PeteHski

My 2019 mtb has standard 175 cranks as you might expect. It also has a fairly low BB like most modern trail bikes and pedal strikes are always a real threat. I would really prefer 170 or even 165 cranks on this bike. It has plenty of low gears for climbing, so don't think loss of crank leverage would be any issue at all. Only thing stopping me from changing is the cost. It's not a bad enough issue to make it a must-do change. But if I was building from scratch I would go shorter.

KerryIrons

The "logic" of "crank length should be proportional to leg measurements" has been around for a LONG time, and lots of people have turned that "logic" into a formula for determining crank length. Only one problem: the research doesn't support it. One key feature that is often ignored in these discussions is the duration of muscle contraction that is controlled by cadence. It just may be that there is an optimum here, which is why there is a fairly narrow range of cadence for optimum performance. Longer cranks tend to mean lower cadence, moving you out of that optimum range. Crank length has been a point of debate since the introduction of the "safety" bicycle in the late 1800s, and there have been all sorts of fads in that regard.

There is no reliable formula for predicting crank length. There ARE lots of formulas out there, but they are just figments of the imagination of their purveyors. No one has ever done a study that shows how crank length should relate to anything.

You will find no high quality data to support any particular crank length as being better than any other. This is true whether or not you correct for leg length, femur length, etc. On the other hand, you will find lots of anecdotal or low quality data to support all kinds of conclusions, and more theories than you can shake a stick at. A rider's response to changes in crank length is 1) highly individual, 2) dependent on riding style and the event (TT, climbing, crits, track racing, etc.), and 3) most important, highly adaptive. This is why it is so hard to study the effect of crank length.

A 2008 study by Jim Martin, Ph.D., from the University of Utah shows zero correlation between crank length and any performance factors.

Camilo

I think that the concept of KOPS is valuable because it gives a reasonably objective place to start when deciding where one likes the saddle and a good place to match for another bike. I like mine quite a bit behind KOPS, but KOPS is where I started, and it's easy to get new bikes/saddles to the same position fairly easily.

I didn't follow any of the links in this thread, but what I remember from fairly credible sources on the WWW is similar to what you have written. Since I personally can tell absolutely no difference between 170, 172.5 and 175, of course I agree! But seriously, this has really helped me as I put together bikes because I just don't worry at all about crank length whether I'm buying a new or used crank. I used to obsess with finding a 172.5 just because it's "medium". Then I found a 175 which worked fine, and then a 170 that also worked fine. I haven't tried a 165 though. It affects the saddle height of course as I try to keep the saddle - pedal spindle height consistent.

I definitely looked for a shorter crank arm for my wife as I was putting together her gravel bike last spring. The frame is marketed to use both 700c/29 and 650B/27.5 heels. The idea, I believe, is that people who use the smaller wheels do it to get higher volume tires (this frame: 700c takes 42mm, 650b takes 2.1"/53mm. The idea is that you'd end up with about the same diameter wheel going with a larger tire in 650b than what you get with a smaller tire in 700c. But I wanted to lower the standover a bit for my short wife so I went 650b, but with the same size tire I put on my 700c - 35mm. This lowers the standover and the chance of pedal strike, so I thought that 165mm cranks would help a little. She really likes the smaller wheels so now I'm ready to take the plunge to find her a high quality 650c road bike although she is happy with the set up on her 700c XS frameset.

But again, the bottom line is neither of us notices crank arm lenght except as it affects saddle height adjustment.

Litespud

I've run 172.5s on everything since I started to get "serious" in the mid-90's. I used a size chart on the Colorado Cyclist website that correlated inseam with crank length, and 172.5 was what it recommended, so I went with it, knowing no better. However, it's always worked for me, and I see no reason to change. I even managed to find a 172.5 XT crankset when I built my first and only MTB (mountain biking never really "took") and I ran 172.5s on my fixie - never had an issue with pedal strike.
As I see it, crank length choice should come down to biomechanics rather than any perceived performance gain - what sort of flex one's knees and hips are comfortable with. Whatever mechanical advantage that crank length gives or takes away is compensated for by the gearing - it all evens out in the end - short cranks allow faster cadence, but the reduced torque generally require a smaller gear, while longer cranks allow more torque (facilitating a bigger gear), but also tend to impose a lower cadence. The only time crank length might be critical is when you're climbing something steep in your lowest gear, when the ability to use a different gear is taken out of the equation - the increased torque you can generate with longer cranks might be the difference between staying on the bike vs walking. Pretty much every other time, crank length is balanced by gear, so pick the length that's most comfortable.

Kabuki12

I have a lot of vintage steel racing bikes and all but two are 170’s . One is 165 but it is mainly my daughter’s bike but I have ridden it and the other is my ItalVega that has 175 cranks. I have a long inseam and most of my bikes are 60cm+ with the ItalVega at 63cm. I honestly can’t tell that much difference but the 175’s seem to climb a bit better and it has the same gearing as most of my others with 170’s. Going up 5mm’s will change your ground clearance so check BB height.

bruce19

Here's a GCN take on the subject:


As far as I can tell, there is a theoretical difference. However, I doubt I could tell the difference. Maybe others can.

