modern braking improvements - tires or brakes?

[rks838]

http://blogs.edmunds.com/straightline/2009/04/il-track-tested-2009-bmw-750li-vs-2009-mercedes-benz-s550.html?tid=edmunds.il.home.photopanel..4.*

So these 4,600lb+ sedans are now stopping in distances of less than 110ft, which three years ago was the 911's distance. It's still phenomenal, but how can cars like this do it now? It is an advance in tire compound, or (I would guess this isn't the answer) an improvement in the brakes or ABS systems? Curious, thanks.

[Z07]

Both. You can only brake as hard as the brakes allow you and you can only stop as hard as the tyres will let you. At lower speeds it's more down to rubber (increased deceleration forces) at higher speeds more down to brakes (absorbing kinetic energy).

The heavier the car, the more KE you need to absorb to create the deceleration force (more brake work). The lighter the car, the greater the deceleration force, for a given set of brakes and the more the coefficient of friction comes into it.