This all-new BMW Z4, complete with a folding metal roof for the first time, is 100kg heavier than the last one. Some of that is down to extra safety kit, better materials, the heavy strengthening stuff to give increased torsional rigidity (which is up by a quarter over the last Z) - all the usual improvements you get from one generation of car to the next. But most of it is down to that roof and all the associated mechanisms.
Don't be too cynical about all this though. BMW has been careful to use as much aluminium as possible, so the roof itself only adds 30kg. BMW claims it's the lightest in the segment.
That certainly isn't much. A big shopping trip could add that sort of figure. Oddly, though, you can still feel it when you put the lid up. Maybe it's partly psychological, but with the roof up there seems to be more roll in the corners, more of a porpoising effect at the rear of the car when you turn in. The centre of gravity feels higher and it can be slightly unsettling, especially if you hit a crest mid-corner. Without doubt, the Z4 is better to drive roof down.
With the wind in your hair, all that extra weight drops into the boot, right over the rear tyres. This means everything feels far more secure. And you never worry about scuttle shake. I only drove it on Spanish roads, and they're hardly the roughest in the world, but other than feeling the wind, you can't tell the difference roof up or down.
For all this, though, it's not the last word in precision handling that people might expect from a BMW. With the 302bhp rear-wheel-drive twin-turbo 3.0-litre, it sometimes feels like you're driving a dragster - the nose is light and wafty, and reluctant to bite on that initial part of the corner. There's a delay to the initial turn-in that isn't as bad as full-blown understeer, but a Porsche Boxster feels more clinical, more ready to obey your every instruction instantly. A similar criticism can be levelled at the Z4's brakes - they're strong but there's too much dead travel at the top.
The steering is, for the most part, perfectly acceptable and vastly better than on previous BMW roadsters. There's a depth of feel here that was missing before, a far greater sense of what the car is doing underneath you. You can also drive by the seat of your pants far more than before - there's a huge amount of feedback through the chassis. But you're sitting on the pivot point and the nose is so far away from you that it feels like the front end isn't reacting as quickly as you are.
Where the Z4 excels is with the engine. We only got a chance to drive the 35i, but it's all you'd ever want. The Z4 is only available with straight sixes - this 3.0-litre twin-turbo, a naturally-aspirated 3.0-litre and a 2.5-litre. For the time being, BMW isn't talking about an M version. If it does come, it'll be interesting to see what those boys do with the folding hard-top. Don't forget, they refuse to use run-flats because of the handling and ride compromise.
It's difficult to see what extra an M engine could bring to this car over the 35i. Zero to 62mph only takes 5.1 seconds with the dual clutch transmission - that's blisteringly quick and 0.2 seconds up on the Boxster S. The twin-turbo tech also means that you're never searching for extra torque or responsiveness as it's delivered instantly. There is no lag.
But this is about more than pure figures and driveability. The 35i has got so much character that the SLK 350 feels like a Toyota in comparison. The noise is the highlight - you start off with a low rumble, almost like a V8, before it becomes much harder edged as the revs climb. Above 6,000rpm it really starts to wail. There's a commitment here, it feels and sounds like a racing car.
Change gear with the DCT and there's a lovely pop and burble as the engine does clever things to the torque to allow you to change cogs. You don't get it as much with the manual gearbox because retarding the torque isn't necessary. You do that with your right foot. Reason enough, then, to pay the £1,810 extra for the DCT which is only available on the 35i. Other than slightly confusing paddles (why can't we just have separate up and down paddles?), the DCT is worth all that cash. It works well as an auto because it's smooth and relaxed, yet it does hardcore driving with as much commitment as you'd get in the manual.
The DCT is also the only transmission option which gets BMW's Adaptive M Suspension, which means electronically controlled dampers. You get a three-setting toggle Sport button in every Z4, which controls the steering, throttle response and stability control - if you also get the £930 M suspension, it controls that as well. The new Z is now a softer car in all bar Sport +, but you'd simply never use that setting in the UK. Never mind your fillings, it'll shake your hair out.
It's better to put the gearbox in sport, because if you leave it in normal it defaults to too high a gear all the time - OK for cruising, but not much else. Sport means the 'box is always in the correct gear for the exit of a corner and the car feels much tauter. You can even set sport to be the default every time you turn the car on, but you'll need to do that in the iDrive system. It might be a pain to navigate your way to doing this, but it'll be worth it. Promise.
Crucially, in both normal and sport, there's far more compliance than on the last Z4. The ride is still a bit bobbly, but there's less crash and bang on rougher roads.
The interior is much smarter as well. This Z4 has a similar quality to the fit and finish that you get in the Boxster, but the design is better. In the last Z4 it felt very much like the engineers decided where to place buttons by the time-honoured method of shutting their eyes and pointing randomly. More thought has been put into the interior this time, and it balances practicality and minimalism well.
The roof operation is faultless. It only takes 20 seconds to retract into the boot, about on a par with others in this class. You can't do it on the move - safety legislation stops this because the third brake light isn't visible when the roof is being folded - but that's no great problem.
More problematic for some will be the lack of bootspace. The Z4 has less room for luggage than the SLK, but boot volume isn't actually too bad - with the roof folded there's easily enough room for a weekend bag. The trouble is, if you don't give BMW an extra £510,you don't get the comfort access option... which is another way of saying you can't actually get your bag out of the boot when the roof is down. All comfort access does is raise the roof slightly so that access is easier. A rip-off, then.
The Z4 might not be the first BMW to get a folding metal lid, but for a roadster, that purest of concepts, it's still a radical step. However, you can't ignore the sales figures - in hot or less-secure countries, BMW reckons this roof will quadruple sales volumes.
And as far as hard-tops go, the Z4 is a neat halfway house between the SLK and Boxster. It looks good roof up or down, it's well finished, the engines are great and it's comfortable enough to be an everyday car. But for clinical driving quality, you'd still have a Boxster.