Air suspensions are used in place of conventional steel springs in the passenger cars as well as in heavy vehicles such as buses or trucks. It is widely used in semi – trailers and trains (mainly passenger trains). The main function of air suspension is to provide for a smooth and constant ride quality but in some cases it is used as sports suspension. Vehicles that use air suspension todayrange from Maybach, Rolls-Royce to Audi, Subaru, Tesla, among others. Citroën nowfeaturesHydractive suspension, a computer controlled version of their Hydro pneumatic system from of air suspension which further, includes sport and comfort modes. Some air suspension designs feature height adjustable suspension controlled by the driver which is suitable for making it easier to clear bumps or clear rough terrain. Tesla Motors offers an optional "Active Air Suspension" on its Model S which lowers the vehicle for better aerodynamics and increases its range.Common air suspension problems are Airbag or air – strut failure,Air – line failure,Air – fitting failure, Compressor failure or Dryer failure.