Tuesday, January 10, 2012

Crank arm length

My new Sram Quarq (linky) power meter and crankset has arrived! I'll review the power meter itself once I get some time on it, but I wanted to first visit crank arm length, as it came up with my order.

I have been running 175 mm crank arms since my first mountain bike in 1994. When I switched to road again in 2006, my then new road bike came with 175 arms so with the 2 custom builds I've had since then I used 175 arms again. There was no science behind it, it was simply what I was used to.

I've since learned the mountain bike came with 175 cranks because that's the industry standard and the road bike had them also because of standards. The road bike was a 57 cm frame and that seams to be right at the cross over from 172.5 to 175.

This has all come up because my power meter arrived by accident with 172.5 mm arms. It then came to a surprise to many that I was using 175 cranks. I'm 5' 9" with proportional legs and my current bike measures as a 56 cm frame. Industry standard would indicate 172.5 cranks. So I started thinking maybe I've been using the wrong length this whole time. I began a day long quest into the subject. I consulted my coach, my bike shop and dozens of online posts and articles and found out some interesting facts, wives tales and opinion. I ended up going with the 172.5 mm arms and here's why.

  • Several studies sited no change in power output among crank arm lengths from 150 to 180 mm.

  • I'm focusing on crits this year and it will be nice to have some extra cornering clearance.

  • When crit racing there is a lot of leg speed changes and gear changes. Shorter arms should make those cadence changes quicker and "winding" up for the sprint should also be faster.

  • If, as some studies showed, crank arm length should be a function of femur length and frame size, 172.5 fits in line with that thinking.

  • The minor loss of torque can be made up for in economy.

  • And finally, by most personal accounts I read no change could be felt with the 2.5 mm difference.

According to my power test, I was spinning the 175 cranks without a problem. I thought that was my answer, but the more I delved into it, since I now have the 172.5's in hand, I may be splitting hairs.

We'll see how this goes. I'll get some miles in and let you know what I discover. See you out on the road!

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