Whether you’re just looking to lower your car or you’re after the ultimate high-performance handling setup, the right springs and dampers or coilovers can help you achieve exactly what want. In this guide we take a look at suspension components and examine how they work.

Guide from Performance BMW. Words: Gerry Speechley, Elizabeth de Latour   Photos: Eibach, ST Suspensions, KW, Ground Control, BC Racing

Suspension guide – what are suspension springs?

In a very basic view of the suspension system, the springs support the weight of your car and control the ride height and its vertical motion over bumps. They do this by storing the energy used to compress them by bumps in the road and then releasing it back into the car in a controlled manner thanks to the dampers.

Suspension guide

Suspension spring rate

The spring rate, usually referred to in lbs/in (though you’ll also see kg/mm), dictates how much a spring will compress depending on the load applied. For example, a spring with a linear rate of 250lbs/in simply compresses one inch for every 250lbs load applied to it. So a load of 125lbs will compress it 1/2” and a 500lbs load will compress it 2”. A spring with a progressive rate gradually gets stiffer as the load increases, so a 150/500lbs/in spring will compress 1” with a 150lbs load on it, but may need 250lbs to compress the next inch and then 400lbs for the third inch until it reaches 500lbs load to move another inch. Looking at it the opposite way, as the load increases, let’s say 150lb at a time, the spring compresses less and less until 150lb may only compress it a further 1/4”.

The spring rate of any given spring is determined by the wire diameter, material and number of coils (which relates to the overall length of the wire). The larger the diameter of the wire the higher the spring rate, but the more coils the spring has the lower the spring rate. The spring rate can be made progressive on a single coil spring by tapering the wire so that the thinner wire has a lower spring rate than the larger diameter wire. On most high-performance applications where a variable spring rate is required, the system of ‘stacking’ coils of differing rates on a coilover shock absorber is used, which allows both progressive and digressive suspension set-ups.

Suspension guide

Progressive springs

Progressive springs are created by altering the coil diameter as opposed to the wire diameter for ease of manufacture and cost. On coilovers, however, a progressive rate is achieved by stacking two or more springs on top of each other. A progressive spring rate starts soft and then ‘progressively’ gets harder so that as the suspension starts to travel, the spring rate is soft and comfortable, soaking up small bumps and keeping the tyres in contact with the road. As load increases, the spring rate also increases allowing for control in more spirited situations. This brings us to the use of a stacked pair of springs for optimum performance using a lighter spring for soaking up smaller bumps, known as a ‘tender’ spring, which can be fixed-rate or progressive, and which is designed to fully compress into coil bind and then allow the main spring to deliver the final spring rate. There is also another type of spring used in a stacked spring setup called a ‘helper’ spring, which has a very low spring rate and is designed to retain the main spring onto the spring perches during full suspension travel only or when the suspension is ‘hanging’, having virtually no effect on the spring rate of the combination.

Suspension guide

Height-adjustable springs

If you’re happy with a fixed drop in ride height then lowering springs are great but if you want height adjustment you’d normally go down the coilover route. However, if you’ve got a car with electronically adjustable dampers, like those found on most modern performance BMWs, and you want to retain these for their functionality but also want to lower your car and have adjustable ride height then you need a height-adjustable spring (HAS) kit. Now, obviously, you can’t adjust the height of the spring itself as such as a coil of metal does not have settings you can just select, so these kits actually use adjustable spring perches. These are threaded and use the same sort of collar adjuster that you’d find on a coilover setup and offer the same easy adjustment method and a wide range of adjustment, allowing you to obtain the perfect ride height on more of a budget.

Suspension guide – what do dampers do?

When your car hits a bump, whether it is a sudden shock or a slow undulation, energy from the wheels is transmitted to the suspension springs which compress, storing the energy until the bump is passed and then the spring returns it by extending back to its original installed length. As the spring does this, it will extend past its original position until the car returns the energy to the spring and this will set up an oscillation in the car and it will continue to bounce, uncontrolled, for a long while. These uncontrolled bounces will significantly affect the handling of the car so there needs to be some way of controlling the spring, and this is the function of the damper. It effectively resists the
fast movement of the spring by absorbing the energy of the spring, usually through heat absorption, and dampers are there to stop the spring from bouncing uncontrolled.

How do dampers work?

A damper is a tube filled with hydraulic oil and with a piston rod positioned inside it – at one end of the piston rod there is a piston valve with holes in it and these holes dictate how quickly the oil can pass through them and therefore how quickly the piston can move. By altering the size of the holes in the piston and by stacking discs on either side of the piston to generate a pressure drop, the bump and rebound resistance to movement can be adjusted. The hydraulic oil needs to be temperature stable or the thinning of the oil due to increases in temperature during use will result in diminished damper efficiency, and they must also have an anti-foaming additive because any aeration of the oil will have a drastic effect on the dampers’ effectiveness because the trapped air will be compressible.

Dampers can use a monotube or twin-tube design – a monotube damper contains a piston housed within a single tube and this type of damper has several positive features. First of all, the damper can be mounted at almost any angle and because the tube containing the oil is in direct contact with the surrounding air it helps to cool the oil inside the damper more effectively, which means you can drive harder for longer without any drop-off in suspension performance. A twin-tube damper is effectively a tube inside a tube with a valve at the bottom. The inner tube functions similarly to the monotube damper with a piston and orifice valves, but the outer tube acts as a reservoir of oil.

Suspension guide  – what are coilovers?

If you’re looking to go low and/or want the best handling upgrade, coilovers are what you need. Coilovers are just coil springs fitted over dampers, hence the name, but they offer more than a simple spring/damper combo with even the most basic coilovers coming with height adjustment via threaded sections on the coilover housings. Most coilovers will offer some form of damping adjustment and, as the price increases, so does the level of functionality offered by a setup. With most coilovers the lower you go, the more you’re reducing spring travel by compressing it and the more you’re stiffening your ride, something to bear in mind, but some use shortened coilover strut housings, so even at maximum lowering the remaining spring travel is not affected.

Pretty much all coilovers will offer one-way damping adjustment – this usually combines compression and rebound behaviour into one adjuster, dictating how quickly the piston in the damper will move through the fluid within. Increase your front damper rates and you’ll up understeer by increasing rear-end grip; increase rear rates and you’ll increase front-end grip but your car will be more inclined to oversteer, so getting the balance right is crucial. A two-way adjustable damper will let you independently control compression and rebound settings; compression (or bump) dictates how the damper will react to the initial encounter with a bump – too firm and the ride will be harsh and the car might even lift off the ground in extreme circumstances. Too little damping and the car might dive under braking and roll heavily through corners. Rebound, meanwhile, affects how quickly the damper returns to its normal height – a high rebound setting will cause the piston to move a lot more slowly and can even cause the car to leave the ground, especially during cornering, as can often be seen on touring cars; too low, and you’ll get soft, wallowy suspension. There are also four-way dampers, where you can adjust high- and low-speed compression and rebound independently of each other.

While coilovers are never the cheapest suspension upgrade option, the lowering range they offer along with the improvement in handling means that they are the best choice if you’re looking for the best way to lower your car and enhance the handling at the same time.

Suspension guide: Passive, semi-active, active, adaptive suspension – what are they?

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…

passive car suspension

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.

adaptive semi active car suspension

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…

fully active car suspension

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.

OEM active adjustable car suspension

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.