Weight affects how your vehicle handles and performs when you take it out for a spin. Whether you’re an ordinary driver, someone who uses their vehicle for work, or a racing aficionado looking to squeeze every ounce of performance, you want to keep track of your vehicle’s weight. In particular, you need to check its sprung weight and unsprung weight. Individually, sprung and unsprung weight affect different aspects of your vehicle’s road performance. If the ratio is skewed toward one, it can drastically change your driving experience, usually for the worse.
What Is Sprung Weight?
Sprung weight refers to the percentage of a vehicle’s weight that is supported by springs in its suspension. Depending on the type of suspension, air springs, Belleville springs, or coil springs could be bearing the load.
Along with shock absorbers and other suspension parts, these springs mitigate the force imparted by shocks and vibrations when your vehicle goes across uneven surfaces. They protect various components from getting shaken apart. Most importantly, they smoothen the ride for the occupants of your vehicle.
Most of a vehicle’s total weight counts as sprung weight. Below is a list of parts and systems that contribute to sprung weight:
- Chassis
- Engine
- Transmission
- Body
- Interior
- Driver and passengers
- Cargo
What Is Unsprung Weight?
Any weight that isn’t carried by the suspension springs is considered unsprung weight.
Since the parts that are considered as unsprung weight aren’t protected by the suspension springs, they receive the full brunt of the shocks and vibrations caused by driving on bad roads and uneven terrain. Getting jolted constantly can damage wheel hub motors, wireless pressure sensors, and other relatively fragile parts.
Examples of parts that are considered as unsprung weight include:
- Tires
- Wheels
- Wheel bearings and hubs
- Wheel hub motors
- Brake assemblies in most vehicles
- Axle assembly with solid drive axle/live axle
- Wheel hub motors in electric vehicles
Sprung Weight vs Unsprung Weight
Both sprung weight and unsprung have different effects on a vehicle. The amount of each can change your driving experience, and altering their ratio could change your vehicle’s handling and performance.
What Does Sprung Weight Do?
Sprung weight influences traction. The weight bears down on the wheels and tires, helping ensure they stay in contact with the ground.
However, excessive sprung weight can negatively affect your vehicle. More weight increases fuel consumption and negatively affects handling. If a significant amount of sprung weight is located near the top, the vehicle is also more likely to roll onto its side.
What Does Unsprung Weight Do?
While unsprung weight makes up a far smaller percentage of the vehicle’s total weight, it usually exerts a disproportionate amount of influence on the vehicle. That’s because most of it is also rotational mass.
Rotational mass is made up of parts that need to be accelerated or slowed down whenever the vehicle’s speed changes. It includes brake rotors, the drive shaft, wheels, and tires.
To move a pound of rotational mass, you need thrice the amount of energy to move an equivalent amount of sprung weight. That’s why many racing drivers replace the aforementioned parts with performance parts that weigh less. They can reduce the momentum required to move those parts.
Sprung-to-Unsprung Weight Ratio
Most manufacturers strive to balance the amount of sprung vs unsprung weight. They want to keep sprung weight high to improve traction, especially if it’s an SUV or truck that’s supposed to go off-roading.
At the same time, manufacturers try to minimize unsprung weight for better handling and better performance.
Any land vehicle that possesses a suspension will have both sprung and unsprung weight. In addition to automobiles and trucks, these concepts apply to suspension-equipped vehicle trailers.
Semi-Sprung Weight
If you take a closer look at the parts that make up sprung and unsprung weights, there are some items that fit both definitions. On one hand, they are connected to a component that counts as a sprung weight, such as the transmission. On the other hand, they are also attached to a wheel, which is unsprung.
These parts are considered semi-sprung weight. They move alongside the wheel, but they do not go as far as the unsprung part.
To get an idea of how a semi-sprung component works, let’s look at a half shaft. One end is connected to the transmission and the other end is attached to the wheel.
When the wheel moves up or down as it goes over a rough patch of ground, the end attached to it follows suit. The shaft’s central part will also move, but it won’t go as far as the wheel. Finally, the shaft’s opposite end doesn’t move because the suspension springs isolate the transmission from the wheel’s movement.
Below is a list of some parts that count as semi-sprung weight:
- Suspension springs
- Control arms
- Shock absorbers and struts
- Several other suspension parts
- Half shaft
- Some drive shafts
- Steering knuckle and several other steering parts
Weight Versus Mass
While many people use the word “weight” when referring to sprung and unsprung weight, others choose to use mass for greater accuracy and reliability. That’s because a solid object’s mass does not change no matter what happens while weight can change according to certain factors.
If you remember your high school physics classes, weight is a measurement of the force of gravity pulling down on an object. If the gravitational force changes, so does the weight. However, the mass of the object remains the same. Therefore, it’s more accurate to refer to it as mass rather than as weight.
