A lot more goes into braking than simply having enough clamping force to lock the wheels. In a situation when you completely lock the wheels, you're trading th

A lot more goes into braking than simply having enough clamping force to lock the wheels. In a situation when you completely lock the wheels, you're trading the friction between the brake pads and brake discs, for the friction between the tires and the road surface. In other words, instead of the hydraulic clamping force on the discs, you're relying solely on the weight of the car. The interface between tire and road is not nearly as good as the interface between brake pads and brake discs, which wear to match each other exactly, as they're used. So as a good rule of thumb, you never ever want to lock up your brakes. That's why anti-lock brakes are standard. (The main exception is when driving on relatively deep and loose surfaces, such as sand or deep snow, where a locked tire builds up material in front of it.) Undersized discs and pads have less thermal capacity, so they will tend to overheat and lose braking ability. A lot of lower-spec and economy cars will be perfectly capable emergency braking from full speed once, but you absolutely have to let the brakes cool off afterwards, especially if it has unventilated discs and rear drum brakes. That's fine, as long as you're aware of the limitations. In comparison, a larger midrange car (such as a VW Passat) can reasonably handle repeated emergency braking situations without losing any significant amount of braking power, thanks to larger discs and pads. There is also the matter of pedal feel, how confident the brakes feel or if they feel mushy and imprecise, and how the ABS system works. So yes, of course the contact patch matters, but only if the brakes are adequately capable.