When shopping around for a new compact crankset, you’ll notice that crank arms come in various sizes. What size crank arm do I need is a common question. Most manufacturers make crank arms that vary in size from 165mm to 180mm.
In general, a rider with longer legs (inseam) will want a longer crank arm and a rider with shorter legs will want a shorter one. 170mm seems to be a very common size for riders with an average inseam. There are several bike fitting experts that claim to have formulas for figuring out the proper crank arm length. Peter white claims that crank arm length should be, “18.5% of the distance from the top of the femur to the floor in bare feet.” This is probably a good starting point, but doesn’t take into consideration the enormous variation in body types out there. Your best bet is to head into your local bike shop and request the opinion of a bike fitting expert. That way, if they set you up with a compact crankset that just isn’t working, you can complain and they’ll probably swap it out for something that’s a better fit.
The other consideration is riding style. A shorter crank arm is going to allow a faster cadence and a longer crank arm is going to allow for more leverage and thus more power. Consider what kind of rider you are and what kind of rider you want to be before making the decision.
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