This past week someone told me that I should get longer crank-arms. They said that I’m always riding in the higher gears and quickly top out going down hills. Being a mechanical engineer, it made sense to me that a longer crank-arm would produce more torque with the same effort. So is he right?
Like most things in cycling, there are crank-arm length guidelines, but no real consensus. A lot of it is personal preference and feel. However, being an engineer, I need numbers. So I found a formula for crank-arm length. It is your inseam length multiplied by 0.21.
Here are the calculations for my 32.5 inch inseam:
32.5 inches X 25.4 mm/inch = 825.5mm
812.8 X 0.21 = 173.4mm
Therefore, the 172.5mm crank-arms that came with my 56 cm Trek bike are pretty close. You can find the size of your crank-arm stamped on the inside by the pedal. Crank-arms can come in sizes 165mm to 180mm. However bike shops usually only stock 170, 172.5, and 175. I could move up to a 175mm, but there is a physical risk of having them too long.
According to Dr. Andy Pruit of the Boulder Center for Sports Medicine, “With too long cranks, injuries can occur at the top of the pedal stroke when the knee and hip are forced into tight angles. It increases stress on the patella, and the hips may even rock side to side to compensate.”
I injured my patella in a cycling class last year, so I need to be cautious. The real answer is maybe. Ideally I need to get fitted at a bike shop. There are so many factors to consider, like saddle height, which I will cover in a future tip.
Here is a video on what length the pros choose.
Quote of the week: Sprint for flash, climb for cash.