Four Wheel drive and Stability Control

One of the criticisms of the 911 up to 1989 was that it was old fashioned technically. The 944 had introduced ABS before then and the 911 was deemed to be lagging behind. All this changed with the introduction of the 964, when ABS became standard on all 911's. Once sensors were fitted to work out individual wheel speeds - several other possibilities to control other aspects of car dynamics emerged. A further step forward was then taken with the introduction of a 4-wheel drive alternative - the Carrera 4.

Because the gearbox is in front of the rear engine, it was possible to fit a drive shaft passing forward to the front of the car where it drives another small differential to the front wheels. Set with a power split of 31% front & 69% rear, computer controlled transverse and longitudinal differential locks alter this split if it is sensed that front, rear or opposite wheels are starting to slip under power -giving greater control in difficult driving conditions. To help get going in slippery conditions an alternative diff lock can be selected to drive all wheels together up to about 30kph (20mph). We have noted that this model also has longitudinal and lateral accelerometers fitted to measure sideslip and rotation of the car. These also are involved in the dynamics of the computer programme.

The 993, additionally, has an automatic brake differential system fitted (ABD) that applies the individual brake to any wheel that is slipping (up to 44 mph) - without locking it of course.

In addition to "ABD", the 993 turbo and Carrera 4, are equipped with permanent 4 wheel drive driven via a viscous multi disc clutch to provide similar all round 4 wheel and drive control.

The 996 Carrera 2 also has a traction control system that detects any rotational speed differences between the front and rear wheels and reduces engine power accordingly to avoid unnecessary rear wheel spin.

The Boxster has the rear engine in front of the gearbox, so a 4-wheel drive version would be difficult to conceive. Instead Porsche have introduced a system that is becoming common amongst many other expensive cars. It reflects the Scandinavian Rally driver's style (and indeed Michael Schumacher's) using "left foot braking".

One of the main reasons that a car can become unstable occurs when fear of a skid or a crash results in the driver taking their foot off the accelerator to reach the brake -because this results in the weight distribution and the load on all the tyres, changing suddenly. To control a car more effectively on difficult surfaces, some drivers have learned to keep the throttle steady while feeding a little brake on with their left foot - preventing any sudden changes in the weight distribution while slowing the car carefully.

Stability control systems do this by computer - but even more effectively by varying the amount of braking on different wheels to suit the circumstances. This is similar to the result of a viscous drive or differential controls because it reduces the torque being transmitted by an individual wheel.

The sophistication of computers allows further adjustment of engine power etc to result in a fantastic system to stabilise the car. The Boxster uses this type of system called "Porsche Stability Management". If (for example) the front wheels of the car drift on a bend - the rear wheel on the inside of the bend is braked. If the rear swings out -the front wheel on the outside is braked.

None of these systems will prevent an idiot - who is driving far too fast for the conditions - from crashing - but they make a huge difference to stability in unexpected road conditions or accidents.

For's and against's.

Included in the arguments for these 4 wheel drive and stability management systems is the added safety for inexperienced drivers or unexpected conditions. It enables drivers to get away with more as the car compensates.

There are however quite a few arguments against. They may invoke a false of security or confidence. Drivers who are used to handling fast rear wheel drive sports cars may not like the intervention of the computer in deciding how they drive the car and may indeed compensate incorrectly themselves - being unfamiliar with the feel or the resulting feedback. Although they are very reliable mechanical and electronic systems, eventually they may become less so (perhaps after 10 to 15 years say) as parts wear and wiring connections become corroded, giving different feedback to the system - beyond the understanding of the computer. If this happens they may prove very expensive to trace the faults and to repair - eventually steering preferences (and possibly values) towards two wheel drive alternatives.

Finally - the performance figures for 4 wheel drive and 2 wheel drive variants are often quoted the same - but our own dynamometer tests disprove this, showing the 2 wheel drive versions to be potentially faster on the road - not unexpectedly considering the extra weight and friction of the additional 4 wheel drive components.

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