The K4 had a drag coefficient of 0.196 at a time when contemporary saloons were around 0.5Cd, but experimenting with aerodynamics without the use of computational fluid dynamics meant research was a case of trial and error.
In order to make the car stable at speed, there had been experiments with fins. “A single fin, with or without slots, has little effect on stability and may even be detrimental,” wrote Bradley. “To be of any use, the fin must have excessive dimensions. Two fins without slots is no better. They have a tendency to deviate the vehicle from a straight course, and they have a lifting effect at the rear.
“The best results are obtained by a pair of slotted fins, really consisting of a fin and a rudder. The system is executed in Plexiglas.”
The K4 had a 3.5-litre six-cylinder engine, producing 88bhp. The engine was equipped with a dual carburettor, “part of which is used for normal running and the other part coming into action during the last movement of the throttle pedal”.
A four-speed gearbox driving the rear wheels was augmented by a planetary overdrive. “Road tests by the French revealed the remarkable advantages of the overdrive,” Bradley remarked.
Even though the engineers tested on roads comprising hills, heavy traffic and built-up areas, the K4 still returned impressive fuel economy. At 77mph and 1950rpm, it was measured at 24.4mpg; at 35mph and 900rpm it improved to 32mpg. “Certainly much better results could have been obtained on the German autobahnen,” mulled Bradley.
Due to the poor state of the K4’s tyres, the engineers were unable to test the car at its maximum speed of 90mph. The car also had automatic tyre pressure regulation to modulate rolling resistance, although some parts of the system were missing and the engineers were unable to verify its effectiveness.
The fate of the K4 is unclear, but it’s likely it was dismantled by the French engineers curious to learn all of its secrets. The philosophy behind the car remains as important to BMW as ever, with 2009’s Vision Efficient Dynamics concept hailed as a modern equivalent to the K4 and in turn inspiring the i8.
12 April 1946
Previous Throwback Thursdays
16 May 1987 - Ford Escort XR3i Cabriolet
17 October 1981 - The £12,000 baby Aston Martin
16 January 1985 - The launch of the Sinclair C5
15 April 1960 - Porsche's four-cylinder roots
17 August 2004 - The Honda NSX's last hurrah
11 October 1986 - Hyundai's second UK market foray
15 March 1980 - Triumph's TR7 Drophead
13 February 1991 - Mercedes F100 predicts future car technology
16 April 1997 - A modern 'Blower' Bentley
19 June 1991 - Volkswagen Polo G40 tested
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