I’ve built up a new bike with 170mm cranks and I’ve been getting knee pain.
My old bike had 175mm cranks and knee pain was never an issue.
Reckon it’s down to crank arm length or just coincidence?

Did you adjust the saddle height to compensate?

Could be more to do with saddle position? I jump between bikes with different crank lengths and can’t tell the difference.

If it’s a new bike it’ll take a few goes to get saddle, bar and stem height correct anyway I would of thought.

I did but I don’t think it was the cranks, I think it was failing to raise the saddle to compensate and then going out on a stupidly muddy XC ride wearing knee pads (which have a habit of causing my patella to mistrack).

Yep seat post height the same, the seat post angle is different so I’ve pushed the saddle back as far as it will go to try to get the same foot position as my previous ride.
I think it hurts when pedals in the horizontal position, like my foot needs to be slightly further forward.
Maybe I’ll put my old 175 cranks on for a couple of rides and see if that makes a difference.
Seems illogical that 5mm would make any difference!

When you say “seat post height the same”

Do you mean: same amount of post out of the frame, or same relative to lowest pedal?

Shorter cranks are like smaller steps, they should be easier on your knees…

I find this useful when I’m setting up my bikes:

http://www.bikeradar.com/gear/article/take-care-of-your-knees-part-2-17445/

ahwiles i tried to set the new bike up the same as the old bike so the distance between the pedals and the seatpost is the same as is the reach.
The only difference is the geometry of the seatpost and the ‘5mm’ less extension my knee is doing on the horizontal part of the pedal stroke.

“pushed the saddle back as far as it will go”
This doesn’t sound right. I know people do it, but it seems like the frame is too small/short. The angle your are then placed between your saddle and cranks is a bit slack. Plus ime you tend to break saddle rails when its set all the way back

Have you checked the Plumb line from the knee bump to the right side of your right knee has a vertical line down through the pedal spindle. Measure when pedal is at the o’clock.

I got knee pain going to longer cranks on a roadie but chainring size is bigger as well. Think it was because the longer lever meant you felt like you could run bigger gears and it wasn’t great on the knee.