From this graph of the open-loop response for 10 cm step disturbance, we can see that when the bus passes a 10 cm high bump on the road, the bus body will oscillate for an unacceptably long time(~50 seconds) with an initial amplitude of 8 cm. People sitting in the bus will not be comfortable with such an oscillation due to the large overshoot and long settling time.
The solution to these problems is to add a feedback controller into the system to improve the performance. The schematic of the closed-loop system is the following, which will be discussed in much more detail in the controller design sections.