The BMW 4 Series Convertible might feature an unfamiliar number but its talents shouldn't come as any surprise. Jonathan Crouch tries the 420d diesel variant.
Ten Second Review
The best-selling 4 BMW 4 Series Convertible model will be a diesel: this one in fact - the 420d. The engine under the bonnet may only have four cylinders but it develops a healthy 190bhp - and much more pulling power than the equivalent petrol version. It's even refined enough for owners to properly enjoy the roof-down vibes.
The logic seems impeccable. BMW is giving the saloons and estates in its range odd numbers and the coupes and convertibles even ones. We've been comfortable for some time in the knowledge that a 6 Series is effectively a 5 series coupe, so that philosophy now extends across the range. The current 4 Series is what would have been a 3 Series Coupe. Now we have the 4 Series Convertible and we can start to appreciate that this isn't just a change in nomenclature; there's a clear shift in philosophy too.
The 4 Series Convertible is a more aggressive and overtly sporting model than the old 3 Series drop top ever purported to be. Yes, you could buy convertible versions of the fire-breathing M3, but to do so was merely to pronounce that you'd missed the point of an M3 quite sorely. Things may well prove very different when dealing with a 4 Series. It's got a bit more about it. Let's try the big-selling diesel version, the entry-level 420d.
The chassis of the 4 Series Convertible is based on that of the coupe but the springs, damping and axle characteristics have all been tailored for the open car. By modifying and fine-tuning variables such as the camber angle, the track and the roll centre, BMW has optimised agility and steering accuracy. The front axle and suspension makes extensive use of aluminium to save weight, such as in the spring struts, wishbones and swivel bearings, which provide a substantial reduction in unsprung masses and improve suspension response. Adjusting the position of the control arms from their arrangement on the latest 3 Series saloon gives the car a 19mm lower roll centre, making it feel better tied down. Servotronic power steering is standard, with Variable Sports Steering offered as an option. A six-speed manual gearbox is fitted as standard with an eight-speed automatic available if you pay a little extra.
With over 60% of buyers likely to favour a black pump BMW, we chose the 420d variant to try, a car with neearly the same output (190bhp) as its petrol-fuelled 420i counterpart but offering nearly 30% more torque, 380Nm making itself keenly felt from low revs. The most obvious stats don't immediately bear this out - both cars make 62mph from rest in around 8.1s (only fractionally slower than their Coupe counterparts) on the way to a top speed of just under 150mph - but you really feel the diesel's extra torque out on the road.
Design and Build
The 4 Series Convertible is one of the few folding hard top cars that manages to look good whether the roof is up or down. The roof itself helps here. By choosing a three-piece roof over a two-piece item, BMW has achieved a shorter stack length when folded, which in turn which avoids that distended rear end that so many folding hard top convertibles exhibit. There's an inherent rightness about the basic proportioning too. The overall length of 4,638mm has been extended by 26mm; the wheelbase has grown by 50mm and, together with the wider front track, this gives the car a low and wide stance on the road. The shorter overhangs, long bonnet and set back passenger compartment give the car an elegant profile.
The roof can be lowered in just 20 seconds when the vehicle is travelling at speeds of up 8mph. Improvements in soundproofing reduce wind noise by up to 2dB. An electro-hydraulic loading assistance system pivots and positions the folded hard top to ensure maximum utilisation of the 220-litres of storage space. When the top is up, the boot has a volume of 370-litres, 20-litres more than in the previous 3 Series Convertible. With the optional through-loading system, a flat and level storage area can be utilised when the backrest of the rear bench seat is folded down.
Market and Model
You'll need a budget of around £37,000 for the 420d diesel, so you'll need to allow for a premium of around £2,500 over the equivalent 420i petrol version - with a further £1,500 needed if you want the 8-speed automatic paddleshift gearbox rather than the standard 6-speed manual. Equipment runs to leather heated seats, xenon headlamps, LED rear lights, a parking radar, as well as climate control. Satellite navigation is standard on most versions, but the adaptive dampers are a box you might want to tick on the options list as well as the sport steering.
And options? Well, we could take or leave the sports seats, but the stereo upgrade is worth giving a listening to. BMW will also tempt you with a range of connected services, a head-up display, and some neat driver aids but you'll need to keep an eye on the asking price. Buyers also get the option of a wider wind deflector, while there's also the three-temperature Air Collar, which is a blatant crib of Mercedes' Air Scarf system but a welcome addition nevertheless. The optional wind deflector is now smaller, lighter, more effective and easier to use. When it is not needed, it can be stored behind the rear seats to save space.
Cost of Ownership
Can this really be seen as an efficient car? After all, it carries around a very inefficiently heavy roof mechanism. A BMW 320d saloon weighs 1,495kg: this 420d Convertible weighs 1,755kgs. Those are the facts - and they go much of the way towards explaining why metal folding roofs have fallen from favour the in current, efficiency-obsessed automotive design. Having said all of that, the Munich maker is keen to point out just how much effort has gone into minimising the downsides of this approach with this car. A whole range of weight-saving measures, including use of aluminium in areas like the suspension, has meant that most 4 Series Convertible variants are up to 20kgs lighter than their predecessors. As a result, this car looks better on the scales against its direct soft top rivals than you might expect. Yes, it's still around 100kgs heavier than a rival Audi A5 Cabriolet, but it's actually now around 70kgs lighter than a Mercedes E-Class Cabriolet.
All of which explains why this 4 Series manages to be so class-competitive when it comes to running costs. The manual gearbox 420d Convertible we tried manages 58.9mpg on the combined cycle and 127g/km of CO2, improvingh to 64mpg and 116g/km if you go for the automatic version. In overall terms for this diesel drop-top, you're talking about figures that are about 5% worse than you'd get from a rival Audi A5 Cabriolet 2.0 TDI 177 or Mercedes E220 CDI Cabriolet.
The BMW 4 Series seems to be a carefully managed piece of product development, especially in 420d guise. This first version of the Convertible merely introduces the theme, teasing at what our assumptions might be. Make no mistake, this is no half measure. Audi and Mercedes will look at this car and wonder how on earth they can come close to beating it, yet one suspects there's a lot more to come from BMW in the next few years on the 4 Series theme.
The only possible flaw in the plan is that BMW becomes a victim of its own success, that buyers will exercise a certain contrarian nature and choose a rival that's objectively not quite so talented merely in order to buy something different. In this sector, cars are as much about what they say about the owners and there are some buyers who are rather put off by the default choice. As problems go, it's a nice one to have. BMW can congratulate themselves on a job well done.