Question: I have 170-mm crankarms on my road bike and 172.5 mm on my mountain bike. Is it correct to have longer MTB cranks than road cranks? I ride both bikes regularly and notice that my legs are often sore when on the road bike. Could it be from the crank length difference? — Laura M.
Coach Fred Matheny Replies: Mountain bikes traditionally have longer crankarms because extra length supposedly increases leverage for slow-cadence climbing.
Longer cranks have at least one disadvantage, however. They can cause knee problems because they make your knee bend more at the top of the pedal stroke and extend more at the bottom.
If you ride your bikesan equal amount but feel soreness only on the road bike, check your total position to make sure something isn’t out of whack. Get assistance from a local cycling coach or bike shop. For do-it-yourselfers, detailed fit and position instructions are in a couple of RBR eBooks, Andy Pruitt’s Medical Guide for Cyclists, Bike Fit, and Bike Fit 101: Your Toolset for a Great Bike Fit.
Because your mountain bike cranks are 2.5 mm longer, start by lowering its saddle 2.5 mm. This will result in the same knee extension on both bikes at the bottom of the pedal circle. This assumes that you currently have the top of both saddles the same distance from the crank axle.