The BMW roadster tradition
As new as it is, the Z4 represents a
long-standing BMW tradition. The Bavarian automaker’s roadster lineage
begins in 1935 with the 315/1 and 319/1, two versions of an energetic
little two-seater powered by engines that were small, yet had 6
cylinders. (Even then, engine smoothness and sound were BMW priorities.)
In 1936, the tradition became a legend with the 328 roadster, which
began as a successful racing car, went on to become a beloved sports
car, and finally became one of the great collectibles of its era.
Another great BMW classic was the 507, of which only about 250 were built in the mid- to late 1950s. Today, this stunningly designed, V8-powered roadster commands high six-figure prices at collector-car auctions. Elvis Presley owned one while stationed in Germany with the U.S. military.
Virtually unknown in America, yet also important to BMW’s roadster lineage, was the Z1 – conceptually amazing, with a fiberglass body and electrically retracting doors. The Z1 was expensive, built in small numbers, and offered only overseas.
In 1996, BMW introduced a roadster that would be accessible to many more customers: the Z3. Designed and engineered to be produced at moderate prices, the Z3 was an instant, international hit. Over its seven-year production span, it evolved from a single 4-cylinder model to a line of 6-cylinder roadsters and a coupe. It will be remembered as a relatively simple, elemental, fun-to-drive machine of typically high BMW quality and unique style.
With its design inspired by the classic 507, the Z8 made its debut in 2000 as BMW’s highest-performing, highest-technology roadster. It continues through 2003, completing a limited production run and commanding the respect of those who test-drove for the media and those with the means to own it. No one at BMW would be surprised if the Z8, like so many of the BMW roadsters before it, becomes a significant collector automobile.
The Z4 not only takes its place in this illustrious historical line, but brings the design and technology of the more popular-priced BMW roadster to a level that, in many ways, is comparable with what the limited-production Z8 offers.
Aerodynamics: Outstanding for an open-bodied vehicle
It is far more challenging to achieve efficient aerodynamics with an open-body car than with a closed one; yet BMW’s designers and aerodynamicists achieved major progress. With its softtop in place, the Z4 has an aerodynamic drag coefficient (CD ) of just 0.35. And extensive development has minimized drafts around the faces and upper bodies of Z4 occupants when the top is down: In its May ’03 issue, Road & Track reported that “at a constant 75 mph with the side windows down, we found the wind only tugging lightly at our cars.”
Smooth, powerful 6-cylinder engines
In a sea of V-6 engines, BMW swims almost
solo with its unusual – but in BMW’s opinion superior – inline-6
configuration. The Z4 roadsters are powered by two versions of the M54
6-cylinder engine family; of the 3.0-liter version powering the Z4 3.0i,
Automobile Magazine (August ’03) wrote that “Its torquey, 3.0-liter six
is always ready to respond.”
In addition to its inline six cylinders, this brilliant engine architecture features:
With all these elements of engine architecture shared, two versions of this powerplant power the two Z4 models:
5- and 6-speed manual transmissions
While the 2.5i model’s standard transmission is a 5-speed manual, standard in the 3.0i is a 6-speed unit. Weighing no more than the 5-speed, this “gearbox” incorporates refinements for even greater driving pleasure (and BMWs are already known for excellent manual transmissions):
Optional STEPTRONIC 5-speed automatic transmission
Available for both models is the 5-speed
STEPTRONIC unit that has garnered repeated praise from auto critics.
Like all other current BMW automatics, it incorporates –
“Third way”: The Sequential Manual Gearbox
BMW currently offers two types of
Sequential Manual Gearbox (SMG), both of which apply
electrohydraulically actuated, electronically or driver-controlled
shifting to a 6-speed manual transmission. Available on the
super-performance M3 models is the very elaborate DRIVELOGIC version,
which offers drivers a total of 11 shift programs. The version offered
as optional equipment for Z4 roadsters (and, for ’04, the 3 and 5 Series
as well) is engineered for performance, convenience and (above all)
driving pleasure; it offers a total of four shift programs:
In either mode, the Sport program is selected via a button on the console.
There is no clutch pedal. The driver selects the desired operating range (N, R, D, S = Sequential) with a console-mounted selector lever, and can execute manual shifts with that lever or with two “paddles” on the steering wheel. After starting the engine (which requires putting the lever in N and applying the brake pedal), the driver moves the lever to the right; this selects the Sequential mode, in which each tap of the lever or paddle(s) shifts the transmission up or down one gear. Moving the lever to the right toggles the unit to its Drive mode, in which shifts occur without the driver’s intervention. To revert to Sequential shifting, the driver can toggle the lever to the right again (and simply tap it toward “+” for an upshift or “–” for a downshift) or toggle one of the steering-wheel-mounted shift paddles. By pulling either steering wheel-mounted paddle, an upshift is executed. Pushing either paddle with your thumb produces a smooth downshift.
In either the Drive or the Sequential mode, an instrument-cluster display indicates the gear currently engaged. In Drive, a “D” is displayed next to the gear. When the Sport program is activated, an orange indicator light next to the Sport button illuminates.
Though SMG does offer automated shifting, it is not meant to serve as a conventional automatic transmission; that role is played by the also available STEPTRONIC automatic. Just as with a manual transmission, power is interrupted for shifts – though in hard, performance-oriented driving the shifts can be stunningly quick. SMG’s appeal lies in these considerations:
Z4 running gear: This roadster is glued to the road
The Z4 raises enthusiasts’ expectations
of how a sports car should handle. Starting with a brand-new
body/chassis structure, BMW chassis engineers developed a sports-car
platform that is almost literally glued to the road. The basis for this
remarkable platform is a body/chassis structure with exceptional
stiffness for a roadster; it achieves 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:
In general terms, the Z4 suspension system applies concepts familiar from the 3 Series. 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. Here are the particulars:
Strut-type front suspension. with notable features:
Central Link rear suspension, a multi-link concept. The Central Link from which the system derives its name is a large, curved longitudinal arm, pivoted directly ahead of the rear wheel’s vertical and horizontal centerpoint on a large 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 contributes to remarkable handling and riding comfort, yet is simpler than many multi-link concepts. Salient features include:
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 inherently sporty and the standard run-flat performance tires are relatively stiff, the available sport suspension (included in each model’s Sport Package) does not employ firmer springs, shock absorbers and anti-roll bars as is customary with BMW sport suspension. Instead, its only difference is a 15-mm (0.6-in.) lower ride height.
Every Sport Package-equipped Z4 comes with Dynamic Driving Control, which provides a Sport button on the console that selects –
Electric power steering: innovation with significant benefits
The Z4 incorporates an electric power
steering system: the steering is assisted by an electric servo motor
rather than the conventional hydraulic pump. Among the benefits of this
The servo motor applies its assist to the upper portion of the steering column; its control electronics are in a housing mounted directly to the motor.
Generously dimensioned brakes
The Z4 3.0i gets ventilated rear discs,
and both models have larger-diameter rear discs than those of their Z3
predecessor. Equipment is as follows:
BMW has applied two refinements to the handbrake mechanism. One is a self-adjusting actuating cable, reducing the need for periodic adjustment; the other is a newly designed cable linkage that ensures equal handbrake force on both rear wheels.
Run-flat tires, standard