BMW 6 Series Gran Coupe: the long and short of it

It’s easy to get hung up on details. Take the BMW 6 Series Gran Coupe. A longer wheelbase than the Coupe? Why, obviously that means it’s less agile.

Simple engineering principals at work there, see: the further the wheels are from the centre of gravity, the more languid the car will be in response.

Yup, because everything in life is so black and white.

I put this to Jos van Aas, BMW Group driving dynamics director (cool job title). There was no marketing-led hyperbole in his answer, just words based in, well, simple engineering principals. The details of which may surprise you.

Yes, the 6er Gran Coupe has a longer wheelbase. However, because the extra length is all added at the rear, it enjoys a better, more neutral weight balance than the standard 6 Series. Therefore, the longer wheelbase actually makes it, from simple engineering principals, a better car.

BMW is a company that takes great pride in the 50:50 weight distribution of its cars, citing it as a key brand definer. As the Gran Coupe is closer to this ideal than the Coupe, then, it is thus arguably more of a BMW than the Coupe itself.

But that’s not all. By we journos sweating on the details about length (as if that matters), we overlook another major one. The Gran Coupe chassis might be marginally less agile than the Coupe, but the difference is minute – of much more significance is the variation in engine weight between a six-cylinder and an eight-cylinder model.

The difference in weight is huge, said van Aas (on paper, the V8 is 115kg heavier than the straight six petrol, presumably mainly in its engine). What’s more, this extra weight is also centered on the front axle, far from the car’s centre of gravity.

“The difference in agility between a six-cylinder and an eight cylinder car is far, far greater than any difference between Gran Coupe and Coupe.

“The effect on the moment of inertia is much more appreciable here.”

Besides, there are benefits to the longer wheelbase, he adds. It is inherently more stable at speed, and the additional neutrality also pays dividends in terms of handling confidence. And how much extra does added confidence give, compared to some infintesimal improvements in turn-in response?

The combination of longer wheelbase and lighter engine thus makes the 640i Gran Coupe perhaps the purest, best-driving, most finely balanced 6er of all, then. Better, even, than the forthcoming new V8 M6 coop? We shall see…

Fag packet analysis suggests that the Gran Coupe will be compromised for drivers by the fact its wheelbase is longer. Only detail analysis with insight reveals the true picture though…

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