JimFE

maxusa

Yes, because the brake caliper design wraps over the rotor edge, the larger diameter rotor pushes the caliper with pads out by 15 mm ((390 - 360) / 2) from center. A new bracket or spacers are needed. The contact patch does not change, but the increased radius (leverage) provides the relative reduction in force to achieve the same torque (other factors remaining constant).

However, if we introduce other variables such as what @kenneyd has done with the PowerStop pads, the result may be different. For example, if the new ceramic pads have lesser friction relative to the original pads, then the 390mm rotor upgrade will help to offset more pedal force necessary to maintain decent modulation ability.

billvp218

I get what you are saying, but I'm curious about the particular design of the big brake kits in the thread (or others with similar setup). It seems possible that the pads could still end up located in the same spot, or slightly moved up to be centered where the extra 15mm space ends up half above and half below the pads. I'm not a brake expert by any means, so perhaps there is something about the way the calipers are set up that a bracket wouldn't possibly result in the pads reaching lower ... is that the case? If not, consideration of how the brackets are set up could be a differentiator for those looking to upgrade.

maxusa

The W212 brake caliper design wraps around the rotor edge. This means that it follows the outer edge of the rotor. The pads inside the caliper have basically a fixed position with some minimal play to accommodate rotor/caliper alignment and wear issues. This wraparound caliper design dominates the auto industry. There are nuances like the floating vs. fixed calipers, number of pistons, hydraulic vs. electric, etc.

I have driven cars and motorcycles with the barrel drum-style brakes, but these are much older designs without the rotor/caliper.

billvp218

so at risk of stating the obvious, the caliper itself does not change - the bracket is to reposition the caliper ... so that means the pad wouldn't move its relative position within the caliper from stock, and thus it likely moves 15mm up (if the caliper needs to move 15mm, vs. possibly also using some free space that was already there)

kevm14

There is no brake upgrade in the world that moves only the caliper and not the pads. I can't even understand the confusion.

maxusa

Correct.
Correct. The caliper is the pads' housing. It is strong and heavy because the rotor wants to rip the pads out when braking. Pads need a solid home to stay in place - strong caliper.
Correct. To be more precise, the caliper/pad assembly moves out (from the wheel center) to match the new rotor radius, exactly by 15mm. You need a new bracket for this. Pads generally move inside the caliper only to clamp the rotor... and a few mm play for alignment and wear. It is impossible to reposition the pads on their own.



Hope this helps.