The SH51 SPD single-release cleats are recommended for all mountain bike (MTB) SPD pedals except PD M858: release the cleat by twisting the heel outwards. In layman’s terms, this is the cleat that will fit 99% of SPD-compatible pedals excluding the PDM858. The SH52 SPD single-release cleats are recommended for PDM858 pedal: release the cleat by twisting the heel outwards. In layman’s terms, this is the cleat that will fit all of the same pedals as above (SH51) and ALSO the PDM858. Unless you have the PDM858, Wiggle would recommend opting for the more popular SH51 option. The SH56 SPD is designed to work with all open design pedals: allow your shoe to be disengaged by rolling or twisting the foot in any direction Depending on your requirements, Shimano MTB Cleats should have an option that can provide a solution for you.