DavidL wrote:- For the testing here of temperatures here mounting thermocouples inside pads although would be cool and get lots of hits on the net, is probably over complicating it a little. An infrared or hand-held probe on the rotor as the bike comes to rest should suffice for trying to determine if brake fluids are going to boil or not. We are just testing if it is 100, 200, 300 or 400 deg. C, not trying to design a system here.
Sorry, pad thermocouples is just the way I usually do things (in my day job as a brake test engineer
), so it's the first way I imagine doing the test. Especially since I can just pinch the sensors and logger from work.
Infrared on a polished metal surface is horribly unreliable. Emissivity is lousy. Contact might work, but you have to consider that there's very little thermal mass in a bike rotor, so bringing an ambient temperature sensor into contact with it - even a small one - is going to suck heat out pretty quickly as it comes to equilibrium.
One thought I just had is a rubbing thermocouple. We use them occasionally. They self-heat to some extent, depending on contact pressure, but that's only a real issue when you're trying to measure wear rate at 50°C and need to cool the damn thing down between stops. At the temps we're aiming for here, a rubbing thermo would do nicely.
- Mt Buninyong isn't really any where near big enough to do a proper heat test.
There's not a whole lot of thermal mass in a bike brake, so it's not going to take very long at all to heat up as much as it's ever going to.
Also, be aware that the Road Nationals course only climbs half of Mt Buninyong. There's another 150m elevation in 2.5km to be had on the top half of the mountain. In total, I could drag a single disc brake for 300 vertical metres over 5.5km down a full descent of Buninyong... and that should be more than enough to cook the hell out of a brake.
Besides, if Mt B isn't big enough... I have to travel a long way to find a mountain that is. Buninyong is about as big as it gets in my backyard.
- Brake fluids are available up to well over 300 deg. C which is going to be very hot at the rotor as in automotive applications rotor temps can be 5 times fluid temps which is going to be well beyond happy working temps of bike rotors. (though vauge physics here, I am trying to suggest that IF the fluid was to boil, it would most likely be a secondary failure)
"5 times" is not meaningful when you're measuring on a scale with an arbitrary zero point, but your point is fair - the same point I was making earlier. If people are worried that fluid is going to boil, then they should be really worried about the temperature of other parts of the brake system. Worst case, fluid isn't going to boil until a couple of hundred degrees, possibly much higher. Unless you're continuously dragging the brake for long enough that the entire system comes to equilibrium, the working parts of the brake will be much hotter.
In automotive terms, the effect is more pronounced because the system is so much bigger. I run tests where the brake is quite hot for a long time (200-300° at the rotor for maybe an hour, spiking to 500°), and the backplate won't get to 100°... let alone the caliper body and fluid. But that's with a 10mm chunk of non-conductive friction material insulating the hardware from the real action. Thinner metallic pads on bike brakes won't be so lucky, but you're still not going to see fluid temperature anywhere near the pad/rotor interface temperature.
I'm used to dealing with pad materials that don't change appreciably in friction until rotor temps upward of 400°C, and some that are only mildly troubled by 800°C temperatures that will cause permanent microstructural damage to a cast iron rotor. Plenty of world wide experience on mountain bikes says that it's possible to fade a disc brake; so even if we only get fairly average friction materials, we're still talking working temperatures of 400°C+ at the limits of what cycle disc brakes can do.
Somebody else can do the validation testing of plastic rotors at those temperatures for me.
Though some testing data would be great to see, especially with a couple of different rotor designs.
I think operator behaviour is going to be a much bigger determinant of operating temperature than any hardware variable. If I drag a brake down a steep hill for several minutes (like the celebrated crasher in that celebrated magazine story did), then I'm going to get my brakes much hotter than if I pulse the brakes, or alternate front and rear, or sit up and roll free and let the aerodynamic drag moderate my speed between brake applications, or do anything at all to moderate my brake temperature.
Same in a car. I can fade my brakes and crash if I try hard enough. Doesn't mean that disc brakes are a bad idea in a car - it just means that I'm an idiot.
tim