All you ever wanted to know about dampers, plus a bit more. Following on from last month’s look at springs, now it’s time for an in-depth journey into the world of dampers. Words: Gerry Speechley Photos: GAZ, KW, Bilstein, ST Suspensions, BMW, Audi, QA1, Intrax.
DAMPERS: WHAT DO THEY DO?
When the vehicle hits a bump, whether it is a sudden shock or a slow undulation, the vehicle transmits the energy from the wheels to the suspension springs which compress, storing the energy until the bump is passed and then the spring returns the wheel to its correct position by extending the spring back to its original installed length. As the spring does this, it will extend past its original position until the vehicle returns the energy back to the elongated spring and this will set up an oscillation. As such, the vehicle will continue to bounce, uncontrolled, for a long while. These uncontrolled bounces will significantly affect the handling of the vehicle, so there needs to be some way of controlling the spring.
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. A damper therefore is a resistance to movement that can be adjusted either by its original design or by the user to optimise the vehicle’s suspension setup.
HOW DAMPERS WORK
You will be familiar with direct acting telescopic dampers as they are on the majority of modern vehicles and although designs vary the principles are very much the same. The extension and compression of the damper simply alters its external length between suspension mounting point and the vehicle chassis with the motion controlled by the transfer of hydraulic oil through calibrated holes.
In these direct acting dampers 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. 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.
These dampers are more reliable than the older lever type dampers because the internal operating pressures are much lower but the design still needs particular attention to the sealing method of the damper to piston rod and the bearing to rod contact where the rod is usually hard chromed and the bearing sintered iron with a small volume of the damper oil used to lubricate the bearing.
The twin-tube damper works similarly to the mono-tube damper except with a few differences. The damper is effectively a tube inside a tube with a valve at the bottom. The inner tube functions similarly to the single tube damper with a piston and orifice valves although the outer tube acts as a reservoir of oil. The ‘base’ valve functions in three ways: during both damper compression and rebound the valve allows a controlled amount of oil into the reservoir chamber, while on slow damper movements it controls a ‘bleed’ valve to reduce the damping effect. These tube type dampers form the basis of almost all performance suspension setups including the inserts and replacement suspension legs in the McPherson strut design, where the damper is integral to the main suspension leg, and the very popular coilover arrangement where the damper body is threaded and so the coil spring sits around and on the damper tube.
The monotube damper, as its name implies, contains a piston housed within a single tube. The tube has an attachment at one end to mount it to the vehicle, usually with a rubber mount to isolate the damper from sudden shocks and to allow slight misalignment of the damper during travel. The piston is connected to a rod that extends out of the cylinder, through an oil seal to the other end mounting.
The piston is fitted with reed valves in the body covering oil holes through which the oil can flow as well as a bump stop to limit the extension against the seal. The rod is protected externally from dirt ingress that can corrode it, debris impact damage and from collecting dirt that will accelerate top seal wear with either a rubber bellows or external tube that extends to below the seal level.
There is also a free moving piston at the bottom of the damper that seals a small chamber containing an inert gas (usually nitrogen). When the damper is filled with oil, this gas is compressed so that during compression of the damper rod, as the oil travels into the upper chamber, the free moving piston moves to keep the contained oil volume constant because the oil itself is incompressible. This gas pressure also pressurises the oil to compress any air pockets or bubbles trapped within the oil to reduce or eliminate its compressibility.
On the compression stroke, known as the ‘bump’ stroke, the movement of the damper piston down causes the oil in the lower chamber to open the compression reed valves and bypass the piston through the holes, absorbing the energy as it does so, providing the damping force. As the damper is then extended during the rebound stroke, the other reed valves open to allow the oil to transfer back into the lower chamber.
By altering the size of the holes in the piston and by stacking discs either side of the piston to generate a pressure drop, the bump and rebound resistance to movement can be adjusted. Absorption of the energy is converted by the oil into heat and so an oil with a stable viscosity is needed to keep the damping effect constant with temperature.
This type of damper has a number of positive features in that the damper may be mounted at almost any angle and because the tube containing the oil is in direct contact with the surrounding air, the thermal transfer of heat from the oil, through the tube to the atmosphere provides an efficient method of dissipating the heat. However, in some forms of motorsport where there are extreme levels of travel or the vehicle suspension is subject to continuous violent actions (such as in rallying) the tube may not be able to dissipate the level of heat transmitted to the oil. In these cases the damper may have a remote oil reservoir fitted to increase the oil capacity and the surface area to dissipate the additional heat.
ELECTRONICALLY ADJUSTABLE DAMPERS
In recent years manufacturers have made their dampers adjustable either manually or, more usually, electronically with the use of solenoid valves in the damper to alter the damping rates. Often seen as ‘comfort’ or ‘sport’ settings, these systems remotely change the damper rates using a solenoid valve to control the bypass rate of the hydraulic damping oil. An early BMW example would be the 7 Series and 8 Series of the late ’80s and ’90s.
