We saw in the last section how Hydragas suspension uses a sealed container of nitrogen gas to make a spring. Air suspension works on the same principle, only using air instead of nitrogen.

Why Use Air?

  To be honest, you could use any gas you wanted in a suspension spring: As long as it can be put into a sealed, flexible container under pressure, you can use it to absorb energy as it is loaded and the container deflects, compressing the gas within it. The limitations are that your choice must remain consistent at different temperatures, and not try to do anything nasty (like explode) as it's compressed. On this basis, nitrogen is a good choice - as we said previously, it's an "inert" gas, so things like compressing or heating it aren't too much of a problem. Air is not inert, given that it contains things like oxygen, but it's plenty stable enough for what we want to do with it.

  However, there is another factor: Cost. Air has a massive advantage in this respect, because it's free and available in effectively limitless quantities. Practically, this means that if you want to add air to a spring, you can just grab some more from the atmosphere, and if you want less, you can just vent it out. This is why air springs aren't totally sealed units: They normally have pumps attached to them to adjust how much air a spring contains, thereby altering it's stiffness. The actual material of the spring "bag" contributes almost nothing to the load-handling of the spring, other than to contain the pressure inside it. These bags are made from highly flexible material such as a flexible plastic or rubber, often with reinforcement fibres running through it.

  We mentioned before that using compressed gas in a spring gives the advantage of making it rising-rate. With an air spring attached to a pump, you get both a rising-rate spring and almost infinite adjustability. For example, a normal spring, even if it has some progressive properties, has to be stiff enough for the maximum load it's going to carry, and this can sometimes mean that it's too stiff when the vehicle's unladen. Being able to simply pump air in or out of an air spring means this is not an issue, so you can accomodate both an unladen vehicle (not much air, soft spring) and a fully-loaded one (lots of air, firm spring), as well as anything inbetween.

  This is why lorries, coaches etc have long used air springs, as there is a big difference between loadings depending on whether they are full or empty. Air springs can also be used in the same way to raise and lower a car's suspension at will, whether for a lowrider that needs to clear speedhumps, or a 4x4 that needs to be raised for off-road use and lowered on-road. The prime advantage is always the ability to adjust without any parts being swapped, allowing one suspension set-up to cope with several (sometimes opposing) requirements.

  On some vehicles, like a Range Rover, more sophisticated systems are used, which can also raise/lower each corner individually to keep the vehicle level over torturous terrain, and reduce the chances of scraping the underside on something - which is a pretty high priority in a £60k vehicle. Such systems can be controlled automatically by computer, or (most commonly in specialist aftermarket systems) by manual controls operated by the driver, with an option to run automatically. They also make getting in and out a lot easier, as the vehicle can be set to sink all the way down in an "access mode" when the ignition is off. We'll be looking at the uses of electronic controls in suspension systems in a later section on active suspension, So I'm not going to add any more details here.

  Although air suspension works well on it's own, it can also be used in conjunction with an existing spring (e.g. a coil or leaf spring) to provide adjustability, which is a common aftermarket option. Such systems can be either linked up to a compressor permanently, or have to be adjusted with a seperate pump depending on the load being carried, which makes for a much cheaper system. In most cases, these are designed purely to level the vehicle when heavily loaded rather than to do anything clever.

  The only real fault with air spring systems is that they can leak, whether from the spring, the pump, or the line connecting the two. Although most modern systems are reliable, the cost of replacing components (especially compressor pumps) can be high. For this reason, on some vehicles that are known for experiencing problems with their air suspension systems (e.g. Second-generation Range Rovers), a small industry has developed around adapting them to use more traditional springs such as coils. This pretty much sums up the advantages and disadvantages of air springs, particularly computer-controlled systems: When they're working they're very effective, but when they break it can be an expensive hassle.

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