SOFT-TOP German roadsters such as Porsche’s now-classic Boxster and the Audi TT live on, but BMW’s redesigned Z4 two-seater has gone the way of its four-seat 3 Series Convertible stablemate – not to mention its most direct rival, the top-selling Mercedes-Benz SLK – by adopting a two-piece aluminium roof.

BMW’s first retractable hard-top roadster, the E89-series Z4, therefore replaces both the E85 Z4 Roadster, launched in 2002, and the E86 Z4 Coupe, which replaced the Z3 Coupe in 2006. In total, more than 180,000 examples of the previous Z4 were sold.

Production of the second-generation Z4, which debuted at the Detroit motor show in January and is now on sale in Australia just a month after its European launch, has shifted from Spartanburg in the US to Regensburg in Germany, which last year produced 300,000 examples of the 1 Series and 3 Series, including the 3 Series coupe-convertible.

BMW’s South Carolina plant will now make only X models, including the next X3 – but not the new X1, which goes into production at Leipzig mid-year and will go on sale in Australian in February, priced from about $55,000.

Sales of the current Z4, which ceased production last year, have slowed to a trickle with just five sold in the first three months of 2009 – 82 per cent down on the same period last year. Last year BMW Australia sold just 129 examples – more than 50 per cent down on 2007 figures.

BMW Australia won’t give specific Z4 sales forecasts for what it describes as a challenging year ahead, but admits its newest two-seater will not emulate the sales success of its predecessor, which posted a top annual figure of 460 sales. It says it will be happy to sell 400 examples in its first full year on sale. Mercedes-Benz sold 369 SLKs in Australia last year.

The larger, heavier MkII Z4 Roadster brings all-new exterior and interior designs, and introduces BMW’s iDrive cockpit interface system and seven-speed double-clutch transmission (on the twin-turbo flagship) for the first time.

According to BMW, the roadster is 148mm longer at 4239mm and 9mm wider at 1790mm, yielding 20mm more shoulder room and 43mm more elbow room, yet rides on an almost identical wheelbase (up 1mm to 2496mm). The latest Z4’s turning circle is a tight 10.4 metres and its aerodynamic drag is 0.34Cd (0.35Cd for the top-shelf turbo).

Despite being 8mm lower overall at 1291mm, BMW claims there is 5mm more headroom under the electro-hydraulically retractable roof, which opens or closes in 20 seconds when the vehicle is stationary.

Apart from added security and refinement, the folding metal roof improves visibility via a 52 per cent bigger rear window and a longer side glass area – via four windows – above doors that are 26mm longer. BMW also claims all-around visibility is up by 40 per cent.

However, the hard-top adds 30kg to overall weight gains of up to 100kg, thanks to extra equipment and the larger body, which BMW says is also stronger than that of the SLK and Boxster. Kerb weights range between 1480kg and 1600kg, spelling weight increases of up to 195kg. Unlike the old Z4, however, weight is distributed 50/50 front/rear.

The Z4 boot accommodates 50 more litres than before – a respectable 310 litres with the roof up (enough to store two 46-inch full-size golf bags using the through-load system), shrinking to 180 litres when the lid is lowered. There is also a new netted storage facility behind the seats that can swallow up to 15.5 litres.

Prices for the more verbosely – and misleadingly – named sDrive23i, sDrive30i and sDrive35i were announced at the Melbourne motor show in February. There is no SLK55 AMG-rivalling M version for now (the circa-$130k Z4 M Roadster and Coupe continue to be available) but, as in Europe, Australians will have the option of three Euro V emissions-rated four-cylinder petrol engines.

The range opens with the entry-level 150kW/250Nm 2.5-litre sDrive23i manual, priced at $86,200 – up $8000 up on the smaller, lighter (1285kg v 1480kg) and more powerful (160kW) 2.5si it replaces ($78,200) and up more than $7000 on Audi’s most affordable TT Roadster, the $79,114 2.0 TFSI S-tronic manual. The TT Coupe 2.0 TFSI manual is more than $9500 cheaper still at $69,581.

However, the least expensive Z4 is $2320 less expensive than the base SLK200K ($88,520) and almost $35,000 less pricey than Porsche’s newly upgraded Boxster ($113,000). Other roadster rivals include the Alfa Romeo Spider ($69,990 for the 2.2 JTS manual) and Nissan’s aged 350Z ($76,990).

The 23i claims 0-100km/h acceleration in 6.6 seconds (up from 6.5 seconds for the 2.5si), a 242km/h top speed and average EU fuel consumption of 9.2L/100km (up from 8.4L/100km) and CO2 emissions of 215g/km. Fitted with a conventional six-speed auto with steering wheel shift paddles, which adds $2308 to the manual price, the figures are 7.2 seconds, 239km/h, 8.9L/100km (down from 9.0L/100km) and 207g/km.

