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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OK, I’ll be the contrarian here. As I am 6’-2” mostly leg and a masher ( not a spinner) I have learned that I prefer 180 mm cranks. However, 180 is becoming more scarce and impossible to find for fat bikes, for example. I recognize that most people are not long legged 6-2 but if everyone starts using 170 and shorter then it stands to reason that manufacturers may stop making even 175mm. 175 may be too long for most, but some of us are taller than most.
Great point. You probably won’t benefit from shorter cranks. However, for shorter riders like myself moving to shorter cranks is a huge help. The bike industry needs to offer a wider range of crank sizes for all riders.
Looongtime mtber…newish E-mtber. Running full sus like most these days. At 6’1″ and growing shorter daily😁 I suffer from “Fred Flintstone syndrome” with long torso and short legs. In general: when on the fireroads/flat singletrack on XC endeavors, the 175s feel most efficient, especially when “locked out” on suspension like a hardtails. When on techy singletrack, the 170s strike less and feel quicker. On my Emtb the sweet spot are the 165s, plus I can pedal lower in the saddle on my dropper without the knees hurting. Probaibly, the pedal-assist motor compensates for my inefficiency and allows for more fun in low-rider style of riding. Also, for the E-mtbs the suspension and heavy motor inherently creates more pedal strike opportunities. The shorter cranks help me…a lot💪
No biomechanist or race-contender here, but inseam/leg length, full sus v. hard tail, and rough (trail enduro) v. smoother (trail XC) terrain appear to be important variables for mtb use…all lean toward shorter cranks with shorter legs especially when using suspension on most singletrack trail conditions IMHO🤟.
Good points. Thanks for the comments. I definitely had less pedal strikes on my old hardtail that I did on my full suspension prior to swapping out the OE 175s for 165mm ones, though the hardtail came with 170mm cranks, which might have helped as well.
My bottom line is 155mm cranks are the way to go!
I do believe the bicycle industry got crank length all wrong, just like the way they got HTA and Reach length all wrong for decades!
I still have bikes with 155, 165, 170, and 175mm crank length. They all work but I prefer the 155mm over all others. Originally I bought some 155mm Canfield Brothers Cranks back in 2015 just for the ground clearance, but what I found out eventually found was they have much more benefits than just extra clearance.
With 155mm Cranks:
1) I do not need the seatpost at an ideal location to be comfortable, I actually can still lay down some decent power and be fairly comfortable with the dropper fully slammed.
2) I can ride the bigger gears easier. Torque is really dependent on gearing and NOT crank length as most people assume.
3) for people with knee issues, shorter cranks are a god send as you are less likely to tweek your knees due to making smaller circles
4) more ground clearance.
Like I said, I can make any of the crank lengths work and be happy with them if I did not know any better. However, having tried them all, I can easily say 155mm is the way to go. I am 170cm tall (5’7″ with shorter legs). I only have 2 sets of 155mm and one set is fairly heavy, the setup is about about 1/2 pound heavier than my 175mm SRAM XX1 DUB Crankset on my Ripmo. If there were lightweight XX1 or RF Next Cranks available at 155mm, I would be all over them.
Informative article. More reason to build my own bike rather than let the manufacturer decide on the build spec. LBS bikes don’t typically come with pedals; I argue they shouldn’t come with seats either since I spec my own. Maybe you’re on to a trend leaning toward more customization from direct-to-consumer brands, or at the LBS.
Great point. I would love to see manufacturers give rider options for things like handlebar width, crank size, saddle width and style, suspension and brakes, and make it part of the MSRP. Having a good bike fit is more than just choosing the right frame size.
Are you aware of any set guidelines that correlate rider height or inseam length with recommended crank length?
There are several different formulas out there but I don’t know how accurate they are. Ian Buchanan said the only definitive rule for him was that riders under 6ft shouldn’t be on 175mm cranks. You aren’t going to do harm to yourself by going to shorter cranks. You just might not get the optimal performance you want. I recommend you talk to a professional bike fitter. Crank length is important but it’s one of several variables that comprise a proper bike fit.
Zinn has some info on how to calculate optimal crank length: https://zinncycles.com/custom-cranks/
I just switched from 175mm to 170mm cranks, and I can definitely feel the difference. Both of my knees have had popping and some pain, and been getting worse, but I can already feel that reversing. I’m 5’8″, so it makes sense that 175mm was too long for me. I’m a huge fan of shorter cranks so far.
Good deal. Thanks for the info. Using the formula Zinn has, the 165mm cranks are perfect for me so it makes sense why they felt so good when I switched to them.
I am just over 6ft tall, would 165mm cranks be too short? Right now I have the stock 175mm cranks on my bike. Definitely looking to go to 170mm, but wondering if 165mm would be beneficial.
