The weighting game: How vehicle mass reduction has evolved

Following the global fuel crisis in the 1970s, vehicle mass reduction became a critical focus area as the automotive industry looked to improve fuel efficiency. Since then, increasingly stringent emission and safety standards, EV range anxiety and driving performance demands have ensured it remains a hot topic!

Reasons for vehicle mass reduction

Political events in the Middle East led to the 1973 fuel crisis followed by the energy crisis of 1979, both of which saw limitations on fuel supplies and record high fuel prices. These events precipitated several developments by motor manufacturers, all aimed at reducing fuel consumption – engine downsizing, aerodynamic drag reductions, and most importantly, mass reduction.

Lighter vehicles require less energy to propel, consequently reducing fuel consumption and associated operating costs. Given the rising costs of fossil fuels, fuel efficiency soon became a key selling point for automakers.

At the same time, the US Clean Air Act and similar vehicle emission standards in other key markets compelled automakers to seek innovative ways to reduce vehicle weight. Consuming less fuel, lighter vehicles emit fewer pollutants, minimising their environmental impact and aiding regulatory compliance.

Compounding the challenge of reducing vehicle mass are two recent market developments – consumer preferences moving from traditional sedan and hatchback body shapes to SUVs, and the move to electrification.

By design, SUVs are heavier than their sedan equivalents, making weight reduction a necessity to achieve competitive fuel consumption and emissions standards compliance.

With EVs, the relationship between battery size and vehicle range is critical as manufacturers balance the weight and cost of batteries with consumer demands for acceptable driving range. Lighter EVs require less energy to propel, allowing for longer driving distances on a single charge and thereby addressing one of the primary concerns of potential EV buyers – range anxiety.

Benefits of vehicle mass reduction

To better understand the importance of vehicle mass reduction in reducing fuel consumption and emissions, a basic understanding of the physics involved is necessary.

Newton’s Laws of Motion state that the acceleration of an object is dependent upon two variables – the net force acting upon the object, and its mass. Applied to motor vehicles, acceleration is increased if the force acting on the vehicle is increased. However, if the mass of the vehicle is increased, acceleration decreases. Translated into real-world driving conditions, the greater the mass of a vehicle, the greater the tractive force required to achieve the same level of acceleration as a lighter vehicle.

With tractive force (the force developed at the contact patches of the driven wheels) being dependent on engine output and gearing, it follows that heavier vehicles will need to be “driven harder” or require higher engine outputs to perform at the same level as lighter vehicles. Assuming similar levels of engine and drivetrain efficiency, this means increased fuel consumption and emissions.

Of course, physics does not only apply to vehicle acceleration but also to deceleration, change of direction, and the forces sustained in an impact. A heavier vehicle will require more braking force to achieve the same deceleration as a lighter one.

While vehicle brakes are designed to cope with this, it does mean braking closer to the limits of traction, a factor in low-grip conditions. Similarly, more force is required to change the direction of the vehicle, making it less responsive to steering inputs and less agile. Contrary to popular belief, mass reduction can also enhance safety. A lighter vehicle carrying less momentum will generally experience lower forces during the impact.

In motor racing, running a car as close to the minimum weight limit has been long accepted as key to improving lap times as reduced weight means quicker acceleration, reduced braking distances, and more agile handling characteristics.

In high-performance road cars this also aligns with consumer preferences for a sportier and more responsive driving experience. Weight distribution also affects vehicle handling and mass reduction can be applied selectively to improve weight distribution for improved responsiveness and handling.

How vehicle mass reduction is being achieved

Vehicle mass reduction is being achieved in many key areas including materials, optimised design, and advanced manufacturing techniques.

Material substitution – where manufacturers replace traditional steel components with lighter alternatives such as aluminium, magnesium, and composite materials – has been taking place for years. Correctly utilised, these materials offer comparable strength and structural integrity at a fraction of the weight, enabling significant mass reduction without compromising safety or performance.

