JamesShort wrote:
Chuck Branscomb wrote:
MikeWhitney wrote:
James, excellent explanation. I honestly could not find one single hole in your explanation. That's a compliment.
We should get a beer sometime.
I agree -- great write-up.
The only thing I would add is that there is a radiation heat transfer component that also comes into play in an increasing manner as rotor temperatures rise. Anything the rotor can "see", from a radiation view factor standpoint, that is at a substantially lower temperature will exchange heat with the rotor surface.
One other thing to add that is not directly related to this discussion about pads but brakes in general, is that using an interstitial material between the pad backing plate and the piston(s) can substantially reduce conduction heat transfer into the pistons/calipers. Commonly thin Ti plates are used due to the low conductivity of Ti. The thickness of the plate is very small, so the Ti part isn't that significant, but by introducing the plate, two thermal contact resistances are added to the heat conduction path to the piston(s). The additional benefit is that the potentially very hot backing plate back surface is shielded, from a radiation standpoint, by the Ti plate (which will be at a lower temp) which helps reduce heat degradation of the dust covers. The benefit of Ti plates increases substantially as pad thickness is reduced (and therefore the backing plate is operating at higher and higher temperatures as a function of pad thickness reduction).
Excellent point on radiation. Definitely should have entered the conversation but I guess I didn't mention it because my rotors haven't glown red while autocrossing yet
. Sure there is radiative heat xfer between any two objects of varying temperature but the constant (epsilon?) in front of (T_1^4 - T_2^4) is pretty small and despite being a difference of 4 powers, radiation is probably small in relation to convection/conduction......well in autocrossing. Maybe radiation becomes an issue when you fly through the Monaco chicane with your wheels almost locked up
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One question about this titanium backing plate....why would you want to limit the heat transfer into the caliper etc? The caliper is mass for getting rid of heat at the pad/rotor interface? Well it would keep fluid colder which is important in prolonged racing... I guess in pure racing, they have such cooling capacity with ducting and 2 piece rotors that they are more concerned with point 3 (ie cooling) than they are about heat capacity of the overall system.
Yeah, it's commonly used on street and track cars at track events as it significantly lowers brake fluid temps and especially protects dust covers from thermal degradation. As pad thickness declines, the heat transfer to the caliper/fluid increases substantially (since the pad material is a fantastic insulator), so Ti shims allow increased protection as pads wear. For the most part, at track events, you want to minimize fluid temperature as much as possible -- especially at high brake duty cycle tracks. You really don't want to impose a high heat transfer into the caliper mass -- it's better to depend on the rotor (mass for transient) and convection+radiation cooling capability than to design the caliper to play a significant role.
The importance of brake cooling on track is massively more than at an autox as you know, so that was where my comment on radiation was headed. A maximum braking effort at the end of VIR's 4000' back straight from 140mph to 40 mph in a 4000lb sedan on R-comps will quickly highlight that part.
Same for the end of front straight, perhaps 130mph to 45 or so. Rotor temperatures can reach well into the range where radiation becomes a factor that can't be ignored.
BTW, using the Ti plates there with the M5 makes a huge difference in survivability of piston dust covers...even with the Stoptech big brake kit and PFC01 pads.
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) because you folks probably appreciate it more.