Why Is Unsprung Weight So Important?

Manufacturers, circuit racing enthusiasts, and Mazda MX-5 owners love to tell us about the benefits of minimizing unsprung weight and how that makes a car superior, but what are they on about?

Technically, unsprung weight is a misnomer and a colloquialism, as physics buffs everywhere will quickly point out. Weight only changes depending on gravitational force, but mass is always constant wherever in the universe it exists. Hence, we should be talking about unsprung mass to be scientifically accurate, but unsprung weight is a more common term. Moreover, Earth's gravitational forces are not in flux, and since we doubt anyone is planning to follow a Tesla to space, we'll stick with the latter term for the most part.

In order to understand why unsprung weight or mass matters, we have to know what it is. To do that properly, we will also discuss semi-sprung weight and sprung weight.

Unsprung mass refers to anything on a land vehicle that is not held up by the suspension of a vehicle. Basically, it's anything directly connected to the rotational assemblies, moving up and down as the wheels do. This includes the wheels and tires themselves, outboard brake calipers, discs and pads, wheel bearings, hubs, hub motors, and even axles. If it's part of the wheel assembly and below the springs and shocks, there's a good chance it's part of the unsprung mass.

Even tracks, as seen on Volkswagen's crazy quad-axle Type 2, form part of the unsprung weight.

Sometimes, the differential can be unsprung, although it can also be integrated into the chassis, thus making it a part of the car's sprung weight. More on that later.

Simply put, anything that is part of the wheelhouse's rotating assembly is unsprung weight, but not all unsprung weight is necessarily part of the rotating assembly (the brake calipers, for example, do not rotate but do move up and down with the wheel).

Semi-sprung, partly sprung, or hybrid weight are all terms used to refer to the items that are directly connected to both the unsprung and sprung elements of a car. For example, the springs, the control arms, the half-shafts that connect the transmission to the wheels (also known as sideshafts or drive axles), the shock absorbers and struts, and some steering components can be classified as semi-sprung components because the sides of these parts connected to the unsprung elements will move in tandem with the wheel, while the sides connected to the chassis or other sprung parts will stay relatively static.

A good example of this is a control arm, which moves on the outboard side where it's connected to the wheel as this rises and falls, while its inner side remains in the same place relative to the chassis.

If a component's outboard side moves along the vertical plane during suspension compression independently of the inboard side connected to the chassis, it's semi-sprung.

Simply put, sprung mass refers to everything that is wholly supported by the car's suspension. Examples include the body itself, the chassis or frame, the engine, the gearbox, and so on.

These are the largest and heaviest individual components, and sprung weight is, therefore, always greater than unsprung or semi-sprung weight.

The term "unsprung mass" was coined by Albert Healey, a mathematician who worked for the Dunlop tire company. His lecture titled The Tyre as a Part of the Suspension System was made in November 1924, and ever since, engineers have been working to improve ride comfort and handling through the weight reduction of specific components.

So how does the relationship between the weight of one group of components in relation to another translate to benefits in the way the car handles?

The heavier something is, the harder it is to change its direction while moving, especially when it's rotating.

Why does this matter in the context of unsprung mass? Because everything unsprung on a car needs to move relative to the road underneath you. When you drive over a bump, the wheel assembly needs to move up and down to absorb the impact, and the lighter this entire assembly is, the quicker it will move and the easier the suspension will cope with. If the wheel assembly can rapidly adjust to a changing road surface, then less deflection gets transferred to the chassis, meaning the car as a whole will be less unsettled by bumps.

Yes, reducing the unsprung mass of a car will effectively make the car ride more comfortably.

There are other benefits, too, like being able to change direction quickly. A lighter wheel assembly on the front end will be able to turn quicker, meaning sharper turn-in. Similarly, the lighter a wheel assembly's rotating mass, the less work the brakes have to do to slow it down, and the easier it is to accelerate.

Something as simple as unsprung mass impacts not only ride and handling but even accelerating and braking ability.

So does that mean that every car should have the lightest possible unsprung components? Yes and no; it's all relative. The benefits of unsprung mass are best expressed as a percentage of the overall mass. For example, the 23-inch wheels on the latest Range Rover would have a horrible effect on the ride and handling of the Miata we mentioned at the outset, making it lethargic and slow to react to direction changes. Yet the same wheels have a far less significant impact on the luxury SUV.

A 20-pound wheel on a Miata would be 1% of its weight, while a, let's say, 40-pound wheel on a 5,000-lb Range Rover is less than 1% of its weight. Therefore, the RR has better unsprung weight values proportionally, and that's why the 23-inch wheel is not uncomfortable, as JLR boss Nick Collins (then chief engineer) explained to Road & Track in 2021.

Similarly, the Miata's lighter wheels, tires, and suspension (assuming they could be swapped) would be too light for the heavy RR and would make its handling twitchy. Things need to be correlated so they work harmoniously.

Without getting into too much detail, weight placement within the scope of unsprung mass can also have an important role. The closer you can keep the bulk of the mass to the center of rotation, the easier it will be to control, which is why a wide wheel and tire combination can have an adverse effect compared to a narrow setup, as there's more weight located farther away from the center of rotation. But rotational mass is, perhaps, a topic best dealt with in detail some other time.

The answer is lighter unsprung components, such as brake calipers, wheels, control arms, et cetera, but the easiest of these to change is the wheels. A set of lightweight alloy wheels can be bought off-the-shelf and create a noticeable difference immediately. And don't even get us started on carbon fiber wheels, which are lighter still.

Braking components can easily reduce weight in a number of areas. Carbon-ceramic discs are much lighter than standard steel items, and aluminum calipers are way lighter than the mass-produced iron items used on most road cars.

But, before you go and get lightweight everything, remember that all elements of a car's suspension are intertwined. Your car's stock suspension is developed to handle the mass of the car, both sprung and unsprung, and making significant changes to either would require you to adjust spring and damper rates too. You should do a lot of research before altering anything, and make sure you're aware of any knock-on effects you could have from reducing weight in the wheelhouses.