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.
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:
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.
I have a 39" inseam and put homemade 195mm cranks on my mountain bike and 205mm on my street bike. The increase in performance was exhilarating and the improved comfort indescribable. I had been monitoring performance with bike computer odometers on both bikes before making the change and observed about a 15% increase in all areas. Last week the crank on one bike broke and I returned to the old cranks. My computers returned to the old performance levels and I feel I'm riding a child's trike! I've heard these "crank length makes little difference" arguments many times over the years and have always found those making such claims to be people of normal stature who can't truly conceive of the issues confronted by us outside the norm. It's also possible that manufactures wish to maintain a "crank length makes no difference myth" to preserve minimal parts manufacturing and that race committees want to prevent tall people from having an advantage on the race course.
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.
One size does not fit all when it comes to crank lenght, and if you are under 5'9" or are a time trial racer, it is important to go shorter. This can increase muscle activation and firing rate.
Imagine 40% more power pulses and a longer duration power pulse for each power stroke and there are momentum aspects and efficiency aspects that need to be understood and addresses and words like torque and horsepower don't cover totally what's happening in the cycling pedal stroke.
Think of muscles as engines, the more of the muscle you can use, the less you damage damage or drain the others. Shorter cranks allow you to change to a broader power range from 90 to 125 watts instead of say 80 to 90 watts with 175 length cranks.
However, watts generated depend not just on crank length but ridability and position. So end of day only real time ABBA type testing, varied skill sets, different events, and time to adapt can tell much. But you have to know why and what happened. A peer reviewed study that doesn't control for this kind of variability won't give viable real-time results.
I've found clearly that short cranks are faster . The studies, including the one by Jim Martin who I knew, I don't believe are relevant. I don't believe it adequately controlled for all of the variables like the power phase of stroke, how to pedal, and gearing changes? Other factors may be aerodynamics frame geometry, standing and sitting rate, and shifting rate? Every one of these things I studied for 15 years out of interest. I had crank sizes from 150mm to 190mm. I was a pro racer and outputs were high. I performed best with cranks between 150 to 155. I am 5x7".
For maximum efficiency, I suggest you slam your seat back and lower your bars and bicycle if possible. I've tested it on a velodrome and found that your gear ratios need to be .8 inches smaller (one front tooth) smaller for every 1mm shorter crankset to keep the same foot speed.
You need to increase foot speed also, using a significantly small gear allows you switch to a horizontal pedal stroke, never push down. Pushing down cycling is beginner and ridiculous. Pederson really is off on this. You pedal a Nike swoosh down and through horizontally. This is so important for you to grasp, probably the best advice you could ever have, "Nike swoosh" will engage more motors. Its stomping down but right into a clawing back to you with a flat shoe. This makes short cranks come to life, And moving seat back at least 1.5 inches.
Do not lift up the backside with the heel raised. Many newcomers think this faster. and point the heel and toe on the way up. Keep shoe horizontal all the time. Kick up and over and down and through. With shorter cranks, keep a high cadence, put a giant cassette like 13 x 28t on, shift a lot and grab gears and wind it out like a two stroke engine.
The key is far greater gluteal and posterior chain use. Lower position, with your butt back, and shorter cranks are key.