BMW 6 Series Convertible
V8 engines and head-up displays will be more important to some consumers than to others but all of them want style, comfort and refinement – and these were the main targets for BMW’s latest big soft-top
- Published in Vehicle Development.
Cruising ability is essential but the car is expected to be just as good in the corners too
When your target customers can afford more than €83,000 on what might be their second or third vehicle you can assume that their expectations are already pretty high.
When your product is a luxury 2+2 convertible the challenge is that little bit greater: with the roof up it must be as quiet as a sedan; roof down you must have room in the trunk for two sets of clubs because many of your customers play golf.
Benchmarks for the second-generation BMW 6 Series involved a number of vehicles but project director Frank Ochmann stresses that only certain attributes or features of each were of interest – targets weren’t set from any one model since none is considered a direct rival. “We looked at the Jaguar XK, Maserati Grancabrio, Mercedes SL and the Porsche 911 but they’re not direct competitors,” he says. “Each has its own strengths. We compare aspects, but not the whole concept – we have to go our way.”
The starting point was the architecture BMW designed to maximise volume of scale in its large cars, first used in the 5 and 7 Series sedans.
Complex modules such as the double wishbone front suspension are shared, together with much of the electronic architecture and some of the body-in-white.
At 2,855mm, the wheelbase is 75mm longer than the old model’s but is 133mm shorter than the 5 Series; keeping them exactly the same would have benefited manufacturing and production but aesthetics were considered more important.
“The intention was to have perfect proportions – when you view the car from the side it shouldn’t look too long, but we went to the limit concerning space for the rear passengers and in the trunk,” says Ochmann. “This is very important – if customers go to the golf course and take a friend they want room for two golf bags. This is a real advantage compared to some competitors.”
The body had to be stiff too: consumers will not tolerate scuttle shake, vibrations or noise caused by a lack of stiffness. Local torsional stiffnesses are up to 50% greater than the outgoing model’s. Much of this is due to better design and materials for the sills and roof compartment, plus additional reinforcement behind the two rear seats.
Most of the optimisation was done digitally but there were still some surprises when the first prototypes were tested at BMW’s proving ground at Miramas in the south of France.
“We took some stiffening bars which we could add to the vehicles,” says Ochmann. “But we found that the cars were so stiff already that if we fitted these bars the ride would become too harsh. We got the balance right.”
How a convertible rides and handles is important, but so are aesthetics, especially with the hood up. The original model had a fabric roof – the concept was retained, rather than going for a folding metal hard top.
The trade-off between weight, stiffness and cost is complex but Ochmann says that there were three main reasons for staying with fabric: existing customers like it, it enabled the stylists to create a coupe-like profile, and packaging was better.
The roof system was developed with supplier Valmet. Optimising its design, and the vehicle’s sealing system, was done mainly in the virtual world. But not all aeroacoustic effects can be fully modelled, and the effects of any error states can be hard to predict so physical testing plays an essential role.