JohnDThompson 

I run 165s on my fixed gear bikes to minimize pedal strike. I put 155s on my wife's bike to mitigate knee joint issues.

PeteHski

I think this video is a good summary of the reality here. Standing start sprints in an excessively high gear being about the only advantage of longer cranks! For most riders there is no real downside to running shorter cranks and quite a few potential advantages.

PeteHski

Coming back to the original question, 2.5 mm difference is very marginal and unlikely to make any real difference at all. It is 1.5% difference in both torque and pedalling circle. To put it in perspective, moving just 1 gear up or down the cassette will be 7-15% difference in torque. So roughly a factor of 10 difference. You can clearly feel the difference between 2 gears that are 10% apart, but could you feel a 1.5% change?

Another way of looking at it is that you would have to go from 172.5 to 155 cranks to see a 10% reduction in torque i.e. the equivalent of just 1 gear on the cassette.

burnthesheep

It depends how impinged you are on hip angle. If you're riding a tri or TT bike and you have a really low stack height, then that undetectable difference of 2.5 or 5.0mm could be a game changer.

For a person riding for enjoyment on a road bike, I doubt outside of it assisting with bike fit in regards to your total height and length of your upper and lower leg......it's not anything game changing.

All this junk about longer cranks and shorter cranks and gearing is moronic. Shift the gears that come on your bike! Or change them to suit! It's a bit of a troll thing that constantly comes up on this forum that gets a bit old.

Polaris OBark

First Week impressions

Last week I decided to try a 165mm crankset, going from 172.5 mm. That is about a 4.3% difference, which I would expect to be hard to detect. This was mainly to help avoid pedal/crank strikes when off-road. I want to see if I can live with the change. If there is any advantage to my joints, all the better. In other words, my goal is to have fewer strikes without impeding my cycling in some other way.

Briefly, I can't really detect any difference when just riding along. Above 10% grade, where I am most comfortable in my lowest gear (30T x 34T), I think I can detect it. Between 15% and 22%, it is unambiguously harder to climb. Subjectively, it seems a lot more than 4.3% harder (which is the reduction in moment arm, and therefore the amount of increased force I would have to apply at 90°, so much so that I have to wonder if something else is going on, like I haven't managed to re-position the saddle optimally). My legs feel it, and my knees hurt a little bit. However, out of the saddle, I find pedaling a bit smoother throughout the circle. Climbing off-road, where I have less of an ability to stand, I found I was really struggling to make it up some steep inclines that previously caused me no problems (unless I had a pedal strike).

I have a 29.5" inseam, which is short for my height (5'9").

Overall, I am not at all convinced that this is an improvement, but I want to try further adjusting the saddle position before giving up.

am8117

My question was not about gearing but eg comfort, injuries, etc anything beyond the obviously implied eg need to change saddle height or higher / smaller risk of pedal strikes.

Polaris OBark

I only just saw this link now. One of the interesting things is he suggests not moving the saddle, at least initially. I moved mine up 2mm only (not 7.5m), and haven't changed the forward/aft position (and since it is a Brooks, I don't have a lot of options with for/aft adjustments).

Polaris OBark

Yeah, I thought that characterization as a troll thread is a bit unfair. It's not like you titled it "Crank Lenghts [sic], infertility and prostate cancer".

GhostRider62

I have ridden 170, 172.5, 175, 177.5, 180 and 200 mm cranks. In my opinion, if a rider has tight hip flexors, they will move all over the saddle coming over the top and not have that smooth look on the bike and might be better with shorter rather than longer cranks from a comfort perspective. At the knee, long cranks make the knee angle more acute and this puts more shearing force. In short, a compelling argument could be made for riding shorter cranks for some riders, especially short riders (say under 6 foot)

At 6'4'', I tend to prefer 172.5 and midsole cleat placement, which lowers stress on the knees.

There are several studies out there showing no difference in maximum power over a very large range of crank lengths. One selects the gears that put the appropriate forces at the muscles.

As to OP's question, 2.5 mm change is nothing. I doubt anyone who tests blindly would be able to tell the difference nor would it show up quantitatively (assuming the position on the bike was kept constant)

ofajen

I’m wary of using longer cranks that increase my “step size” and require a larger knee bend at the top of the pedal stroke. I’ve used 170s since the 70s.

I didn’t like the 175mm cranks that came on my RockHopper but they also gave me a wider tread factor, so I don’t know that crank length was what I didn’t like.

Otto

Trakhak

Maybe consider trying a cassette with a larger lowest-gear sprocket to make up for the difference in crank arm leverage. Might have to be quite a bit bigger for the 22% grade, though. Perhaps some posters who (unlike me) are not math illiterates could crunch the numbers.

Polaris OBark

Thanks, but with a 46/30T crankset and 34T or 36T cassette, I am as low as I can go with Ultegra Di2 (officially the rear derailleur is limited to 32T).

My main point is the difference in torque should, at the maximum point where the force is perpendicular to the lever arm, should only be 4.3%. So I think something else must be going on.