Crank lengths - Road/Tri vs MTB

Scott Alexander

Dear EN Haus,

Just picked up a new mountain bike, a Specialized Crave Comp 29er (L/19" frame).

After I got it home, I noticed it had 175mm crank arms, vs my road/tri bike (quarq) that has 172.5mm crank arms.

FYI, I'm 6'1" with a 32" inseam, if it makes a difference.

I realize that 2.5mm (doubled to 5mm) isn't THAT significant of a length, but will there be any real significant difference, except a higher seat position (ex, distance from crank center) to accommodate the longer crank arms?

I'm thinking it's not worth trying to push the LBS to get me a 172.5mm crank to match my existing quarq crank length.

Sincerely,

MTB Novice

John Withrow

Don't have the real answer for you but at least a comment. I ride 170mm cranks on my road/TT bike. My Fat Bike came with 175’s (I have no idea what size is on my mountain bike since I never even looked at them). Upon investigation, I learned that the crank that came on my fat bike (SRAM XX1) only comes in a 170 or a 175 with all of the bigger frames having the 175. To be honest, I notice it a bit when I'm riding on the roads, but not at all on the trails. I came from the world of Triathlon where every single thing on my P5 was dialed in to the exact millimeter. When you're sitting in the exact same position aiming for the right balance between aero and power, all of this stuff matters. When riding a mountain bike, it's totally different... I'm not even sure if my seat height is right within a few centimeters. The biggest 2 reasons I went with a shorter crank on my TT bike were 1) to get lower in the front without having my knees hit my chest and 2) to have a higher cadence at the same effective foot speed. Neither of those 2 things apply on a mountain bike.

With all of that said, I wish I had the 170’s on my Fat Bike, but I'm doing the IM on it and will often be leaning over onto the front handle bar. In the woods, my pedal hit a rock or the ground on a few occasions (which probably still would have happened with the shorter crank). I think the one advantage of the longer crank is a bit more leverage at super slow cadence, like hopping a big rock or log while going 3mph uphill. You will be doing this a lot more than crouching over your handle bars to try to break 30mph...

If they'll swap it with no hassle, then I se no harm in it. But if they won't, prolly not worth much of a fight. But at least Google the type of crank that comes on your bike first to see if it's even made in a 172.5 so you don't look like an idiot if you walk in there and demand one.

Al Truscott

FWIW, I have 165 on TT bike, 170 on road, 175 on MTB, and just yesterday ordered new road bike with 167.5. I don't notice any issues going from one to another. I think the value of shorter length is (a) to allow faster spinning and (b) lower profile ... so knees aren't hitting elbows in aero position. Neither of which is an issue with 29er, right. I'd stick with the 175s for my MTB, for the small strength advantage at slower rotations?