Xplora wrote:Yeah, the Road.cc (?) reviewer put himself into the meatwagon using the lightest CX rotor available and then blamed the discs for his failure to descend safely. It could very well be that a huge 200mm rotor could be ideal for big descents. Or not.
Early stage R&D is hardly the time to bag the tech, that's for sure.
Two things for certain:
1)
Energy in (kinetic / potential energy) = energy out (heat).Can't be created or destroyed, and all that.
If the rotor can't dissipate the heat generated by the brakes, it will get hotter. Hotter things are better at dissipating heat, so whatever the disc (be it 140, 200 or 700mm), it will reach an equilibrium temperature. The less effective the rotor is at rejecting heat to atmosphere, the hotter that equilibrium temperature will be.
2)
Disc brakes can operate at much higher temperatures than rim brakes.I work in automotive braking. We have brakes in the lab working just fine with the rotors glowing, well over 600°C. That's with resin-based pads, on hydraulic brake systems, which is what 100% of cars use.
I don't know much about sintered metal, but I know they go there for motorcycles, trains and industrial applications where the brakes get really hot.
I also don't know anything about the friction materials currently used in bicycle disc brake pads, but I really find it hard to believe we're pushing hard against the temperature limits of the best friction material technologies in existence. As a very crude indication... I've never seen a bicycle disc brake glowing red like I regularly see car disc brakes.
By comparison, rim brakes. Alloy rims, filled polymer pads. Alloy melts at <700°C... loses plenty of strength before that. And it's a very good thermal conductor, so whatever temperature your braking surface is getting to, your tyre bead and tube will be in contact with almost the same temperature. Rubber isn't good with temperature... especially when it's trying to contain 100psi air.
So, combining - a disc brake system WILL get hotter than a rim brake system, because it isn't as good at rejecting heat. But a disc brake system is MUCH better at coping with high temperatures than a rim brake system - and IMO has the potential to be significantly better than the current implementation.
As above, I'm not convinced the friction materials currently in use are all that good. And that's aside from all the work that could be done to improve brake cooling. Why can we cool the processor of a laptop computer, but can't cool a (significantly hotter) bike brake? Calipers don't even have cooling fins yet...
tim