Lighter by design

Jon Lawson

With an emphasis on weight reduction, this is how one sports car manufacturer set about improving one of its best sellers. Jon Lawson reports

Launched in April, McLaren’s 750S comes with impressive official performance figures. From a standstill, 60mph can be achieved in 2.7 seconds. The power-to-weight ratio is 587PS/tonne, and its dry lightest weight is just 1,277kg - light for a supercar. In fact, it’s the lightest and most powerful series production vehicle ever made by the firm at its HQ in Woking, Surrey, in the Southeast of England.

Sandy Holford is Chief Engineer for the 750S. He says the original brief was, “to build on the success of the definitive supercar – the 720S – with sharper dynamics, updated design and increased power and performance. To consolidate our position as building the lightest car in its class with the best analogue driving experience for any supercar.”

Understandably, the project started with a long, hard look at the 720S. Holford continues: “We set attribute targets for the 750S using each area of the 720S as the baseline. Our goal was to improve across the board, with particular focus on reducing overall weight, increasing performance and driving enjoyment through enhanced driver engagement and improved driver-centric cockpit and controls.”

The team scrutinised every aspect of the model, and quickly identified what could be made lighter and what could not, allowing appropriate allocation of resources. For example, the engine itself was difficult to make substantially lighter, but improvements with the stainless steel exhaust system led to a 2.2kg weight reduction over the unit in the 720S. Holford explains, “It was a complete new muffler design for the platform with major input from our NVH team. We set out our package requirements with respect to volume and outlet position, and the engineering team optimised the design and use of materials and technologies like hydroforming to minimise the weight.”

Body work

Another area where weight has been restrained is the body. It’s a very complex shape, designed with aerodynamics in mind, but it is not heavier than the 720S. The company has vast experience with composites, having introduced its first carbon fibre chassis into Formula 1 in 1981 with the MP4/1.

For construction of the new car, Holford notes, “The body is comparable on weight to the 720S. We do not use a single process to produce all of our carbon fibre components, each component is evaluated individually to ascertain the most appropriate production process to achieve the required properties and production volume. The car utilises leading manufacturing technologies and materials within our supply base to optimise the component design within the framework of the vehicle’s air management. Within programme design reviews and utilisation of CAE tools, the team optimised the system whilst keeping to the stringent weight targets.”

It was important for McLaren to be able to offer an open-topped version, traditionally a problem for sports car manufacturers due to the amount of strength – and therefore weight – needed to be put into the floor.

Not with the 750S, however. The Spider variant has a DIN weight of 1,438kg, 30kg less than the 720S Spider. “The only weight added is for the retractable folding hard top mechanism,” observes Holford. “That is due to the strength and rigidity of the carbon fibre monocoque used in McLaren supercars, meaning that additional reinforcing is not required – just a rollover structure on the Monocage II-S used for the Spider in lieu of the overhead structure on the coupe’s Monocage II monocoque.”

This means the Spider is only 49kg heavier than the coupe version.

Everything, everywhere

The team really pushed the boundaries with their attention to detail, Holford explains: “The instrument display is a different design to the 720S and weighs 1.8kg less. The windscreen glass is thinner, saving 1.6kg.” The new carbon fibre-shelled racing seats are a combined 17.5kg lighter than the base seats in a 720S.

In fact, there are further options for seating, with the passenger seat coming with rails or fixed in place. Super-lightweight carbon fibre racing seats can be specified, these are a third lighter in comparison with the same seat shell produced using conventional carbon manufacturing techniques. Each shell weighs just 3.35kg.

The suspension is a new design, softer at the front and stiffer at the back than the outgoing model, with a new method of tuning: adjusting the accumulators in the struts. “The four damper assemblies together are 2kg lighter – the weight is saved in the spring used,” says Holford.

The wheels also received a lot of attention. Saving 13.8kg over the previous design, the new 10-spoke ultra-lightweight forged units are the lightest ever fitted as standard on a series-production McLaren. Holford explains, “The team were tasked with a design to push the boundaries of spoke and rim thickness through CAE modelling, optimising the ability to accommodate impact and stiffness targets for NVH and dynamic targets. And blending an aesthetic design at the same time.” Titanium wheel bolts are optional.

The weight targets have been met, with 30% of components new or changed, and the manufacturer has such confidence in the design that the three-year unlimited mileage warranty can be bumped up in 12- or 24-month chunks until the car’s 15th birthday.