Why are there only 170 and 175mm lengths for the crank arms of MTBs? I would have guessed that touring bikes would benefit from far longer crank arms than those, suitable for mountain riding.
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There is some natural limitation to the crank length defined by your body dimensions, which is the height difference between the lowest and the highest possible pedal position for a given leg length.
The lowest possible position is the one where you just can reach the pedal with your foot at the crank's bottom position (plus some additional height to prevent that you have to stretch your leg to reach the pedal). You can set that height by properly adjusting your saddle.
Now comes the important part: If you turn the crank up now to its highest point, the limitation is your knee angle. If it is lower than a certain angle (don't know what's the exact limit, but 90 deg might be already critical), you will have problems to apply a reasonable force to the cranks AND you will get knee problems sooner or later.
The maximum crank length is now half of the length between top and bottom position where you can reach the pedal on bottom and have a reasonable knee angle on top position. I would guess (without any figures at hand) that for most people 175mm is fairly close to their maximum crank length while 180mm might already be too long for many of them. You surely will be able to get longer crank arms as well but they might be not on the mass market for the aforementioned reason.
MTB crank arms usualy come in 165, 170, 175 mm. Some people (usually PROs) can have access to more fine grained lengths (e.g 167.5) or ever smaller sizes such as 160mm.
The reasons for such "micro-optimization" are:
Today MTBs for trail riding need to have a lot of front and rear suspension and very low bottom brackets. This makes it easier to hit rocks with the pedal and that is not desired.
Also note that ground clearence problems are in addition being solved with very thin pedals (thin pedals also solve other issues).
So, saving 5mm from the cranks and 5mm from the pedals may not seem enough but it can actually reduce the times you hit the ground.
Note that downhill specific bikes will usually have 165mm cranks.
proper fit to rider height
A short rider (with short legs) will pedal more efficiently on a 170mm crank than a 175mm one.
Thus small MTBs usually ship with 170mm and medium ones with 175mm.