The mass supported by this WheelCollider.
Vehicle simulation uses the sprung mass model where it would decompose the full mass of the car supported by all wheels at once into a set of sprung masses supported by each individual wheel. Then it would work out all the suspension and tire forces for each wheel-mass pair and, as the last step, it would integrate all forces applied to the vehicle's body. In the simplest case of a perfectly symmetric car with four wheels, all sprung masses would be equal to a quarter of the car's mass. This property is automatically computed based on the car's configuration.