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.
You guys seem close to getting it. One size does not fit all and if you are under 5foot x9" or are a time trial racer, it is important to go shorter. But so far you don't really seem to have proper guidance on how. Its about muscle activation and firing rate yes.
Imagine 40% more power pulses and a longer duration power pulse for each power stroke. Re,ember Work is what we are talking about.. There are momentum aspects and efficiency aspects that need to be understood and addressed. Took me a while actually because cycling is caught up in this current trend of fancy bicycles and the look, rather than hard smart work. So words like Torque and Horsepower don't cover totally whats happening in the cycling pedal stroke.
Think of muscles as engines, the more you can use, the less you damage the others, or drain. These short cranks allow you to change to a broader power range from 90 to 125 instead of say 80 to 90 with 175s.. Watts tell some but nothing about 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. This Exact problem is why this whole Peer reviewed study and blind leading blind "science" rarely gives viable real time results.
I found clearly short cranks are faster, and the studies, including Jim Martin who i knew, EDS, were not relevant, and are not. No study can say shit about so many variables..what height, what about the watts and the acceleration Work aspect, Dynamic Power? How about the power phase of stroke, what about how to pedal, what about gearing changes? What about aerodynamic possibilities? Can the bicycle be 2 inches lower to the ground? What about the standing and sitting rate, 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. Yet I performed best in 150 to 155. I am 5x7". I could lower handlebars 3 inches and found you only move the seat back. Don't raise it, don't think its Logical, because logic is far more concepts than you and i Were aware of.
Studies and cycling are almost all worthless because the person giving them have no clue of optimization and variables. Also old training and circles changes because of an learned movement. I can't put it all down, but slam your seat back (not scientific but slam it anyway), lower your bars and bicycle if possible. I tested it on a velodrome, all sorts, I was very interested for over 20 years and decided to 15 years back. Gear has to be .8 inches smaller (one front tooth) smaller for every 1mm shorter crankset to keep the same foot speed.
Also shortening cranks isn't the solution to your knee problem. You're doing something wrong, Q-factor, no tights in cold, no warm-up, damaged badly from early years, pushing down only on your stroke (if you bounce at rpm then there is the solid evidence, aka technique. You're looking for a patch. Shoe and cleat position cause issues. Also being to far forward and curved sole shoes with toe pointing cause the problem most often.
You need to increase foot speed also, using a significantly small hesr.masnyou move back 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. I guarantee 155 is magic if you keep cadence in the 108-125 range and climb above 95 if possible. You didn't need a 12 or 11 tooth cog or a 53 tooth ring. You can go as fast as you want by spinning right past them. 40 percent more power per minute basically, maybe not quite as much, but you get two hits for his 1 basically. Or 3 for his 2.. Put a giant cassette like 13 x 28t on. Shift a lot and grab gears and wind it out like a two stroke. You lug and you're lost.
The key is far greater gluteal and posterior chain use and they often are not used by triathletes especially. Bicycles can be lower and position on top of that. I call it my masters specific position. Short cranks rule, far more stable, a joy to stand and I will never ride 170s again. Feels like a truck after driving a Ferrari f1 at 8000 RPMs and climbing!
There is a lot there if you wade through it, I happen to be a fanatic about that topic, few even considered it until the cocky triathletes got into it. First aerobars and then funny shifting, now going to short cranks, a smart move whose time has not come...why you ask, a long story but I'll tell you..later. For now, its because you have to know how to set them up and how to maximize, most simply are not aware of how to pedal with them. It will open up a can of whoop ass if you do it right... it just happened to interest me. Results later.