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The New BMW Z4 roadster
2003-07-15 - By Philip Byrne
Roadsters have played a key role in BMW's history, and the Z4 is set to write another chapter in the open-top story.
Since the birth of the first example in 1936, the famous BMW 328, all roadsters have shared common proportions and design principles - long bonnet; low, set back driving position; long wheelbase with short overhangs and a fabric soft top. The new BMW Z4 is no different. It shares all of those traditional principles, but takes the roadster experience full throttle into the 21st century.
Design:
 A real head turner, the Z4's principal styling feature is its bold double waistline. The upper waistline extends from the headlamp across the top edge of the bonnet, dipping through the door handle before rising to create the muscular rear wheel arch.
By contrast, the lower waistline extends under the door before rising up at the rear of the door. Joining the two waistlines is a near-45 degree crease in the front wing that displays an innovative combined side repeater and BMW roundel.
The dramatic styling and a choice of four non-metallic and six metallic colours compliment the car's proportions, offering a purposeful and visually impressive car. The long wheelbase of 2,495 mm (98.2 inches), and overhangs that are 33 mm longer at the front and 2 mm longer at the rear than the outgoing Z3, stretch the Z4 to 4,091 mm (161.1 inches), with a width of 1,781 mm (70.1 inches).
But it is not just the exterior that commands attention; the interior is a triumph of ergonomics and is designed to balance its comfortable and roomy cabin with a driving position that feels cocooned and low to the ground. There is a real 'connectedness' to the Z4.
From the driver's perspective, the dashboard combines design purity and traditional roadster practicality. The dash binnacle housing a circular speedometer, rev counter, fuel and temperature gauges dominates the view, with robust but elegant switchgear falling easily to hand.
Both the dashboard and low-slung seating position ensure the driver feels connected to the Z4, with the seat location 20 mm nearer the tarmac and 40 mm further back in the wheelbase than for a Z3 driver. The standard Sports seats envelop the occupants and are available in four different cloth and leather combinations, with an angle and reach adjustable steering column also fitted as standard.
Whilst enjoying a sports car persona, the Z4 also offers practicality, with large door pockets, a ten-litre capacity storage box between the seats and a 260-litre boot - enough for two sets of golf clubs. The capacious boot size owes much to BMW's innovative variable soft-top storage compartment. With the roof down, 20 litres of storage space is taken up by the convertible roof. When the roof is raised, a lever in the boot collapses the soft-top compartment leaving the roof storage space to be used as boot space.
The BMW Z4 also holds a world record - the fastest retracting fully automatic convertible roof in the world. Proving handy for the UK's occasional downpours, it retracts fully in less than 10 seconds with a simple push of a button and comes standard on both the 2.5i and 3.0i cars. It also features an inventive convertible roof mechanism. The roof structure is made of weight-saving magnesium and aluminium and the reinforced front panel of the fully lined fabric roof folds flat, which means no need for a tonneau cover with the added benefit of reduced wind noise in the cabin.
By the end of 2003, a hard top will also be available, featuring an integral roof spoiler. This makes the car look more coupe-like, and increases down force, improving aerodynamics.
Engine:
At launch, the Z4 features either 2.5i and 3.0i power plants carried over from the 3 Series range - the best 2.5-litre petrol engine in the world, according to this year's International Engine of the Year awards. The 2.5i (2,494 cc) roadster produces 141 kW / 192 bhp at 6,000 rpm and 245 Nm of torque at 3,500 rpm. It sprints to 62 mph in a more than respectable 7.0 seconds before topping out at 146 mph.
Its larger brother, the 3.0i, displaces 2,979 cc, producing 170 kW / 231 bhp at 5,900 rpm and 300 Nm of torque at 3,500 rpm. That's enough to blast the car from 0-62 mph in 5.9 seconds and on to a limited top speed of 155 mph.
Although the engines may be reassuringly familiar, much of the technology is brand new.
Introduced as a world first, Engine Sound System (MSS) is fitted as standard to the Z4 3.0i. BMW acoustic engineers have worked on a system that delivers enhanced engine intake noise directly from the intake manifold into the cockpit, making the driver feel more connected to the car's lightning performance.
Adding to this connectedness is the Z4's all-electronic throttle butterfly, which delivers maximum engine power spontaneously as the throttle butterfly adapts itself to any given driving situation.
The Z4 also benefits from Driving Dynamics Control (DDC), previously only seen on cars from BMW Motorsport. DDC is operated using the Sport button on the centre console and once activated, alters the throttle pedal mapping for faster accelerator reaction times, and the power steering response for a more direct and dynamic steering response. It also positively recalibrates the point of gear change if the car is fitted with the optional automatic or Sequential Manual Gearboxes.
