EVO BLUEPRINT
WEIGHT WEIGHT, OR LACK OF IT, IS CRUCIAL TO A GREAT DRIVING EXPERIENCE. WE ASK THE EXPERTS FOR THEIR VIEWS ON HOW THE INDUSTRY IS TACKLING THE GROWING CHALLENGE OF KEEPING CAR WEIGHT TO A MINIMUM
Industry experts from Lotus, Prodrive and JLR's SVO division are asked about weight and how it matters in a car
HAS LEGISLATION MADE CARS HEAVIER OVER THE LAST 10-20 YEARS? IF SO, WHICH REQUIREMENTS HAVE ADDED MOST WEIGHT?
Steve Fletcher, director of body/trim/chassis and CAE (computeraided engineering) at Group Lotus: “Most notable are crash test requirements. In the last 20 years there has, quite rightly, been an uplift in global technical safety standards and a huge campaign of consumer safety testing.”
Ryan Vann, chief engineer of vehicle performance and technology at Prodrive: “Safety has contributed, particularly with increased side impact testing, roof crush loads and small overlap impact testing. Deflection of the car body structure into the occupant area needs to be minimal, so the body is physically bigger to provide the space.
“Multiple airbag systems add more weight. Compared with the 1999 original, the 2019 Focus is 200mm longer, 200mm wider and 200kg heavier but has a better safety rating. Consumers want better features too. And hybrid powertrains and driving assist systems only add weight.”
Dave Pook, director at VEDynamics and Life110, ex-JLR SVO: “Regulations for emissions control and crash. The crash part is more obvious as cars carry more airbags and structure for crash protection.
They are a lot safer than they used to be; having a 50kmph accident and being able to walk away is assumed now. But cars are heavier mainly because they are so much bigger and packed full of tech and features.”
Aluminium weighs almost three times less than steel and is often touted as the metal for lightweight construction, so why do so many production cars with an aluminium chassis or monocoque not deliver an appreciable weight saving?
“In general, I would disagree,” says Fletcher. “There is an appreciable weight difference in aluminium-construction vehicles, but it’s not twothirds. A competitive aluminium sports car monocoque is circa 200250kg, in steel this would be 300-350kg. Although aluminium is a third of the density it is also a third of the stiffness modulus; the main advantage is gained and recovered through section modulus [linked to the physical size of a beam]. For high-volume cars these section sizes can compromise other vehicle attributes such as ingress/egress and vision angles.”
The weight advantage, aluminium versus steel car bodies, is 30-40 per cent, he says, adding that this saving is then often used to increase content for the consumer rather than give a weight saving.
“You need to use the advantages of aluminium in the car’s design,” says Pook, “using castings for complex parts and take away what is effectively redundant material and strength you would have from steel where it is not needed. It is also worth noting that an aluminium body, weighing around 200kg, is only a small part of a car’s overall mass. There’s no point making it lightweight if you then just hang heavy parts from it. A quote attributed to Colin Chapman resonated with me: ‘Lightweight design is more important than material.’ The Alpine A110 is a shining example of this approach, a ground-up design that focused on making the car lightweight. The achievement of this car, with a 1100kg kerb weight, should not be understated. It’s a huge step.”
“Generally speaking, aluminium is not as strong or stiff as steel for an equivalent structural cross section,” says Vann. “However, increase the size of the cross section and its strength and stiffness match a steel one whilst saving weight. Aluminium is more expensive and can be tricky and expensive to join and so tends to suit bigger, premium cars where cross sections can be bigger and the material cost borne by the price of the vehicle.”
Several OEMs claim a 40 per cent body weight reduction, he says, but if the aluminium body of an A8 weighs 241kg, he calculates that a steel-bodied Audi A8 would be 100kg heavier. “Noteworthy,” he adds, “is that the 2018 A8 is about 200kg heavier than the 2002 A8 [both used aluminium], similar to the Ford Focus example.”
Carbonfibre is more increasingly common in production cars but mainly for addenda, yet BMW used it to make the structure of the i3 back in 2013. Why hasn’t it been more widely adopted?
“Put simply, cost,” says Fletcher. “Carbon offers excellent lightweighting opportunities through its high strength, good specific stiffness and directional nature. You can achieve something like 50 per cent weight reduction versus steel. The issue is cost and scaling. F1-style autoclave processes are very expensive and only suitable for low-volume applications, like the Lotus Evija. With non-autoclave technologies tooling costs increase dramatically but the piece price drops, making them appropriate for higher-volume applications, but the raw material costs are still very high.” There is a point where vehicle price and production volume make carbon viable, he says, but it’s narrow and very sensitive to volume changes.
“For mass production, composite-material body panels must be made in about 60 seconds or less, precluding traditional hand lay-up,” says Vann. “Shortening the manufacturing cycle time requires special tooling that combines heat and pressure, and that requires large investment by the car maker.” He says that producing the carbon substrate itself is energy-intensive, “potentially creating lots of CO2 emission if green energy is not used, which can undo the CO2 savings from using less fossil fuel to propel the vehicle over its lifetime”. Carbonfibre is also difficult to repair, he adds.
Motorsport has traditionally pioneered weightsaving techniques. Are there any currently employed that might find their way into production cars?
Tim Kearney, director of vehicle Integration at Group Lotus: “There are lots, but they need to be implemented with care. Motorsport is a
fantastic development environment but it has a singular focus: lap time. We need to adopt technologies with a more balanced attribute outlook. Our customers want cars that are quick but also quiet, efficient, comfortable and convenient. An area we follow closely from motorsport is systems integration, particularly battery integration to the vehicle structure.”
John McQuilliam, engineering director at Prodrive Composites: “One interesting area is natural fibre-based composites. For instance, the regulations for GT4 require body parts that are not on the road car – spoilers and wings, for instance – be made with natural fibres. This has demonstrated that natural fibre composites can compete with traditional carbon and glassfibre.”
“The holistic thinking and aggressive lightweight targets for motorsport are things that could transfer to road cars,” says Pook. “When a car gets heavy the problem snowballs, as components need to get stronger, it gets heavier again, and so on.”
Given that batteries for EVs typically add 200-300kg, where are the weight savings that will offset this coming from?
“In the short to medium term BEVs cannot be lighter than a conventional ICE variant for equivalent power and range,” says Vann. There will be continued gains from multi-material bodies, he says, but short-term weight savings will come from the batteries too. “Each year sees an increase in cell energy density and volumetric density, so for a given capacity the weight of battery cells reduces and the battery enclosure becomes smaller and lighter also. Medium term, a move to solid-state batteries will not only further increase energy density but may reduce the battery cooling required, reducing battery system cost, weight and size. Lastly, the ability to fast-charge batteries in ten minutes could also reduce most peoples’ requirement for a large, heavy battery.”
“Integration,” says Kearney. “Most current products are ICE vehicles adapted to EV. In the future we will see the propulsion components more fully integrated into vehicle structures, carrying load and providing primary structural reinforcement. There are also significant weight opportunities in advanced materials and architectures. We [Lotus] are leading an engineering partnership with the UK government’s support to develop a nextgeneration lightweight Battery Electric Vehicle architecture.”
“As an EV has regen and puts the energy it used to get going back into the battery as you slow down, the overall mass is much less important,” says Pook. But he also thinks that while currently there isn’t the need to take mass out of electric vehicles, this will change over time as people understand the impact of having so much material in a car, and the compromises it brings.
They will come to the realisation that they want them to drive better, he thinks. That’s a hope we all share, I expect.
WHEN A CAR GETS HEAVY THE PROBLEM SNOWBALLS