The mid-range sDrive30i, with a slightly less powerful 190kW/310Nm 3.0-litre petrol six, will cost $98,100 – $6700 more than the outgoing 3.0si ($91,400), $2673 more than the TT Roadster 3.2 V6 quattro S-tronic ($95,427) and $5110 more than the Alfa Spider 3.2 V6 AWD ($92,990).

The 30i manual sprints to 100km/h in an alleged 5.8 seconds (auto: 6.1 seconds) and a 250km/h top speed with both transmissions. Again, fuel economy is better in automatic guise (9.2L versus 9.0L/100km), as are CO2 emissions – 215g versus 210g/km. But while the auto’s fuel economy stays the same as its predecessor’s, the manual’s is up more than half a litre from 8.6L/100km, to be just 0.2L/100km short of the new twin-turbo Z4 flagship.

Top of the new Z4 tree will be the sDrive35i, powered for the first time by BMW’s 225kW/400Nm 3.0-litre twin-turbo direct petrol injection inline six, and priced at $116,900 as a manual.

That is over $17,000 more than the 200kW turbo-four TT S Roadster ($99,452, with the 225kW-plus TT RS yet to come), but just under the 224kW SLK350 V6 ($115,637) and well below the upgraded 228kW 3.4-litre Boxster S ($140,400).

A version of the M3’s seven-speed M-DCT automated manual transmission is a 35i option, for $3500 more at $120,400, in which guise the flagship Z4 offers claimed 0-100km/h pace of 5.1 seconds and returns 9.4L/100km average fuel consumption.

That is better in every performance respect than the manual, whose vital numbers are 5.2 seconds, 9.8L/100km and 228g/km. Priced within $10,000 of the still-on-sale Z4 M Roadster (1410kg, 252kW/365Nm), the 1600kg Z4 SDrive35i auto is just 0.1 second slower to 100km/h and almost 3L/100km more efficient.

In terms of safety features, all new Z4s come with twin front and side/thorax airbags, a pair of fixed rollover hoops, an electronic stability (DSC) and traction (DTC) control system with anti-lock brakes (ABS), cornering brake control (CBC), electronic differential lock, cruise control with brake function, run-flat ‘safety’ tyres with pressure indicator, Servotronic electric power steering, and two three-point seatbelts with pretensioners and force-limiters.

Starting with the base 23i, all Z4s come standard with automatic wipers and headlights, anti-dazzle rear-view mirror, front and rear parking sensors, warning triangle, first-aid kit, ‘Kansas’ leather seat trim with SunReflective technology, electromechanical handbrake, dynamic drive control (DDC) variable maps for both engine and steering, bi-Xenon headlights with washers, welcome and follow-me-home lights, rear foglights and LED indicators.

There are also two heated buckets seat with fixed head restraints and manual height, slide and recline functions, a height/reach-adjustable multi-function leather steering wheel, climate-control air-conditioning, power windows/mirrors, heated rear glass, a trip computer and BMW Professional AM/FM/CD sound system with USB interface, aux-in and six speakers.

The 23i rides on start-spoke 17-inch alloy wheels with locking wheel nuts and 225/45-section tyres, and comes with 300mm brake discs front and rear.

The 30i, instead, rides on 17-inch ‘Turbine’ alloys with 22/45 front and 255/40 rear tyres, while the mid-range Z4 also adds 330mm front brake rotors, electric seat adjustment with driver’s memory, a BMW Professional satellite-navigation system with folding 8.8-inch monitor ($4500 for 23i buyers) and voice activation ($700 on the 23i), and an 11-speaker 245-Watt sound system ($1500 in the 23i).

While the enlarged kidney grille of the 23i and 30i feature black vertical bars within a chromed bezel and a black air inlet grille, the range-topping 35i features matt aluminium bars for both grilles. Similarly, the 35i features silver instead of black door sill plates, twin outboard tailpipes instead of a single exhaust outlet.

Other additional Z4 35i features include the Comfort access keyless entry/start system with remote roof operation, sports seats, adaptive headlights, high-beam assist, 18-inch alloy wheels with 225/40 and 255/35-section tyres, 348mm front and 324mm rear brake discs all round and anti-dazzle wing mirrors.

Options include Ivory White Nappa leather with sports seats and Fineline Anthracite wood trim ($3930 or $2600 for the 35i), 10mm-lower adaptive M suspension with adjustable damping ($2600), 18 and 19-inch alloy wheels (the latter ranging between $1400 and $3900), a heated steering wheel ($400), TV in addition to sat-nav ($2250), lumbar adjustment ($640) and metallic paint ($1840).