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There are some length options for the crank arms - like 170mm, 175mm.
What is the difference for the rider? In which way does it effect the commute?

Tried looking at Selecting the right crank length, didn't get a comprehensive answer.

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Obviously, it affects the length of the leg "stroke". Generally, taller people with longer legs would benefit from longer crank arms, though in practice this bit of "tuning" is never done outside of the pro arena. Note that, on a given bike, it also affects ground clearance and cornering ability, and clearance between toe and front wheel while turning. –  Daniel R Hicks Jul 1 at 23:10
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Thanks @Moz. Hopefully, some day my great English will be good enough :) –  Alexander Jul 1 at 23:27
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I was getting a niggling pain in my left knee, after I'd been cycling for a few hours. I went to the physio and we discovered that one of my legs was an inch shorter than the other. I do some exercises now to keep it loose, but I couldn't help wondering whether having different crank lengths would have made a difference. Trouble is, that's an expensive experiment. –  PeteH Jul 2 at 6:32
    
@PeteH, you might be better off with a thick inner sole or some kind of packing at the cleat. A shorter crank might help when the short leg is at the bottom of the stroke, but be worse at the top of the stroke. A professional bike fitter should be able to help. –  James Bradbury Jul 2 at 10:13
    
@JamesBradbury thanks, to all intents the physio was a professional bike fitter. Very worthwhile, I'd recommend it to anyone who gets niggles while riding. –  PeteH Jul 2 at 10:35

2 Answers 2

If you keep the rest of the bike the same a shorter crank gives more ground clearance at the bottom of the stroke, and that's going to be the main thing most people notice. You're unlikely to notice a change in power output unless you're shorter than, say, 1.7m, which would put you firmly on the down-slope of the power curve (ie, the part where longer = less power). You may notice a change in comfort or pain levels due to the increased joint articulation from the longer cranks. IMO most people would be better off switching from 170mm cranks to 160mm or 165mm cranks.

At the high performance end the power output varies in interesting ways. Since power = torque x speed, and torque = force x radius, a longer crank means more torque but also more leg movement to produce it. And vice versa for shorter cranks. In practice most athletes have a fairly flat relationship between average power and crank length in the range 150-170mm, but it's also something that is little studied.

A study by Martin (2001) suggests that it doesn't really matter:

Power produced with the 145- and 170-mm cranks was significantly (P < 0.05) greater than that produced with the 120- and 220-mm cranks. The optimal pedaling rate decreased significantly with increasing crank length, from 136 rpm for the 120-mm cranks to 110 rpm for the 220-mm cranks. ... The optimal crank length was 20% of leg length or 41% of tibia length. ... Even though maximum cycling power was significantly affected by crank length, use of the standard 170-mm length cranks should not substantially compromise maximum power in most adults.

I found some thought experiments and this interesting summary/bibliography. (edit to add) Ian Sims at GreenSpeed has long had an interest but I can't access their site right now, so here's an A2B magazine post. It's worth noting that Ian has changed his position completely, based on evidence that he mentions in that article. Previously he was a fan of long cranks, now he's suggesting that short ones work better.

Edit by mattnz replacing Mσᶎ's link: PowerCranks have an interesting sales push with useful links and discussion. They also provide test results showing advantages of short cranks. Apart from being benificial to people with poor flexibilty and knee problems, short cranks provide major racing benefit from improved aerodynamic position. They also discuss why its a myth you need long cranks for climbing.

(moz again) My experience is that shorter than conventional cranks work better than I expected and reduced my knee pain somewhat when riding hard (long tours or fast commuting for 40+ km/day). I'm 1.8m tall and ride 155mm cranks because those are the shortest I could get without paying a price premium. My partner is 1.5m tall and prefers 145mm cranks. Going up to the 165mm cranks on my load bike feels like a significant jump, but I haven't tested them because I don't have access to a power meter.

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I found on 145's I could maintain higher cadence and speed (therefore poweroutput) for longer and have since gone down to 165 on my bike (MTB), I am 1.75cm / 65kg. My personal feeling is crank length is understudied and '1 size fits all' suits the manufacturers ad shops best, so "the rule about crank lengths is no one talks about crank lengths". –  mattnz Jul 2 at 0:37
    
@mattnz I suspect I am in the same position, but power meters are still too expensive and I don't have a consistent enough regular ride to even estimate. I really should have a go at this next time I've got access. I wonder if I can make TriSled sponsor some research. –  Mσᶎ Jul 2 at 1:27
    
@mattnz the powercranks stuff is interesting and I think their page is a useful summary, but I linked rather than quoting because they are very much pushing the BUY OUR PRODUCT. I tweaked your wording and fixed spelling, hope that's ok. –  Mσᶎ Jul 2 at 2:32
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The Martin et al. study looked only at max power but the results have been the same for submaximal power: over a wide range of crank lengths, submaximal power does not appear to be affected. That means you can choose crank length on other criteria, including personal preference, clearance or cornering issues, fitting or range of motion issues, or even because a crank is on sale, without worrying that it will decrease sustainable power. It also means that changing crank length won't increase sustainable power. Which is to say, it's been studied a lot. –  R. Chung Jul 2 at 5:10
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It's been studied quite a bit but usually from the perspective of muscle shortening speed so searching for "crank length" won't always find the right studies. But perhaps this little empirical experiment is closer to what you had in mind: bikeblather.blogspot.fr/2014/07/… –  R. Chung Jul 7 at 6:53

It affects several factors:

The range of motion during the pedal-stroke

Time per revolution (as a result of the increased radius)

and the maximum torque a rider is able to put on the crank

The majority of riders only pay attention to range of motion. For taller people and those with longer legs, longer crank arms are necessary to generate a larger range of motion in the thigh/hip. This uses more muscles and feels less cramped. However, shorter riders might benefit (if they have the power) from longer crank arms for the sake of increased torque. However, torque is generally ignored since the increase is quite minimal (only heard of a few pros doing this). Most people will be fine in th 172.5mm range. If you are tall then I would say it is an absolute necessity to get 175mm crank arms (I myself am tall and feel cramped when riding anything shorter). Hope that helps.

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Can you please add some calculations on the time per revolution? Although it's bigger distance, yet is more easy to move, as the moment is bigger. –  Alexander Jul 1 at 23:34
    
What means "maximum torque" / "increased torque"? –  Alexander Jul 1 at 23:34
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@Alexander - "Torque" is the measure of angular force. The same pressure applied at the end of a longer lever arm results in more torque, and this translates into more force provided to the wheels. –  Daniel R Hicks Jul 1 at 23:50
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Altough this is the common argument and theres formulas for leg length vs crank length, there is evidence that such formulas really are unreliable. –  mattnz Jul 2 at 1:50

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