The Mountain Bike Crank Length Discussion

So, did you try a smaller chainring with a shorter crank?

I'm 6'1", fairly long inseam and spent most of my youth racing XC (Semi-Pro) and Road (Cat 1/2). I prefer 175's on bikes I plan on pedaling, so XC, trail and even some aggressive trail and all-mountain bikes. I have been running 170's on my enduro bike and I don't dislike it but I'm also not too concerned with pedaling performance on that bike. I have tried 170's on an all-mountain bike (SC Hightower) and really didn't like it, felt like the lack of torque required a smaller gear coming out of fast, flowy, pedaly downhill corners and created a situation where I'd want to fit a downshift in between corners where there wasn't enough time for one.

Being in SoCal, the need for pedal clearance isn't too much of a concern when compared to other parts of the US. There's also nothing but med-high speed corners here, very few tight corner trail sections, so I think most of that lends itself to longer cranks working well.

But who knows, maybe if I stuck to them long enough I'd adapt, being in my early 40's now I've developed a LOT of muscle memory to 175's.

If anyone is curious to continue the experimentation, you can get a lot more clearance at no additional cost by cutting off the ends of your handlebar, too. Sometimes as much as 60mm of added clearance on either side!

Does that help with pedal strikes as well?

On a more serious note, here’s an interesting tidbit from an article on Velo about Tadej Pogacar’s stubby little tour-winning 165mm cranks:

"The study folks cite the most when it comes to crank arm length is this one from Martin & Spirduso. There, they compared maximum sprint power between sixteen trained cyclists, each using a range of crank arm lengths: 120, 145, 170, 195, and 220 mm.

In short, the study found that the crank arm length with the highest output was a measly 145 mm, while the lowest power outputs were found at the extremes of their tests (120 and 220 mm). Even still, that difference in power wasn’t much, at just 3.9 percent between the best 145 mm length and 220 mm cranks. Power outputs didn’t change all that much, largely because riders adapted to the crank arm length they were riding.

From that data between the sixteen cyclists, the study concluded that a rider’s ideal crank arm length should be 20.5% of leg length or 41% of tibia length.”

Now, I don’t know how on earth they arrived at the 41% number, and I can't view the full text to read their full methodology for arriving at that number [Note: after re-reading the abstract, their sample size was n=16 "Trained cyclists," so probably they deliberately chose 16 cyclists with significant variations in height], but just for giggles I measured my tibia length to see how that number fared relative to my own anecdotal experience. I have very long legs and prefer 170mm cranks on my road bike, which clearly couldn’t be correct (too short), but when I measured my tibia at 16.5” (41.9 cm), that number would lead me to an “ideal” crankarm length of 171.8mm. That's spooky close to the number I arrived at on my own after riding 175, 172.5, 170, and 165mm cranks.

Curious what other people's "ideal crank length" is according to this 41% number, vs. what crank length they've settled on over time. Here's a link to the simple method I used to measure my tibial length, measuring from the most prominent point of the medial malleolus on the inner ankle to the medial condyle on the inner knee (tibial plateau).

I’m at the 155mm crank party exclusively for the dh benefits and think 165 pedals up better at my 5’9” height. When Greenland and Dak were shown to have gone to 155 from 165 and knowing how much better 165 are for dh and low bb enduro bikes than 170-175 lengths it is not a big gamble to go short. Flat pedal rider( they stick out farther leaning the bike over in a turn) with no ragrets

I ran 180 cranks on my singlespeed for 20 years because the internet said my long legs needed that. I now run 165 and my knee pain, along with pedal strikes are gone. Never trust the internet... unless it is Vital 😁

Yeah...it solves the torque difference and your bike has a permanently lower top speed....woo

I'm not sure I agree with that statement as simple as that.  I personally have never felt that issue on the top end switching from 170 x 32 to 155 x 30T.  I run a 10x51 cassette for perspective.  Now I don't ride a lot of roads with a 1x12 drivetrain, so YRMV.  

With shorter cranks I've found you naturally increase your cadence compared to longer.  To keep power the same it's said a 10cm change in cranks would require a 6% increase in cadence to keep the same speed.  If your normal cadence is 80rpm, you'd increase to 85rpm. Regardless of crank size, your human power output remains the same.  

At the top end, let's say you are pedaling for 35mph with a 32T.  You'd be doing 125RPM to do that speed.  To do the same with the 30T, you'd have to go to 132RPM.  I don't think thats much change personally, and not outside of the realm of unnatural.

I'd really focus on crank length changing is primarily for fit.  I found this was the best benefit, essentially at turning over the pedals at the top is easier and creates less knee pain.  Obviously pedal clearance is nice too on the MTB, but I think its the secondary reason you'd want to switch. 

