Shimano Cycle Pedal Foot Cleats
SPD SH51 Versus SH56 Differences.

• A choice of Usage, Performance And Safety !
• Take the cleats that come standard with the shoes ?
• Or take the other cleats which my shop was willing to swap to at no charge?.
• With decades of toe clip reflex, in an emergency, I fancied minimising the chance of falling off , feet still locked in with a different mechanism.
• One also fancies higher performance cleats - but at what if any extra (or possibly lesser!) risk ?
• I couldn't find detailed analysis, not on wikipedia, not on Shimano web,
  one other web referenced below knew what the usage difference was, but not Why
  another web referenced below only spotted the less important of 2 differences.
• So I bought both, measured differences, analysed, with both, am free to later choose to install alternate cleats in shoes on different days, for alternate types of forest dirt track & main road tours etc.
• The 2 (main) Shimano SPD cleat types
  • Require a choice between maximum power Versus easier un-clipping.
  • The minute differences in metal cleats, create differences in release mechanism between rotate only, & {rotate &/or up} to release.
  • SH56 (Silver) could be better on tour days inc. dirt paths with groups of other bikers with fatter tyres, not used to clips, who are careless of tree roots & ruts etc, & for newcomers to SPD cleats;
  • SH51 (Black) better for road racing days & people more experienced with SPD cleats, & people wanting more power by pulling up on pedals as well as pushing down, & people on mountain bikes up mountains who dont want their bike falling away from under their feet when they jump over rocks etc.
  • There'll be less horizontal flex with SH51 (Black).
  • Understanding the differences & commonalities could also help on where to watch for wear etc

• SM-SH56
• As sold with my shoes @ www.sportscheck.com 2012_07_02
• Allows horizontal, vertical & diagonal escapement. & slack/ wobble to allow for knees moving etc.

• MTB cleat is MUCH smaller than the road cycling cleat
• SPD is 2 hole
• SPD-SL is 3 hole
• SH51 (Black) - single-release
• SH56 (Silver) - multi-release

There's a 2nd more important difference, only easily seen by unscrewing cleats from shoes:

The SH56 (Silver) at the back, at the top (when cleat is viewed same way up as it would be cycling), has about (comparing with SH51 (Black), 1.5 mm (measuring horizontally) of metal shaved off at an angle of 45%, each side of the centre. On my new SH51 (Black), moving the spring loaded lever of new pedal with pliers, one can see the lever pulls Very tightly over the top of SH51 (Black), on metal that would be absent on the SH56 (Silver). This locks the cleat tightly on SH51 (Black), no lateral horizontal play, (as felt by hand, maybe some with foot in shoe though) but I imagine over time some wear & slack will return, as at the front the cleat mounts loosely, & does Not have a centre position. (Some cyclists like some slack for knee movement, for racers with another manufacturer Look, there's 3 settings of slack, 0 upward).

It would seem, if ever the cleats screws to shoe became loose, & pedal release lever became distorted & rode up over rear of cleat, you'd be trapped, potentially fatal. The man in the shop said keep the the screws tight. I'd also think: monitor for abrasion on vertical at rear, at top edge, & if you see any, fit new cleats. That goes for both SH51 (Black) & SH56 (Silver)

SH56 (Silver)
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SH51 (Black)
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  SHOES SH-M087G