How Does Unsprung Weight Affect Performance?

Oftentimes when discussing the weight of a vehicle we only focus on the curb weight, or the weight of the vehicle without any passengers or cargo but including full fuel and fluids. While this simplifies things and makes it easier to compare the weights of differing vehicles, it also glosses over factors that influence how this weight affects the way these vehicles drive.

One of these main differentiators is sprung vs. unsprung weight.

What is Sprung and Unsprung Weight?

When discussing the “sprung weight” of a vehicle, it refers to the mass of the vehicle that is supported by the springs of the suspension system (hence the name sprung weight). 

So, that means that unsprung weight is any of the remaining mass not supported by the suspension system. This leaves the wheels, tires, axles, springs, wheel bearings, and a few other components as the only parts of your vehicle that are unsprung.

If unsprung weight only makes up a fraction of the curb weight, why does it matter so much? That comes from the different ways sprung and unsprung weight affect the physics of your car. Higher unsprung weight contributes more inertia to the suspension, making it more difficult for your suspension to dampen the forces from uneven road surfaces, bumps, dips, and everyday driving. Though 50 pounds of unsprung weight and 50 pounds of sprung weight sound like they should affect the vehicle near-equally, the unsprung weight has a much larger negative effect on almost all facets of the vehicle. This includes acceleration, braking, and handling, as sprung weight makes the suspension’s job more difficult in every direction. 

Rotational Mass

Another factor that ties in directly with sprung and unsprung weight is rotational mass which plays just as big of a role in how your vehicle handles. To understand rotational mass, think of a merry-go-round. When the ride is stationary you don’t feel any external forces pushing you. However, when the ride is active, you’ll feel a slight outward force. This occurs as the mass of your body gains outward G-force while rotating around a fixed point, in this case, the center of the merry-go-round. Any mass that is rotating will act heavier than it is, and this is directly affected by the distribution of weight towards the edge. Due to the spinning motion, these objects effectively weigh more than they truly do. 

For an example more relevant to cars, consider a wheel. Say your stock wheels weigh 40 pounds, while the aftermarket option you’re looking at comes in at 35 pounds. This may not sound like a large difference, only 20 pounds less across all four corners. But with rotating mass, you have to factor in the extra force required to add/remove inertia caused by the outward force of the rotating object. 

So while only technically dropping around 20 total pounds of static weight, the actual effect on the performance of the car is amplified, making it feel like you shed multiple times that amount of weight in reality. This principle is true for wheels as well as the following rotating components: driveshaft, brake rotors, clutch, and tires. 

This concept is why lighter wheels can be one of the most significant modifications you can make as the benefits of less rotating mass are spread throughout the chassis. It’s also important to consider this in conjunction with unsprung weight, as both affect components like the wheels or tires. 

How to Reduce Unsprung Weight

Wheels

Maybe the easiest modification to make to reduce your vehicle’s unsprung weight is lighter wheels. Most OEM wheels are constructed of cast alloy, often with additional weight to ensure the imperfect cast is strong enough to deal with everyday wear. Many aftermarket wheel options are made from a stronger manufacturing process, such as flow-forming or forging, removing the need for bulky designs and allowing for lighter and stronger wheels.

With wheels acting as both unsprung weight and rotational mass, shedding weight from your wheel setup can be one of the most effective modifications despite being a relatively simple replacement. 

Struts/Shocks

Another heavy component that could be used to save some weight is the strut/shocks of the suspension. The OEM versions of these are generally bulky and heavy to maintain durability, however, aftermarket performance suspension options like coilovers can help you save on unsprung weight in all four corners of the car. 

Brakes

Brake rotors are the metal discs pushed against the brake pads to create the friction needed to stop our vehicles. These are often bulky and can be replaced with lightweight aftermarket options to both reduce unsprung weight and improve cooling performance. 

Suspension Services at Alex’s Autohaus

Whether you’re looking for a performance suspension setup or just to have your vehicle maintained, the expert Euro vehicle technicians at Alex’s Autohaus are your trusted choice for European auto repair in the Salt Lake Valley. Give us a call or schedule online today!