Originally Posted by
tomato coupe
Yeah, his argument is too simplistic. You can't just look at the heat generated in one braking event. You have to consider the number and frequency of braking events, and how efficiently the rotors dissipate heat.
In theory they can dissipate it much better because you can allow them to get much hotter. The hotter something gets the better it is at spitting out heat (it's the 4th power of the absolute temperature). Rims can only get so hot before the tyre blows off so that dictates the range you can work in.
There's no reason why you couldn't allow disks to become red hot but you need a pad material that works both at that temperature and also when the brakes are cold.
But it seems in practice disk brakes on road bikes do have issues with overheating. It looks like what happens is you reach a temperature where the pads stop working properly so the brakes lock up.
I don't like them on road bikes. The top of the fork. And the SS bridge are just much better places to transfer braking forces, on the centreline of the frame and in places where it's strong anyway. With a disk it's all happening at the end of one fork leg, the worst possible place, which means the whole fork has to be made bigger and stiffer. On an MTB this is a non-issue because the fork needs to be big and stiff anyway.
They're also heavier and less aero. Neither of these matters for MTB. The brake feel however is much better which does matter for MTB but less for road. And rim brakes on an MTB get covered in mud.
Then there's the speed of wheel changing which matters for racing. And don't get me started on thru axles.
If they actually worked a lot better on mountain descents they might be justified for pros and you would actually see them gaining time. But it doesn't look like they are achieving this yet.