SOME successful designs seem to go on for ever, one such being the BMW 5 Series.

With BMW just announcing it has sold two million versions of the 5 Series, making it the world's most successful business model, it is interesting to take a look at how the car evolved.

Since the start of production in 1972 the 5 Series is one of the decisive factors in BMW's success story.

I have driven many over the years, but I can never forget my first acquaintance with this model in the early 1970s when it was very new to the UK.

This was the E12 model which was produced from 1972 to 1981.

It started something of a BMW craze in the UK and I recall everybody wanted one, because in those days its styling was as sharp as the suits we wore and it made a somewhat Germanic statement of business success.

Roll up to a meeting in a BMW 5 and it seemed to say a lot more than a Ford Granada.

The car I drove was in striking red and powered by the six-cylinder petrol engine of the day. It was fast, silky smooth and took no prisoners.

Modern 5 Series vehicles are noted for their tenacious roadholding, but back in the 70s the 5 Series of the day demanded skill if you wanted to push it to its limits.

It could lose adhesion if cornering hard on gravelly roads and you had to be ready for the almost imperceptible twitch that told you it was going to pick its feet up if you fed it too much power.

But that being said it was a noble beast and stamped the BMW brand firmly on the list of big hitters in the UK car market.

The 5 Series was posh, youthful and powerful and made the rest of the market sit up and take notice.

And its momentum has never ceased.

Last year alone more than 347,000 customers worldwide opted for a BMW 5 Series saloon, a 5 Series Touring or a 5 Series Gran Turismo.

These days the series owes its extraordinary popularity partly to the wide range of engines. A broad spectrum of powerful and efficient four, six and eight-cylinder petrol and diesel engines are available.

These ranges from the BMW 518d to the dynamic M5 delivering 560 bhp.

But the one constant factor is that the mainstream 5 Series has remained rear-wheel-drive and only recently have we seen ones with BMW's X-Drive intelligent four-wheel drive.