The main function of a car suspension is to maximize the friction between the tires and the road surface, to provide steering stability with good handling and to ensure the comfort of the passengers. Different body types of vehicles have different suspension setup requirements. Automakers spend many man hours trying to bring you the perfect blend of comfort and handling. We usually talk about the weight of the car itself, and how the car handles. But then there is the unsprung weight that not many know about.
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The suspension of the vehicle suspends the body/cabin so that you can have a comfortable journey. All the bumps, humps, and vibrations of the road are suppressed by shocks and springs. The weight of the car above the struts (spring and shock absorber) is called sprung weight whereas anything is known as unsprung weight. Unsprung weight or mass in any vehicle is the weight of all the things hanging below your strut. That includes the weight of your wheel, tyre, brake assembly, some suspension parts, lug nuts, steering and other bits and bobs. Or, in other words, everything that has to be dampened by the shock absorbers is called unsprung mass.
Unsprung weight in racing cars
Rule of the thumb is, the unsprung weight is kept as low as possible. For race cars, that is true in every aspect. The overall weight of the car is kept as light as possible. But the special focus is given to keeping its unsprung weight as low as possible. In normal cars, the durability and longevity of parts are the focus. Manufacturers make them in order to be long-lasting than anything else. But in automotive racing, that is not the primary concern of the team. Whether it is Super V8, Endurance Racing, DTM or even something as exclusive as Formula 1, the weight reduction along with the unsprung mass is the priority here.
Unsprung weight in road cars
As mentioned above, in normal passenger cars, automakers mostly focus on the longevity of the suspension parts. But that doesn’t mean unsprung mass cannot be messed with. One of the ways you can easily mess everything up is by installing massive cheap alloys. Alloy wheels are supposed to be lighter among other things. But it is a common practice here in Pakistan where we change the wheels as soon as we take delivery of our brand new car. A 14-inch steel wheel is going to be lighter than cheap 16-inch alloy wheels. So if you are thinking about upgrading your alloys and tyres, keep this aspect of the upgrade as well. You are increasing the unsprung weight and hence putting extra stress on your suspension parts including the bushings. And that extra stress means the bushing might give up sooner than expected.
Adverse affects of increasing unsprung weight
On the other hand, if you increase the unsprung mass, you are also affecting the handling of the vehicle. When going over the bumps at high speeds, it is suspensions job to bring the tyre back on the road and maintaining the contact between the rubber and the tarmac while also dampening the vibrations. If you have increased the mass, it will make the suspension work longer and harder to keep the vibrations at bay. So if you are going for bigger wheels, always opt for alloy wheels by some reputed company. Cheap Chinese alloys are heavy and can be dangerous to use.
It is better to stay stock if you are running a stock vehicle. Changing tyres and wheels to a larger size will change how the car handles and it can have adverse affects on your daily driving. But if you are upgrading, always go for high-quality aftermarket parts.