Correct crank length for mountain biking?

Hello, Mountain Bike Action:
At 5’7″ with a 29-inch inseam, should I use 170-millimeter length crankarms instead of 175s?

-Ken, with a low center of gravity

A: Most small bikes come equipped with 170-millimeter cranks. You could reap one benefit of the smaller crank arms, and that is better pedaling clearance. However, should you run out and buy a new crankset? It is just not worth it. Mountain bike fitting tends to be more personal preference than hard and fast rules like road bike fitting. That’s because mountain bikers don’t plant themselves in one position and pedal for hours. We are continually sliding up, sliding back, getting out of the saddle and using more than a little body English–and that is all on a simple singletrack. You would discover that shortening your crankarms five millimeters wouldn’t make a huge difference in normal riding situations. We knew of a rider who installed two different size crankarm lengths by mistake and didn’t notice it for weeks.

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