I am not a bike fitter so I would definitely recommend you consult with one before making any decision to go with shorter cranks. That being said, you are in the height range for 175mm cranks, so the only reason for you to go shorter is because you are either getting a lot of pedal strikes when you ride or you are experiencing knee or hip pain after riding. Shorter cranks will benefit you in those cases. Also, shorter cranks can give you a higher cadence and better acceleration. There has been no evidence to indicate that going with shorter cranks is detrimental to riders, so there is no downside to trying them as long as you are willing to pay for them.
I experimented with 152mm cranks. I liked the shorter length, but the cranks themselves weren’t high quality and loosened while a few hundred miles.
Going back to 170/175mm cranks made it feel like the circles I had to spin were really exaggerated. I could tolerate 165mm, but vowed not to ride any longer. The moment higher quality short cranks are offered, I’m going back.
Thanks for the comments. The hidden gem in the Martin study is that max power was generated with 145mm cranks, so it’s not surprising you liked the 152mm cranks.
We are all thinking from the perspective of power, which should be the primary concern. However, cranks also determine the length of the base or platform our feet and legs use to support us while standing on downhills. I wonder when (how short) your idea Varaxis becomes substantially detrimental to our stability going down technical and/or high-speed descents. Again, this is obviously to some degree relative to height. I’m guessing that the current crank lengths are already too short for optimum support but still fine obviously. Hopefully, there is only a minimal effect as optimal crank length is redefined but there is a point somewhere where it would be substantially detrimental.
In theory you can regain some of that stability by dropping the bottom bracket height more. If you have 150mm cranks and a 15mm lower bottom bracket height and still have more ground clearance.
Trailcraft is what you’re looking for – solid direct mount cranks that use a shimano 24mm BB. Looks like all their stock is reserved for their kids bikes now, though.
I’m just over 5’10” and have the 152mm ones on my bike. Tried the 140mm as well for a few months and concluded that was OK with them when seated but they felt just a little too short when standing.
My 152mm crank experience is with the Trailcraft DM.
Even with the preload-cap and pinch bolts tight with medium threadlock, the NDS crank slides outboard. It has straight mini-splines.
Doesn’t changing gears solve any problems with lever arm torque? It feels like the complaints laid out in the article become problems when the gear choice is wrong (which is made worse by the large gaps between cogs on mountain bikes). Go for high cadence, and you can quickly accelerate when necessary, and you probably avoid the knee problems since there is less force on the knee. My feeling is that crank arm length should be chosen for what makes you comfortable, not because of arguments about performance or lever arms.
I understand your argument to a point. However, optimizing your comfort and fit on the bike results in optimum performance. Gear changes won’t do that. Check out Chloe Woodruff’s Instagram story on her journey to shorter cranks.
Crank arm length, pfft!
Torque capability, rider inseam, Rider strength, typical cadence of said rider regardless of ratio in use.
Now enter the pizza sized cog in the rear with 16′ of chain… And the RD that is always grinding the ground next to the rear tire!!!
Great article and discussion. Thanks Richard!
Thanks. I’m glad you enjoyed it.
When I bought my new Scott Genius my LBS suggested I go to 170 cranks. I am 5′ 10″ and 68 years old so I was nervous about the loss of leverage. I did the research before going shorter. I went with the 170. The fit is much better. If I had the money I would try 165.
Kudos to your bike shop for making that recommendation. If only all bike shops were that accommodating. I just bought a new bike from my LBS and they wouldn’t swap the OE 175mm cranks for 170mm ones unless I paid extra $$. I hope we can move to a place in the future where you can choose things like crank size and it be part of the bike price without paying additional money, a “short person tax” if you will.
Glad to see this article! I’ve currently got 175mm cranks, and while I don’t have any real knee pain I would like to reduce the chance of rock strikes and what not. I think I’m going to try 165.
Thanks for the comments. I really have enjoyed using the 165mm cranks on my full suspension bike. I have virtually no pedals strikes now, which helps me have a lot better flow on the trail. I don’t think you’ll be disappointed if you make the switch.
I think you should look at the biomechanics of crank length. If your thigh muscle is strongest over an arc of x degrees, then your crank length should accommodate that. Shorter riders should have shorter cranks, taller riders longer. Remember when you rode that kid’s tricycle and you had no strength? Same principle, your thigh didn’t have the arc to deliver power.
At 6’8″, I love my 200mm cranks, the difference between “normal” cranks is dramatic for me. Of course it means I ride custom frames with a bottom bracket 1″ higher, but the rest of the frame keeps the back pain at bay.
I’m 6’2″ with longer legs, but I put 170mm cranks on my last bike a few years ago. The upside is definitely fewer pedal strikes. The downside is it feels like the bike has one less low gear. For normal 2-3 hour trail rides I prefer the shorter cranks, but for a couple 10 hour endurance events I put the 175mm cranks on, and I think I was better off for it.
