What is the difference between sprung weight and unsprung weight?

Car fans, especially those who race, sometimes talk about "sprung" and "unsprung" weight (or weight). What do these terms mean?

The spring is the suspension component that holds the vehicle and protects it, passengers and cargo from impacts. A car without springs would not be very comfortable and would soon fall apart from shaking and bumps. Horse-drawn carts have used springs for centuries, and as far back as the Ford Model T, metal springs were considered standard. Today, all cars and trucks run on leaf springs.

But when we say that a car "runs on" springs, we don't really mean the whole car. The portion of any car or truck supported by the springs is its sprung mass, and the remainder is its unsprung mass.

The difference between sprung and unsprung

To understand the difference, imagine a car moving forward until one of its front wheels hits a bump big enough for that wheel to move up towards the body of the car. But as the wheel moves up, the car body may not move much or not at all because it is isolated from the upward moving wheel by one or more springs; the springs can compress, allowing the car body to stay in place as the wheel moves up and down underneath it. Here is the difference: the car body and everything that is firmly attached to it are sprung, that is, isolated from the wheels by compressible springs; the tires, wheels, and anything attached directly to them are not sprung, meaning that springs don't keep them from having to move when the car goes up or down on the road.

Almost the entirety of a typical car is a sprung mass because almost every part of it is firmly attached to the body. In addition to the body itself, which includes all other structural or frame components, the engine and transmission, interior and, of course, passengers and cargo.

What about unsprung weight? The following are unsprung:

• Tires

• Wheels

• Wheel bearings and hubs (parts on which the wheels rotate)

• Brake units (on most vehicles)

• On vehicles with a continuous drive axle, sometimes referred to as a drive axle, the axle assembly (including the differential) moves with the rear wheels and is therefore unsprung.

It's not a long list, especially for cars with independent rear suspension (i.e. not a solid axle) the unsprung weight is only a small fraction of the total weight.

Semi-sprung parts

There is one difficulty: some weight is partially sprung and partially unsprung. Consider, for example, a shaft attached at one end to the transmission, and at the other end to the wheel ("half shaft"); when the wheel moves up and the case and transmission don't, one end of the shaft moves and the other doesn't, so the center of the shaft moves, but not as much as the wheel. Parts that need to move with the wheel but not as far are called partially sprung, semi-sprung or hybrid. Typical semi-sprung parts include:

• The springs themselves
• Shock absorbers and struts
• Control arms and some other suspension parts
• Half shafts and some cardan shafts
• Some parts of the steering system, such as the steering knuckle

Why does all this matter? If most of the vehicle's mass is unsprung, it's harder to keep the tires on the road when driving over bumps because the springs have to exert more force to move them. Therefore, it is always desirable to have a high sprung to unsprung mass ratio, and this is especially important for vehicles that must handle well at high speeds. So racing teams reduce unsprung weight, for example by using lightweight but thin magnesium alloy wheels, and engineers try to design the suspension with the lowest possible unsprung weight. This is why some cars, such as the 1961-75 Jaguar E, used brakes mounted not on the wheel hub, but on the inside end of the axle shaft: all this is done to reduce unsprung mass.

Note that unsprung mass or mass is sometimes confused with rotating mass because some parts (tyres, wheels, most brake discs) fall into both categories and because riders want to reduce both of them. But it's not the same. The rotating mass is what it looks like, everything that needs to rotate when the car is moving forward, for example the steering knuckle is unsprung but does not rotate, and the axle shaft rotates but is only partially unsprung. Less unsprung weight improves handling and sometimes traction, while reducing rotating weight improves acceleration.