Updated: Oct 16, 2021
We often hear that lightweight wheels are preferable. Theoretically, reducing unsprung mass may aid in a vehicles handling.
Reducing rotational mass should also result in quicker acceleration and stopping of the vehicle.
But what is the truth to this speculation? Let's see.
Table of Contents:
What Are Lightweight Wheels?
Lightweight wheels (also known as lightweight alloys or lightweight rims) are a wheel that has been designed to be lighter than stock wheels by the usage of lighter materials and optimised designs.
Lighter wheels are usually made from an aluminium or magnesium alloy.
Some very expensive hyper-cars such as the Koenigsegg have carbon fibre wheels.
What Is Unsprung & Rotational Mass?
In a vehicle, the unsprung mass is the mass of the moving suspension components, wheels or tyres, as well as any other components that are directly linked to them.
Unsprung mass is constantly being moved by the suspension, lighter unsprung mass allows the suspension system to work more effectively.
The term rotational mass (rotational inertia) refers to the components of a vehicle that must be accelerated or decelerated as the vehicle's speed changes.
Items that spin, such as the driveshaft, brake rotors, wheels, and tyres, are included in this category.
Accelerating rotating mass is about three times more difficult than accelerating sprung weight.
Sprung mass is the mass of the vehicle supported by the suspension, which consists of the body as well as any additional components contained inside or connected to it.
Below is a list of common components that affect the unsprung mass.
Wheels & Tyres
Shock Absorbers & Springs
How to Reduce Unsprung & Rotational Mass?
Reducing the weight of any component which isn't supported by the springs will reduce unsprung mass.
The major parts are the wheels, tyres and brakes.
An easy to reduce unsprung mass is to make the wheels lighter by reducing their size, either width or diameter, providing the car is designed for that size of a wheel, or installing lightweight wheels.
However, changing the size of a wheel will also alter handling, gear ratios and characteristics of the vehicle, you will likely need to recalibrate the speedometer too.
Remember, if you're looking at changing your wheels, installing wider wheels and tyres will increase unsprung mass.
Read this article to learn more about the advantages and disadvantages of fitting wider wheels and tyres.
Changing to lightweight brake callipers or rotors will also reduce unsprung mass.
Another cheap and easy way is to use lightweight aluminium lug nuts and bolts.
What Materials Are Wheels Made Of?
There are many different materials used in the construction of wheels, most wheels are a mixture of various materials, as listed below in order of heaviest to lightest.
Wheels are also made using various techniques such as gravity casting, high and low pressure die casting and forging.
Each method of manufacture have specific benefits and drawbacks; however, pressure die casting or forging is the best option for strength and reliability.
Steel is the most common material if a car doesn't use alloy wheels.
It is often used on older vehicles and is very cheap to produce, spare wheels are often made from steel.
The density of steel varies based on the alloying constituents but usually ranges between 7.75 and 8.05 g/cm³.
Titanium is almost never used for wheels on its own, but it is sometimes used within other alloys or as concept wheels.
The density of titanium is 4.50 g/cm³, around 40% less dense than steel.
Aluminium is the most common material used for wheels on modern cars.
Almost all modern alloy wheels have a component of aluminium in them.
Aluminium has a density of 2.70 g/cm³, which is approximately one-third that of steel (66% less dense) and a much lower density than most frequently encountered metals.
Carbon fibre wheels are more commonplace on hyper-cars and very expensive cars.
Although the density of carbon fibre is very similar to magnesium, it is slighter denser still.
The density of carbon fibre is around 1.75 to 1.93 g/cm³.
Magnesium alloy wheels are commonly used in high-performance cars.
Lightweight wheels are usually a magnesium and aluminium alloy.
The density of magnesium is around 1.74 g/cm³ which is around 78% less dense than steel.
Strength vs Density
The least dense material used in wheels is magnesium, followed closely by carbon fibre.
You're probably thinking; "why use carbon fibre wheels if magnesium is lighter?".
Actually, there's more to it than just density, strength of the material also needs to come into the equation.
While magnesium is technically lighter and less dense than carbon fibre, it is not as strong as carbon fibre.
This increased strength compared to magnesium means you can use less material while maintaining the same strength, creating an overall lighter wheel.
Benefits of a Lighter Wheel
The benefits of a lightweight wheel mostly stem from its reduction on unsprung and rotational mass.
The benefits of a lighter wheel are listed below.
Rotational mass has a huge affect on acceleration, as mass that spins is around 3x harder to get moving than unsprung mass.
Because of that, reducing rotational mass via fitting lighter wheels will help drastically in improving a vehicles acceleration.
The small reduction of the vehicles overall mass also has a slight impact too.
Unsprung mass is around 2x harder to move than sprung mass.
Therefore, reducing unsprung mass has more of an effect on the effectiveness of the suspension than reducing sprung mass.
It can help improve the suspensions' ability to track an uneven road, absorb bumps and abnormalities better and therefore improve cornering and track times.
The same way rotational mass affects acceleration, it also affects deceleration.
Bringing a spinning object to a stop is harder than something that's not spinning.
Therefore, fitting lightweight wheels will improve braking distances and times.
Drawbacks of a Lighter Wheel
A lightweight wheel has minimal drawbacks, there are only a few.
One disadvantage is the decreased strength of lightweight wheels compared to steel or stronger alloys of wheels.
This could lead to cracks and failures of certain lightweight wheels when going over bumps, potholes or uneven terrain.
Magnesium alloy wheels are also less reliable in terms or corrosion than other alloys such as aluminium.
Lightweight wheels are also expensive.
Should You Get Lighter Wheels?
If you're after the very best and are using your vehicle for racing then lightweight wheels may be a good option to consider.
But they are expensive and there are better options for improving performance for the same price, such as coilovers or lowering springs.
If you're already after some new wheels, it may benefit you to pay slightly extra for some magnesium alloy wheels compared to aluminium alloy wheels.
Overall, lightweight wheels are good to have but they have a small percentage of impact for improving handling and track times.