A lightweight racecar may have unassisted braking, but they still use discs.
Rim brakes haven't been used on a car for almost 100 years. Ask about drums though and he would undoubtedly still come down on the side of disks, although perhaps not on the rear where they can often overcome the available grip. If you asked about assisted or unassisted the answer would be unassisted. We are talking more about the amount of usable braking force available here, not really the method of obtaining that force. None of the cars in, say Formula 600 or the Jedi classes, run assisted brakes. You can have too much force, even for a Jedi.
Not so far.
Surely a racecar has a small contact patch with a limited amount of grip aswell? Why don't they use rim brakes? You'll probably find that the answer is pretty close to the reason that discs are better on bikes too.
Not nearly so small as a bike. But yes, it is limited. And they don't use rim brakes because the predecessor to disks was drums. Rim brakes went out with wooden wheels.
On the car, which would never have had rim brakes, the alternative, drums, were not efficient enough to overcome the grip available, especially with the advent of modern tyre compounds. The disk has other advantages over the drum for a car too, they are lighter and less prone to overheating. These are not advantages that disks have over rim brakes.
There is no point having brakes which produce breaking force far in excess of the available grip. This is why on a car, even with unassisted disks, the amount of breaking force available is reduced in times of reduced grip, either by changing brake cylinder ratio, or the leverage available at the pedal.
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