but for many larger brands, the average medium-sized mountain bike will have longer crank arms than any road bike in the line-up.

If pedal clearance isn't seen as a factor, why are we still seeing relatively longer crank arms on many mountain bikes, and are we possibly starting to see a change? We spoke to Alex Rafferty, MTB Communication Manager at SRAM.

“I think pedal clearance is one of the most significant factors in recent changes to crank length choice. It wasn't that long ago that most mountain bikes had a 175mm crank on them. This is still the preference for many riders and what they feel comfortable on. For others, pedal clearance and bike geometry developments have progressively led to shorter crank lengths. We all know longer-lower-slacker, right? 

DH bikes are typically built with 165mm cranks, and the Enduro category has followed suit, with many or most riders running 170 mm and shorter cranks. I mean, there are e-bikes out there with 150mm cranks on them too. Without a doubt, most bikes have gotten longer, lower and slacker - whether that's a cross-country or downhill bike. So to accommodate these geometry developments, cranks are slightly shorter for clearance without losing any potential advantages. 

These lower BB bikes combined with gnarlier terrain require more clearance to avoid pedal strikes and ultimately crashes. That's the most significant reason for cranks getting shorter on mountain bikes. Power output advantages for longer crank length sizing is marginal at best. There are other factors at play though - bike size and rider ergonomics play a part. We're used to seeing larger bikes of the same model with longer cranks accommodating taller riders with long legs. But more recently, all of the bikes in the same range would have the same crank spec. After all, the BB height is typically the same, right? Product managers responsible for the build and specification of a bike are progressively leaning towards shorter cranks across the size range. I'd side with there being more advantages to having a sensibly shorter crank length (165-170mm) than there are disadvantages. Road and mountain bike. 
 

On the other hand…Anecdotally I had a conversation with an incredibly talented and exceptionally tall rider who firmly believes the longer crank specification (175mm) allows a more stable platform to shift the bike fore and aft to aid in balance. "Imagine if your feet were side-by-side, it would be more difficult to dynamically shift your bike forwards or backwards underneath the body, and the larger spacing between your feet also give you a more stable base to stand." I can get on board with that idea too, but I still like to keep my pedals and cranks more clear of the ground. 

At SRAM we present the products the rider needs now and also what we believe they'll need in the future as bikes develop to even more capable machines. Everyone has a choice and a preference. We're here to help people along the way to what suits them.”

So while component manufacturer SRAM will deliver what the customer requests, there are some more progressive bike companies that are making bigger changes. Mason Cycles, based in Coombes, near Brighton, recently released the RAW mountain bike, after many years of producing acclaimed road and gravel bikes. As standard for all sizes of the RAW bike builds, 170mm is the default option. Is Mason ahead of the curve? We spoke to Matt Nightingale, the tech expert at Mason to discuss the subject further.

“While I can not speak for other brands, I certainly think that standard crank length has not yet caught up with the progression we've seen in mountain bike geometry. With the trend towards greater stability and a lower centre of gravity for cornering performance, bottom bracket height has been getting lower and lower over the last 10 or so years, but crank lengths have remained largely the same which is of course going to reduce ground clearance.

There is also the argument that the higher cadence encouraged by shorter cranks arms is better suited to road riding, whereas for mountain biking, and especially off-road climbing, the lower cadence and better control of torque you get with a longer crank arm is beneficial. The science is pretty moot on this point though!

Whilst we specify 170mm crank arms as standard on all sizes of the RAW, we do encourage our customers to change this to suit their needs, preferences and fit. Crank length, along with stem length and bar width is all customisable via our online configurator. That said, I would always err towards the shorter crank lengths on modern mountain bikes to increase ground clearance and reduce pedal strikes. With the RAW, the bottom bracket drop is at the progressive end of the spectrum for stability and handling purposes, so a shorter crank is beneficial to allow you to pedal through the chunk. The RAW is designed to be pedalled, it's not just a sled for the downhills!

As we look ahead and potential change, there is a supply and demand situation here. If the big bike brands continue to order longer cranks in large numbers then we can't expect to see a change any time soon. As more riders become clued into the benefits of shorter cranks, especially for off-road use then the big names will start to take note and maybe we'll see more off-the-peg bikes coming with cranks at 165mm and shorter.

For individual choices, we usually recommend going for a professional bike fit prior to purchase, this really helps with all aspects of fit including crank length. We have one supported endurance rider that is 193cm (6'4") tall and rides a 155mm Rotor crank. The major component brands have not caught onto this yet, but we're definitely seeing a trend towards shorter crank recommendations from bike fitters, and this is matched by the preferences of many endurance athletes.”

From those we have spoken to, it seems they are all in agreement to an extent that the standard crank lengths we see on bikes are longer than needed, and perhaps we will see this change over the coming years. What no one was able to answer is specifically why we see mountain bikes specified with crank arms longer than road bikes. To me, the answer is simply down to tradition and the bike industry that is often slow to change. 20 years ago disc brakes were starting to become more popular, yet it took some time to be widely accepted and geometry and components seem to follow the same pattern.

We would love to hear from you, and find out what other riders think. Do you simply use whatever is supplied on the bike? Or perhaps you have experimented with various lengths and found a sweet spot?

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