What is it?
We’ve already driven the range-topping BMW 435i convertible, now it’s the turn of the car which will make most of the volume - the 420d. Aside from the engine, this is much the same car; a lower, longer and wider prospect than the model it replaces, and one that continues to sport a retractable metal hard-top roof.
From launch, the familiar 181bhp 2.0-litre unit will be the only diesel engine available with the convertible, and its frugality affords the car 55.4mpg economy and 133g/km CO2 emissions in conjunction with the six-speed manual gearbox. Select BMW’s eight-speed automatic, and they improve to 58.9mpg and 127g/km.
There are five trim levels to choose from, starting with the SE sampled here (although not on UK roads). The mechanics of a life cycle change means the 420d ends up with more standard kit than its predecessor - including the current 3 Series’s large infotainment screen, iDrive controller, Drive Performance Control, 17-inch alloy wheels and heated front seats.
The price difference is a modest £495. But that still sees the model start at £36,675 - almost £2500 more than the equivalent Audi A5 cabriolet 2.0 TDI SE. The example tested also featured a number of options - BMW’s Professional media system, 18-inch wheels, adaptive suspension - hiking its final total to £43,850.
What's it like?
Much like the 435i - hardly short on quality, but overall a disappointment. The blame for this is not difficult to spread around. The bulk of it, appropriately and inevitably, must be placed on the vast, mechanism-festooned roof which barely fits in the boot and weighs the dynamic equivalent of a planet.
Watching it slowly unfold from its overstuffed hiding place, a lengthy, 20-second process which plays out with with all the irksome methodicalness of an elderly scout leader opening a map, one wonders why BMW insisted on persevering with the three-piece tin-top.
Its benefits are negligible - under it, the 420d is no more refined than a contemporary fabric roof - and the penalty unavoidable.
The 320d SE weighs 1495kg. The 420d SE weighs 1755kg. That extra burden is not easily hidden. The 435i buries it under 302bhp of straight-six concealer; the 420d, even with a healthy 280lb ft of torque available, is less able to do the same - especially when the four-cylinder engine’s well-known shortfall in refinement is taken into account.
However, the vocal, gearknob-rattling motor is at least game; the suspension tune is not.
BMW’s engineers have certainly beavered away underneath to shoulder the strain - ultra-stiff control arms and axle subframes have been slightly repositioned, pivot points adjusted, the camber angle and roll centre fine-tuned - but even in the most aggressive of its four modes (Sport+), the conspicuously doughy result never settles into quick progress like the 3 Series.
Clearly, a more benign, agreeably cushioned car was the intention here, and driven in a manner more befitting a mobile sun lounger, the 420d is at least comfortable.
Only the meanest of surfacing infringements will dial themselves into the seat backs, and the roof-down interior is acceptably blustery rather than tempestuous. But too much of the former flair is gone; push on, and the car can no longer push back.
Should I buy one?
If buying a convertible is merely a four-up vanity exercise or a convenient way of keeping the comb-over flat, then the 420d’s newly inherited interior - typically first rate, easygoing attitude, solid residuals and likely very decent economy will certainly appeal, and duly make BMW’s entrant a match for its similarly fettled opposition in this segment.
But the car feels even less like the current 3 Series than we expected, and in our book that’s more than just a shame. Just as the folding roof has been allowed to intrude on the model’s usable boot space (reducing it to just 220 litres - less than you’d get in a TT roadster) so has it mercilessly eroded the marvellous handling identity.
That, above all else, is why the 320d is a five-star car - and why the 420d convertible certainly isn’t.
BMW 420d convertible SE
Price £36,675; 0-62mph 8.2sec; Top speed 146mph; Economy 55.4mpg; CO2 133g/km; Kerb weight 1755kg; Engine 1995cc, 4 cyls, diesel; Power 181bhp at 4000rpm; Torque 280lb ft at 1750-2750rpm; Gearbox Six-speed manual