SHOCK TECH

Shocks (or more correctly dampers) still seem to be little understood, and that even applies to top level race teams! The following text will hopefully give some insight into the different types of dampers and how they work, as well as their benefits and disadvantages. We will look at how adjustable dampers work, and how they can be tuned to suit your car.

DAMPER TYPES

All current vehicle dampers operate using oil (principally) and fall into two categories – gas monotube and twin tube. Added to dampers are springs of many different types.

Twin tube dampers are the type usually found on production cars, and are made by companies such as Monroe, Boge, Sachs, Delphi, Gabriel etc. Adjustable versions of twin tubes are also made by Avo, Spax, Gaz, Leda etc. Koni make adjustable twin tubes, as well as gas monotubes.

Gas monotube dampers are made principally by Bilstein for production cars. Adjustable gas monotubes are made by Ohlins, Penske, Dynamics, Sachs, Koni amongst others. Nitron only manufacture gas monotubes.

GAS MONOTUBES

Monotube dampers have only one tube in the body construction, i.e. the tube you see from the outside has the piston running on its inner surface. All Formula 1 dampers are gas monotube. Mass produced road type dampers are a twin tube design due to the fact that they have a second smaller tube inside the outer tube and it is that smaller tube that has the piston inside it. By necessity the piston is much smaller, they are heavier and the heat dissipation properties are poor.

A gas monotube has the room to incorporate a larger piston. A small increase in diameter gives a very large increase in surface area – this translates to a much larger volume of oil being used by the piston for a given rod stroke to create the same total damping resistance, so the oil won’t get so hot. This gives you a damper that is more resistant to fade and will give consistent results over a long period of use i.e. endurance racing. Using a larger volume of oil for a given shaft movement also means a gas monotube damper is more sensitive to very small shaft movements.

A damper is essentially an energy converter, mainly converting the kinetic energy in the spring into heat energy using the damping oil. This heat has to escape and gas monotubes do this very effectively as they have the hot oil in contact with the outer tube exposed to the air. Gas monotubes are also lighter and their construction allows for a greater use of aluminium which allows the heat to dissipate more easily.

Gas monotubes are filled with high pressure Nitrogen which allows them to run in any orientation (unlike twin tube dampers that can only run rod up due to their internal valve structure). Running a damper with the light piston rod end attached to the wishbone helps keep the un-sprung mass element to a minimum, which in turn will improve the un-sprung mass’s response time to a disturbance.

So to summarise the benefits of gas monotubes over twin tubes:

• They are lighter
• They dissipate their heat better
• They are far more sensitive to small shaft movements
• They produce very accurate damping matches when built
• They give consistent damping results during long periods of use
• They can be run inverted to reduce un-sprung mass

HOW DO DAMPERS WORK?

All current racing dampers work by forcing oil through a constriction to produce a resistance to its flow. The amount of resistance is the damping force, and as a result the energy that the damper is absorbing is converted into heat energy.

There are numerous ways in which this resistance is achieved, but the accepted method for a serious performance damper is with the use of deflecting shims. The piston attached to the end of the piston rod has a series of complex ports running through it.

Oil flow in bump

Oil flow in rebound

These ports are then covered with a stack of shims of different diameters and thicknesses, and the whole assembly, with shim stacks both sides of the piston, is clamped together. Different shim stacks produce different damping values and characteristics that vary with shaft speed. The shims can also be pre-loaded to produce increased low shaft speed damping values. The ports vary on either side of the piston, and the shim stacks will also be different on the bump and rebound faces. Every application will require a different shim stack design, and usually each user will require one tuned to them.

As the piston moves through the oil, oil is forced through the ports, deflecting the shims, producing the damping resistance.

The ingress of the rod into the damper body will require something inside it to compress; if the body were totally full of oil, there would be no room for the piston rod to enter. There is a second piston of the same diameter inside the body tube, without ports and not attached to the piston rod (i.e. fully floating), on one side of it is the damping oil and on the other side is high-pressure nitrogen gas. As the piston rod enters the body tube, this gas charge compresses by the same volume. This gas charge also stops the oil from cavitating, a condition where the oil will produce gas bubbles when worked hard, in the same way as a submarine’s propeller.

In a twin tube type damper, the oil and air are just all in the damper casing together; some twin tube manufacturers refer to their dampers as gas filled where they pressurise the air volume already in there with nitrogen, this is a method really employed only by large volume damper manufacturers (Boge, Sachs, Monroe etc.). You can easily tell this type of damper from a non-pressurised one by pressing the rod in, if it pops straight out with some force it is gas pressurised. It does help to a certain degree to resist cavitation. Some twin tube adjustable manufacturers (Avo, Spax, Gaz etc.) refer to their dampers as ‘gas’ dampers, in their case the gas is an inert substance that is held in a bag (resembling a crisp packet), this bag is incorporated into the damper during the build process and allows for ingress of the piston rod, but it is not under pressure, hence the rod will not come out on its own.

