Wheel collider in the air problem

Hi All,

I have a problem (or at least I think it is a problem). I have a car with 4 wheel colliders . Only the 2 rear wheel drive the car that is I apply motorTorque and brakeTorue to them them (and only them). I can drive forward as well as backwards (using negative motorTorque). When I drive back (thus apply negative motorTorque and gain enough) and suddenlt brake (apply brakeTorque to the rear wheels as well) the front of the car goes up in the air (including the 2 front wheel of course). Is it normal behavior ? and if not how can I fix it?

Thank u in advance