Influence of Suspension Stiffness and Vehicle Speed on Passenger Vehicle Ride Comfort over Random Road Profiles

Ride comfort is crucial to passenger vehicle design because it affects occupant comfort, stability, and suspension performance. We developed a 7-DOF (seven-degree-of-freedom) spatial full-vehicle dynamic model in MATLAB/Simulink to investigate a passenger car’s vibration response to ISO 8608:2016-classified random road profiles. The spatial model represents coupling effects among the four suspension corners by accounting for vehicle-body bounce, pitch, and roll motions, as well as the vertical motion of the unsprung masses, unlike quarter-car and half-car models. The model is compared with simplified vehicle vibration models, and its capabilities are discussed in relation to recent full-vehicle models reported in the literature. The vehicle response is evaluated for road classes B–E, vehicle speeds ranging from 20 to 80 km/h, and suspension stiffness values ranging from 0.5 to 1.5 times the nominal stiffness. Ride comfort is evaluated using Root mean square (RMS), body displacement and vertical acceleration based on ISO 2631-1. Results reveal that road roughness affects vibration response more than vehicle speed. Raising suspension stiffness from 0.5 K to 1.5 K at 80 km/h on a class E road decreases RMS body displacement by 32.8%, while increasing RMS vertical acceleration by 43.8% (from 0.680 to 0.978 m/s2). These results show the inevitable trade-off between vibration isolation and body-motion inhibition. Because it captures coupled bounce, pitch, and roll responses, the 7-DOF spatial model represents vehicle-body vibration more accurately than vertical-only models, making it better suited for suspension parameter assessment under different road and speed conditions.