BMW Z4 prototype 2018: first drive of new roadster

The engineering behind the new BMW Z4

To understand why the new Z4 is so different from its predecessor, consider all the major bits taken from the old one. “The only thing we carried over was the name,” says Dietrich.

Well, that didn’t take long.

The Mk3 Z4 is built on a new architecture, developed by BMW specifically for sports cars and shared with the upcoming Toyota Supra. Elements of the platform will also be used on future models, starting with the next 3 Series.

When they started work, the Z4 development team came up with a series of technical specification requests, and BMW management made Dietrich “very happy” by agreeing to all of them. As a result, the M40i has been developed as a package, starting from the ground up, with the use of the Michelin Pilot Super Sport tyres also fitted to the M3 and M4.

The pre-production cars we drove were fitted with optional 19in wheels, but that’s about the only option M40i buyers will have in terms of dynamics. So there are performance brakes, electronic adaptive dampers, adjustable suspension with MacPherson steel struts at the front and a multi-link set-up at the rear, and a limited-slip differential.

Up front, power comes from the twin-scroll turbocharged 3.0-litre six-cylinder unit also found in the M240i. Here it produces somewhere around 350bhp and 369lb ft, driven to the rear wheels through an eight-speed automatic gearbox.

The M40i is the only version of the Z4 that BMW’s engineers would talk about, but expect models badged ‘20i’ and ‘30i’ as usual – and don’t expect a full M version, or a hybrid.

A key target was to ensure the chassis was as stiff as possible, to offset the inevitable compromise that comes from not having a fixed roof. Dietrich says the torsional rigidity is more than 30% better than the previous Z4, calling it “the stiffest open car we’ve ever made. It’s stiffer than the M4 coupé”.

The team used the M240i as a closed-top reference, and Dietrich says that at every point of the M40i where forces are brought, the car is stiffer than that model – although the absence of a fixed roof means it’s slightly less stiff overall.

The new Z4 is slightly longer than the previous one, although the wheelbase is marginally shorter and it’s around 70mm wider. It’s lighter, too, the extra weight of the latest safety systems offset by reverting from the previous Z4’s folding hardtop to a soft-top, for “a pure roadster interpretation”, as Ederer puts it.

The canvas roof saves 60kg; overall, the car is around 50kg lighter and, crucially for its handling, has a lower centre of gravity. Of course, with a renewed focus on dynamics, BMW doesn’t want to shift too far back the other way, and although the Z4 we drove wore a camouflage livery, a sneak peek at an undisguised version confirmed that it’s a fine-looking roadster indeed, closely following the Z4 Concept previously shown.

Behind the wheel of the BMW Z4

That was the canvas on which the driving dynamics team had to work. The canvas on which we get to evaluate their progress is the demanding Miramas handling circuit, which features a good mix of corners, bumps and dips, as well as the winding mountain passes and faster roads in the surrounding Provençal countryside.

In late May sunshine, it’s an ideal location in which to drive a cabrio, so it’s telling for the car’s focus that much of the M40i’s dynamic development wasn’t done here but at the Nürburgring, where the weather is rather less conducive to top-down cruising.

The M40i features three main driving modes – Comfort, Sport and Sport Plus – along with a custom option. It’ll cruise happily and benignly in Comfort, soaking up even the bumpiest roads we could find with relative ease and composure. Yet push it harder and the M40i will still offer good response and sharp, precise steering when called upon.

Switch to Sport mode and things step up a gear substantially. The suspension stiffens, without becoming bone-shakingly brutal on bumpy roads, while the steering tightens, the engine becomes more responsive, each gear is held longer and the sound coming from the twin exhausts becomes more sonorous. That makes the driving experience more involved and rewarding, the M40i responding well to being pushed.

Sport Plus is, as you might expect, more focused still. Positive signs, then, especially if there’s more refining to be done.

It’s all about the fundamentals and, in this case, that started with the brief. “Everybody knew we wanted to make a sports car,” says Dietrich, “and BMW management has let us do what’s necessary to make a car like that. It’s an honour to work on cars like this.”