By: IOL Motoring Staff
Munich, Germany - When the two-door Three became the 4 Series BMW promised us an M4 convertible - and here it is, sharing the M3 sedan and M4 coupé's superlative three-litre M TwinPower Turbo straight six - good for 317kW at a screaming 7300 revs and 550Nm all the way from 1850-5500rpm.
It also shares the coupé's impressive acceleration and fuel-consumption figures: 0-100 in 4.4 seconds (with the optional seven-speed M Double Clutch transmission; standard is a six-speed manual that's two-tenths slower to 100km/h) and 8.7 litres per 100km in the combined cycle.
But the drop-top M4 is also very much its own car, with a graceful 'barchetta' (boat-deck) silhouette when the top is down (even the rollover protection system and the radio/navigation antenna are now hidden away below the waistline) and a dramatic, pillarless glasshouse when it's up.
As per the Four Series convertible, the three-piece folding metal roof opens at the touch of a button in less than 20 seconds, while travelling at up to 18km/h.
THE LIGHT STUFF
The twin, mutually intractable problems of convertible design are weight and rigidity; the electric motors, struts and frames of the mechanism make even a fabric-top roadster unavoidably heavier than its tin-top sibling, while folding hard-tops are even worse in that respect.
Then, without an integral roof to complete the body-shell 'box' a convertible naturally flexes more than the equivalent coupé - which usually means the maker has to use softer rear suspension to reduce 'scuttle shake' - and the more underfloor bracing you add to stiffen the body the heavier the convertible becomes.
It's a no-win situation - but BMW has done its best to force a draw by making most of the extra bracing that goes into the M4 convertible out of rigid, extremely durable but ultra-light carbon fibre.
Both coupé and convertible also have an aluminium bonnet and front side panels, a carbon fibre front suspension brace and a one-piece carbon-fibre propshaft.
All of which helps to keep the convertible's kerb weight down to 1750kg, about 60kg lighter than the previous M3 convertible.
CIVILISED OPEN-TOP MOTORING
The M4 convertible's folding metal hardtop is significantly quieter inside than the previous model, thanks to a sound-absorbing headliner that reduces wind noise by up to 2dB.
It also closes automatically when you operate the boot latch, so that you can get to whatever's in the boot; when it's down, however, the folded roof takes up 150 litres of boot space, reducing luggage capacity from 370 to 220 litres.
The optional wind deflector is wider than before, and there's a new neck warmer (likewise optional), adjustable through mild, warm and toasty settings so you can drive with the top down no matter how cold the weather - as long as it's not actually raining!
The interior has all the M-specific kit - the round white-faced instruments, leather steering wheel with chrome trim and colour contrast stitching, M gearshift lever, door sill finishers and an M driver's footrest.
Likewise, the driver and front passenger's leather-trimmed sports seats with recessed head restraints and integral belt guides are electrically adjustable - but only the convertible has an illuminated M logo set in to each head restraint.
BUILD YOUR OWN
The M4 Convertible comes as standard with 18” forged-alloy rims wearing 255mm front and 275mm rear rubber; available as an option are 19” hoops in either grey or black with the same width of tread.
Inside, customers can select from four colours for the standard fine-grain Merino leather upholstery; another nine options are available to order from BMW Individual.
The standard Active M Differential reads the traction control, steering, throttle position and yaw sensors to optimises rear-wheel traction, while a newly-developed electromechanical steering system provides a precise link between driver and car, adjusting the level of steering assistance according to the car's speed.
The M4 Convertible has M compound brakes as standard, but customers can also order theirs with even lighter and more effective M carbon ceramic brakes and gold-painted callipers.
The M4 convertible will be released in South Africa in October 2014; prices, as always, when they get here.