There are really two ways in which a damper may be made to adjust, one is by producing a variable valve bypass bleed, either with a needle and jet arrangement or by a series of different diameter bleed holes, the other way is to provide a variable preload to the main valve arrangement so it requires a different pressure to open it. Dampers that have more than one adjustment will often use a combination of methods.

WHAT IS BUMP / REBOUND RATIO?

When a damper is compressed this is known as the compression stroke or ‘bump’, when it is extended this is known as ‘rebound’. The amount of force that the damper generates during these movements is usually different in each direction and is often stated as a ratio, for example a 3 : 1 ratio means the rebound force is 3 times the bump force. However, things are never this simple. As the damper movement speeds up the force it generates increases, if the bump and rebound forces were totally linear it would be possible to plot an exact bump to rebound ratio. Generally, damping forces do not increase at a constant rate with an increase with speed, and so the bump to rebound ratio varies for different shaft speeds.

Looking at a typical damping graph here, it can be seen that the damping forces do not increase at a linear rate, and the bump and rebound forces also increase at different rates. This means that at the slower speeds the damper would give a 1 : 1 ratio, this increases more to a 3 : 1 ratio at medium speeds, increasing further as the speeds get higher.

Each vehicle will have a unique damping graph shape which will have been developed through extensive testing on the track and on the road.

WHY IS ADJUSTABLE DAMPING REQUIRED?

Damping forces that work well in the dry will most probably be too stiff for wet conditions. One set of dampers can therefore be used to adjust to wet conditions if they have the ability to have their damping forces reduced. Similarly, some drivers may prefer the car to be stiffly set up, whereas other drivers will prefer to have their car slightly softer. Adjustable dampers allow you to do this.

A well sorted circuit car in the hands of a competent team will have different damper settings for different circuits, as well as different springs. So in the same way you change your diff ratio for different conditions (circuits), you also change your damper settings, even though the changes may be subtle.

Multi-adjustable dampers will allow you to fine tune the damping forces generated at different damper speeds for a certain set of conditions, but in reality unless you are competing this feature will most probably be unnecessary.

Drivers that use their car on the road as well as the track (i.e. track days) will usually find the settings that suit them for track work, and will then back them off for road use.

Fixed rate dampers will have their damping optimised for one set of conditions, this can of course be changed by rebuilding the damper’s shim stack.


WHAT IS A REMOTE RESERVOIR FOR?

All dampers require something to compress internally to allow the rod to enter the body, in the case of a gas monotube this is achieved using a floating piston and a compressible volume of nitrogen.

A damper with a remote reservoir either on a hose or piggybacked onto the side of the shock allows the length of the main shock absorber to be reduced by moving the floating piston assembly to the side, and a much larger volume of nitrogen may be used which has a more consistent pressure during the rod stroke.

As the piston rod is pushed into the top of the body tube, an identical volume of oil is displaced into the remote reservoir. Placing valves in the head of the remote reservoir canister allows us to vary the damping rate in compression as oil is passed through it during the compression stroke. An additional benefit is the ability to cool the nitrogen by mounting the reservoir where there is cooling airflow and away from sources of heat, this will keep the nitrogen at a more constant pressure and help with consistent damping. Remote reservoirs also allow us easier access to the compression valve adjustment knobs.


WHAT IS HIGH SPEED / LOW SPEED COMPRESSION DAMPING?

As a damper can only move in and out, it can only generate forces in compression or extension. The forces it produces can vary as the shaft speed varies, and can vary from compression to extension. A damper engineer will work at producing suitable forces for the vehicle in question, if necessary rebuilding the shim stack to produce the forces that are required. An adjustment system will help, as turning a damper control knob is a lot quicker than rebuilding it with a different shim stack. An adjustment system that can allow for independent adjustment of low and fast shaft speeds in compression can be of real benefit.

Low shaft speeds can really be thought of as the chassis moving around, i.e. diving, rolling and squatting, and combinations of these. As a car is driven through a corner, the chassis will take a variety of different attitudes as it goes from straight to straight. These are all low speed shaft movements as far as the shocks are concerned.

High shaft speeds are where the wheels encounter something that rapidly disturbs their steady state, and the chassis will hopefully remain stable, i.e. a bump.

In reality, the shocks will see a whole combination of speeds, as the car goes balancing its way on the limit through a corner, whilst taking a kerb on the way. Independent adjustment of high and low shaft speeds in compression will allow the chassis to be fine tuned to cope with these situations to the optimum degree.