Automotive Engineer

If you can afford a Bentley Mulsanne you can certainly afford to pay somebody to drive it for you. This being the case you’d probably never have to change a wheel yourself either. 

Which is unfortunate, in a way, because then you’d never notice that the spare sits in a well not pressed from steel but woven from carbon fibre in both omni and directional patterns to give exactly the right strength and stiffness required.

Of course, owners are more interested in the fact that the V8 develops more than 1,000Nm of torque, the smoothness of the ride, or that the interior seems as quiet at 250km/h as it is at 100km/h. But the mix of materials used to make the vehicle is fundamental to delivering those attributes. They also determine weight and emissions. The Mulsanne weighs over 2.5 tonnes and, although its 6.75-litre engine will take it to 296km/h, it pumps 393g of CO2 through its two tailpipes every kilometre.

Weight reduction would certainly help to reduce emissions but maintaining the body stiffness required is challenging. A weak shell is detrimental to NVH, and the suspension attached to it cannot perform its springing and damping functions correctly.

“From a body-in-white point of view, if you take size and mass into account – which can be calculated in a lightweight index – we are actually among the lightest designs for our stiffness,” says Dr Ulrich Eichhorn, Bentley’s board member for engineering. “But if we were to halve our body-in-white mass we would take around 200kg out of one of our heaviest cars. That would undoubtedly be a big difference.

“On a very well-equipped, highly powerful car, there are a lot of masses – that drives the requirement for high static stiffness to achieve the dynamic stiffness we want. We do not tolerate any shakes or resonances of any kind in our cars. The levels we’re aiming for are very high.”

So Bentley’s body structures are made from steel because the dominant attribute is inherent stiffness. Aluminium is much lighter – its density is around a third – but its elastic modulus is around a third too, so simply substituting one material for another doesn’t help because you’d need to use a lot more.

Optimising section design can solve that problem but the vehicle architecture has to allow for that from the outset because it affects packaging. Castings and extrusions can take up a lot of space. Luxury carmakers want that for the features – not body structures. This is why there are no all-aluminium Bentleys.

“We concluded that we wanted to use an intelligent mix of materials,” says Eichhorn. “So steel or carbon fibre where stiffness, crash strength or durability is the main target. If not fulfilling those roles, such as the fenders, then aluminium – usually superformed. Or other materials, as appropriate.”

For the Mulsanne that includes a trunk lid made from sheet moulding compound, which features integrated antennae for the infotainment systems, a pressed aluminium hood and more superformed aluminium in the doors. Superforming works well for Bentley because, although its products work to high margins, production volumes are low, especially for the top of the range Mulsanne.