2006 BMW Z4 M Roadster - Road Tests

Proof that more is not a monolithic concept.BY TONY SWAN, PHOTOGRAPHY BY AARON KILEYJune 2006

Is more always better? We confront this question regularly, and when it comes to horsepower, we can usually be relied on to answer with a resounding, “Bring it on!”

But we do temper this response with a few provisos. After all, more can’t be a one-dimensional deal. That was the lesson of the muscle-car era: cars with an abundance of go but not much ability to change directions and almost no inclination to stop. You get more go, you need more stop. More rubber. More roll stiffness. More chassis rigidity. More of everything.

Meet the new M roadster, the Z4-based sequel to the Z3-based original. With all the required elements of more, including more chassis stiffness, a more-up-to-date suspension system, and more brakes.

BMW excels as no other in the realm of inline-sixes, a heritage dating to its first Sechszylinder back in 1933 — 1173cc, 22 horsepower, propelling the then-new 303 series — and the heart of more here is a 3.2-liter DOHC 24-valve straight-six, with BMW’s Double VANOS variable valve timing and an 11.5:1 compression ratio, the same engine that propels the M3 coupe and convertible. The only distinction is max output, which is down three horsepower, due to slightly more restrictive exhaust plumbing, a consequence of the Z4’s smaller dimensions.

This is not the newest of BMW sixes. The block is iron rather than the aluminum-magnesium metallurgy employed in the sixes that were launched in the latest 3-series sedans. But even so, it delivers plenty of thrust: 330 horsepower at 7900 rpm — just 100 rpm short of redline and the fuel-cutoff rev limiter — and 262 pound-feet of torque at 4900 rpm. That last may sound a little peaky, but most of the torque is available from about 2500 rpm right up to max.

It’s enough punch to propel the M3 coupe to 60 mph in 4.8 seconds and through the quarter-mile clocks in 13.6 seconds at 105 mph, according to our comparo of May 2003("Compact Adrenaline-Delivery Systems"). That’s pretty brisk, but the 3277-pound roadster is 117 pounds lighter and edges the M3’s progress to 60 (4.6 seconds, which is 0.3 second quicker than BMW’s official forecast) and through the quarter (13.2 seconds at 107 mph).
As always, mass is the enemy, and by way of illustration it’s worth noting that the Z3-based M roadster we tested in August 2001 ("Topless Toys") weighed 3141 pounds, enough of an advantage to make it a 10th quicker to 60 and through the quarter, even though it had 15 fewer ponies under its hood.

However, all this parsing of 10ths gets to be academic.

The M roadster can hunker down and get out of the starting blocks with the best in its class, and if driving a stoplight-wars winner is one of your priorities, we’d be surprised if this car ever proved to be a disappointment.

But how about the other elements of more? That’s where the previous M roadster needed some help — it finished fourth of four in that 2001 comparo — and that’s precisely what it gets in this next-gen offering. In particular, the Z4 chassis is distinctly stiffer than its Z3 predecessor, and the M suspension elements bolted to it are designed for heavier duty. The front track is a half-inch wider than the Z4’s, the forged-aluminum lower control arms locating the bottom end of the struts are essentially the same as those employed in the M3, the hefty steering knuckles are unique to this car, and the M roadster eschews the Z4’s electric steering for a hydraulic-power-assisted rack-and-pinion setup that delivers distinctly better feel, particularly at lower speeds.

At the rear, a multilink arrangement replaces the Z3’s semi-trailing arms, enhanced in the M edition by stronger wheel bearings and heavier subframing, the latter to accommodate BMW’s variable differential lock, which evens out power delivery. The final element in power delivery, as well as turning and stopping, is a set of meaty Continental ContiSportContact tires, 225/ 45Z front, 255/40Z rear, on 8.0-by-18- and 9.0-by-18-inch aluminum rims.
Every element of the suspension tuning — damping, spring rates, anti-roll bars — is distinctly firmer than the Z4 setup, even Z4s equipped with the optional sport suspension. What it all adds up to is a jaunty little roadster with the instincts and reflexes of a cheetah. Turn-in is instantaneous and precise. Body roll is minimal, and transient response seems even more eager than the M3’s, even though the M3 coupe is far from reluctant in this game.

