MichaelB wrote:Can someone say "That's not a disc ....."
...that's a cheese grater!
As mentioned elsewhere, I work in an automotive braking test lab.
There's a fashion, on "modified" street cars (fullysickmoite), to fit slotted and/or drilled brake rotors. There's no shortage of internet (and old-wives... ahem... old-timers) expert opinions on how much improvement these slots and holes make to your braking, and why. The explanations always include the word "outgassing", which is a word I've never seen used other than in the context of slotted/drilled rotors, and which I have never seen used in any academic publications on the subject. But I digress.
On a brake performance dyno, time and time again, I've seen demonstrated that the only measurable differences [1] between a plain rotor and a slotted rotor are in pad wear rate and noise. The slots just shave friction material off the pad every time they pass. Chew them up and turn them into brake dust. No change in performance at all. And of course, every time a slot passes the pad, you can hear it.
I understand that on bikes, they also need to cut material out to save rotor weight - the "drillium" principle. But I look at those rotors and imagine how the pads deform under load. The inner and outer edges of the pad are fully supported by a solid bit of rotor, so they'll compress slightly under caliper load. The centre of the pad will bulge out relative to the edges... until one of the grater blades swings through, shaving the pad surface clean with its sharp edge. When the brake is released, the compressed edges will relax - I reckon you'd end up with measurably concave pads
tim
[1] except for one case, where a car-maker wanted to offer slotted rotors as a "dealer option" on their boy-racer models. As they would be fitted on behalf of the manufacturer, the car had to be proven to meet Australian Design Rules on delivery, which meant a validation brake test. Everybody assumed it would just be a formality, because everybody knows that slotted rotors are just a silly cosmetic thing. Everybody was surprised when the slotted rotors
reduced brake performance, repeatably, by more than the ADR acceptance limit. The car maker changed their mind about offering "genuine" slotted rotors...