Some of the more common examples where components – broadly categorised as plastics – are now commonly used are vehicle bumpers, wheel arch liners, certain body panels, interior trim, and fuel tanks. Under-bonnet use of plastics has also expanded to include engine covers, splash shields, cooling fan shrouds, air-cleaner boxes, inlet ducting, and, in some cases, even inlet manifolds.

Aluminium is now commonly used for road wheels, engine blocks, cylinder heads, oil sumps, and other engine and suspension components. Radiators, previously made from copper and brass, now typically feature an aluminium core with plastic tanks.

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High-strength steels are increasingly being used in key structural areas where their increased strength allows a reduction in material thickness and weight. While cost and time-consuming production processes limit the use of carbon fibre-reinforced composites in mass-produced vehicles, they are now widely used in low-volume, high-performance vehicles where their combination of strength and low weight provides significant benefits.

The incorporation of these materials has been supported by innovations in manufacturing processes, offering increased flexibility in design and assembly, and incorporating combinations of aluminium, steel, and composites, reducing weight while minimising production costs.

Mass reduction – innovative approaches

Some manufacturers have been implementing particularly innovative approaches to achieve further improvements.

Introduced in 2013, the BMW i3 was the first mass-produced car with a passenger compartment and body made of carbon-fibre reinforced plastic (CFRP), while door panels were made of hemp mixed with plastic for further weight reduction.

Assembled using adhesives only, it was mounted onto an aluminium drive module which incorporated the powertrain, frame, battery, and suspension. Its kerb weight ranged from just 1 195 to 1 290 kg depending on battery specification, an excellent achievement for a four-seater BEV and comparable to similarly sized ICE vehicles.

Shortly thereafter, BMW introduced a new 7 Series model featuring a multi-material construction combining steel, aluminium, and carbon fibre-reinforced plastic (CFRP) – known as “Carbon Core”.

Aluminium is extensively used in the chassis and body panels, with high-strength steel in key areas to increase rigidity and crash protection. CFRP was used in critical structural components such as the roof panel, transmission tunnel, and various reinforcement elements to further reduce mass while enhancing structural stiffness and crashworthiness.

The result was a vehicle 130 kg lighter than its predecessor, with reduced fuel consumption and emissions and improved handling and agility. For the current iX BEV, BMW built upon this approach, using CFRP components for the side frame, rain channels, roof frame, cowl panel and rear window frame to create a light and exceptionally strong “Carbon Cage” passenger cell.

Tesla has been particularly innovative when it comes to weight- and cost-reducing manufacturing processes, including commissioning the world’s largest high-speed die-casting machines dubbed Giga Press.

Measuring 19.5 m × 5.9 m × 5.3 m and weighing 430 tonnes, these machines allow for the casting of the Model Y rear underfloor in a single piece, replacing the previous assembly of 70 different metal parts. In addition to weight saving, it also reduces production time and improves consistency. Tesla has plans to ultimately extend this approach to casting a complete one-piece underbody.

SA-born designer Gordon Murray utilised a different approach to weight saving with the GMA T.50 supercar. In addition to the more obvious use of carbon fibre for the monocoque and body panels, thinner glass, and forged aluminium suspension components, Murray benchmarked every single component for the lowest possible weight.

During the early development phase, weekly ‘weight watcher’ meetings were held to ensure that no part was heavier than it needed to be. This approach was particularly effective as with three seats instead of the typical two, and a naturally aspirated V12 engine, the vehicle weighs in at just 986 kg and provides similar performance to considerably more powerful supercars weighing up to 50% more.

Efforts to reduce vehicle mass have been ongoing for many years with all vehicles currently in production benefitting from some of the innovations in materials, design, and manufacturing techniques described. With benefits extending beyond fuel efficiency to encompass improved handling and safety, extended range for electric vehicles, and reduced emissions, vehicle mass reduction will remain a key focus area and continue to drive innovation in the industry.

Find the full feature in the May issue of CAR Magazine.

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