Quote Originally Posted by Wasp View Post
How do I measure the crankarms?
If you want to know what length they are, just look at the backside - almost all cranks have the length stamped/cast into the back side (aka inner side) somewhere. It'll be a number like 165, 170, 175, etc. This is the distance in mm center to center between the crank spindle and the pedal spindle.

However, I suspect you're asking how to measure for your correct size. Start by adjusting the saddle height to get good leg extension at the bottom of the pedal stroke. Your knee should remain slightly bent and you shouldn't need to "ankle" (tilt your foot) to reach the pedal. Now position the crank to check the top of the pedal stroke. Check the angle made by your knee. If this angle is too small, you can't make power at the start of the pedal stroke. This means the cranks are too long. If the angle is too large, you're giving up the potential leverage of a longer crank, and you're not using your muscles to their fullest. If you search the web, you'll find some recommended knee angles (for the top of the stroke). However, there's a significant personal preference issue here. You might simply prefer using a particular range of knee motion. And this might even change as your fitness level changes.