I don't get it. Why would anyone want to use SPD pedals—such as PD-M520, PD-M8000 (left of the image), PD-M9000, PD-M9100, ...—when SPD pedals with platforms—such as PD-EH500 (right of the image), PD-M9120, ...— are available?
Within the same design line and in one snapshot for comparison (the two models pictured), the latter is even lighter—383g instead of 425g.
We can use rotational symmetry as a guideline while discussing pedal designs. Rotational symmetry is how many times one can rotate a pedal until it "looks the same" before one full revolution is reached.
The Crankbrothers pedals have SGo4 (belong to a symmetry group of order 4). If the present orientation of a pedal is not right, you need to spin it by no more than 45° one side or the other, and you'll be ready to engage.
The SPD SL, the Look Keo, and the Look Delta pedals belong to SGo1. There is just one position for clipping in, but that's not a disadvantage, because in all three the rear of the pedal is heavier than the front. The position of the pedal can be predicted, and this can be done (crucially) while looking up rather than down.
(In my limited experience, an SGo1 pedal will not always "lean back and be ready for clipping in." That possibly has something to do with the viscosity of the lubricant inside as well as how cold it is. These issues are also a topic for another question.)
Like a rectangle, SPD pedals belong to SGo2. As with the Crankbrothers' SGo4 (which resembles a square), we cannot predict their resting position. Since the SGo2 pedals could be at an arbitrary orientation, the rider may need to rotate a pedal by as much as 90° to one side or the other until the pedal is ready for engaging (and the rider would truly like to be able to clip in without looking).
The platform SPD pedals have a built-in weight because the platform side is heavier so they're always ready for clipping in (leaving aside viscocity, ambient temperature, as well as factory quality control). Plus they provide the option to use regular shoes or boots.
So, once more, why would anyone want double-side SPD rather than single-side SPD with a platform on the other side? Aerodynamics?
I am curious about the following two questions. I do not intend to ask them imminently. You are more than welcome to elaborate on either/both of them and ask.
- Suppose I buy a pair of SGo1 pedals. Since the rear is heavier than the front, I expect them to lean back to just the right position, but one of the two in the pair I bought doesn't. Should I return the pedals? Should I heat it and try again?
- Shimano has two models (PD-M8020, PD-M9020) that have SGo2 symmetry (we can't predict their orientation), yet that have a large frame. Suppose I don't care about the risk of impact (because I'm using SPD to ride on snow/ice and not in forests) and will (hopefully) always be clipped in, what's the purpose of their existence? In particular, does the frame make it easier to clip in without looking?
- My clothes, boots, and wheels have reflectors. My bike has front/rear strong lights. Yet the tell-tale sign for motorists that a cyclist is ahead is the characteristic up-down motion of the pedals. Do I really need to be concerned if I use either PD-M8000 or PD-EH500 (neither has reflectors), or does only the PD-T8000 make sense in an urban environment?