Aaah, the fabled "outgassing" that is so often talked about on Interweb forums (and the real-world gatherings of enthusiasts that predate them), but which has never been observed in the sheltered world of brake engineering.
I've had good pads and *****ty pads that have nearly burnt our dynos down because they get too flamy.
Actually, it's only relatively good pads that get that hot. *****ty ones tend to self-regulate. They get slippery at high temperature, which puts an effective limit on how hot they can get. Can't get any hotter if they're not absorbing any energy. W00t, we're back on the Papal Decrees of Thermodynamics thread.
Anywho... the conventional "wisdom" of slotted rotors is that they clean the decomposed friction material off the surface of the pad, so you have good fresh material ready to do the work. But the decomposition/combustion products that you get at the surface of friction materials are part of the material design.
Most modern materials - especially high-performance ones - are scorched in production (sat on a 600° plate for ~2 minutes) to get this surface chemistry started, rather than waiting for it to develop in use. Without a scorch layer, performance is shyte.
If your salami-slicer rotors scrape the scorch layer off, you're losing the good stuff.
All the testing I've done confirms that friction levels are lower, and less consistent, on slotted rotors. And the pad wear rate is just ridiculous.
One particular Australian OEM wanted to sell slotted rotors in their blingbling "genuine spares" shop, which meant they had to get them through ADR testing that requires equivalent brake performance to OE - no better, no worse (within 15%). Everybody assumed that would be no problem, because everybody knows that slots are just a cosmetic thing with no performance benefit. Turned out they failed the ADR brake test, because the fade friction went through the floor. So there are no slotted rotors in the blingbling department.
Of course, that's using normal road pads. But I've seen the same thing using higher performance pads too. Ones that will quite happily turn rotors translucent red and burn all the seals out of calipers until the brake fluid pours out to cool the pads down.
I'm always confused when I see slotted rotors being used by people who should know better than I do what the effect is. Wondering whether there's something that they know that I don't. I'm yet to see any evidence to suggest that they are used for anything other than cosmetic reasons.
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