I have ridden ISIS cranks on my 36er as long as 175mm and as short as 125mm, and on my 29er as long as 170 and as short as 125.
The longer cranks worked awesome for climbing and descending, the increased torque for slow/power moves on tech trails was incredible, but they don’t ride as smoothly as a shorter crank. One day I switched cranks up and down a bunch of times on my 36er, going from 165 to 170 to 175, then back down to 165. That day I ended up “holding” at 170’s.
I rode this past winter with 150’s on my 36er, it made sense for the conditions as it was too wet to get enough traction to prevent “overtorquing” the wheel which leads to spin out. Once the season warmed and the trails started drying out, I’m back to 165’s and loving the climbing power.
With a good head of steam, I can spin up shorter steep climbs without resorting to a longer crank, but if I need to slog up a steep hill, esp on technical off road trails, a longer crank keeps me riding where shorter cranks would make me walk. I also find that a longer crank gives me more control on technical downhills, though at the price of inefficient spin at high speeds.
I have swapped crank lengths back and forth for a few years, my current pattern is short cranks for flatter and wetter trails, long cranks for dry trails and steep climbing. Maybe if I lived out West where the trails are a more even grade, then I’d consider staying with 150’s on all my munis, but for low speed tech moves and abrupt climbs, a long crank is useful. I don’t tend to seek out flat trails, so spinning for miles on easy to intermediate trails is not something I do, but if I did, then I might run a shorter crank or swapping pedal positions more often.
We have these “crank length” discussions every year, so clearly there are folks on both sides of the fence. From an physics standpoint, a longer lever decreases the amount of effort needed to move the wheel, so clearly a longer crank increases your pedaling power, but for some folks this increased power is gained only at the loss of pedaling efficiency and perhaps a loss of balance (?). Keeping in mind that the standard bicycle crank is 175mm long, we are not using “long cranks” per se, but then a unicycle is not a bicycle.
If you have only ridden shorter cranks and you wonder whether a longer crank will improve your climbing, well then, just get some longer cranks and give it a try. For $5-10 you can get a set of inexpensive cast Al QuAx 170’s from UDC.
But be prepared to lower your seat, to be a bit more “dynamic” in your body and how you move on the uni. Also, give it some time, ride them for a few weeks so you can adjust to the feel and re-establish your balance. There is no magic to lengthening or shortening cranks, it’s a “balance” to find the right length for your needs.
I no longer think that wheel size means as much when choosing cranks, it’s more a question of what you want to do with that wheel. If you ride your 29" or 36" for muni, then why wouldn’t you run a long crank just like your small wheel counterparts?
There are no rules for what works best, it’s all trial and error, riding experience plays a part, but in the end it is personal preference. For instance, I have been told that 29ers are not appropriate for “real muni” and yet I ride a 29er for real muni a few times each week. When I first tried riding muni on a 29" I was a dismal failure, yet a few years later the 29" is my small wheel and I ride tech muni on a 36er!
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