jroyero

I just started out cycling so I'm getting experience. But i wanted to know if there is a benefit from having a crank-set with a longer "arm" lets say the biggest one being 175 right? I think there is being the formula of Archimedes of the level. The longer the level the easier to move objects. I'm 5'8" with an inseam of 29 1/2", what should length of crank arm should i get? Or, How can i figure out what should be a correct size for me?

rhenning

I wouldn't for your height go over 170 which is kind of the norm. The only advantage to longer is on a mountain or touring where you get more leverage/torque pedaling at slow speeds.. Roger

HillRider

Actually, the longest (although custom made) cranks are about 220 mm but 180 mm is the longest regular production length. 175 mm is very common on mountain bikes while the old road standard was 170 mm.

High line road cranks (Dura Ace and similar) are available in 2.5 mm increments from 165 to 180 mm while mid-line cranks (like Ultegra and 105) are available in 165, 170, 172.5 and 175 mm lengths and lower line cranks usually just in 170 and 175.

That said, there is no good formula to determine the optimum for every rider. Longer cranks do give more leverage but tend to inhibit cadence so "gear mashers" tend to like long cranks while "spinners" prefer shorter ones. Some riders are sensitive to minor changes in crank length while other (me for example) are pretty indifferent.

I ride 170 mm cranks on most of my bikes and I'm 5'9" with a pants inseam of 30" and a "cycling inseam" of 32.6". However, I have one bike with 175 mm cranks and really don't notice much difference.

The subject of crank length vs rider height/leg length has been studied fairly often and the results seem to indicate a very poor correlation. Lennard Zinn, a custom bike maker and maker of the very long custom cranks I mentioned above, did a rather extensive study trying to develop a formula relating optimum crank length to rider dimensions and came to the conclusion there wasn't one.

So, try different lengths to see what you like but don't expect any great gains in efficiency or speed by varying the crank length.

Metzinger

The Dutch have a saying about this:
"If any parts of your crankarms are below the surface of the water, you should head for higher ground."

dabac

There is a difference in having longer cranks. Whether it's a benefit, a drawback or a non-issue is open to debate and personal preferences.

But the longer the lever, the longer the end of the lever has to travel. For cycling this translates into a longer stroke of the leg, and some knees object rather strongly to being put to work at "extreme" angles. My legs are definitely happier with shorter cranks.

A net search along the lines of "bicycle fit" or "crank arm calculation" or something should yield something useful.

HillRider

The only problem with these calculations is that they are based on the author's prejudice or on some contrived mathamatical formula or "rule of thumb" (or "rule of leg" in this case?) and never seem to stand up to empirical testing, as per Lennard Zinn's results.

Read them, see what they say, then try different crank lengths yourself and see what feels the best.

As I mentioned above, I'm not real sensitive to crank arm length in the 170 -175 mm range but prefer 170 mm since that's what I'm used to and ride 95% of the time. Mostly it's a matter of habit and familiarity. YMMY