These had the option of EDC (Electronic Damper Control) where the spring rates were very soft with a high spring preload so that on the ‘comfort’ setting the soft spring rate matched with a low damping rate produced the comfort setting but selecting ‘sport’ really stiffened-up the damping rate giving the feel of a more sportsorientated car, although the spring rates were not altered and remained soft. EDC was used in the E9x M3 and E60 M5 and BMW still offers various adjustable suspension systems across the current range, such as Adaptive M suspension on the M3 and M4, and Variable Damper Control, which all perform a similar function in allowing you to adjust the suspension stiffness based on the road conditions and your driving style.
A more recent development in damper technology is the magnetorheological suspension system. Commonly known as magnetic ride, these are a 21st century solution to the compromises of the old design adjustable dampers. To achieve improved vehicle dynamics, handling and control, we need to increase spring and damper rates. However, this would compromise comfort, so in come the magnetic ride dampers.
Usually coming with a number of driver selectable base settings, such as ‘comfort’, ‘sport’ or ‘race’ the vehicle uses sensors to fine-tune the dampers. These dampers can continually adapt to driver and vehicle inputs including vehicle roll, yaw, speed, steering and braking by self adjustments made in milliseconds using a magnetorheological (we will call magnetic) damper oil, which contains millions of microscopic particles in a special hydocarbon oil. Within the damper body is an electrical coil that is connected to a control module. When the vehicle and driver inputs indicate a higher damper rate would be favourable, a voltage is applied to the coil, which aligns the particles in the fluid, effectively increasing its viscosity and therefore damper rate. By increasing the current through the coil, the fluid viscosity can be further increased effectively producing an infinitely variable damper that can respond to requirements in milliseconds.
DAMPING DONE RIGHT
Now we have looked at the basic types of damper, we need to consider exactly what the damper does and how altering it will affect the vehicle’s handling capability. First, we need to understand how the damper controls the spring oscillations from under- and over-damping, to critical damping.
Without any form of damping the vehicle would continue to bounce in the form of a sine wave until the small resistances in bushes and the shock absorption in the tyres eventually reduces it. If the damper is too weak, then the suspension will continue to bounce for too long. This is underdamping and causes the tyres to lose contact with the road as the weight of the vehicle moves upwards. If the damper is too strong, then the suspension will not recover in time to absorb the energy from the next bump. This is over-damping and will again cause the wheel to lose contact with the road Finally, the correct level of damping will be where the suspension recovers quickly, keeping the tyres in contact with the road but controlling body roll and pitch.
SPRING AND DAMPER RATES
If we have a fixed damper rate, as in most vehicles, and then increase the spring rate during lowering of the vehicle, then the spring will be too strong for the damper to control and so the effective damper rate will decrease causing the ‘bouncing’ down the road that’s often seen on cars with extreme lowered suspension where only the springs have been changed.
This brings us to the first major relationship between suspension components: the relationship between spring and damper rates. It must be remembered that the damper does not affect vehicle ride height; even a gas pressurised damper will have a negligible effect on ride height because the damper is simply a device to restrict the speed of suspension movement, it offers no vehicle support. The faster the movement applied to the damper, the higher the damping rate, so in effect the slow roll of the body during cornering is controlled by the spring rate acting against the weight transfer of the body to the outside suspension, and the resistance offered by the antiroll bars (sway bars to our North American friends), not the dampers which only control the roll speed.
There are numerous ways that we require varying degrees of damping, both in compression and rebound depending on the weight and geometry of the vehicle in question and the degree of comfort versus performance that the driver expects from the car. If the rebound damping rate is very high, the suspension can ‘load up’ until the spring is fully compressed as repeated impacts compress the spring further and further because the damper is restricting the recovery of the spring to its normal installed length.
Most aftermarket suspension kits that come from reputable suppliers come with dampers that are matched to the new spring rates and are aimed more towards the performance area of the car’s ability rather than the comfort zone. These kits are the most popular due to there usually being no setting-up required; you can just install and enjoy with maybe an alignment recommended to get the most from the kit.
However, going a stage further, we get to the adjustable ride height and adjustable damping coilovers (or adjustable dampers suitable for adding to conventional uprated coil springs). This type of damper can be adjusted with a combination of both compression and rebound damping adjusted together (or rebound only) to suit lowered and uprated springs, or adjusted ride height in the case of the popular coilovers, as well as going even more performance-related with less comfort than many drivers would find acceptable.
From here on, the dampers start to get more technical with the compression and rebound damping adjustable separately in two-way adjustable dampers or even separate high speed and low speed damping rates on both compression and rebound on four-way adjustable dampers. So what is the effect of each adjustment? Well, as the damping rates are increased on the front of the car in both bump (compression) and rebound, the grip at the rear of the car increases causing an increase in understeer. This increase may be desirable if the car was oversteering before the adjustment, therefore making the handling more neutral. If the damping rates are increased on the rear of the car in both bump and rebound, then the opposite occurs and you increase front grip but increase the tendency to oversteer.
