Why are cars so heavy?
Have you ever wondered why are cars so heavy? it seems like each year that goes by cars seem to get bigger and heavier! The average weight of a recently purchased vehicle in the United States reached 4,329 pounds in 2022. This figure exceeds the 1980 average by over 1,000 pounds and reflects an increase of approximately 175 pounds within the last three years alone. In essence, more than a third of the average weight of an American car has been added in the past four decades, a trend further intensified by the shift to electric vehicles (BEVs).
Conventional automobiles like the Honda Accord, Toyota Camry, and Ford Taurus have experienced substantial weight increases, ranging from several hundred to as much as 650 pounds, over the past two decades. Similarly, sport sedans including the BMW 3 Series, Mercedes-Benz C-Class, and Audi A4 have undergone comparable weight gains, reaching up to 550 pounds in certain cases. Even the relatively compact Toyota Corolla and Honda Civic, known for their lighter builds, have seen significant increases in curb weights. This trend is attributed to consumer preferences for safety, comfort, and convenience features, shaping the evolving landscape of vehicle specifications in recent decades.
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 referred to as 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.
When did cars start becoming heavier?
Mild steel was able to provide a perfect balance of strength, formability, cost, and design flexibility that the automotive industry needed during these early years. It was not until after the gas shortages of the 1970s when the fuel economy standards were adopted in North America, that automakers began to look at increasing the efficiency of their vehicles [4]. However, this trend would not continue for long.
The surge in vehicle weight gain initiated in the 1980s is partly attributed to the introduction of new safety regulations. The inclusion of airbags, emphasis on crash-test ratings, and the adoption of sturdier body-in-white structures contributed to the increase in the weight of vehicles. Simultaneously, advancements in construction techniques and the utilization of stronger materials diminished engineers’ concerns about weight, leading to a shift where efficiency became less of a primary focus. This lack of focus on vehicle mass is not good for the environment, it’s not good for resources.
Enforced Safety Features Contribute to Increased Mass
In compliance with federal regulations, newer vehicles must incorporate a range of safety-focused technologies (such as anti-lock brakes, stability control, and tire-pressure-monitoring systems) and equipment (including airbags, laminated glass, and door intrusion beams). In addition every few years the federal government raises the bar for automakers to meet crash safety standards.
Although these safety measures are crucial, they inherently introduce additional weight to the vehicle. This may involve the integration of new systems or components and the reinforcement of existing parts to enhance the resistance to body structure deformation during a collision.
Added Weight Due to Comfort and Convenience Features
While convenience features are not a recent addition to automobiles, modern vehicles, including entry-level models, are equipped with an abundance of features aimed at enhancing driving ease and convenience. Today’s consumers expect a full array of power and luxury amenities (such as heated/cooled seats, sound deadening, and rear climate control) along with the latest technologies (Bluetooth connectivity, infotainment systems, real-time navigation). Once again, the compromise for increased convenience is the added weight—sometimes quite substantial.
Why is the mass of a car important?
Attributing to Sir Isaac Newton’s first law of inertia (“an object at rest tends to stay at rest”), moving a heavier object (a vehicle) necessitates more power. Regardless of the engine type or fuel efficiency, the drivetrains need to work harder and consume more fuel when accelerating all this additional mass. This underscores the common understanding among sports car enthusiasts that weight poses a significant obstacle to performance.
A bigger and heavier car is better for vehicle crashworthiness, however, it is worse for vehicle fuel efficiency. For that reason, automotive engineers are constantly balancing vehicle crash performance and overall vehicle mass/efficiency.
How the Added Weight Incurs Costs
The additional costs associated with increased weight extend beyond the fuel pump or charging station, resulting in heightened fuel expenses. As articulated by Newton’s law (“an object in motion tends to stay in motion”), a heavier vehicle necessitates larger brakes, bigger tires, and a more robust suspension system— that ironically contribute to additional weight. Encumbered by this extra mass, these elements are prone to quicker wear and tear compared to their counterparts in lighter vehicles. For manufacturers, who must pass on heightened production expenses to consumers, oversized parts incur greater costs than their smaller equivalents.
To counterbalance the augmented mass, automakers have been compelled to enhance drivetrain power. Fortunately, advancements in engine technology have managed to offset the need for progressively larger engine displacements. For instance, a 1990 Toyota Corolla with a 1.6-liter engine and automatic transmission achieved an EPA mileage of 22 mpg city/30 mpg highway, while its 2010 counterpart, equipped with a significantly larger and more powerful 2.4-liter engine, maintains identical fuel economy. However, it prompts contemplation about the potential savings if every vehicle on the road could miraculously shed 500 pounds.
How do you cut weight on a car?
