For a start, SPDs probably weren't designed with that much upward force in mind - you'll be putting far more force on them than you ever would in just pedaling.
I agree with silasmellor about the twisting.
BTW, managed to get a bargain set of brand new SPD pedals, complete with cleats of the bay the other week for less than the price of a new set of cleats. Worth looking out for.
For a start, SPDs probably weren't designed with that much upward force in mind - you'll be putting far more force on them than you ever would in just pedaling.
I agree with silasmellor about the twisting.
BTW, managed to get a bargain set of brand new SPD pedals, complete with cleats of the bay the other week for less than the price of a new set of cleats. Worth looking out for.