Going back to the two-way adjustable dampers, the bump setting will affect how the car reacts to the initial encounter with the bump. Too much damping and the ride will be very harsh and the car may even lift, increasing its ride height and the excessive damping may even reduce the body roll, while insufficient bump damping can cause a soft bump response, diving during braking or lifting during acceleration and excessive body roll in cornering, especially on the inside front entering a corner or on the outside rear exiting the corner. Turning now to the rebound settings and their effect, if the rebound setting is too high, when exiting a bump the wheel may leave the ground due to the slowing of the reaction of the spring to maintain road contact. The inside cornering wheel can be held up during fast cornering (seen often in Touring Cars) and the ride height may be lowered by restricting the ability of the spring to return to desired ride height. If the rebound damping is set too low, then we return to the example of the suspension bouncing to the natural frequency of the spring and the suspension compressing during acceleration causing loss of traction.
So to see how the damping rates affect handling we can summarise as follows: to increase oversteering characteristics we can decrease front bump or rebound or increase rear bump or rebound settings, or, to increase understeer, increase front bump or rebound or decrease rear bump or rebound settings.
Manufacturers tend to design a car with a tendency to understeer as it is a far safer option for inexperienced drivers and far easier to control than a car that oversteers on the limits of handling or road conditions.
Going a stage further and we are entering the realm of professional motorsport and Formula One with the four-way adjustable damper. In this case, the damper speed (the speed at which the damper compresses and extends) has different damping rate settings, user adjustable, for high and low speed damping in both bump and rebound. To define these ‘speed’ settings we should mention the damper speeds relating to the terminology.
Low speed damping refers to damper speeds of static to 50mm per second and is controlled by bleed holes or discs in the damper piston, a bypass around the piston, or valving in a bypass jet or adjustment needle. Medium speed damping is damper speeds of 50mm to 200mm per second controlled by the disc stacks either side of the main piston and high speed damping of 200mm or more per second being controlled by the piston port size and the disc stacks. The diagram above shows how the damping rate changes with damper speed and how the valving produces a distinct area of low and high speed damping rates. Each colour trace is the damper ratio from different settings. The upper traces are the compression graphs and the lower traces the rebound settings.
In the low speed damping settings, the dampers control the small bumps, undulations and changes from acceleration and braking affecting the pitch (front dive and rear squat) of the car. The high-speed damping settings control the larger or more violent bumps. With these settings the different vehicle dynamics can be adjusted in different situations and points of a turn.
With a corner approaching, we begin to brake and the vehicle transfers weight from the rear tyres to the front. If this transfer has insufficient front bump damping or too much rear rebound damping then the sudden transfer of load to the front tyres will induce a loss of grip. If the bump rate is too low, or the rear rebound too high, then braking effect will be compromised as the rear dampers reduce the downwards force on the rear tyres and they would lose grip.
As a corner is approached, the vehicle is subjected to an increasing braking force with the introduction of increasing steering input. This combination will put the outside front damper into low speed bump and the inner rear into low speed rebound. As the corner is entered, the braking is reduced and the steering angle increased as the car turns. This puts the outside rear damper in low speed bump and the inside front in low speed rebound. Once into the corner, vehicle speed is stable but the steering is at maximum input. At this point the weight of the vehicle is transferred to the outside of the turn, so both outside dampers are in low speed bump and both inners are in low speed rebound.
As the vehicle starts to exit the corner, the opposite happens to the vehicle as the steering angle decreases and approaches and passes straight ahead. As the vehicle accelerates out of the corner, the weight is transferred back to the inside and rear of the car, putting the outside front in low speed rebound and the inside rear in low speed bump. At this point, the rate of weight transfer can be adjusted with the damper settings. At the optimum setting, this transfer will be fast enough to maximise traction without breaking the tyres loose due to excessive bump damping.
Almost all of the low speed damper rates are used to control the vehicle’s weight transfer during braking, acceleration and cornering and we need to set this up to maintain as much traction from the tyres as possible. If too much weight is transferred to just one front tyre in the corner, the total grip will be reduced. As the difference in load on the two front tyres increases, so does understeer. As can be seen in the description of the damper conditions above, the dampers are asked to perform different functions to optimise the handling, even in just the forces exerted in a single corner, so we still have to compromise, even at this stage of four-way damper development.
Out on the road the low speed damping reacts immediately to suspension motion by damping the movements in both bump and rebound, reducing the speed of the motion to keep the wheels and tyres in contact with the road. The answer is adaptive damping, as seen on most modern electronically adjustable damping systems, where adjustments may be made to all the systems by a computer receiving data inputs from sensors mounted around the car measuring pitch, roll and yaw as well as the suspension reactions and driver inputs from the steering and throttle, all whilst driving to optimise the suspension at all times. BMW has even developed a system called Active Comfort Drive, which scans the road ahead and adjusts the air suspension settings accordingly.