LEARNING how to do folding hard-top roofs with the 3 Series Convertible served BMW well.

It meant it could launch a folding hard top Z4 relatively easily – simply because it had acquired so much knowledge.

What’s more, the shift over to the folding tin top is another reason for the car’s production relocation to Germany.

The Regensburg plant where it’s built has now become BMW’s official ‘centre of excellence’ for folding hard-top roofs.

It’s so well geared up, the supplier is even located there on site, an engineer insider told me.

‘This gives us the best possible means of achieving high quality levels.’

Engineering a folding hard top isn’t the work of a moment. Even one such as the Z4’s, whose lower 2-panel count reduces the number of joins and joints to worry about.

The problem come in areas you probably don’t even think about. Such as?

•    Packing the roof into the boot without the lardy bum of a Peugeot 307 CC. ‘A close working relationship with the designers is vital.’

•    Making it watertight – ‘this is the key. The biggest challenge is not to get the roof moving, it’s to keep the water out.’

•    The need to like coffee. ‘I was able to work closely with the expert from the 3 Series Convertible project – we could have coffee, chat about our weekends, and how to get the Z4 roof watertight. We did this a lot…’

‘Once you’ve got it watertight the first time, you then need to do it again. Repeatability is of huge importance.

‘Our approach is to always take one car at random and test it for water tightness.’

Passing the tough BMW test involves putting the car in a high-pressure shower. Literally. ‘It’s a severe test. At the moment, every car is being tested. In time, tolerances will be such that we can test a certain percentage.’

What’s a watertight test pass rate for BMW? 95 percent ‘right first time’. Which means that, basically, every single Z4 has to be built to be watertight.

‘We’re almost there…’

BMW Z4 development background

BMW Z4 chassis secrets

BMW Z4 photo stream on Flickr