I'm 5'6" with a 29.5" inseam (measured by the book method) and was riding a 170mm crank on my primary bike without much problem. Picked up a vintage MTB with 175mm crank - any ride over 10 mies left me sore in the hips and knees. Meanwhile, I upgraded the primary bike to 165mm crank and found riding to be much easier and more efficient. While my cadence was higher, it was more natural and With the helps of the LBS, the vintage MTB was converted to a 165mm crank and all was well. While 170mm works, 165mm is more comfortable and natural for me...