Vehicle Lightweighting 101
In the ever-evolving automotive industry, vehicle light-weighting has emerged as a key strategy for enhancing fuel efficiency, performance, and sustainability. By reducing a vehicle’s gross vehicle weight (GVW), manufacturers can improve gas mileage, extend battery range, and enhance acceleration, braking, and handling. Additionally, lighter vehicles have greater towing and hauling capacities, as they are not burdened by unnecessary mass.
HSSs, composites (e.g., CFRP), and advanced materials (e.g., metamaterials).
This process involves using advanced materials such as aluminum, carbon fiber, magnesium alloys, high-strength steel, and polymer composites, as well as innovative techniques like material substitution, material elimination, and design optimization. As the demand for more efficient and environmentally friendly transportation grows—especially in hybrid and electric vehicles—lightweight is shaping the future of automotive engineering.
What were old car bodies made of?
Since the dawn of the 20th century, auto manufacturers have been experimenting and using sheet metal primary steel, and aluminum to construct the frames of their vehicles. However, steel quickly became the material of choice for automotive body design. These early passenger automobiles primarily used mild steels, also called low-carbon steels. Early racecars relied heavily on aluminum due to the weight savings, however, it was cost-prohibitive for large-scale implementation on passenger cars.
How much energy is required to move a car?
For an automobile to be able to propel itself forward, the powertrain must overcome various resistance forces acting against it. The power required to drive the vehicle can be represented by the following:.
P required = Prolling+ P acceleration +P gradient +P aerodynamic
Where P required is the total power required to drive a vehicle the rolling resistance a vehicle must overcome is defined as,
Prolling = (m vehicle +m payload) *g*cos (αroad gradient) *f rolling resistance *v vehicle
Where m stands for the mass of both the vehicle and the payload it is carrying, g is the gravitational acceleration of gravity, αroad gradient is the gradient of
the road, f rolling resistance is the coefficient of rolling resistance and V vehicle is the velocity at which the vehicle is traveling.
the acceleration resistance is defined as,
P acceleration = (e gear *m vehicle +m payload) *a vehicle*v vehicle
Where e gear is the inertia coefficient for the i-th gear, and a vehicle is the vehicle acceleration.
the gradient resistance is defined as,
P gradient = (m vehicle +m payload) *g*sin (αroad gradient) *v vehicle
the aerodynamic drag the vehicle must overcome is defined as,
P aerodynamic = .5ρair*Cd*A cross-sectional*(v vehicle – v wind)2 *v vehicle
Where ρ air is the density of the air, Cd is the coefficient of drag of the vehicle, A cross-sectional is the cross-sectional area of the vehicle, and v wind is the velocity of the wind. Apart from the aerodynamic resistance, the power required to overcome all the driving resistances is directly proportional to the mass of the vehicle. Since the body-in-white (BIW) is one of the largest contributors to overall vehicle mass, automakers have been looking at ways to reduce vehicle mass by adopting higher-strength materials that could withstand the required loads while lowering overall vehicle mass. The figure below is a typical unibody BIW construction
What challenges do automakers face when designing lightweight BIW structures?
Up until recently, the high cost associated with implementing lightweight BIW solutions, reduced the large-scale adoption because the average customer was not willing to pay a premium for weight reduction in their automotive. However, as the threat of global warming starts to become more prevalent, CO2 reduction and resource efficiency have become the top priority of many consumers as well as world governments. For example, the European Commission is looking to reduce CO2 emissions to 75 g/km by 2030, down from the current standard set in 2020 of 95 g/km. The three most mainstream solutions for enabling lightweight BIW are available:
- significant usage of high-strength steel (Gen 3 steel)
- a higher employ of lightweight metallic materials, such as aluminum and magnesium
- a combination of lightweight metallic materials and composites like carbon fiber for structural parts
The dichotomy automakers face is enabling a BIW design that meets the increasingly stringent performance requirements such as modal, acoustics, and crash performance while enabling an overall BIW mass reduction.
different types of aluminum alloy in 2030; (c) Honda Pilot frontal crash test (from the (IIHS)), where an extruded aluminum bumper; (d) New Mercedes AMG SL with high aluminum content in the bodyshell architecture
How does weight affect EV range?
