I'm not really sure why this standard was chosen, but it could easily be piston areas of both sides although the "piston area" of the non-piston side of a sliding caliper isn't real obvious. So it is a math thing and not indicative of any design superiority of either design.
As to why a fixed caliper vs. a sliding caliper, let me dig out my soap-box.....
Sliding calipers are an economic to mfg design, but they are an inferior braking solution at the outer limits. For an appliance type vehicle they're fine. Until they quit sliding, then not so much. Later designs, like the Exploder RDB's are better at shrugging off grit & dirt so they don't lock-up and fail to slide. I think that the GM caliper sliding design is superior to the Ford design. When I was more frequently involved with brakes I saw the Ford design not easily moving a lot more often than I saw the GM design doing so, but both suffer when not maintained. A thin smear of dielectric silicone grease on the sliding surfaces goes a long ways. Finely detailed and well maintained a sliding caliper usually wears out the inner pad by somewhere between slightly faster to moderately faster. Poorly maintained and the wear difference can be really significant. The culprit is the friction in sliding the caliper subtracting from the force on the outer pad. When you see the outer pad worn out and the inner is still serviceable you know the caliper isn't sliding much if at all and it is bad enough that it is causing the the outer pad to drag excessively.
Sliders flex a LOT more than a well designed fixed body caliper. The way that caliper flex was measured when I was doing design work in brakes was to use a block of aluminum between the pistons. We strived to use a block that was a snug fit between the pistons when they were fully inserted in the caliper body. Then a master cylinder was bolted to a very rigid bracket that allowed the use a big bolt to move the m/c piston. Attached to the m/c piston was an LVDT, a position sensor, and on the output port there was a pressure transducer. There was a braided SS flex hose used to couple the caliper to the m/c assembly. A data acquisition system gathered up the pressure sensor and LVDT outputs and we imported that into Excel where we could plot pressure vs. travel distance.
Before each test the far end of the hose was plugged and the m/c was run up to max output pressure. Then we would run the caliper flex test. In Excel we used the plugged hose data as our relative zero so that in flex in the hose etc. wasn't included in the results. I do not recall specifics (it was ~15 years ago!), but as a general rule sliding calipers flexed roughly twice as much as the worst (most flexy) of the fixed body calipers.
That flex shows up as excess pedal travel and a slightly more "mushy" feeling brake pedal. Most people won't notice this or don't care, but some will and do. Or say they do because they've spent a big chunk of cash on such systems.... ;D
As Tobin mentioned above, the whole thing is scalable, with a smaller piston area caliper you simply use a proportionately smaller piston area master cylinder. I feel that it is much more important to get the front areas ratio correct first. Can then work out how or what to do to get the overall brake system balance i.e. the front to rear balance. It is most ideal when the ratio of front to rear caliper piston areas is such that minimal proportioning correction is needed, but that isn't always possible and is what proportioning valves try to address.
), I pulled the trigger without double checking if these will fit on a stock 15" steel wheels. Does anyone have any experience with installing these up against OE steelies?
