Everything you’ve ever wanted to know about dampers but were afraid to ask… We delve into the dark and mysterious world of suspension tuning to shed some light on just how performance dampers actually work… Words: Jamie King.
Suspension tuning has been somewhat of a dark art for many years, as it is much more involved than you might think. Far from just bolting on a set of lowering springs, getting the right suspension setup requires careful consideration of a number of factors, the finite adjustments of which can, and will, have a huge effect on how the car behaves. That’s why suspension tuning is a very specialist subject, and why manufacturers spends millions developing it.
However it’s not a case of one-size-fits-all; suspension setups are always somewhat of a compromise. Ever fitted uprated springs and dampers and found the setup either too harsh or too soft? Or that on some roads the car handles fantastically and on others it’s horrific? Well, that’s all to do with how the dampers behave, and that’s all down to their specific characteristics and the way they are built.
Here we will take a closer a look at the different types of damper available, how they work, and how they affect a car’s handling. Armed with this knowledge you’ll be able to ensure you get the right kit for your car and, more importantly, your needs. Then, and only then will you get the handling characteristics you’ve always dreamed of…
What is a damper?
Often incorrectly called shock absorbers, the damper doesn’t actually absorb any of the shock from running over an uneven surface. That’s the job of the springs. What the damper does, however, is control the oscillation of that spring to give control over the ride quality. Dampers that are too soft will cause the car to pitch and roll all over the place because the damper won’t be able to react quickly enough to the oscillation in the spring. Dampers that are too hard will cause a very bumpy and uncomfortable ride as they restrict the oscillation and therefore shockabsorbing effect of the spring. The art of suspension tuning is getting this balance just right for the intended use.
Bump and rebound
When talking about performance dampers there is lots of jargon to contend with, but the main terms to understand are bump and rebound. These are actually really quite easy to get your head around; bump refers to the damper’s behaviour when the spring is being compressed, and rebound refers to the damper’s behaviour when the spring expands again after being compressed. Different damper types will control both bump and rebound effects in different ways.
High speed and low speed damping
You will also hear quite a lot spoken about both ‘high speed’ and ‘low speed’ settings when talking about performance dampers. Within a damper you will find a piston and a cylinder. When this piston moves up and down within the cylinder, movement of oil above and below the piston is known as the ‘high speed’ settings. However, the piston itself is also able to allow oil to pass through it; the movement of oil through the piston is known as the ‘low speed’ settings.
At first it may sound odd to need both but imagine you are on a race track and you run over a kerb. The fast and aggressive compression of the damper means the piston needs to move the oil very quickly, whereas if you are driving round a long sweeping corner the load put on the damper is much more progressive and the movement of oil is much slower. Being able to control both high and slow speed settings gives you more control over the damper’s (and ultimately the car’s) behaviour. Most fast road kits feature an adjuster that controls the high-speed movement of oil, while the low speed settings are fixed – specified during the initial build stage, which is why it is vital to tell the suspension specialist what you want from the car. However, some top-end motorsport applications will also have adjustable low speed settings, too, with the pinnacle of dampers offering four-way adjustment (high speed bump, high speed rebound, low speed bump, and low speed rebound).
There are two main types of damper design: twin-tube and mono-tube. Twin-tube, as the name suggests, features a secondary tube to allow movement of fluid within the damper body. Whereas in a mono-tube damper the damper body itself is the ‘tube’ filled with fluid. Twin-tube dampers have been around for years, and most standard production dampers use this design. Inside the outer body are two tubes, a smaller high-pressure capillary tube, and a precision-honed pressure tube.
The piston fits inside the larger of the two tubes (often referred to as the cylinder to avoid confusion), and has precision machined holes that allow the oil within the damper to flow through it. When the damper is compressed on the bump stroke, the piston is forced into this cylinder. The oil then flows through the holes in the piston (the rate of which is controlled by the shim stacks used – we’ll cover that later) and up the cylinder into the piston rod guide at the top of the unit. The rod guide has a hole cross-drilled in it, which allows the oil to flow through it and into the capillary tube at higher pressure. The capillary tube then flows back down the length of the damper and into the damping control assembly at the bottom. When the pressure in the capillary tube exceeds the preload on the adjuster spring, the oil is allowed to bleed off into the rest of the damper – thus controlling the rate of damping.
On the rebound stroke, as the piston is travelling back up the cylinder, the pressure above the piston forces oil through the capillary tube and into the adjuster screw. This is how a single adjuster can control both the bump and rebound settings.
On a mono-tube damper the body itself becomes the tube, and the piston is an exact fit for the inside diameter of the body. This allows the use of a larger piston than in a twin-tube damper, and a larger piston offers more damping control because it displaces more oil. Also a mono-tube damper is usually able to react quicker than a twin-tube damper.