As the United States accelerates its shift to electric vehicles (EVs), automakers have a heightened incentive to prioritize streamlining their vehicle lineups. The correlation between heavier vehicles, necessitating larger and costlier batteries, poses a financial challenge due to the combination of increased weight and reduced efficiency. Car manufacturers unable to effectively manage vehicle weight may find it challenging to remain competitive in terms of pricing and driving range.
As a result, there is a huge push to improve aerodynamic performance and reduce overall vehicle mass. on average a BEV is 33% heavier than a traditional internal combustion engine vehicle (ICE).
Unlike an ICE vehicle where the majority of the mass comes from the BIW in a BEV, the majority of the mass comes from the battery which is used to move the vehicle forward. If automakers are not careful this can quickly turn into a spiral, where the battery is enlarged to increase the vehicle range, however, this increased mass makes the vehicle less efficient. As a result, the battery needs to get larger to meet the range targets. Which results in the vehicle becoming heavier and less efficient once again.
Is aluminum a substitute for steel?
With an increasing desire for lowering overall vehicle mass to improve vehicle efficiency, vehicle structure material selection is changing from mostly mild steel to a combination of materials that include higher strength gen 3 steel, non-ferrous metals, such as aluminum, and polymer composites such as carbon fiber.
Aluminum is one-third the weight of steel, it is also three times more compliant than steel but possesses a lower tensile strength. If you have 2 identical parts one made from steel and one made from aluminum both being subject to the same loads to achieve the same performance both the modulus of elasticity (E) and moments of inertia (I) must be balanced, see Eq.2.
E steel I steel = E aluminum I aluminum
(Eq2)
However, E steel = 3 E aluminum if the two parts have identical geometry, the aluminum part will be 3 times weaker than the steel part. For this equation to be true the mass moment of inertia for the aluminum part must be 3 times greater than the steel part see Eq.3.
E steel I steel = E aluminum 3I aluminum
(Eq3)
What thickness of aluminum is equal to steel?
On average an aluminum part has to be 1.5 times thicker to achieve comparable performance to steel, however increasing the thickness of a part is usually not the best way to achieve comparable performance. The worst thing an engineer can do is just throw mass at a part thinking that the added mass will help increase load carrying capacity of the aluminum part. Fallacies such as this usually result in the aluminum part being as heavy or heavier than the steel part, thus losing any weight advantage the material provides.
To get weight savings from an aluminum part and retain the performance requirements, engineers must look at how to maximize the mass moment of inertia of the aluminum part. Proper design changes in the geometric configuration of the aluminum part allow it to retain the mass advantages the aluminum provides while achieving comparable structural performance to a traditional steel part.
Is aluminum body better than steel?
Not every part in the BIW will lend itself to be made from aluminum or another lighter-weight material. However, advances in technology are making lightweight BIWs more common in the industry. Federal regulations are only accelerating this change. Vehicles such Audi R8, Telsa Model S, and Chevrolet Corvette as made predominately from aluminum bodies however they are still very expensive and out of the reach of most consumers.
Ford shocked the world when it switched its F150 to an all-aluminum, becoming the first unitarian vehicle to be offered in all aluminum. However, these vehicles are far and few in between, the bulk of the vehicles currently made in the automotive industry still rely heavily on a traditional steel BIW. As technology continues to advance new welding techniques such as RRSW and EASW will only accelerate this change. Most automakers do not like dealing with aluminum since welding aluminum is much more challenging than working with steel, however, RRSW and EASW provide OEMs with a way to incorporate aluminum into their BIWs without needing to deal with the hassle of directly welding aluminum.
Once this technology becomes widely adopted by the automotive industry multi-material bodies will start to become more and more common. This will lead to a more structurally effective design where all the structural elements and load paths can be made from Gen 3 high-strength steel, while less critical components are made from aluminum or other lightweight materials. Thus, enabling vehicles to meet all structural requirements while still reducing overall vehicle mass.
