Air, juice, hydros, bags, whatever you choose, active suspension systems have been around almost as long as cars have existed. Here’s a brief guide to what you need to know…
Passive, semi-active, active, adaptive, what?
While things like suspension arms and bushes are used on all suspension designs, when it comes down to the job of controlling the vertical movement of the wheels, there are a few variations. These can be divided up in to active, passive, semi-active, and adaptive suspension. It sounds complicated, and while they certainly can be, understanding what they do isn’t too hard at all, so here goes…
What is passive suspension?
Passive suspension is what most cars come with from the factory, and it’s what almost all race cars are fitted with too; it’s the good old gas/oil fitted shock absorber and coil spring combo. Passive suspension is light, simple, and effective, but passive basically means it not adjustable or adaptable to changing situations without stopping and physically adjusting parts yourself.
What is semi-active/adaptive suspension?
Semi-active and adaptive are basically the same thing, and is certainly the most common type when it comes to production vehicles with trick suspension designs. While there’s a huge amount of variations, they all do the same basic thing, which is to enable the shock absorber to be stiffened or softened while on the move. Many cars with semi-active suspension allow manual in-car adjustment via switches. The main function of all designs is to help prevent body roll when cornering, squat under acceleration, and dipping under cornering, without the need for stiff suspension like most aftermarket kits would give.
Differentiating between ‘semi-active’ and ‘active’ is hard, as exactly what constitutes what depends who you listen to. While most people consider anything that offers in-car ride height adjustability to be ‘active’. So this covers aftermarket air and hydraulic kits, not to mention things like the hydropneumatic system fitted to many large Citroens for decades. However, some people consider ‘active’ suspension to be purely systems with fully automatic height adjustability. Which brings us neatly on to…
What is fully active suspension?
As mentioned at the end of the semi-active section, exactly what is active is open to debate, but one thing is for sure, it means ride height can be adjusted, either manually via in-car switches, or automatically via a series or sensors and often a hugely clever ECU.
Height adjustable suspension is used for countless reasons, from the aftermarket hydraulic and air suspension kits we all know and love, self-levelling systems that use it to prevent the system) kit, yum! vehicle from being lower to the ground when carrying a heavy loads, systems that facilitate ride height changes to allow the vehicle to traverse differing surfaces, or even on things such as buses and trailers to simply allow easier loading or unloading. Active kits can vary massively in speed and effectiveness, and be adjusted with air, hydraulic fluid, and in some sophisticated systems even electromagnets, but the result is the same; easy and super-quick height adjustability.
What is OEM active adjustable suspension?
While standard air suspension often bares little resemblance to the aftermarket stuff, you might be surprised to hear that it’s been around since almost the dawn of the car, well, over a hundred years!
It was the 1950s that really brought about the biggest changes in active suspension though, with some production cars automatically adjusting and self levelling regardless of load or driving surface, and most of these kits were also in-car height adjustable. The most famous setup is the Citroen hydropneumatic system, which used a mix of hydraulics and compressed gas, and they still use this same basic design to this very day on some vehicles. Many manufacturers have used the system too under licence, including Rolls Royce, Maserati, and Mercedes. From the 50s there have been literally hundreds of vehicles, from cars to busses, trucks, and trains, with some form of active air suspension.
In the ’90s there was a trend, especially on large Japanese sports cars such as the Toyota Supra and Soarer, to have semi-active systems fitted, though it’s been quickly forgotten about and many owners don’t ever realise, as by now most have been replaced with aftermarket passive suspension kits. Since 2000, active suspension technology has really taken off on production cars, with even many small hatchbacks having a ‘sport’ button to stiffen the suspension, and many vehicles use modern computer technology to adjust the suspension height and stiffness in a fraction of a second to suit any surface and driving style.