Ross wrote:barefoot wrote:[1] My day job is testing automotive disc brakes. I am yet to see slotted or drilled car rotors improve anything other than bling factor. All they do is cause pad wear, as the edges of the slots shave the surface off the pads. We saw in a recently discussed article, disc brakes in CX failing in mud due to excessive pad wear. I reckon this was largely due to abrasive debris (ie mud) getting into the rotor slots, and getting into the brake contact area that way. I'd like to get rid of the slots. On principle, I
bought a set of Avid "solid sweep" un-slotted rotors, but haven't fitted them yet... damn they're heavy with all that steel where the slots used to be!
I changed the disc rotors to slotted (not drilled) on a previous car I had and could notice a big difference in the braking. I can see your point about the brake pads wearing (I didn't have the car long enough or do enough miles to tell) but I guess that's the price you pay for better performance.
Many performance type cars such as V8 Supercars run a slotted rotor, I'm sure they have tested them and compared them to non-slotted rotors. Obviously V8 Supercars have different braking requirements than a road bicycle.
Did you change anything other than the rotors? I've seen that happen a few times - people go from a set of cheap and nasty pads on plain rotors to higher-friction pads on slotted rotors, and attribute the braking improvement to the rotors
Not meaning to start a pissing competition, but I've got about $6m worth of brake dynos humming away in the room next door. When I've tested plain and slotted rotors back to back with the same friction, I've not seen a difference. Caveat: under normal test conditions.
By "normal test conditions", I mean a standard performance test procedure that includes a "fade" sequence of 15 moderate stops from 100 km/h to zero, back to back. Brake temperatures up around 500°C. Now, rotors don't start glowing until upward of 600°, and I know race cars frequently get theirs lit up into the cherry-ish colour range, upward of 700°C. We don't test at those temperatures very often, because honestly, it's just not relevant. I'll cheerfully admit ignorance of that end of brake performance, where slots may well make a measurable difference. It's the only explanation I can make as to why they, and other serious players who really ought to know [1], persist with slotted rotors.
However... I'm yet to see a bicycle disc rotor glowing red hot either, so I'll reclaim some relevance.
One particular local car builder, at one stage, wanted to offer slotted rotors as a dealer option for their wings-and-blings models. Since it was a new car option, they had to prove that the brake performance was adequate to meet ADR. Rather than pay for a comprehensive ADR test, they went for a quicker "equivalence" test - where they just had to show that the optional brake system performed within 15% of the standard system throughout the test. No problem, because everybody close to the industry knows that slots make no difference (at least under the of conditions in an ADR equivalence test). Everybody was surprised when the slotted rotor REDUCED the fade friction by more than 15%. Many modern friction materials incorporate the products of their own chemical decomposition under temperature as part of the friction couple. By scraping this combusted layer off the surface of the pad, the slots were removing a vital part of the material. Wearing the pads unnecessarily fast AND reducing performance... but they look sweeeeet
. That dealer option didn't go ahead.
Anyway, that's an unusual result. Like I said, most often, the only discernible difference on dyno is the wear rate.
tim
[1] as opposed to certain local street-racer-spec modified taxi builders, who fit slotted rotors to their vehicles purely for aesthetics, knowing full well that it gives no performance benefit. I've heard snippets of discussion between engineering and marketing people from these places...