What are the disadvantages of aluminum cars?
Auto manufacturers can also cut mass by choosing a lighter-weight, lower-density, material. For example, switching from steel to aluminum would reduce the weight of a component. However, it is crucial to be cautious with the joining techniques, since certain materials such as steel and aluminum cannot be welded together. When switching materials, it is also of utmost importance that the structural integrity of the system is not diminished. Steel is a popular option in the automotive industry because of its relatively low cost. By switching from steel to aluminum you will also increase the cost of the component you are manufacturing anywhere from 60 to 80 percent. Vehicles that incorporate aluminum into the design of the vehicle structure can also experience problems with the paint and premature corrosion.
What are the practical considerations for the selection of materials for different automobile components?
Before alternative materials such as aluminum become more mainstream, they need to prove they are comparable to steel in the following areas: structural performance, manufacturability, cost, and ease of assembly. The joining techniques used to assemble these multi-material BIWs must also be rapid, robust, and reliable. Joining is an important consideration for automotive engineering while designing a vehicle structure. This is a result of joints being commonly the weakest areas in the BIW and is often the failure initiation location.
As automakers look to transition from a mono-material BIW to a multi-material BIW, joining turns into an even more important factor. This is because the different materials might not be compatible with each other. Some of the biggest factors engineers must monitor when designing a multi-material BIW are material surface characteristics, dimensional variations, thermal expansion rate, corrosion, and assembly stresses.
According to various investigations and mechanical analyses, it is possible to design and assemble a multi-material BIW design in a cost-efficient way that also reduces vehicle weight. Structural components, such as the Rocker and B-pillar, must be designed using high or ultra-high-strength steels to ensure crash safety. However, by designing the surrounding parts out of a lighter-weight material, you can design a vehicle that is structurally sound and is lower in mass compared to a steel BIW. Thus, the joining of dissimilar materials is going to be an essential aspect of future car design and manufacturing.
Why is the use of high-strength steel desirable in a car?
The method most traditional automakers use to reduce mass is replacing a metal alloy with a higher-strength steel alloy, the increased strength of the material enables auto manufacturers to decrease the gauge (thickness) of the component. Advances in technology have led to the creation of advanced high-strength steel, high-strength low alloy steel, ultra-high-strength steel etcetera.
The benefit of incorporating these materials is they have excellent properties and considerable potential for improving a vehicle’s crash safety performance. If the main load-bearing components in the BIW are upgraded you could achieve your desired crashworthiness performance while also enabling a reduction in overall vehicle mass.
However, substituting these materials could cause weldability issues if the gauge difference between the two mating parts is too large. Another potential weldability issue is that high-carbon steel and low-carbon steel have different melting temperatures, which could complicate the welding process. Depending on the alloy used, the material cost of the substituted metal could also significantly increase the cost.
Conclusion
In conclusion, vehicle lightweighting remains a critical focus for the automotive industry as manufacturers strive to improve fuel efficiency, reduce emissions, and meet increasingly stringent regulatory and consumer demands. The transition from traditional materials like cast iron and steel to advanced lightweight alternatives—including light alloys, high-strength steels, composites, and other innovative materials—has proven to be an effective strategy for weight reduction. However, challenges such as cost, manufacturability, and recyclability continue to shape material selection and structural design.
While significant progress has been made, further collaboration between academia and industry is essential to develop more adaptable and high-performing lightweight materials, ensuring their seamless integration into future vehicle designs and large-scale production. As the push for sustainability intensifies, continuous advancements in material science and engineering will play a pivotal role in shaping the next generation of lightweight, high-efficiency automobiles.
Definitions/Abbreviations
| BIW | Body-in-white |
| BSI | Body Side Inner |
| BSO | Body side outer |
| EASW | Element Arc Spot Welding |
| FMVSS | Federal Motor Vehicle Safety Standard |
| OEM | Original Equipment Manufacture |
| RSW | Resistance Spot Welding |
| RRSW | Rivet Resistance Spot Welding |