Great article. Long overdue. I have been riding 165 mm cranks for the past nine years, First to relieve knee pain And second as a big rider to reduce pedal strikes. I would not go back to longer cranks. Point worth mentioning here though, shorter cranks are not particularly easy to source. Most of the big manufacturers have a few variants of some of their cranks But nothing approaching their full lineup. Right now I’m running race face conch C six carbon cranks in 165
Thanks for the comments. I am now using 165mm RaceFace Aeffect cranks. I hope this article will prompt more companies to offer a wider range of crank sizes.
Very true. Maybe this article will encourage more manufacturers to offer a wider range of crank lengths. I am using 165mm RaceFace Aeffect Cranks and love them.
For those who want to experiment, here’s a starting point:
Trailcraft
127, 140, 152, 160mm
Spawn Cycles
89mm, 102mm
127mm
Brood Styx Cranks
Prevelo
Heir Cranks
Suntour
XCT Jr
Zeron 1-X
Canfield
AM/DH Cranks
SRAM
NX 1x Cranks
Great information. Thanks for sharing.
So a ‘professional’ bike fitter states that anyone under 6 foot shouldn’t be on 175 cranks? Really?
We seem to be going through a particularly bad phase at the moment where journalists peddle (ha!) Fashions.
This is yet another, “everybody stop, you’re doing it wrong, you should be doing it like this” non-isssue, and there is always the crowd that laps up every bit and follows the latest fashion.
To be fair, the full quote is “I wouldn’t recommend MANY people under [six feet tall] ride a 175mm crank on their mountain bike.” (emphasis added) Hopefully it is clear this is Buchanan’s personal opinion.
My overall reading of this article is that there is no such thing as “one-size-fits” all for crank arm lengths which is what I hear you saying as well. Everyone will need to either experiment, or work directly with an expert who can look at all of their individual factors.
It reads better now (edited since I posted I believe).
But still, I think riding style outweighs any perceived idea of ‘correct’ from a bio- mechanical viewpoint.
Consider what a shorter crank length means: a faster cadence. This means that the pedal will return sooner towards any obstacle, which in turn means that there is less time to get over an obstacle before the pedal comes around again.
More leverage created by a longer crank means a higher gear, slower cadence and more distance travelled per pedal revolution.
With this in mind, there is the argument that longer cranks reduce pedal strikes.
To visualise: consider tight single track with a single deep rut (created by walkers, maybe 1 foot wide), and deep enough that pedals will strike the top of the rut regardless of crank length.
Now, you can position the cycle to one side so that the lower most pedal descends into the rut, then move the cycle to the other side so the other pedal can enter the rut. Easy enough, but then add complexities of roots, short climbs and prickly bush obstacles and then timing becomes paramount. Too fast a cadence and you won’t clear any of the above combined obstacles without severe (momentum halting) pedal strike.
In this situation, shorter cranks exacerbate the difficulties of traversing the obstacles.
The rest (or all) surely is preference.
All good perspective and what you’re saying does make sense.
It’s a serious accusation in journalism to say an article was edited without notice following publication. I can confirm that the post has not been edited or changed by anyone other than me (editor in chief) since March 16 @rshinn 11:56am. Note this was three days before the publication date; articles are scheduled days, sometimes weeks in advance.
The words MANY and ANY are just one letter off so it’s possible you misread it the first time? I mean no disrespect to you but suggesting a post may have been edited is not fair or accurate to the writers, editors, or individuals being quoted.
Comment edit: I just realized my comment might not be clear, sorry! No one has edited the article since March 16 @rshinn 11:56am. I was the last one to make an edit.
No problem, I may well have misread it and quite happy for that to be the case. For what it is worth, I had no issue with the idea of an edit, because the message (or exchange of ideas) is always of greater worth than the messengers, ie, you or I!!
At 5’2″, going from 170mm to 165mm cranks made a huge difference on how stable I feel when I’m out of the saddle.
I think you make some excellent points for smaller cranks, but as some of the other comments point out you overlook the biomechanics of the taller riders and the fact that gear range of the modern 12speed is better for spinning than mashing the pedals. I’m a taller rider and raced BMX in the 80s and I still prefer 180mm cranks on my single speeds. 175s feel just right on my geared bike. Overall though I think you’re spot. Thanks.
Not including Canfield in this article seems like a huge oversight. Canfield has been building and espousing the benefits of short MTB cranks for nearly a decade. Only brand I know of building high-end AM/DH cranks shorter than 165 (all the way down to 150).
I’m 6’4″ and love my 165s.
https://canfieldbikes.com/collections/cranks-chainrings