For most of the summer I have been running 170mm long cranks in an effort to figure out if there was anything to this whole "Crested Butte Philosophy" regarding the use of shorter cranks on 29"ers. (If you want to know more, see this post from earlier in the summer.)
Well, I have spent a ton of time on the single speed and have a few good rides on the geared rig. The two applications do have different results which I found interesting.
The single speed is where i think the major difference is to be found from 170mm to anything longer. I used to exclusively run 177.5mm or longer on my single speeds and going to the 170mm felt really different at first. As I rode it more (and exclusively as my main SS rig this summer) I found the initial strangeness to have disappeared. However; I also have noted an increase in my ability to scale steeps that I normally would have crapped out on. The 170mm crank seems to get around to it's power stroke a little sooner for me, therefore I lose less momentum in my pedaling "dead spot" and I can keep those big wheels rolling easier. Of course, spinning like a whirling dervish is achieved easier as well, so running the super low gear I have, (34 X 22) is easier on me when I'm on the flats. I actually commuted on this gear a few times. Talk about spin fests! This was only possible because the smaller circle allowed a higher cadence without bumping up and down off the saddle, which a 180mm crank set would have had me doing for sure.
In conclusion, I'm really sold on the 170mm for single speeding and I'll try it on another rig soon. The 170mm on the geared rig was a bit different though.
<=== ....and on a single speed.
The geared set up was a far more transparent situation. I never did notice anything different here, but almost all my geared experiences are on 175mm crank sets, so perhaps this is why. The 170mm cranks on the 29"er worked just fine and were so easy to just hop on and ride without anything noticeably different that I forgot they were 170mm cranks for several weeks!
It may have proven more useful to have noted what gearing I was using in different situations as compared to a longer crank set on the same terrain. That is a possibility as I have 175mm and 180mm geared set ups here that I can cross compare. I think through the fall I might try to get that set up once or twice to see where there might be a difference. Certainly there is no discernible difference in terms of feel for myself. The length was indistinguishable from a 175mm crank set in my mind.
So, the results so far are inconclusive on the geared side, but on the single speed side, I do think there is an advantage for me. Not only can I power through hills easier, but I have better pedal clearance on trail obstacles too. That's a bonus! I'll have to set up a test on the geared side that scrutinizes the gear choices a bit more closely to see if the length is affecting my gearing choices, and also if certain gearing choices feel different between crank sets.