The oil still flows through the piston in exactly the same way and is still controlled by the shim stacks used. The difference is in the adjustment. Instead of having the control valve at the bottom of the damper, the adjustment is made from the top.
The piston rod is gun-drilled and has a shaft running through the centre. Attached to the end of the shaft is either a needle valve, which alters the amount of oil allowed to pass through the centre of the piston, or a spring that alters the amount of preload on the shims, and therefore controls the oil flow through the piston.
Some high performance mono-tube dampers have remote reservoirs. They work in the same way as a single-bodied damper but rather than all the oil all being stored in the one body there is, as the name suggests, a remote canister. This has several advantages over a single-bodied damper, such as the ability to carry more oil which helps with cooling and degrading issues (especially on endurance race cars).
However, the main advantage over a single-bodied unit is that a remote canister allows adjustment of the high and low speed settings on the rebound stroke, too.
Most performance dampers have an element of adjustment built into them. This can vary from simple rideheight adjustment on coilover-type dampers to four-way adjustment of both high and low speed settings on both the bump and rebound of the damper.
Ride height adjustment is fairly self-explanatory and is controlled by the length of the spring. In traditional kits this just means a shorter (or lowering) spring and with coilover-type units you can effectively alter the length of the spring by adjusting the spring platform height.
The bump and rebound adjustment can start to get a bit more complicated however. Most fast-road applications will be a twin-tube design with a single adjuster found at the bottom of the damper body.
This adjuster works in a similar way to a bleed valve. When the piston moves either up or down the rate at which the oil can flow through the piston determines how stiff it is. This depends mainly on the shim stacks or valves used (specified at the build stage) but if you allow some of the oil to bypass this piston you can alter the stiffness of the damper. This type of adjuster sits at the bottom of the damper. It comprises of a blanking plug, a spring, and an adjuster screw. The screw allows more or less preload to be put on the spring, and when the oil in the capillary tube reaches a pressure that exceeds this, the blanking plug is forced back and the oil from the capillary tube is allowed to bleed past, therefore giving control as to how stiff the damper is.
This setup controls both bump and rebound settings with a single adjuster; for individual bump and rebound control a second adjuster is needed. The bump settings are controlled in the same way as before with an adjuster at the bottom of the damper body, but to control the rebound settings another adjuster is introduced at the top. The piston rod will have a shaft running through its centre with either a needle valve or a spring and a plate at the end of it. The needle valve will allow more or less oil to flow through the centre of the piston, therefore bypassing the shim stacks and giving a softer setting. The spring and plate setup will allow more or less preload to be put on the shim stacks themselves, therefore altering the rate at which the oil can actually pass through the piston.
Mono-tube dampers can be controlled in the same way, with either a single adjuster controlling both bump and rebound at the same time or individual adjusters top and bottom. However, dampers with remote reservoirs allow you to add a further element of control – both high and low speed control of the bump settings.
The rebound settings are adjusted in the same way as normal but the bump settings are controlled by a combination of a needle valve (for the low speed settings) and a spring (for the high speed settings). When the damper is compressed oil is forced into the remote canister through either the needle valve or the sprung valve. Which route it takes depends on the speed at which the oil needs to flow. High speed damping (like running over a kerb) requires the oil to move quickly, and as the oil won’t be able to pass through the needle valve fast enough the pressure will build up and cause the spring to open. Controlling the rate at which the spring opens determines the high speed damping settings. Likewise, low speed damping (like driving round a long sweeping corner) requires the oil to flow much more slowly so pressures will be lower, the spring won’t open, and the oil will be forced to move through the needle valve. Controlling the rate the oil can move through this needle valve determines the low speed damping settings.
It is possible to control the rebound settings in the same way, resulting in a four-way adjustable damper. These are the pinnacle of damper technology but are so expensive and complex they are rare even in motorsport.
We’ve established that a within a damper there is a cylinder, and within that cylinder there is a piston. We’ve also established that as the damper operates this piston moves up and down, and as such oil is required to pass through it.
It does this by passing through small holes drilled into the piston. To control the rate of flow through the piston, and therefore the rate of damping, small sprung steel shims are used to partially cover these holes. These shims will bend as oil passes through. The amount they bend, and therefore the amount of oil that can pass through, will depend on the size and thickness of the shims used. A thinner larger diameter shim will bend more easily than a thicker smaller diameter one. In reality, a combination of varying thickness and diameter shims are used to give progressive control over the oil flowing through the piston, known as the shim stack. Typically this will include a large diameter thin shim at base of the stack, followed by a number of shims of decreasing diameter but increasing thickness. Oil will quickly and easily pass the first shim but it will become progressively more difficult for the oil to pass through the shim stack, therefore progressively increasing the damping force.
