Magnetic force

Traditional vehicle suspensions are optimised for comfort or handling performance — but not both.

With most cars, every bump can be a jolt to the kidneys, and every corner can make passengers feel as if they are about to fall over. It's a compromise consumers have come to accept depending on their expectations of a car.

That age-old conflict between ride and driving dynamics may be a thing of the past if a hi-tech magnetic damping system fitted to the latest Audi TT is anything to go by.

Unlike conventional shock absorber systems the TT's pistons do not contain oil, but a magneto-rheological fluid in which microscopic iron particles are suspended. When in a magnetic field, the particles align themselves in the direction of the magnetic flux.

The electromagnetic coil is integrated into the damper piston in such a way that when it is energised, the magnetic flux runs transverse to the admission ports in the piston. As it moves up and down, the aligned iron particles cling together and create resistance in the suspension fluid. This changes the fluid's viscosity within milliseconds, providing a softer or firmer ride. The driver can choose between a 'normal' and 'sport' setting with a simple push of a button on the console.

The greater the energy applied and the stronger the magnetic field, the greater the attractive force between the iron particles. This means overall greater resistance and damping power.

The brain of the car is an electronic control unit, which communicates with the vehicle's sensors and other chassis controllers using complex algorithms to determine the prevailing driving situation in milliseconds.

'Magneto-rheological fluids are miraculous,' said Olivier Raynauld, manager of controlled suspensions at Delphi, which provided the TT's semi-active suspension. 'They can do things that have not been possible before.'

The technology is not entirely new. According to Raynauld, magneto-rheological fluids were invented in 1947, but it took 30 years to make them into practical fluids that would maintain iron particles in suspension and not settle. This was accomplished by coating the particles with a chemical substance that is patented by technology company Lord Corporation.

Delphi collaborated with Lord over the next 15 years developing the fluid for vehicle dampers. it introduced its MagneRide damper, which remains the only available semi-active suspension system with no additional moving parts, in the US in 2002.

The first European production vehicle to offer MagneRide semi-active suspension technology was the Ferrari 599, which went on sale last summer. MagneRide suspensions have been introduced in the Audi TT and are to be introduced in the Audi R8 in July.

Semi-active systems, although available in cars for years, have often been expensive and complex, depending on variable valves in the dampers, along with separate control systems, hydraulics and pumps, all of which had to be accommodated in the limited space around the axles. This increased the weight and cost of the cars.

In addition to space and weight advantages, Raynauld said the system has a longer lifespan than normal dampers. 'This system is supposed to last the life of the car — which is 300,000km (190,000 miles),' he said, adding that normal dampers have to be changed two to three times during this time. 'There is no maintenance with Delphi's dampers because they are so simple there isn't much to break.'