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Braking: Slide-rail vs. piston calipers

Floating "slide-rail" calipers are a fact of life. They are OE on practically everything now and as usual the marque fans will yell "The brakes on my [fill in the blank] are great!" Sorry. If they are any type of sliders, they are not "great."

Simply, slide rail calipers work by moving a single large piston -- and the caliper frame -- to press the pad against one side of the rotor, while in opposition pulling the other side of the housing and pad to the other side of the rotor. The whole 5-6 lb. assembly moves. In addition to that, the piston covers just a portion of the pad, while leaving the rest of the pad unsupported by anything but its backing plate. The motion of the rotor causes the pad to flex in on one unsupported side, and out on the other, leading to pad taper. (See drawings below of slide rail and 6-piston versions of Wilwood calipers. The dotted-line circles show how much of the pad is used by the respective pistons.

The biggest problem with slide-rails centers around the most important issue in braking: release. That you can "lock them up anytime I want" is NOT a good test of a braking system. On the contrary.

The average driver is right in thinking that weight (unsprung weight in calipers and rotors on four corners) and temperature are the critical factors in braking. The article here in the University on brake pads looks at some of those considerations. But the real issue in performance braking is not the clamping force but release and how quickly and precisely you can take the force OFF the rotors to keep that knife edge of balance. Slide rail calipers just don't -- can't -- release rapidly and smoothly enough.

Look at the system: with the application of pressure through the hydraulics, slide rails must move the caliper against both sides of the rotor. To release the clamping force they must move the same distance, back. On the other hand, pistons move a fraction of that distance to clamp and unclamp the pad to the rotor. The result translates into lots more finesse.

Most of the time, slide rail calipers contribute to significant pad taper, again due to the basic geometry of the system. The inherent flex of the rail system causes the pads to interface with the rotors unevenly --'tilting' or torqueing, rather than clamping the rotor with consistent force along the face of the pad and the rotor. As you will read in the article on brake pads, this has a dramatic effect on performance and feel -- another element in that 'release' issue we're discussing.

Fixed piston calipers -- with pistons on both sides -- are designed to deliver more consistent force across the brake pad. That's why six-piston calipers are more precise than four-piston ones, given the same installation design.

Here, I need to note that radial-mount calipers enhance this intent compared to lug-mount calipers. First, because the radial mounts, that are perpendicular to the diameter of the rotor allow for more leeway in where/how you need to mount them; and second, because the wider the gap between the mounts, the stiffer the caliper -- some of the lug-mount calipers have a 3 1/2" gap, some have 6" -- which again affects how evenly the clamping force is delivered to the pad. But some wheel configurations don't allow for radial mount. And lug-mounts are definitely less expensive.

Going back to the number of pistons and why that's important. I once saw a system with 12 (yes, TWELVE) pistons per caliper. When you ordered pads ($1200 a set) you got 24 tiny little brake pads, each perfectly matched to the face of its piston. When you got the caliper set-up perfectly, this $8500 system was magic. The braking was incredible (each piston was independently applied, on a floating mount rotor). When the alignment was bad, as they say, it was horrid. Ultimately the 12-piston set-up was banned -- the new ruling said you could have just 4 pads per corner.

Piston calipers equal more braking force controllability, which equals greater ability to feather your brakes.

We are gratified that Wilwood has decided to make a commitment to racers -- we think, just in time -- and will continue to develop products like the new Superlites (13" and 14" rotors, and brake fluid with performance numbers as good as the stuff that's twice the price) that meet the exploding demand for leading edge brake answers in road racing, short track and street applications. 'Bout time!

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