There are a number of pedal systems on the market. This is a quick summary of the features that each system provides along with a subjective assessment of each. This is an incomplete list that ignores the less common systems. Additionally, we do not have experience with the most popular road pedal system as created by Look so it is not included here.
We have chosen to not discuss a couple subjects in our assesment of each pedal system. Stiffness was omitted because we cannot decouple pedal flex from the flex taking place in the rest of the power chain. Even if we could detect this flex we would still usually find more interesting aspects to discuss. The size of the pedal platform was also not discussed. We've found that a stiff shoe can and will distribute the pressure evenly to the rider's foot thereby taking pedal platform out of the equation.
Technical: SPD is a Shimano design and is probably the most common pedal in use today and has probably been in use the longest. SPD is targetted towards off road riding and uses a two bolt cleat that is designed for recessed mounting. Most pedals are two sided and offer a stack height of about 17mm. Single sided pedals are less common and can have a lower stack height. SPD offers only minimal float. The design relies on the sole of the shoe to rest on the pedal to provide lateral stability. If the sole of the shoe becomes worn or the rider applies lots of pedal pressure the SPD system can sometimes allow some lateral rocking during the pedal stroke. The cleats seem to last longer most.
Subjective: We've used SPD on and off for many years and we really want to love the SPD pedal system. There are a large variety of pedals. The smallish cleats are probably the most walkable. They are reliable. They shed mud pretty well. Slim even used them for one season of Cyclocross. Despite all these factors they are not our favorite. We dislike the near lack of float and the lateral instability that comes from big efforts or worn soles. We think they are fine for short rides and still use them for commuting but have moved to other systems for more serious cycling.
Technical: The Frog system uses the same 2 bolt mounting pattern as SPD and offers the same options for mounting on walkable shoes. The pedals are double sided and targetted at the off road rider. The stack height is comparable to the SPD system at about 17mm. The Frog pedals offer twenty degrees of free float without spring resistance. This pedal design offers no resistence to the rotation of the cleat when releasing. Like other Speedplay pedals they are relatively light and offer a bearing grease port. This pedal is offered with a choice of three spindle materials; chromoly steel, stainless steel, and titanium.
Subjective: Unrestricted free float is often described like pedaling on ice. This is appreciated by some (including us) and hated by others. Another oddity of this pedal and cleat design is the lack of feedback when engaging the pedal. Sometimes a faint click can be heard or felt. Other times engagement offers zero feedback. We adjusted to this without an issue but understand it could be a deal breaker for others. Disengagement is also void of feedback but we found this difference to be to our liking. (Disclaimer: Although this system is marketted as an off road pedal system we did not give it serious off road use. Our use would be categorized as non racing road riding.)
Speedplay (Zero, Light Action, X series)
Technical: Speedplay road pedals have evolved from through numerous X series pedals to the newer Zero and Light Action models. All of these designs use use a four bolt mounting pattern that is not as common as the typical three bolt pattern. But the Speedplay cleats can still be mounted on typical three bolt shoes with the use of the included cleat adapter. Stack height is 11.5mm or 8mm when used with or without the adapter. The Speedplay road pedal/cleat system also offers the most ground clearance when pedaling through turns. All models offer plenty of free float with the Zero cleat also providing the option to adjust the amount of float. Speedplay road pedals are unique in that the moving parts in the engagement mechanism are in the cleat and not the pedal. This makes them more susceptable to dirt and debris picked up when walking off the bike. Some models are offered in different spindle materials, chromoly steel, stainless steel, and titanium and differing spindle lengths.
Subjective: If you like lots of freefloat then this is the pedal system for you. When switching to Speedplay from a non free float system the first few moments always feel odd. And then we just forget about the pedals and enjoy the ride. Which is a pretty good compliment. We did not prefer, however, the amount of torque required to disengage from the Zero model of pedal. This is their top of the line model and the spring that holds the cleat engaged to the pedal is plenty stiff. We prefer the other models for non racing applications. Speedplay cleats continue the 'on ice' theme when off the bike. We skated a few too many times.
Crank Brothers (Eggbeater, Candy)
Technical: Fundamentally the Crank Brothers pedal system is a refinement of the SPD system. Both use springs to retain smallish two hole metal cleats that are designed for recessed mounting. Both rely on the sides of the shoe for lateral support. Crank Brothers offer some additional features. Reversable cleats allow the user to adjust the amount of angle rotation required to disengage. A major improvement for users over SPD is the Crank Brothers tolerance of mud and debris. For this reason Crank Brothers Eggbeaters are often seen on the Cyclocross race course.
Subjective: We used a set of eggbeaters for both cyclocross and commuting. We loved them for both roles. They were definately different than other systems in a couple ways. While racing, even caked in a variety of flavors of Pacific Northwest mud, clipping in was fast and easy. And, despite their spindly appearance, pedalling unclipped was never a problem. While commuting on them, when pedaling starts with less hurry, we learned that a back to front motion helped the cleat find the pedal more reliably. The cleats are described as having either 15 or 20 degrees of float depending on how the cleats are mounted. What we eventually realized, however, is that the quality of the float was different. The rider's feeling was that the pedal would accomodate any foot angle within the wide float range. And the pedal would help hold that position with a tiny bit of friction from the pedal to cleat interface. This felt completely natural and helped us forget the pedal and enjoy the ride. Another bonus is the shoe support. The round sections of pedal on either side of the retaining wings meet the sole of the shoe and provide excellent lateral support. Unlike most pedal systems, these require periodic maintenance. The cleats are brass, which spares the steel retaining wings from wear, but does not last nearly as long as steel cleats from other recessed cleat pedal systems. The pedals also require periodic regreasing. Which is not difficult nor time consuming. But many other systems seem to run for years without maintenance.