Attachment
The pedal spindle is threaded to match a threaded hole at the outboard end of the cranks. Multi-piece cranks have a 9⁄16-inch (14.2875 mm) hole with 20 TPI (a diameter/pitch combination fairly unique to this application). One-piece cranks use a 1⁄2-inch (12.7 mm) by 20 TPI hole. French pedal spindles use M14 x 1.25 (14 mm/0.55 in metric diameter with 1.25 mm or 0.049 in pitch) threads, and thread loosely into a 9/16 pedal hole. The threading size is often stamped into the crank, near the pedal hole. The right-side (usually the drive-side) pedal spindle is right-hand threaded, and the left-side (usually the non-drive-side) pedal spindle is left-hand (reverse) threaded to help prevent it from becoming loose by an effect called precession. Although the left pedal turns clockwise on its bearing relative to the crank (and so would seem to tighten a right-hand thread), the force from the rider's foot presses the spindle against the crank thread at a point which rolls around clockwise with respect to the crank, thus slowly pulling the outside of the pedal spindle anticlockwise (counterclockwise) because of friction and thus would loosen a right-hand thread. For a short time in the early 1980s, Shimano made pedals and matching cranks that had a 1-inch (25.4 mm) by 24 TPI interface. This was to allow a larger single bearing, as these pedals were designed to work with just one bearing on the crank side rather than the conventional design of one smaller bearing on each side.
Read more about this topic: Bicycle Pedal
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