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Shock Tactics
Classic American issue 178

 
You may remember that last time we began our journey through the various components of the suspension system. In this months exciting installment we move on from the mighty spring and take a look at the role-played by the humble shock absorber. We shall look at the various types of shock, how they work, what can happen when they go wrong and, of course, how to make improvements.

Firstly, what is a shock absorber? Well the name shock absorber is slightly mis-leading; shock absorbers should be looked upon as more of a damper that works in close relationship with the springs. If they were not present the vehicles springs would extend and release the energy they absorb from a bump at an uncontrolled rate. The springs would then continue to oscillate at their natural frequency until all of the energy originally put into them had dissipated. So it is the job of the shock absorber to damp these oscillations making the vehicle more comfortable and easier to control.

In their simplest form shock absorbers are basically oil pumps. A piston attached to one end of a piston rod works against hydraulic fluid in the pressure tube. When the suspension moves up and down, the fluid is forced through small holes, called orifices, in the piston. However, because these orifices only let a small amount of fluid through the piston it can not move as fast as the suspension spring. The effect of this is to slow down spring and suspension movement. Consequently the amount of resistance a shock absorber produces depends on the speed of the suspension and the number and size of the orifices in the piston. Modern shock absorbers are what we would call velocity sensitive hydraulic damping devices - meaning the faster the suspension moves, the more resistance the shock absorber provides. Because shock absorbers work in this way they automatically adjust to road conditions. As a result, shock absorbers reduce the amount of bounce, roll, sway and brake dive.
So is that it, is that all we need to know? Well, if only it was as simple as that. Whilst the basics behind shock absorber function applies to pretty much all shock, there are in fact several different types of shock absorber, each having their own individual properties.

Twin-tube shocks, are what we would normally define as a standard shock. These function exactly as described above, and can be considered as a basic shock absorber. In fig 1 the cutaway clearly shows the inner and outer tubes that contain the oil and air space. If we move in further, we see in fig 2 the piston in the pressure tube.

Fig 1. Twin-tube shock

Fig 2. Twin-tube cutaway
There is usually a simple valve system in the end of the pressure tube, we can see this in fig 3. The orifices in the piston can be seen in fig 4. These are the holes that the hydraulic oil passes through to actually create the dampening in the shock. In comparing twin tube shocks with other types of shock absorber you would find they are considerably cheaper to manufacture, and whilst offering effective dampening of all the different types of shock they offer the least consistent dampening by comparison. Twin-tube shocks are also more susceptible to fade, hydraulic oil aeration and heat dissipation.

Fig 3. Pressure tube

Fig 4. Piston assembly
Gas shocks, contrary to popular belief, offer a ride that is just as smooth as with a twin tube shock. However, the gas charged shock absorbers do provide far superior shock-damping consistency. So how does it do this? Well the rougher the surface you are travelling over the faster your shocks have to respond to damp spring oscillation. In a regular shock this rapid motion forces air bubbles into the oil within the shock, the oil turns into a frothy goo. This aeration of the oil reduces the performance of the shock absorber. Gas charged shocks get around this problem by using pressurized nitrogen in the shock body. The nitrogen, at pressures of 80 to 300 psi, will not mix with the oil and thus prevents the aeration of the oil and provides a more consistent and smoother ride. A selection of gas shocks can be seen in figures 5, 6 and 7.
Mono-tube shock absorbers use a single-wall shock tube to enclose the piston; the shock oil and if used the pressurized gas. These mono-tube shocks are much more precise at dampening than the standard shock absorber. They are made to closer tolerances than twin tube shocks. Because most mono-tubes use a larger diameter piston than their twin tube equivalent they are normally much stronger. Their design also allows for a better divide between the oil and air space, this in turn allows for reduced shock fade due to aeration and better heat dissipation.

Fig 5. Gas shocks

Fig 6. Gas shocks
There are some shock absorbers that are made with an external reservoir. The reservoir mounted on this type of shock isn’t, as many people might think, to hold extra shock oil. The purpose of this reservoir is to house extra air space for use during the shocks compression cycle. Although we say ’air space’ this is usually nitrogen, as used in a gas charged shock. Whilst there may be a small amount of oil in the reservoir it should be primarily thought of as the ‘air space’ for the shock. The way this type of shock is designed means that the main shock body is completely filled with oil. The reservoir is typically connected to the main shock body via a reinforced flexible hose or a metal tube. The shock functions by oil being forced into the reservoir through the connecting tube and against the pressurized nitrogen gas. Because the air space is in a separate reservoir there is less chance of aeration in the oil. This in turn leads to a shock that again provides a more consistently smooth ride.
A standard shock absorber provides its damping by the valve system located at the head of the shock piston; it is this that actually determines the dampening rates. Bypass shock absorbers aren’t all that different in that aspect, they do however add to this standard method of dampening by the use of extra metering valves. So how do these work? Well the additional metering valves are mounted externally to the main shock body. A spanner can be used to adjust the metering valve, changing the compression and rebound of the shock. It should be said that this type of shock is not cheap and is used mainly for high performance and off road applications.

