The Caddy has leaf springs at the back, because it was designed to carry loads, and that is exactly the part that tires from the way it is used. The sign of a tired spring is that the back of the car sits down even unladen (the rear wheel "sinks" into the arch), and a dull clonking or knocking is audible from the rear over bumps. The spring mount and the trailing arm bushes make the same noise. On a heavily loaded fleet car it is effectively a wear item; and a tired rear suspension wears the tyre unevenly too.