to compensate for the heavier steel subframe. The rear suspension consists of a four link design (called "Z-link"), which is similar to the system used by the E38 7 Series. The design minimises unintentional toe angle changes, which increases the stability of the handling. Wagon self-levelling suspension The Touring model was the first BMW model to use air suspension (self-levelling suspension was first used by BMW for the E23 7 Series with a closed-loop nitrogen system that operated in parallel with the steel springs). This "self leveling" system controls the ride height of the rear of the vehicle and is designed to keep the center of the wheel a specified distance from the lip of the fender as the weight of the load in the cargo area varies. Instead of using a traditional coil springs, the system uses pneumatic springs paired with air reservoirs that are pressurised by an air compressor. The system is controlled by two Hall effect sensors at the rear of the vehicle. These sensors tell the EHC (electric height control) if the rear ride height needs to be adjusted and adjust headlight height for vehicles equipped with Xenon headlights. When a door or the rear hatch is opened and then closed the control module will constantly monitor the input signals from the HALL sensors and will activate a correction if the ride height has change greater than 10mm. During normal operation the system stays online but does not adjust to conditions such as potholes. M5 model The M5 model of the E39 was introduced in 1998 at the Geneva Motor Show and was produced from 1998 to 2003. It was powered by the S62 V8 engine. All E39 M5 cars were sold in the sedan body style with a 6-speed manual transmission. Special models Alpina B10 and D10 The Alpina B10 3.2, 3.3, V8 and V8S petrol-engined models were built in sedans and wagon body styles based on the E39 from January 1997 to May 2004. The Alpina D10 was the first diesel model produced by Alpina and was introduced in February 2000. The engine, a 3.0 litre twin-turbocharged unit rated at and of torque, was based on the engine of the 530d model. Protection line The 540i Protection light-armored vehicle was launched in Europe in September 1997 and in North America from January 1998. These models included aramid fiber armor, bullet-resistant glass that is coated