The United States has some of the most stringent crash safety requirements as a result auto manufacturers are always looking at ways to reduce the overall mass of a vehicle without jeopardizing the structural integrity or vehicle performance. A few common methods are used to reduce the vehicle’s overall mass. First would be looking at different materials, for example changing from a steel wheel to an aluminum, magnesium, or carbon fiber wheel, would enable lighter wheels. Thus helping reduce the mass of the chassis. When it comes to the vehicle structure, car manufacturers usually look at changing the part geometry of the component, for example, adding lightning holes. However, this may lead to an increase in manufacturing, part, or labor costs.
These options might be too expensive for a typical passenger vehicle, however, for a racing car, it might not be out of the question. Each gram of a vehicle is worth a specific amount to the manufacturer, for example, a vehicle program such as a Toyota Corolla might not be willing to pay more than $1-2/ gram of weight saving thus reducing the number of lightweight options that it can incorporate. However, a racecar engineering program team might be willing to pay 10+/gram of weight savings to enable a lightweight racing performance.
How much energy is required to move a car?
In a conventional ICE vehicle, less than 30% of the energy that is contained inside of the gas is converted into kinetic energy which moves the vehicle forward. The vast majority of the energy is lost due to inefficiencies in the powertrain/driveline primarily in the form of sound and heat energy.
The energy that makes it to the tires and wheel must still overcome various resistance forces acting against it. This is the reason the mass of a vehicle is important because the mass affects all but the aerodynamic forces that a vehicle must overcome. The power required to drive the vehicle can be represented by the following equations:
Why is reducing weight in vehicles important?
“Apart from the aerodynamic resistance, the power required to overcome all the driving resistances is directly proportional to the mass of the vehicle” [5]. 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 [4]. By reducing the mass automakers can build more efficient vehicles and thus make it easier for them to meet fuel economy standards.
How to make a car structure lightweight?
Up until recently, the high cost associated with implementing lightweight materials reduced the large-scale adoption of these materials into the automotive industry. The average customer was not willing to pay a premium for weight reduction in their automobiles. 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. [2]. The three most mainstream solutions for enabling lightweight BIW are:
- 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
Are lightweight cars better?
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 [2].
Generally a lightweight car results in better fuel economy/range but worse crash performance. On average enabling a weight reduction of 10% in vehicle mass would improve the fuel economy or range by 6-8%. As a result, automotive engineers need to know what structural members can be down-gauged to reduce mass and which can not. Below is a typical breakdown of the function of various elements in a typical automotive structure design.
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 [1]. 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 [1].
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 [6].
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 [3].
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. [7] 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 that could arise is high carbon steel and low-carbon steel have different melting temperatures which could complicate the welding process. Depending on the alloy that is used, the material cost of the substituted metal could also significantly increase the cost.
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 |
References:
- Deb, Chapter 11 – Crashworthiness design issues for lightweight vehicles, Editor(s): P.K. Mallick, In Woodhead Publishing in Materials, Materials, Design, and Manufacturing for Lightweight Vehicles (Second Edition),Woodhead Publishing, 2021, Pages 433-470, ISBN 9780128187128, https://doi.org/10.1016/B978-0-12-818712-8.00011-2.
- Cischino, Elena, Et all. An Advanced Technological Lightweight Solution for a Body in White, Transportation Research Procedia, Volume 14, 2016, Pages 1021-1030, ISSN 2352-1465, https://doi.org/10.1016/j.trpro.2016.05.082.
- Fang, Xiangfan and Zhang Fan, Hybrid joining of a modular multi-material body-in-white structure, Journal of Materials Processing Technology, Volume 275, 2020,116351, ISSN 0924-0136, https://doi.org/10.1016/j.jmatprotec.2019.116351.
- Horvath C.D., Chapter 2 – Advanced steels for lightweight automotive structures, Editor(s): P.K. Mallick, In Woodhead Publishing in Materials, Materials, Design, and Manufacturing for Lightweight Vehicles (Second Edition), Woodhead Publishing, 2021, Pages 39-95, ISBN 9780128187128, https://doi.org/10.1016/B978-0-12-818712-8.00002-1.
- Kumar, Kiran Et all. State of the Art on Automotive Lightweight Body-in-White Design, Materials Today: Proceedings, Volume 5, Issue 10, Part 1,2018, Pages 20966-20971, ISSN 2214-7853, https://doi.org/10.1016/j.matpr.2018.06.486.
- Mallick, P.K. Chapter 8 – Joining for lightweight vehicles, Editor(s): P.K. Mallick, In Woodhead Publishing in Materials, Materials, Design, and Manufacturing for Lightweight Vehicles (Second Edition), Woodhead Publishing, 2021, Pages 321-371, ISBN 9780128187128, https://doi.org/10.1016/B978-0-12-818712-8.00008-2
- M.R. Bambach, Fibre composite strengthening of thin steel passenger vehicle roof structures, Thin-Walled Structures, Volume 74, 2014, Pages 1-11, ISSN 0263-8231, https://doi.org/10.1016/j.tws.2013.09.018. (http://www.sciencedirect.com/science/article/pii/S0263823113002401)
