NEWS: Riding on Air - Air Springs Aren't New

October 4, 2004
View All (5) Photos
0411 Praxis Oversteer

Oversteer is difficult to provoke in the
undresteer-prone Praxis BMW.

Air springs are more than a century old. Long before cars hit the road, the most sophisticated horse-drawn buggies rode on air-filled leather diaphragms. Molded-rubber air springs were developed and patented in the 1930s. Transit busses began using air-filled bladders to improve ride in the 1940s. Automotive applications commenced in the 1950s. Today millions of automobiles and trucks ride on them.
An air spring consists of a cylindrical rubber chamber positioned between a suspension member and the car's body. Those included with the Praxis system consist of four material layers: a synthetic fiber inner liner, 2 body plies, and an outer cover. They make a sensible addition to any tire maker's product line because of the obvious construction similarities. Filling the chamber with pressurized air supports one corner of the vehicle.
Air springs can offer superior isolation from road noise and vibration than steel coils, torsion bars, and semi-elliptic leaf springs. But the major attraction to air springs is their versatility: while a conventional spring's performance characteristics are fixed, the rate and the ride height of an air spring are both readily adjustable.
0411 Praxis Upright V

A Praxis suspension upright.

Add load (additional passengers or luggage in the trunk) to a steel spring and it deflects (settles) according to its spring rate. The same is true of an air spring. But restoring optimum suspension height with the latter device is readily accomplished by adding air from an on-board compressor.
Adding air pressure also increases the spring rate, a feat not possible with steel springs. That means that air springs can be engineered with a low rate for a comfortable ride when normal loads are carried, then readily switched to higher rates for heavier loads.
Another air spring attribute is a constant natural frequency. When the load carried by a steel spring rises, the natural frequency of the vehicle's ride motions is significantly reduced. Ride quality suffers because dampers optimized for one ride frequency provide either too much or too little damping when the load changes. But with air springs, the natural frequency is independent of the load carried so dampers can be calibrated for superior ride and handling characteristics at all times.
The shape of the piston that presses into the air spring determines how much the rate rises with upward wheel movement. Increasing the piston's diameter versus its vertical height results in a higher effective spring rate.
0411 Praxis Air

The Praxis air management system, mounted in the spare tire.

0411 Praxis Ecu

The Praxis ECU mounts in the trunk behind the liner.

0411 Praxis Controller

The Praxis control module can sit on the console or hide in the glove box.

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Photo Gallery

  • Oversteer is difficult to provoke in the
    undresteer-prone Praxis BMW.
  • A Praxis suspension upright.
  • The Praxis air management system, mounted in the spare tire.
  • The Praxis ECU mounts in the trunk behind the liner.
  • The Praxis control module can sit on the console or hide in the glove box.

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