Fig 7. Gas shocks

Fig8. Air shocks
Finally we have the air shock, fig 8. This is probably the most misunderstood shock of all. The secret behind the air shock is the sealed air chamber within the shock. Because this chamber is separate to the shocks required air space, the air shock provides an even smoother ride than the gas shock. There is of course the added advantage of being able to pump air into this chamber to allow load leveling. Air is pumped in via a small hose that comes as part of the installation kit for the shocks, fig 9. A lot of people make the big mistake of using air shocks to improve the ride height of a vehicle with failing or sagging springs. At best this is only a temporary fix.

Fig 9. Air line kit

Fig 10. Macpherson strut
We have separated the Macpherson strut, see fig 10, from our list of shock absorbers because of the way it is integrated into the suspension system. Struts and shocks both function in a similar way, but are very different in their design. The aim of both is to control excessive spring motion; struts, however, also form a structural component of the suspension system. It is possible in some systems for a strut to take the place of two or three conventional suspension components. They can also be used as a pivot point for steering and to adjust the position of the wheels for the purposes of alignment. The top of the strut will usually be held in place by means of a ‘strut mount’, see fig 11. These are normally found fitted to the inner wings on the strut towers. We discussed these in the last article.

Fig 11. Upper strut mount

Fig 12. Bump stop
So now we have a basic understanding of how a shock works and the variety of shocks that are available. We should now take a look at some of the reasons shocks fail and what we can do to make some improvements. So exactly how long should a shock or strut last. Well in terms of miles it is almost impossible to say how long a shock or strut will last. This is because the amount of work they do changes depending upon so many factors such as the condition of the road surface, the load on the shock and of course the way in which the vehicle is driven. By work we mean the conversion of suspension motion to heat. Remember your high school physics, energy cannot be created or destroyed, just converted from one form to another. It is this conversion of motion to heat that causes wear. Because this wear can vary so much, due to driving conditions, we would recommend that you check your shocks and struts at least once a year. If you cover more than 12,000 miles in a year you may want to check them on a more regular basis.
When your shocks or struts are working correctly you will normally find a light film of oil covering the upper half of the shocks working chamber. This is quite normal and is nothing to worry about. This coating of oil results when oil used to lubricate the rod gets wiped from the rod as it travels into the shock or strut chamber. An allowance is made for this slight oil loss and during manufacture extra oil is added to compensate. However, should you find fluid leaking down the side of the shock or strut this is a good indication of a worn or damaged seal, and the unit should be replaced. If the cause of a leaky shock is a damaged seal, then further inspection may be required to find the cause of the damage. Most suspensions usually incorporate rubber suspension stops called "jounce" and "rebound" bumpers, commonly known as bump stops, fig 12 and 13. It is these bump stops that protect the shock or strut from becoming damaged due to topping or bottoming. If these stops are damaged they should be replaced. The rubber stops usually last a good many years, however, if your vehicle takes a lot of punishment in the suspension system it may be worth considering and upgrade to polyurethane stops.
Many struts and shocks, especially for use on off road vehicles, will have a protective dust boot or gaiter, fig 14. This helps to keep dirt, dust and other contaminants from damaging the oil seals. These components should be regularly checked and replaced if they are worn, cracked, damaged or missing.

Fig 13. Bump stop

Fig 14.
Many shock related problems are usually identified by unusual noises originating from the suspension system, sometimes accompanied by excessive bounce, sway, or dive during braking. These symptoms warrant inspection. Any or all of the above symptoms could be caused by a shock or strut that has lost a large amount of fluid, is bent or broken, or has damaged brackets or worn bushings. As always the advice here is to replace the offending item. As a matter of course it is recommended that shocks be replaced as pairs. In fact more and more suppliers will only provide shocks as pairs. Struts may be replaced individually without too much concern, but if finances allow it is worth considering the replacement of both struts. There was a time when it was common place to replace just a shocks bushing, if this was the problem. Again more and more mechanics and suppliers are recommending replacement of the shock as an entire unit. This is not done just to increase the annual profits for suppliers, but does in fact make sense if you think about it. If a bushing is excessively worn, fig 15, there is a chance that excessive stress has also been placed on the shock itself resulting in possible damage to the shock. So changing just the bushes could be false economy. Some more expensive shock manufacturers will supply polyurethane bushes, fig 16, with the shocks as standard. These ‘poly’ bushes will go a long way to increasing the working life of a shock absorber.

Fig 15. Worn or damaged mount

Fig 16. Polyurethane bushes
When inspecting a strut based suspension system you will also need to take into account the upper strut mount, as we have already mentioned. These are a fairly large item containing a rubber doughnut again see fig 11. If this rubber is worn or damaged in any way then, you guessed it, replace it. If you choose not to replace this mount there could be numerous repercussions. Because the mount holds the top of the strut if there is any wear the strut is allowed to move. This movement can have a great affect on the caster and camber angles, not only dramatically changing the way the vehicle drives but also increasing tyre wear.

So just to recap, if you are looking to upgrade your shocks or struts your best and often most economical option is to choose a gas charged product. If you are likely to be using your vehicle for towing or carrying large numbers of passengers on a regular basis you may want to opt for an air adjustable shock. Our advice as always is to talk to your parts supplier and seek their advice if you are unsure. Finally remember to check your shocks and struts regularly and any associated fittings and bushes. This regular maintenance should help give you many miles of trouble free motoring. So go now and smooth out your suspension problems with confidence.