At the time when other makers were executing the idea of a toe clipless style pedal with a cleat locking into the pedal top, or modifying a ski binding system, Shimano was conceiving their concept of a clipless pedal. The Shimano system uses a small, recessed steel cleat, rather than the plastic protruding ones used by others, so there isn't a tendency for them to abrade in use and have to be replaced so often. Another consideration was to make the cleat small enough so that it could recess into the sole of the cyclist's shoe. Recessed cleats allow the rider to walk on the shoes with cleats installed, yet not be bothered by a large plastic protruding cleat or make it sound as though you had horse hooves. Their final consideration was to vary the style of disengagement, and make the retention system adjustable. Shimano has achieved all of these. The cleat itself is just a little over 1 1/4" square and 1/4" thick weighing just 19 grams. Because it's a micro-cleat, having it recess into shoes was fairly simple, and shoe makers other than Shimano make shoes to use with this recessed micro-cleat system. These are referred to as "SPD compatible shoes". Shimano's retention system is adjustable in the amount of tension required before it will release the shoe from the pedal. To vary the style of disengagement, Shimano has two types of SPD cleats, each with tow style of disengagement. The one that comes with all their pedal sets is the SM-SH50 (MTB) or SM-SH70 (Road).
They are Black in color, made of forged steel, and will permit your foot to release only with lateral motion by twisting your foot away from the pedal horizontally. The other cleat is the SM-SH55 (MTB), or SM-SH71 (Road). It is Silver in color and will permit your foot to release with lateral motion, or sharp vertical motion, or even a twisting diagonal upward movement of the shoe. The SM-SH55 is available only as an aftermarket replacement for the SH-50 pair that come as original equipment with their pedals. Incidentally, the difference between the two types is the depth of the notches that the clamps grip on the upper side of the cleat. The notches are considerably shallower on the Silver SH-55 or SM-SH71 set, making the grip slightly less precise and allowing release at various angles. You can adjust the cleat for your perfect comfort because the Shimano cleats, when mounted, are adjustable front to back, by up to 3/4" (depending on the shoe) and laterally on the shoe bottom by up to 3/8". The cleat rests on the pedal inside a steel "saddle" made with four vertical tabs to assist in positioning it. The two rear tabs are bent outward slightly to permit release and accent the movement of your foot, in a way the retention system interprets as "intent to release". The cleat is held in the retention system by a point at the front that slides under a steel clamp. The rear of the cleat has a blunted tip that drops into a rear spring loaded clamp that has a tension adjusting bolt. The tension bolt is turned with a 3mm allen wrench clockwise to increase the spring tension, and counter clockwise to decrease it. There is a detent at each third of a revolution, so you can count the adjustment with 1/3 turn precision and repeat it on the other pedal so they are equally tensioned. There is also a visual indicator of how tight the spring is set, by the movement of a red dot on a nut, on the adjusting screw. This dot moves as you rotate the adjusting screw, for a rough visual sense of the spring tension. As an aside, the front cleat clamp is also spring loaded on the M737 pedal, though it's tension isn't adjustable it makes entry easier. The bottom of each of the points on the cleat is rounded so that it easily finds it's way into the steel "cradle" that holds it. With downward pressure, the steel spring clmap moves aside permitting the cleat to drop in and be held by the clamps. The clipless mechanism itself, isn't nearly as complicated as you imagine it to be, before really examining it. The variations in the pedals have to do with the reversible nature, or lack of it, retention clamp springs and materials. Starting in 1994 Shimano began to sell their SPD pedal sets directly to retailers. They previously had wholesale organizations stock and sell to retailers, creating price competition that held or drove prices downward. It is widely believed that with the direct sales to retailers, Shimano will eventually institute a form of retail price maintenance. It is believed that Shimano will insist everyone sell the pedals at a specific price or not receive future deliveries (as they have tried to do with their SPD shoes for the past several years, which is why you don't see their shoes sold on a mail order basis). The next perceived step will be the cessation of mail order sales. More than a few people believe that Shimano will eventually be dealer-direct, with an enforced price structure for the sales of all parts. When the designers who specify the parts that go on bikes used SPD pedals previously, they had been able to insist that their wholesale organization be able to sell them after- market leading to widespread availability. With Shimano as the only after- market source, there is less reason for designers to use them as original equipment which is why the Ritchey pedals have become so popular for the 1995 & 1996 model years.