Sprung and unsprung weight are phrases used to describe certain components of suspension in vehicles.
Springs sustain the sprung weight of a vehicle. As the wheels go over bumps, potholes, and other obstacles, the unsprung weight travels up and down with them.
Unsprung mass refers to the weight of the:
Anything else directly attached to the wheels is unsprung weight.
Most of the vehicle, including the chassis, engine, gearbox, body, and interior, as well as the passengers and luggage, are classed as sprung weight.
There are additional sections that are semi-sprung (also called partly sprung or hybrid weight). On one end, these elements are normally connected to the wheel, while on the other, they are attached to a springy component.
The spring, for example, is connected to the body and the wheel. Another semi-sprung component is the shaft that connects the gearbox to a wheel.
Shock absorbers and struts, control arms and various other suspension components, drive shafts, and various steering components are included on the list of semi-sprung.
In general, a high sprung-to-unsprung weight ratio is preferable.
With a bigger percentage of sprung weight, the wheels and tyres may be pushed down harder, maintaining them in contact with the road.
Maintaining contact with the road enhances handling and traction.
As a result, designers aim to keep unsprung weight to a minimum in order to optimise handling and steering. That's one reason why many car enthusiasts fit alloy wheels, it minimises unsprung weight and improves handling.
Although not directly related, the differences between rotating mass and non-rotating mass is similar. It requires more energy to accelerate rotating mass than non-rotating mass.
Lighter wheels and tyres need less energy to spin or stop. Reducing the weight of unsprung and rotating mass is a two-in-one combination of performance improvement.