""ideal crank arm length should be 20.5% of leg length or 41% of tibia length."

That matches what Canfield says.  "Geared trail/AM/enduro bike: 20% to 20.5% of inseam length".  https://canfieldbikes.com/pages/mtb-crank-chainring-faq-canfield-bikes

Using an height to inseam ratio of 1:2.2 (what AI searching told me), for a 166cm (5'5" freedom units) person would be 155mm, 177cm / 5'10" = 165mm, 188cm (6'2") = 175mm for simple calculation.  That checks out for me (168cm). 

I ended up running 2 bikes one with 155 and 165 and then after a year determined I prefer 155 and added to both.  That said I don't mind the hardtail with 165mm since I stand up more pedaling on it. 

What chainring sizes are people running with shorter cranks? Maybe I'm a weakling, or maybe we have long steep climbs but honestly a 30t with a 52 rear is borderline not low enough for me with 165s. I hate to think what it would be like with 155.

I went from 30T on 175 to 32T on 170. No issues. 🤷 

How many options are even out there for cranks under 165?

Hope make them in normal MTB or 83mm.

New SRAM cranks from the mechanical transmission group sets are available is 160 and 155, but not in dh width

Not too many.  I expect there to be many in a relatively short time though.  I also wouldn’t be shocked if some brands are speccing diffirent length cranks the same they do droppers for diffirent size ranges.

Bike checks say Laurie Greenland is running 160s, but he runs Saints, maybe we get new Saint at some point with shorter crack options?

Canfield and 5dev both make short cranks with DH spindles. Canfield is the most affordable option.

I went from 165 to 155. Running a 32t and it seems fine. Honestly the smaller circles almost feel more natural and I dont have to pause or ratchet as much so it almost smooth overall on the trail. Just got off a 14 day riding trip and dont see a need to go long again.

Canfield more affordable than hope?

Except for short technical sections where pedaling cadence is really low, length shouldn’t matter appreciably. If you’re are on a fire road or smooth trail, spin a tad faster.

Power = torque * angular velocity(cadence)

Torque = crank length * pedal force

Say you need to maintain 200W for your climb. Say you run 170mm cranks and were putting out 25Nm of torque, your cadence would be 76rpm.

If you shortened your cranks to 165 and maintained pedal force, your cadence would have to go up to 79rpm to maintain the same power.

Short cranks really only make climbing harder when cadence has to drop substantially.

I fully understand how power is generated, i was just asking what size chainrings people were running with smaller cranks and said I was using 30/52 and was finding it to be on the limit. But we do have some sizable, quite steep climbs here.

In a recent video Reece Wilson said he went all the way down to 140mm on his DH bike for some courses! 

I'm on 140mm now, but on an ebike (with stupidly low BB ), so pedalling up isn't really an issue even with a 34t ring. I don't find much difference descending other than the vastly improved clearance.

I know different places have very different types of trails etc, but I raced enduro in the Alps for years with good amateur results on a 30t front with 11-36 cassette. Top end speed was never an issue.

155 with a 34t, but i have a kenevo sl2 so motor goes brrrrrr

Yeah, on an ebike I think I'd be going 155 or even shorter, haven't succumbed to the dark side yet though haha.

165mm 32t on non e-bike

160 34t on e bike

living in the swiss alps, never had problems uphill, so i maybe go up to 160 on normal bike as well.

I'm average-tall (185cm), more in legs than torso, and after a bike fit a few years ago was told to get 165mm cranks because of hip impingement (or at least 'shorter than 172.5mm', and 170 didn't feel like much of a difference). I figured 'long legs' would disprove this... Nope, 40.5cm ish tibia = 165mm cranks. Interesting.

They felt a bit spinny on the road bike at first but fine on the MTB, especially with less tow overlap and increased ground clearance on road/MTB respectively.

Yes, by about $150 plus you're not locked into Hope's chainring mount standard. I've 3 sets of Canfields so that can be taken as a recommendo.

I read your implication as the fact that you found shorter cranks harder (i.e., more force to pedal) and thus were considering changing your chainring due primarily to that. I was just pointing out that with minimal cadence change, your force output is equivalent across crank sizes. Many people do not know the relationships between cadence, torque, and power.

You might be very happy with a chainring swap for any number of other highly valid reasons, but if you can spin freely on your climbs, crank length probably isn’t the driving factor (crank length discussion thread)

 

Cool, I've never even heard of them. No NZ distribution but I could always just buy internationally. 

Dub axle and 3 bolt chainring fitting is nice for compatibility.