Which length crank do I need?

Hi Lee,
I need a new crankset on my Iron Horse Mark III team 5″ trail bike. I’m thinking I would like to get a 180mm crankset instead of the normal 175mm that I have. I ride a lot of steep trails here in Idaho and I think I will get more power. I am a big person too at 6’3′ and 194 lbs. Years ago, I replaced the crankset on my hardtail that was a 170mm to a 175mm and I could not believe what a difference 5mm made. So, why wouldn’t another 5mm be even better? Why are these 180mm cranks so rare? What are the disadvantages? I’m looking and the race face crank. Please explain this crank length stuff. Roadies like their cranks short and some bmx racers long. But, that’s all I know. Thanks
Walt

Hey Walt,

Oh man, that was a can of worms.

There is a ton of debate on this issue, even on the road. When you add gate starts, ground clearance and the other fun variables, it gets even more interesting. Let’s start simply.

Basically: Long cranks yield more torque. Short cranks are easier to spin. The perfect crank length is as long as you can spin smoothly.

I’ve read many times that small people are on too-long cranks and tall people are on too-short cranks. Component and bike makers spec certain length cranks not because they are ideal for your pedal stroke, but because they are easy and cost effective. Why make 180mm cranks when everyone accepts 175s?

The ideal crank length is, of course, proportionate to your leg length. There’s a lot of voodoo on this topic, but bike-fit expert Lennard Zinn uses the formula [inseam in mm x 0.216].

For me: 812mm (32″) X 0.216 = 175mm

Ha! 175mm happens to be the standard length for a medium mountain bike. But I’m average sized.

For you: 889mm (35″) X 0.216 = 192mm

Good luck finding that crank! And how would that monster fit on your bike? Foot hitting the front wheel, pedals hitting the ground, who knows? Want to find out? Lennard makes custom cranks up to 220mm.

Most of the smaller women I coach have a hard time pedaling smoothly with stock cranks. Riders down around five feet tall need 165mm or even shorter cranks. That lets them turn smooth circles without rocking their hips and reaching with their toes.

That’s the theoretical ideal based on leg length. What about the other variables?

Pedaling style

If you mash a big gear, you’ll like longer cranks.

If you spin a smaller gear, you’ll like shorter cranks.

But: From a leverage standpoint, a 180mm crank vs a 175mm crank has an advantage of only 3 percent. If you’re not used to spinning the bigger circle, I bet you’ll lose more than 3 percent of your power.

The Specialized Tricross I’m riding has 172.5mm cranks, and I feel the difference (not in a good way) from my normal 175s. However, after a few more rides it’ll feel normal.

Type of riding

In BMX you accelerate against high resistance then maintain a spin for a very short time. Long cranks make sense here. 180s are standard.

On the road you maintain relatively low power and high RPM for long periods. Shorter cranks are standard.

XC is along the continuum somewhere near road.

4X and DS are very close to BMX.

DH is somewhere very close to the middle.

And let’s not forget ground clearance. I run 170mm cranks on my DH bike — that 5mm can be the difference between clipping a rock or not.

Sheesh. Never simple.

Just pick one

Call me silly, but I think we should learn to spin a certain size circle, then rock that on all of our bikes.

If you’re average height, ride the stock cranks.

If you’re much taller or shorter, do the math and try a crank that fits your legs.

Walt: Definitely try some 180s.

Yuck … worms all over the place!