The beefy steering wheel transmits detailed road information to the driver’s hands without a hint of kickback, even on rough surfaces. The flow of sensory data from the helm is augmented by supplemental sensations coming from the operator’s visceral regions. The shifter for the ZF six-speed manual gearbox (the previous M roadster had a five-speed) delivers exceptionally crisp engagements. The brake and throttle pedals are ideally located for heel-and-toe work, and the bucket seats keep the driver (and passenger) firmly anchored during hard cornering.
The sense of car-and-driver connection is strong here, and the Bimmer’s balance is exceptional. Its shorter wheelbase (by 9.2 inches) makes it feel almost twitchy compared with the M3, but once accustomed to the quicker responses, the driver quickly appreciates the faster footwork. The threshold of the dynamic stability-control system is high — there’s not much point in switching it off unless you’re running for max time at an autocross. With DSC disabled, it’s possible to overcome the substantial grip of the rear tires, although this is oversteer of the progressive, predictable variety, rather than the omigod kind that culminates somewhere in the roadside hedges.

Braking performance is superb. This was a strong suit in the previous M roadster, but it’s even more so in the new one. Immense cross-drilled, vented rotors (13.6 inches front, 12.9 inches rear) yield stopping distances of 152 feet from 70 mph, 10 feet better than the old M roadster and pretty close to race-car braking.
Race car is a term that also applies to this roadster’s attributes as an all-around ride. A cheetah isn’t a house cat, and the M roadster isn’t very well suited to ordinary domestic life. The suspension tuning that produces those feline responses yields a ride that doesn’t concern itself much with comfort. It’s not a go-kart. Like all BMWs, there’s at least a modicum of compliance — but it’ll tell the occupants the dimensions of every expansion joint and concrete patch the tires encounter, and 10 miles of nasty surfaces may be enough to make some owners question their purchase decision.

Our only other significant reservation is cabin noise at highway speeds. The roadster’s double-layered power top is quality goods, but it isn’t immune to wind roar, aggravated by additional air currents around the side mirrors. Why a sound system is important in a car such as this is a bit of a mystery.

Beyond the audio and nav systems, though, the roadster’s cabin is pretty austere, which we find refreshing. No frills, no gadgets, no iDrive. The focus is performance. We’re almost equally divided on the styling — love it/hate it, with exec ed Gillies feeling strongly both ways — but we’re all agreed that the M treatment, with its more aggressive wheel-and-tire package and restrained cosmetic tweaks, lends an element of menace that’s absent in the Z4.

With a base price of $52,995, the M roadster competes in a four-car class with the Corvette roadster, Mercedes SLK55 AMG, and Porsche Boxster S, with the Honda S2000 lurking nearby for guys who discover they might like to hang on to some 20 grand and still have a serious sports car. But of the core quartet, the M roadster is arguably the purest iteration of the old British-sports-car ethic: an open two-seater that can, in a pinch, be raced. That means minimal street-car compromises. And like its Z3 predecessor, that’s what the M roadster represents. We just wonder why it took BMW three years to serve it up.

COUNTERPOINT

STEVE SPENCE

The M apparently stands for “mean,” as in, “When we say the M division builds kicked-up driving machines, we mean it.” But be sure about what you want. If you’ve got 50 grand and want to go racing or hard road-slicing on weekends, this is fine. It’s powerful, with quick steering and a rigid suspension. But if you’re after designer cruising and mirror-gazing at your own divine image, this thing’s got a thumpingly hard ride. I’m at last used to der Bangle’s ostentatious, body-slashing creases, but the interior strikes me as cold, minimalist, unsports-car-like. And really, this is carrying the “fat” steering wheel to extremes.

MARK GILLIES

There’s something missing with this M roadster. The steering’s good, there’s lots of grip, and the M3 engine makes all the right moves, but the sum of its parts isn’t as sweet as a Porsche Boxster’s. It just doesn’t feel as coherent or as connected as it should. In some ways, it reminds me of the old M roadster: The rear suspension feels soft, like it’s set up for traction rather than balanced handling, and you feel like you’re perched on the rear axle, too. Sure, it slides more progressively, but it still feels a bit crude. The M roadster simply reminds me that BMW makes great coupes and sedans, but Porsche is the best German sports-car maker.

CSABA CSERE

Count me in the camp that’s always liked the styling of BMW’s Z4. Yes, it’s untraditional, but if a roadster can’t push the boundaries of conventional style, what car can? Besides, who needs another British-sports-car clone? I’m also a sucker for high-winding engines that gain power with each additional rpm. Although you must shift such rev-happy engines more often, you can more precisely regulate the amount of torque you deliver to the rear wheels. Besides, hearing an engine scream above 7000 revs evokes the raciness that sports cars are about. The M roadster’s deliciously fat steering-wheel rim and slick shifter further the illusion. How could you not like this car?