Keeping the Z4 on the straight and narrow is BMW's Dynamic Stability Control (DSC). Activated when the car begins to lose grip and threatens to slide, it reduces power to the driving wheels at a 5 per cent loss of traction. In extreme circumstances, the system applies the brakes to get the car, and driver, out of difficulty. The Z4 also features Dynamic Traction Control (DTC), maintaining driver involvement in tricky conditions. Activated by turning DSC off, DTC provides the same fundamental reassurance as DSC, but with a greater threshold of 10 per cent loss of traction. The combination of the two gives the driver the safety and security provided by DSC with the same nimble agility of a car equipped with a conventional limited-slip differential.
Transmission:
Every driving style is catered for, as the Z4 is available with four gearbox combinations. The 2.5i roadster comes as standard with a five-speed manual gearbox, whilst the 3.0i has the new six-speed manual gearbox introduced recently on 3 Series Coupe and Convertible models.
Customers can also specify the optional five-speed automatic gearbox featuring Steptronic manual shifting, carried over from the 3 Series range, or BMW's latest six-speed Sequential Manual Gearbox (SMG). Featuring shift-by-wire technology, the driver can change gear seamlessly either using steering wheel mounted F1-style paddles or the reassuringly stubby gear lever. Gear change and clutch actuation are controlled by gearbox electronics and a hydraulic system, allowing the driver to 'flat change' in lightning gearshift times of 150 milliseconds, with absolute precision and consistency. Flattering those who haven't quite perfected the art of 'heel and toe', the system will blip the throttle to deliver perfect down changes, matching engine speed with the selected gear ratio.
The SMG system also has a full-power acceleration mode in its armoury. With DDC activated by pressing the Sport button and subsequently deactivating DSC, the engine's full power is unleashed when the revs reach 4,000 rpm and the system 'dumps' the clutch, guaranteeing the Z4's 0-62mph sprint in 5.9 seconds. For those appreciating the system's full flexibility, SMG also has a Drive mode, acting effectively as an automatic.
Chassis and Body:
The bedrock of the Z4's impressive performance and agility is its high degree of body stiffness and almost perfect 50-50 weight distribution. The new car boasts a torsional stiffness of 14,500 Nm / degree - almost three times as stiff as the outgoing Z3 (5,600 Nm / degree). This is achieved by a number of innovative techniques. For the first time on any BMW, the visible outer A-pillar panel is a load-bearing component, with a reinforced tube sandwiched between it and the inner plate. Additionally, the body shell incorporates a V strut brace within the engine compartment, connecting the two suspension turrets and the engine bulkhead below the windscreen. As a result, the Z4 requires no further vibration damping.
Being a pure driver's roadster, handling has been optimised by reducing the car's unsprung masses, with the share of light alloys on the suspension now amounting to approximately 60 kilos, or 20 per cent of the overall weight. In addition, the rear track has been widened by 40mm. The Z4 3.0i is also fitted with BMW's M Technic sports suspension as standard (optional on 2.5i), lowering the body by 15 mm (0.6 inches). In terms of design, the front and rear axle is a spring strut structure with twin-sleeve gas pressure struts.
A reduction in suspension weight also assists the overall mass. The Z4 2.5i weighs in at a svelte 1,335kgs, with the 3.0i tipping the scales at 1,365kgs. This is 25kgs less than the outgoing Z3 3.0i despite larger dimensions, larger wheels and a six-speed gearbox.
Steering:
Introduced as another first is BMW's new Electric Power Steering (EPS). With no need for weighty hydraulic steering components, the new system adopts an electric motor connected to the one-piece steering column for steering assistance. Using a torque sensor to measure the driver steering input, information is transmitted to the electric motor that in turn provides the desired steering angle to the tie bars. Software is used to optimise steering damper effect and feedback, and when the Sport button is pressed activating DDC, the level of steering assistance is reduced, giving a more direct steering feel. Unlike hydraulic systems, EPS uses no engine power and owners can save up to one litre of fuel per 250 miles (400kms).
Safety:
Occupant safety is controlled by BMW's Advanced Safety Electronics (ASE), with a centrally-mounted safety and information module linked to two 'satellite' sensors located in the left and right B pillar. Each module independently detects and measures any impact and with the choice of four airbags, deploys the appropriate restraint. Information is passed by a 'byteflight' optical network, giving a faster data transfer and consequent reaction time, causing the following:
... Airbag activation
... Deactivation of the electric fuel pump
... Activation of cabin lighting and hazard warning lights
... Deactivating the alternator
... Pyrotechnically breaking the safety battery terminal
Protection of all occupants is important, and should the driver wish to carry a roadster prodigy in a child seat on the passenger seat, the passenger airbag can be deactivated using the ignition key and switch located in the side of the dashboard. Occupants are notified of the deactivation by a light on the transmission tunnel.
Punctures are less of a problem for the Z4 owner as 'run flat' tyres are standard across the range of 16", 17" or optional 18" wheels. Using a reinforced sidewall and a more temperature-resistant rubber compound, the car can be driven safely for up to 90 miles (150 kms) at a reduced speed of 50 mph (80 km/h).
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