Sprung weight includes anything supported by a vehicle’s suspension, whereas unsprung weight is anything not supported by the suspension. While that may sound simple enough, each type of weight has different effects on a vehicle. Understanding those effects is important any time you’re considering making modifications.
What Is Sprung Weight?
Sprung weight is the weight of anything that is carried by the car’s suspension. This includes the body, engine, transmission, computer, lights, cabin, interior components, passengers, and cargo. Sprung weight moves up and down as the vehicle travels over surfaces.
"...weight of anything that is carried by the car’s suspension."
Sprung weight includes everything supported by the suspension, such as the body, engine, transmission, and passengers.
There’s also semi-sprung weight, which includes the parts that connect sprung and unsprung components. For example, control arms and the springs themselves.
How Does Sprung Weight Affect a Vehicle?
Sprung weight has positive and negative effects on a vehicle. A higher amount of sprung weight pushes the vehicle down, granting additional traction. Too much sprung weight can make a vehicle too heavy, lowering fuel economy and negatively impacting performance. Lots of sprung weight on top of your vehicle, like a loaded roof rack, can increase body roll and make the vehicle feel ungainly.
In most vehicles, you want a good balance of sprung vs unsprung weight. Because sprung weight helps with traction, it’s an essential part of vehicle design. More traction is especially important for racing and off-roading.
However, in excess, it can be detrimental for everyday vehicles. If you’ve ever had a car loaded with people and cargo, you probably noticed the vehicle struggling to get up hills. Too much sprung weight will also result in lower MPG and handling that feels much heavier.
How Can You Reduce Sprung Weight?
Reducing sprung weight can be as easy as removing unnecessary items from your vehicle or filling it with less gas. More hardcore drivers will reduce sprung weight by “deleting” their rear seats and stripping their interior of anything unnecessary, like sound deadening and trim. Some components like crankshafts and flywheels can be swapped out as well.
It’s important to remember that sprung weight is mostly beneficial. It helps provide traction by pushing down against the suspension. For a daily driver, reducing sprung weight will hardly be noticeable unless you’re removing tons of cargo. If you’re a racer, reducing unsprung weight will usually have a greater pound-for-pound effect than reducing sprung weight.
What Is Unsprung Weight?
Unsprung weight is weight that is not supported by a vehicle’s suspension. This includes the wheels, tires, brakes, wheel hubs and bearings, solid drive axles, and differentials. If you’re trying to remember what unsprung weight is, it helps to think of your vehicle’s springs. If the springs don’t support it, the weight is unsprung.
"...weight that is not supported by a vehicle’s suspension."
Unsprung weight includes everything not supported by the suspension. Much of this weight is rotational mass, such as the wheels and brake rotors.
It’s important to remember that just because weight is unsprung doesn’t mean it "doesn’t count." Though your suspension is not supporting them, unsprung components still have to be moved by the power of the engine.
How Does Unsprung Weight Affect a Vehicle?
Unsprung weight often has a large effect on a vehicle because most (but not all) of it is rotational. Rotating mass requires more momentum to move, meaning that each additional pound has an exponential effect.
Rotational mass applies to the parts that must be accelerated or decelerated when the speed of the vehicle changes. This includes items that rotate, like the driveshaft, brake rotors, wheels, and tires. Rotating mass is roughly three times harder to accelerate than sprung weight.
This means that dropping just 10 pounds per wheel with the addition of a lighter set of racing-oriented wheels would equate to a reduction of almost 120 pounds of sprung weight. In most drag racing instances, 100 pounds lost equates to a tenth of a second or one car length in the quarter-mile. It’s a small amount on paper, but it adds up if you’re racing competitively.
How Can You Reduce Unsprung Weight?
The most popular modification to reduce unsprung weight is adding lighter wheels. Tires and brakes can also be swapped out for lighter versions. You can change out the suspension itself or your drive axles. Finally, you can opt for lighter nuts and bolts. Yes, even the few ounces you save on your wheels’ lug nuts can add up over time.
The aftermarket offers hundreds of race-focused options that are designed to be as light as possible. Just make sure you’re still getting the stopping ability, handling, and grip you want out of them.
Because of the pound-for-pound benefits of reducing rotational mass, there are a few easy wins if you want to make your car lighter. Minimizing unsprung weight will improve handling as well.
Does Sprung or Unsprung Weight Have More Impact?
Pound-for-pound, a reduction in unsprung weight will have a far greater impact on speed and handling than a reduction in sprung weight. That’s why lighter wheels, brakes, and driveshafts are popular mods on performance and race cars.
Sprung weight is mostly beneficial, due to the additional traction it provides. But most racers will regard just about any weight reduction as a good thing. Just remember that the increased benefits of reducing rotating mass make it the best place to start on your car’s weight loss journey.
Sources: What Is the Difference Between Sprung Weight and Unsprung Weight?, Autoblog | What Are the Differences Between Sprung and Unsprung Weight?, MachineDesign
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