SPDs have small metal cleats that can be recessed into the tread pattern of a shoe and were designed for mountain biking.
SPD-R was Shimano's first attempt at making a road cycling-specific pedal system, since superceded by (the much better) SPD-SL.
The three systems (SPD, SPD-R and SPD-SL) use different cleats and are not interchangable. All three cleat types use different shoe bolt mount-patterns too
Originally posted by rek SPDs have small metal cleats that can be recessed into the tread pattern of a shoe and were designed for mountain biking.
SPD-R was Shimano's first attempt at making a road cycling-specific pedal system, since superceded by (the much better) SPD-SL.
The three systems (SPD, SPD-R and SPD-SL) use different cleats and are not interchangable. All three cleat types use different shoe bolt mount-patterns too