New running gear: This roadster is glued to the road

Expect the Z4 to raise everyone's expectations of how a sports car should handle. Starting with a brand-new body/chassis structure, BMW chassis engineers evolved and adapted the basic 3 Series suspension concept into a sports-car platform that is almost literally glued to the road.

"The correlation between chassis rigidity and athletic handling has long since ceased to be a mystery," wrote Car and Driver in its April '02 issue. BMW knows this: With each new vehicle generation, BMW further advances vehicle dynamics by improving this "stable platform" of chassis rigidity.

Thus BMW's structural engineers have developed a body/chassis structure with exceptional stiffness for a roadster. They achieved 21 Hertz - truly outstanding for a roadster, and close to the 25-26 Hz of today's BMW sedans. A number of specific features contribute to this rigidity:

• Y-form front longitudinal members. Each chassis rail carrying the engine (one per side) branches into a "Y" to form a side sill and half of the central tunnel. In this regard, the Z4 is similar to the Z8 roadster (though the Z8 structure is of aluminum, the Z4's of steel).
• The side sills were designed to achieve maximum rigidity within acceptable bulk. (It's not acceptable simply to make them huge; this would cut into passenger space or make the car too bulky.)
• The underbody (floor pan) was designed to spread its strength evenly over its entire length and width.
• The underbody is further reinforced by two thrust plates, similar to those employed in M3 models. The front one is of aluminum, the rear of steel. Also as in M3s, there are reinforcing braces from the front suspension's strut towers to the cowl area.
• High-strength steels are employed extensively for best strength without excess weight.
• Also to save weight, the hood is of aluminum. In fact, the new body shell, with its increases in size and rigidity, weighs no more than that of the Z3.

The Z4's suspension is a targeted evolution of the 3 Series' award-winning concept. In a February '02 seven-car comparison test, a Car and Driver tester described the 3 Series' handling: "The chassis calibration is simply superb. It is unflappable over all road surfaces, maintaining traction and stability." Now, imagine a further developed system in vehicles almost 300 pounds lighter and with a significantly lower center of gravity, and you get an idea of the Z4's potential.

Strut-type front suspension. Though essentially the same concept as the Z3's front suspension, the Z4 system is distinct in every detail. Among the differences:

• Forged aluminum lower arms, vs. steel, to reduce unsprung weight and thus improve ride and handling on rough road surfaces.
• Hollow strut rods, vs. solid; these weigh 10% less than conventional solid rods.
• Greater positive caster to improve straight-line stability.
• Wider track - by fully 60 mm/2.4 in.

Central Link rear suspension, a multi-link concept totally different from the Z3's semi-trailing arms. The Central Link is a large, curved longitudinal arm, pivoted directly ahead of the rear wheel's vertical and horizontal center point (hence the name) on a rubber bushing of highly sophisticated design. Each wheel also has an upper and a lower lateral arm, for a total of three links per wheel. The system helps give remarkable handling and riding comfort, yet is simpler than many multi-link concepts. Salient features include:

• Track widened by 29-mm/1.2 in. over Z3 models. (All recent Z3s had an exceptionally wide rear stance, so the Z4-vs.-Z3 track increase is not so great as at the front.)
• Increased negative camber angles, compared to the 3 Series, to enhance cornering ability. Z4s have 2.25° negative camber at rest, visible in a slight inward tilt of the tops of the rear tires.
• Extra-firm forward bushings for the subframe that carries the suspension system.
• Aluminum upper transverse links to help reduce unsprung weight.

Overall suspension calibration. To underscore the Z4's sporting nature, relatively firm springs, shock absorbers and anti-roll (stabilizer) bars have been adapted. This means a firm ride and very "flat" cornering.

Because the standard suspension calibration is so sporty, the available sport suspension (included in each model's Sport Package) employs shorter, stiffer springs, stiffer tuned shock absorbers and auxiliary springs. Its ride height is 15-mm (0.6-in.) lower.

Every Sport Package-equipped Z4 comes with Dynamic Driving Control, which provides a Sport button on the console that selects -

• Firmer steering effort (less power assist) via the electric power steering
• Quicker accelerator response via the "drive by wire" throttle system
• In vehicles with automatic transmission, an additional Sport mode.

Electric power steering: an innovation with significant benefits

The Z4 incorporates a new electric power steering system: the steering is assisted by an electric servomotor rather than the conventional hydraulic pump.

Among the benefits of this feature are:

• Facilitates specific tuning of steering to the vehicle - shock damping, on-center feel, return to center position, overall steering feel - via software.
• Vehicle-speed-sensitive power assist (Servotronic).
• Reduced vehicle fuel consumption, because the electric motor operates only when the steering wheel is turned.

The servomotor applies its assist to the upper portion of the steering column; its control electronics are in a housing mounted directly to the motor.