BMW M ROADSTER
Vehicle type: front-engine, rear-wheel-drive, 2-passenger, 2-door roadster
Price as tested: $57,870
Price and option breakdown: base BMW M roadster (includes $695 freight and $1000 gas-guzzler tax), $52,995; Premium package (consists of auto-dimming interior and exterior mirrors, power seats with driver memory, storage package, cruise control, BMW Assist with Bluetooth, and premium sound system), $2500; navigation system, $1800; heated seats, $500; hardtop prep, $75
Major standard accessories: power windows, seats, and locks; remote locking; A/C; tilting and telescoping steering wheel; rear defroster
Sound system: BMW AM-FM radio/CD player, 10 speakers

ENGINE
Type: inline-6, iron block and aluminum head
Bore x stroke: 3.43 x 3.58 in, 87.0 x 91.0mm
Displacement: 198 cu in, 3246cc
Compression ratio: 11.5:1
Fuel-delivery system: port injection
Valve gear: chain-driven double overhead cams, 4 valves per cylinder, variable intake- and exhaust-valve timing
Power (SAE net): 330 bhp @ 7900 rpm
Torque (SAE net): 262 lb-ft @ 4900 rp
Redline: 8000 rpm

DRIVETRAIN
Transmission: 6-speed manual
Final-drive ratio: 3.62:1, limited slip
Gear, Ratio, Mph/1000 rpm, Max test speed
I, 4.35, 4.8, 38 mph (8000 rpm)
II, 2.50, 8.3, 66 mph (8000 rpm)
III, 1.66, 12.4, 100 mph (8000 rpm)
IV, 1.23, 16.8, 134 mph (8000 rpm)
V, 1.00, 20.7, 159 mph (7700 rpm)
VI, 0.85, 24.3, 159 mph (6500 rpm)

DIMENSIONS
Wheelbase: 98.3 in
Track, front/rear: 58.5/59.7 in
Length/width/height: 161.9/70.1/51.3 in
Ground clearance: 4.0 in
Drag area, Cd (0.38) x frontal area (23.0 sq ft, est): 8.7 sq ft
Curb weight: 3277 lb
Weight distribution, F/R: 50.6/49.4%
Curb weight per horsepower: 9.9 lb
Fuel capacity: 14.5 gal

CHASSIS/BODY
Type: unit construction with a rubber-isolated subframe
Body material: welded steel and aluminum stampings

INTERIOR
SAE volume, front seat: 48 cu ft
luggage, top up/down: 8/7 cu ft
Front-seat adjustments: fore-and-aft, seatback angle, front height, rear height
Restraint systems, front: manual 3-point belts; driver and passenger front, side, and knee airbags

SUSPENSION
Front: ind, strut located by a control arm, coil springs, anti-roll bar
Rear: ind, 1 trailing arm and 2 lateral links per side, coil springs, anti-roll bar

STEERING
Type: rack-and-pinion with hydraulic power assist
Steering ratio: 15.4:1
Turns lock-to-lock: 3.0
Turning circle curb-to-curb: 34.3 ft

BRAKES
Type: hydraulic with vacuum power assist, anti-lock control, and electronic panic assist
Front: 13.6 x 1.1-in vented, cross-drilled disc
Rear: 12.9 x 0.8-in vented, cross-drilled disc

WHEELS AND TIRES
Wheel size: F: 8.0 x 18 in, R: 9.0 x 18 in
Wheel type: cast aluminum
Tires: Continental ContiSportContact; F: 225/45ZR-18, R: 255/40ZR-18
Test inflation pressures, F/R: 32/32 psi
Spare: none

C/D test results
ACCELERATION: Seconds
Zero to 30 mph, 1.7
40 mph, 2.6
50 mph, 3.6
60 mph, 4.6
70 mph, 6.1
80 mph, 7.6
90 mph, 9.2
100 mph, 11.4
110 mph, 13.8
120 mph, 16.6
130 mph, 20.1
140 mph, 25.8
150 mph, 32.9
Street start, 5–60 mph: 5.1
Top-gear acceleration, 30–50 mph: 8.9
50–70 mph: 6.9
Standing ¼-mile: 13.2 sec @ 107 mph
Top speed (governor limited): 159 mph

BRAKING
70–0 mph @ impending lockup: 152 ft

HANDLING
Roadholding, 300-ft-dia skidpad: 0.90 g
Understeer: minimal moderate excessive

FUEL ECONOMY
EPA city driving: 16 mpg
EPA highway driving: 24 mpg
C/D-observed: 16 mpg

INTERIOR SOUND LEVEL
Idle: 54 dBA
Full-throttle acceleration: 82 dBA
70-mph cruising: 74 dBA