The shims are held in place by a spacer shim, then a plate of the same diameter as the piston, and then finally a securing nut. The thickness and diameter of the spacer shim also has an effect on how much oil can pass through the piston because it restricts the amount the blanking shims can move. For example a 1mm thick spacer shim will allow the blanking shims to move much more (and therefore allow more oil to flow through) than a 0.5mm thick spacer shim.
The shim stacks are used on both sides of the piston; the topside stack controls the bump settings, while the underside stack controls rebound.
One common misconception with dampers it that the term ‘gas-filled’ means the unit is charged with gas instead of oil. And that is utter rubbish! Think about for a second, it wouldn’t work at all – gas is compressible and a damper by its very nature needs to resist compression. This means that a purely gas-filled unit would offer no damping at all. However, gas-filled dampers do exist and they do contain a small amount of gas but this gas has almost no effect of the rate of damping. Instead it is used to aid reliability and longevity.
Ironically, it is because gas is indeed compressible that is used. Without gas the damper cannot be completely filled with oil and there will need to be an air gap. This is to allow space for the oil that the piston rod displaces as it moves in and out of the damper body. Without the air gap the damper would simply hydraulically lock.
However, an air gap can cause cavitations and aeration of the oil, reducing the performance of the damper.
So a compressible gas is used to eliminate the problem. It can compress enough to compensate for the oil the piston rod displaces and it allows the unit to be completely filled with oil to avoid cavitation problems.
Therefore the amount of gas used in a damper needs only be equal to the amount of oil the piston rod displaces.
On a twin-tube damper a gas-filled bag is inserted into the body of damper. When the piston rod is forced into the cylinder, the oil it displaces causes the air bag to compress. Whereas on a monotube damper the bottom of the unit is charged with gas, and the gas and oil are separated by a floating piston. There is a circlip on the inside of the damper to prevent the floating piston from rising too far up the damper, and the gas is kept underneath this piston. When the oil is displaced by the piston rod entering the damper the floating piston is forced downward, compressing the gas beneath it. As with the gas-filled bag the amount of gas charge only needs to be equal to the amount of oil that the piston rod displaces.
A lengthwise sectional view of an E60 M5’s electronic damper: On the left is the inbound stroke, on the right is the rebound.
Active damping control
Many modern performance cars, including BMW’s legendary M-cars, offer active damping control systems. These types of system can be known by many different fancy-sounding acronyms (in BMW’s case it’s called EDC – Electronic Damper Control) but all are very similar in the way they work, and all have the same goal: to remove some of the compromise between the level of ride comfort on offer and the performance potential when used in anger. We’ve already established that a setup that works well on the road isn’t necessarily the optimum setup for track use, and vice-versa but what if you could have your cake and eat it? Well, that’s what EDC kind of allows you to do.
The way active damping works is very complex in practice, requiring various sensors mounted all over the car and a very clever ECU to interpret all of these inputs and make sense of everything. But in theory the idea is fairly simple when you break it down. The easiest way to understand EDC is go back to looking at how a mono-tube damper works. We still have the shim stack controlling the oil flow through the piston and we still have a gun-drilled hole through the centre of the piston rod. However, rather than having a manual adjuster controlling the oil flowing through the centre of the piston rod (the knob on top of the damper body), this is controlled by electronic valves. This means the amount of oil flowing through the piston rod (and therefore by-passing the shim stack) can be directly controlled by the damping ECU. And this is what allows us to have various different suspension modes. For example, a ‘comfort’ mode might see this valve wide open, meaning less oil needs to travel through the shim stacks, giving a softer setting. A ‘sport’ mode might see the valve half-open, meaning only some of the oil can bypass the shim stack, resulting in a slighter firmer setting. And ‘sport+’ might see this valve fully closed, meaning all the oil needs to flow through the shim stack giving the firmest setting possible.
In reality it’s not quite as clear-cut as that; these valves are continuously being adjusted at fraction of a second intervals, and the modes are more of a base setting for how the ECU tells the dampers to respond. For example, the active damping system can respond so quickly it can detect when you stand on the brake pedal and actually firm up the front dampers to reduce the car’s tendency to dip its nose under heavy braking. The same is true when cornering and accelerating, too, resulting in a suspension system that is always at the optimum settings for the conditions.
Some active dampers can also alter the settings by changing the physical properties of the oil used. These are known as magnetorheological dampers, and use special oils containing metallic particles along with electromagnets to affect the oil’s viscosity. When current to the electromagnet is increased the oil is behaves like a thicker fluid, therefore making the whole damper feel stiffer and firmer. When the current is dropped the oil is free to move around more easily and behaves like a thinner fluid, softening the damper effects.
Active damping systems like these are very good for those who need the compromise between comfort and performance, so are ideal for fast-road cars. However, for race cars and track enthusiasts looking for the ultimate in handling performance without the concern of day-to-day use, these systems tend to be replaced with a more traditional setup designed purely for track use.