Delphi has written a few papers on brake caliper components; surprisingly the design is still a lot of trial and error, so papers like this, where they create a computer model and compare results to measurements from a vehicle, are still fairly leading-edge.
The paper uses multiple braking events during a mountain descent with periodic cooling periods as the simulated case. Design targets are to keep the brake fluid below 140C, which is when wet brake fluid may boil. They point out it may not boil while driving, but up to 10 minutes after stopping, due to heat soak back into the caliper body.
In this paper, front wheel braking power is 7 times that of rear wheel braking. (Don't take the number of 7 as gospel, but it points out that the fronts do most of the work.)
Figure 8 shows measured temperatures; disc at 475C, pad surface at 475C, caliper bridge peaks at 150C, brake fluid at 120C. The fluid temp hits it's peak after the vehicle is stopped. These are with a cast iron caliper and semi-metallic pad. The computer model predicts well, as Figure 9 shows.
First simulation (figure 10) is to change to an aluminum caliper. Now the caliper bridge peaks at 200C and the brake fluid at 160C. The higher heat transfer rate of aluminum over cast iron pulls heat into the fluid, and this would predict a fluid boiling situation.
Second simulation (figure 11) is to keep the aluminum caliper and change to non-asbestos pads. Non-asbestos pads have much lower heat transfer rates, so the disc surface temperature rises to 525C, the pads surface to 500C, but the caliper bridge to only 110C, and keeps the fluid below 100C.
Third simulation (figure 12) is with 2/3 worn down non-asbestos pads. Now the pads conduct more heat, so the disc peaks at 500C, the pads surface at 425C, the caliper at 175C, and the fluid at 150C.
Summary: Use of aluminum calipers will cause higher brake fluid temperatures than with cast iron calipers. Use of non-asbestos pads will produce much lower brake fluid temperatures compared to semi-metallic pads. Worn down pads will allow higher brake fluid temperatures than new pads. Soak-back may boil fluid even after stopping.
What did we get out of this? (1) We're not going to change aluminum calipers back to cast iron, I know. (2) Cool down laps are important. (3) When you shop for pads, maybe the right question is how good of a heat barrier (insulator) are they rather than how much heat can they physically tolerate. Sometimes pads are "rated" as to what temperature they operate best at, which is important, but how much of that heat they transfer is also important. If you had two pads that "work" at 500C, you'd want the one with a lower heat transfer coefficient. (4) Another thing is that worn pads will transfer more heat into the caliper and brake fluid. Some of you knew this from experience, some of us found out by accident, maybe some actually blamed the pads as being no good. But pads that are 2/3 worn should probably be retired from track use.
