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80's and 90's BMW models have long been known as great handling cars.  Part of the reason for that came from innovations on the rear of the cars, in particular the multi-link rear suspension introduced on the E31 chassis called Integral III Axle.

What's the job of rear suspension?

Ideally it should keep the rear wheels flat on the ground at all times - toe & camber should be consistent on both sides during cornering as well as driving straight.  If that is not achieved then the chassis can contribute to increased and uneven tire wear - and reduced stability.  If you imagine the inner wheel lifting up when cornering and the opposite wheel being pushed down into the ground the contact patch between the tire and the road changes - that leads to a difference in grip between sides that will vary as the corner straightens up.

I found an old BMW diagram that illustrates the difference between an old fashioned rear axle and the multi-link.  On the left is the old-school design where fixed wishbones connect the wheels to the subframe.  The diagram here is showing that cornering load on one wheel transmits force across the subframe to the opposite wheel.  Suspension height changes, such as bumps also transmit force through the subframe.  BMW's integral design uses ball joints, bushings and in some cases spherical bearing to connect the hubs to the subframe.  Thus independent rear suspension provides many benefits as forces on one side do not necessarily adversely affect the other - therefore greater stability and traction from keeping each wheel closer to their ideal contact patch.

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Ok, that may not seem very cool so far - the detail is what BMW call elastokinematics - this encompasses the control arm positions, lengths and strengths of the bushings.  This old E31 diagram shows that when under heavy lateral load (cornering) each rear corner can pivot into the corner for increased stability - this is a completely passive system - it all works from the arm pivot positions and their uneven lengths - lateral load leads to the spherical bearing at the rear being pushed and pulled to some extent.  At the same time the wheel is pulled on it's 'up axis' to try and achieve ideal camber.  Now that's some impressive engineering!

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Development

BMW retained the concepts of the Integral Axle throughout the E39 5-Series, E38 and E65 7-series as well as 6-Series and E53 X5.  Although the trailing arm was removed and the shock/spring moved the link concepts remained.

If we fast forward to the present day Integral Axle V was debuted on the modern 7-Series and one of the options was active rear steering (IAL).  The theory between the control arms is largely the same as the previous designs, but here you can see an actuator (number 1 in diagram) is able to actively push and pull the rear hubs as well as passively.  Tighter turning circle & greater stability are the end result.

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And before anyone comments, back in 1991 the E31 850 CSi was also available with active rear wheel steering (called AHK or Aktive Hinterachs-Kinematik which I guess translates as active rear kinematics).  The circle is complete!

 

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About the Author: Bryan McPhail

IMG_2691Bryan is a longtime BMW enthusiast in Florida.


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Written by :
Bryan McPhail

Bryan is a longtime BMW enthusiast in Florida.


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