I am a little confused about why a passenger truck like a Chevy 2500 or a Ford F250 requires brake rotors with larger diameters than a car like a Toyota. I have a degree in Physics, and although I have not used it in 10 years I specifically remember learning that mass does not effect braking distance. The reason is as follows. A car stops due to frictional force between road and tire. When comparing stopping distance between two cars any difference in mass can be ignored because a greater momentum in a car with greater mass will also have a greater frictional force by the same magnitude, since friction equals normal force times co-efficiant of friction (kinetic or otherwise). For example if you have 2 Toyota Camrys and one is made out of aluminum and the other iron and they are allowed to coast to a stop from an initial velocity of 60mph they will come to rest at the same time. I understand that mass distribution plays a role in 18 wheelers, but assuming mass is distributed uniformally why does a truck need bigger brake rotors??? Paul Tippler's College Physics Text shows mathematically that mass cancels when calculating stopping distance. Thanks!