[YSFEJ4SWJUVU6]

>A motorbike will generally [...] brake faster than a car in a straight line (mostly because it's light)

That's generally not true. At best you'd roughly match an average car when balancing your weight perfectly; but typically you can only expect worse results when comparing the two.

[nwallin]

Classical equations of friction gives a constant relationship between weight and maximum acceleration from friction. And that applies fairly well to most pairs of materials.

But it applies less well to rubber. Rubber has significant natural adhesive qualities, so a normal force of zero Newtons gives a non zero acceleration. This adhesion dominates low weight acceleration. This is why performance cars tend to have larger tires. See dragsters for the logical conclusion. Without that adhesive quality, performance cars would prefer smaller tires for reduced air resistance.

The classical coefficient of friction model is one of "spherical frictionless cows in a vacuum" white lies we tell students to prevent them from getting overwhelmed by the hairiest differential equations imaginable.