Originally Posted by IMOA
These days I usually steer clear of these discussions but I've got some time to kill so what the hell.
Short version - CSG_Mike is right, it's the tyres that determine the stopping distance
Medium version (I've got a very long version with all the maths/physics somewhere from back from the days I was racing and helping people build cars but I've lost it so I'll do the abbreviated version)
There's a number of factors which affect how much braking force can be generated by the braking system, the summary is as follows:
Brake pedal - push harder, more braking force
Brake master cylinder - smaller diameter, more braking force
Caliper pistons - more caliper piston (face) area, more braking force
Pad material - higher coefficient of friction, more braking force
Disk diameter - pad further away from centre, more braking force
Car weight - heavier car, more force required
Car speed - higher speed, more force required
However no matter how much force you generate in the braking system that force only does something when it acts on the ground and that is done entirely by the tyres. In effect the tyres can be in two states, rolling or locked and when they're locked they lose a significant amount of their ability to apply that braking force to the road (yes, yes, we're talking paved roads here, not gravel, snow etc)
So basically you can do lots of things to the braking system to increase the amount of force it is able to generate but if that force is in excess of what the tyres can apply to the road it will go to waste. As a result the tyres create the ceiling of what force can be applied so once you are at that ceiling you cannot apply further braking force to the road therefore changes to the braking system will not reduce the stopping distance.
To be very clear, once your braking system has the ability to lock the wheels at a certain speed then the only way to reduce the stopping distance is to get stickier tyres to raise the ceiling.
The most common mistake that people make is to put bigger brakes/better pads on a car and go for a drive, when they press the pedal the car stops quicker. Instinctively you think 'well if the car is stopping quicker when pressing the pedal this hard, when I really jump on the pedal it will stop even quicker so my braking distance has been reduced'. This kind of makes sense but it ignores the ceiling of the tyres, what it really means is that you are now able to reach that ceiling with less force on the brake pedal. So while it feels like the braking distance has reduced, it actually hasn't.
What this means is on a modern street car travelling at 'normal' speeds (ie, the braking system can generate enough force to lock the tyres) the way to reduce stopping distance is to change to better tyres, upgrading other components may help other factors (pedal feel, pedal length, unsprung weight etc) but you won't stop quicker.
On a track car where you are stopping from much higher speeds on stickier rubber it's likely that the standard brake system will not have the ability to lock the tyres so increasing the braking force will reduce stopping distances. Also on a track car since you're braking heavily from high speeds regularly having a braking system which can both handle higher temperatures and dissipate that heat more quickly becomes far more important than for a street car, and that includes street cars used for spirited country roads.
In short, if you want to stop in a shorter distance change your tyres, changing your brakes will only help once you have enough grip that the wheels don't lock at the point you're pressing the pedal the hardest.
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