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.
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).