Unsprung

Unsprung refers to the portion of a vehicle’s mass that is not supported by the suspension springs. In practice, unsprung mass includes the wheels and tires, brake assemblies, axles, wheel hubs, and the lower portions of the suspension components. The remaining mass—the body, engine, seats, and other components above the springs—is called sprung mass.

Why it matters: The dynamics of the unsprung portion determine how quickly the wheel can follow road irregularities. Higher unsprung mass increases the inertia of the wheel and reduces the suspension’s ability to maintain tire contact, leading to a harsher ride and reduced grip on rough surfaces. Lower unsprung mass generally improves ride quality and handling, assuming the rest of the suspension is designed accordingly. However, reducing unsprung mass involves trade-offs with strength, durability, and cost.

Common approaches: To reduce unsprung mass, designers select lightweight wheels (for example, forged or aluminum rather than steel), lighter brake rotors and calipers, lighter axle assemblies and suspension links, and use advanced materials such as aluminum, magnesium, or carbon composites for components below the sprung mass. Independent suspension layouts and optimized dampers also help improve wheel control and ride quality.

Rail and other applications: The term is also used in rail engineering; wheelsets, axles, and portions of the running gear constitute unsprung mass and influence wheel-rail interaction, especially on switches and poorly aligned track.