by Jim Kerr

Sometimes technology is wonderful. Sometimes not! Just before BMW was about to present their Variable M-differential lock to the Automobile Journalists Association of Canada (AJAC) technology panel, their computer crashed. Gone were all the presentation slides and notes (even Bill Gates has experienced this problem!). Fortunately, a paper description of the differential was available, so the show went on. Competing for this year’s Best New Technology award, the Variable M-differential lock is a technology that enhances the performance of BMW’s superb M3 coupe and convertible.

2002 BMW M3 Coupe
2002 BMW M3 Coupe
(Photo: Grant Yoxon,
Click image to enlarge

An understanding of existing torque-sensing limited slip differentials will help us understand the improvements offered with this new variable differential locking system. In the past, limited slip differentials have used a constant basic locking torque. When one wheel spins, the differential locks and sends torque to the other wheel. The differential lock value for current M class BMW’s is 25%.

This system works well, but a problem occurs when one wheel is on very low traction surfaces such as ice or when the car is turned hard in a corner and the inside wheel lifts off the road. Then there is not enough traction provided by the slipping wheel to cause the differential to lock. The traction advantages of this differential lock are limited by the maximum traction torque of the slipping wheel.

The Variable M-differential lock can provide traction advantages even when one wheel is on glare ice. The factor that determines torque transfer is the speed differential of the drive wheels. As soon as one wheel starts to spin, torque is transferred to the other wheel. The greater the speed difference, the more torque is transferred.

To accomplish this, BMW is using a sheer pump located inside the differential. One drive wheel is connected to one part of the pump and the other wheel is connected to the other part. As wheel speeds differ, the pump provides a variable pressure that applies multi-disc clutches preventing the spinning wheel from turning faster than the other wheel.

The pump is a sealed unit that uses high viscosity silicone oil between two sheer discs. As soon as the two sheer discs turn at different rates, the silicone oil is sheered by the grooves in the sheer plates and this generates a pressure dependent on the speed differential of the two plates. Thus, a variable differential lock effect is achieved. As soon as the two wheels are turning the same speed, the pressure of the sheer plates decreases and the differential unlocks.

Conventional limited slip differentials use a little constant pressure on their clutches to preload the differential. This works fine for performance applications but it is undesirable on ice. When two wheels are “locked” constantly together even with limited force, they tend to fight each other when the vehicle turns corners and the wheels have to travel different distances. On slippery, icy road surfaces, the action tends to cause both drive wheels to loose traction. This can have a dramatic effect on lateral stability.

The Variable M-differential lock system continually varies the “locking” of the two drive wheels. During cornering, the speed differential is low enough the differential remains unlocked so lateral stability is maintained. As soon as one wheel looses traction, the differential holds, and the car continues to move. Combined with the BMW M3’s 50:50 weight distribution and their stability control system, the M3 is a superb handling vehicle on any road conditions.

I have had the pleasure of testing both the M3 coupe and convertible equipped with the Variable M-differential lock. These cars are some of the best in the world for all around driving. They are very sporty, quick, comfortable, and luxurious. The new differential in these cars works very well. It is a worthy candidate for AJAC’s Best New Technology award. The award winner will be announced in February.

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