Air spring technology is common on trucks and trailers, and has obvious advantages for off-road cars and commercial vehicles that do a lot of towing. It is also fitted on some luxury cars.
But even though the technology has been around for a while in these markets, it is not until recently that it has become a possibility for vehicles such as high-performance sports cars, ambulances, disabled transport, motor caravans and those carrying delicate loads.
Tyre giant Firestone has provided the missing link by using ride height sensors and an electronic control unit, called Intelliride, to allow the air springs to adjust to different loads, road conditions — or simply driver requirements.
Air springs produce spring force by compressing a trapped volume of air, contained by a sleeve-like reinforced rubber bladder held between two end-plates. The ride-height of the vehicle can be changed by controlling the air pressure inside the spring with an onboard compressor. Also, by profiling one of the aluminium end-plates into a conical shape, the volume of the air inside the spring can be changed, altering the spring rate or the resistance the spring produces when it is compressed.
Pressure-based systems look at air pressure to determine whether the vehicle is level or not. Because air pressure is only one indicator of potential ride height, the systems are not very flexible.
If, for example, an extra 150kg — passengers, luggage and fuel — is added they will actually deflate the springs to reduce pressure, producing a levelling problem.
But Intelliride, comprising an ECU, height-control sensors, air reservoir, air compressor, desiccant air dryer and valve block, automatically restores the preset ride height, no matter how much load is placed in the vehicle.
The driver can choose one of three vehicle heights — raised, standard or lowered— thereby improving road handling and comfort, while also being able to easily unload passengers and goods. The system can be installed in any vehicle with full four-corner air suspension and weighs around 10–15kg more than a conventional suspension system. Driverite, Firestone’s distributor in
Ford recognised the advantages of Intelliride and has incorporated it into a ‘mobility-adapted’ SUV concept car to make adventurous driving more accessible to the disabled. The three ride-height levels allow easier ingress and egress at rest, a middle setting for performance on the road, and a two-inch increase in height for improved off-road capability.
Firestone also has a Full Air Suspension kit for the rear axle of the Volkswagen T4 and the Renault Espace and Trafic. The existing coil springs are replaced with Firestone tapered sleeve air springs, which as well as giving a more comfortable ride, allows the rear chassis to be raised and lowered. This makes these vehicles attractive for the ambulance and disability markets.
But perhaps the most surprising take-up has been in the area of performance cars. Here, Firestone has taken the Intelliride system, replaced standard steel coil springs with air spring units, and ordinary shocks and struts with special adjustable gas pressure struts produced by German suspension specialist H&R, and called the package Praxis.
Featuring three suspension modes — touring, sport and track — Praxis has been installed in the Subaru WRX and the BMW 330Ci. The aim was to produce an intermediate ‘sporting’ car that would provide a comfortable street ride, but at the touch of a button (and without getting filthy under the chassis), would transform into a racing car.
Although the electrically-adjusted shock absorbers are set from inside the car, the transition is not, as yet, all achievable from the cockpit. The H&R dampers, for instance, are adjusted by clicking a thumb wheel at the bottom of each of the struts through a range of 18 settings.
The problem in the past for racing vehicles was that when a car with airbags was lowered, its spring-rate would automatically decrease. A lowered car is good for performance, but lower spring rates certainly are not.
Praxis has a unique air strut that actually increases the spring rates at each corner as the pressure inside each bag, along with the vehicle’s ride-height, drops. The piston of each strut is designed in such a way that as the pressure in the air bag decreases, the bag rolls over the edge of a bell-shaped piston and increases the spring rate.
In touring mode the cars are comparable to those with the original suspension, but when the ride-height is dropped a full 50mm or more into track mode and the spring rates are doubled, they turn into high-performance racing cars, staying flat in the turns and cornering as stiffly as any conventionally sprung racer.
By adding electronic control and monitoring to the versatility and function of airbags, Firestone has achieved a level of performance that was previously impossible for this type of suspension.