Secrets of the M4 Convertible's roof

Is the BMW M4 Convertible's folding roof too complex for its own good?



I've just been driving the BMW M4 Convertible, and I'm reminded of the craziness of the whole roof mechanism. It's actually the same as all other 4-series Convertibles, so this criticism isn't just aimed at M. In fact it isn't really aimed at BMW, it's aimed at the lazy people who buy the car.

Here's a series of photos of it in action. To lower the thing, all the driver has to do is slow down to 10mph, and touch a button. The four windows drop, and the roof unlatches itself. Then the front panel of the roof moves back until it's snuggled against the section that's normally above the rear passengers. The rear section meanwhile is levitated up to lie above it.

Then the boot lid opens, hinged from the back, and this metal and glass sandwich motors into the top of the boot cavity. Finally the lid closes again, so you can open it in the normal way to get at your stuff. Except you can't. The stowed roof almost entirely blocks the boot entrance.

But of course the ever-diligent BMW engineers have thought of this. Press another button and the entire boot-lid rises vertically. And the roof sandwich rises to follow. You don't have to exert yourself: it's all done by a set of electro-hydraulic actuators. This takes many agonising seconds, but at last you can get your bags out. Then you have to press the button again so all the roof and lid contraptions can ease their way down again.

Sure, it works, but surely it could have been simplified and lightened. Does everything have to be powered? Couldn't BMW owners raise a tiny bit of muscle power beyond the onerous business of holding their manicured fingers on a switch for 20 seconds?

A Convertible M4 weighs 250kg more than the M4 Coupe. Because of all the stuff needed to heave to roof about the place, its weight distribution is about 52 per cent on the rear instead of 48 per cent for the Coupe. And when you drop the roof, it goes to 53 per cent rearward. That's a whole lot of junk in the trunk.

Of course the 4-series isn't the only car affected. The Porsche 911 Targa was, in the 1960s, a car where you just lifted out a panel and stowed it. The new one could have been the same: the glass rear window could merely have hinged up manually, and you could have popped the panel in. But no. Instead the whole thing moves out so far that it even overhangs beyond the back bumper. It's entirely powered. And the original idea for a simple semi-open car ends up distorted into the heaviest, costly, most complex 911 body of all.

No doubt you can add your own suggestions of roof-type pointlessness in the comments box below. First one to mention the Honda CRX del Sol gets a prize. Oops, won it myself. The Citroen Pluriel doesn't count, because it had a DIY element.

Author: TopGear
Top Gear is a British television series about motor vehicles, primarily cars, and is the most widely watched factual television programme in the world.