Dear Silak, A car travels along the ground at a certain speed, but the speed of the whole car is not be the same as the speed of the movement or motion of some of its parts. Here, the writer is not interested in the motion of the vehicle as a whole, but the motion created between the wheels and the rest of the vehicle by the suspension system. A good suspension system has a higher degree of relative motion between the wheels and the rest of the vehicle than a bad system, because it is doing more work to ensure a smoother ride.
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silak12connects a vehicle to its wheels and allows relative motion between the two.Relative motion between the vehicle and its wheels is the motion of the vehicle as experienced by its wheels or the motion of the wheels as experienced by the vehicle. These are small motions compared to the motion of the vehicle along the road.