More about AHK (Rear wheel steering) than you wanted to know

mattsimis · Oct 19, 2011

Hats off to the author of this!
http://e31wiki.org/wiki/Active_Rear_Axle_Kinematics

Minimizing the sideslip does not only improve safety in curves. During the development of AHK at BMW, people from BMW Motorsport GmbH (in 1993 renamed to BMW M GmbH) who were developing the 850CSi at that time, were given the opportunity to drive a test vehicle equipped with AHK on the Nürburgring Nordschleife circuit. The drivers had the feeling the car with AHK did not allow them to exactly hit the brake and acceleration points they were used to and complained they could not hit the proper racing line and apex in curves. Despite that and much to their surprise, the lap times of the test vehicle equipped with AHK were consistently noticeably faster. The reason is again with the sideslip angle: The car without AHK develops sideslip in a curve which is still present at the exit of the curve. The car with AHK on the other hand minimizes sideslip making it much more stable on the corner exit. This allowed the drivers to apply throttle much sooner and gaining higher speeds at the straights. Because of the performance edge AHK provided, BMW Motorsport GmbH decided to make AHK standard equipment on the 850CSi.

Nevertheless the 850CSi scored very well in tests which can be attributed mostly to AHK. In a series of tests performed in 1996 by the German Auto Motor und Sport Magazine the 850CSi put up a remarkable performance in the 36 meter (39 yard) slalom, only beaten by the Porsche 911 Carrera and Turbo and the Ferrari F350 sports cars. In the 110 meter (120 yard) moose test the 850CSi beat all other cars despite weighing over 500 kg (1100 lb) more than the other cars in the top 5.

LEINS · Oct 19, 2011

My taking from this is that rear wheel steering on your 850 only kicks in above 25mph and that the rear wheels only ever turn in the same direction as the fronts. Am I right in thinking that this is a different approach to other 4WS systems (eg Honda Prelude, Mazda 626) which turned the the rear wheels in the opposite direction to the fronts up to that speed?

mattsimis · Oct 19, 2011

Yes, my understanding too. Those systems are for low speed manuverability while the BMW system is the opposite, its for high speed sweeping bends and lane changing. Which, biased as we are, is much cooler.

ciars · Oct 19, 2011

Love these threads! Added the link to the Directory Thread in General Chat forum also

mattsimis · Oct 20, 2011

The 850CSi will now be known as the "850CSi - Winner of the 1996 Moose test".

mattsimis · Oct 21, 2011

That Honda system sounds excellent! Why didnt that become standard equipment on all (performance) cars?

More about AHK (Rear wheel steering) than you wanted to know

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