I have been riding mountain bikes for 10 years, but I have never thought about crank length, probably like a lot of people. When I overheard two coaches having a conversation about crank arm length during a recent practice with our local NICA team, the subject piqued my interest. One of the coaches, Michael Maddox, said shorter cranks can make a real difference in on-bike performance and comfort. Coach Mike, as he calls himself, is also a coach with Science of Speed, a local performance testing and endurance coaching company. He has 30 years of high-level racing and coaching experience in cycling, so I knew there must be some truth to his statement.
I asked Coach Mike if the 175mm cranks that came stock on my bike were too long for me. His response was an unequivocal “yes.” From there, I wanted to find out more on the subject of crank arm length and why the lengths have become more standardized. Here’s what I found.
Science favors shorter cranks for mountain bikes
Jim Martin of the University of Utah conducted the seminal study on crank arm length in 2001. He had test subjects ride a stationary bike with adjustable crank arms. He tested a wide variety of lengths, ranging from 120mm to 220mm. The test subjects sprinted all-out for 4 seconds using the various crank arm lengths, and Martin recorded their average power output for each length. Martin found that the max power output of the test subjects only varied by about 4% from the smallest crank length to the largest crank length. Thus, he disproved the argument that longer cranks give riders more power.
In 2010, Paul William Macdermid of New Zealand released a study showing shorter cranks can be more beneficial to mountain bikers than longer cranks. Macdermid focused on the effects of 170, 172.5 and 175mm crank arms on performance measures relevant to female XC riders. Macdermid found that riders reached their peak power nearly 1 second quicker on 170mm cranks than they did on 175mm cranks. That means shorter cranks give riders better acceleration, which can make a difference when passing other riders or trying to clean an obstacle.
Shorter cranks can also have health benefits for riders. In July of 2019, Rick Schultz and Amy Schultz, father-daughter bike fitting specialists and physical therapists, published an article for LER (Lower Extremity Review) Magazine recommending shorter cranks in order to relieve chronic knee and hip pain in cyclists. They argued that longer cranks placed excessive forces on the knee at maximum flexion (top of the pedal stroke). While their focus was on road cyclists, the science applies equally to mountain bikers.
A professional bike fitter weighs in on crank size
I spoke with Ian Buchanan, the founder of Fit Werx in Waitsfield, Vermont to confirm my understanding of the science I found on crank length. Buchanan has raced both road bikes and mountain bikes, and has been fitting riders to bikes for 20 years. Buchanan doesn’t see the logic behind the industry standard of 175mm cranks for mountain bikes.
“Crank length on mountain bikes is pretty random. It seems to be a one-size-fits-all approach with little thought being given to it. If you were going to default to one length and put it on every size bike, then it should probably be 170mm. However, there’s no reason why the mountain bike world shouldn’t have at least 3 different size crank lengths when the road bike world now has 5 or 6 different readily available crank lengths.”
While some argue longer cranks provide riders with better leverage, Buchanan doesn’t believe it’s that simple.
“Cranks are a lever, but they are dependent on levers in the rider’s body. Making things disproportionate doesn’t add up to the best result in most cases. There is more to it than just leverage. You are pedaling in a circular motion, so you have joint movements going on throughout that motion. If people didn’t get joint pain and all there was to pedaling was pushing down, then maybe there would be an argument in favor of longer cranks.”
Besides agreeing with the results of the studies I read, Buchanan also provided a practical benefit to using shorter cranks on mountain bikes: they reduce pedal strikes.
“One of the more common complaints we get from people about riding mountain bikes is that they hit their cranks. For me personally, I hate dabbing a crank when I’m riding, so the benefit of not doing that on a ride alone makes a shorter crank worth my while.”
Speaking generally, he recommends that many riders consider downsizing cranks to a length where you don’t experience frequent pedal strikes when riding.
There is no magic formula for determining a rider’s perfect crank length, but Buchanan uses a simple rule.
“Whether talking biomechanics or simple functionality while riding, I wouldn’t recommend many people under [six feet tall] ride a 175mm crank on their mountain bike.”
How the MTB industry settled on longer crank arms
According to Lori Barrett of ROTOR Bike Components, it began when one major component brand started putting in 175mm cranks as part of their MTB groupsets, and the rest of the industry followed suit. Even though studies have disproved the theory that longer cranks always create more power, riders and the industry have been resistant to change. Barrett, who is 5’4″, uses 165mm cranks on her mountain bike.
“A long crank inhibits my ability to spin a gear up. Since power is generated by a function of force times velocity, and I’m not a power-rider, my best way to make watts is velocity; a quick spin. This means a long crank keeps me from generating power, particularly if I am trying to do it with expedience, like getting up on top of something on the trail. I can tell by my ability to get a bike off the ground if the cranks are too long. And, if I keep riding with cranks that are too long, I get sore hip flexors.”
In the road bike world, there are several different cranks sizes available to riders. The variety of road cranks on the market today stems from road riders being pickier about bike fit because they don’t move around on a road bike the way riders do on mountain bikes.
“Road riders have to be super comfortable for a potential 6 hour stint locked into the same posture. Also, it’s worth noting that there has been a recent interest in shorter road cranks because a short crank puts less strain on the hip flexor, especially if the rider is getting into a more aero position. Fewer MTB riders care about getting aero, and rightly so.”
However, Barrett notes that the mountain bike industry is starting to trend downward in crank length.
“170mm is the new 175mm for OE spec. They result in fewer pedal strikes, and are more neutral for a variety of rider heights. We also see this trend in the aftermarket, although I’d say more riders are interested in upgrades that shave grams or add power to their rigs. We have also had a brisk business in replacement cranks for enduro bikes that have had OE crank failures, still primarily 170mm.”
Will riders see more MTB crank sizes become available in the future? Only if they demand it, says Barrett.
“There’s not many options for high-end, short MTB cranks because the market has not made it so. For ROTOR, we do have some riders, like Chloe Woodruff, who run our 160mm road crank with an MTB axle to get her biomechanics matched up with the right crank length. We also work with the Outlaw MTB freeride kids’ team, and they buy cranks ranging from 155mm to 165mm. For the 155mm cranks, they match our road crank with an MTB axle, but we wouldn’t recommend that hack for bigger riders because road cranks are less beefy in construction than MTB cranks.”
After switching to shorter cranks, I can attest to their benefits
I replaced the 175mm cranks that came stock on my mountain bike with a set of 165mm cranks after reading all the research on the issue. Though the difference in size is seemingly negligible (less than half an inch in length), the difference in performance is amazing. On 175mm cranks, I averaged 2-3 pedal strikes each time I rode my local trails. Now, I can go several rides without a single pedal strike. I also accelerate faster on the 165mm cranks than I did on the 175mm cranks. Best of all, I no longer feel pain in my knees or hips after hard rides. I haven’t collected enough data yet to definitively say that my speed has improved, but the small amount of data I’ve collected so far indicates I am faster on the 165mm cranks than I was on the 175mm cranks.
After experiencing the difference between the two crank sizes, I believe shorter cranks definitely benefit shorter riders like myself.
Don’t let others decide what size cranks you should use
All riders should read the science on the issue and talk to professional bike fitters like Buchanan of Fit Werx to see if they have the right crank arm length on their bikes. Don’t assume the stock cranks on your bike are the right size for you. 175mm cranks might be right for some riders, but they are not right for all riders.
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