4WD MODIFICATIONS - TECH TORQUE
Shock absorbers are probably the least understood components of a 4WD’s suspension. These devices are more properly called dampers and their function is to control spring action.
If you want to understand the function of a shock absorber or damper try driving a vehicle with none fitted to it: it’s a terrifying experience. Springs that aren’t damped oscillate up and down and the vehicle can get out of control at very low speeds.
The first sprung vehicles were horse-drawn and were fitted with leaf springs. At the relatively low speeds possible with such vehicles the inherent self-damping action of tightly bound successive leaves was sufficient to control spring action, but the motor vehicle soon showed the need for better damping action.
Early shock absorbers were simple friction types: ‘clutch’ plates clamped together.
The first hydraulic shock absorbers were lever types, linking suspension action to a piston in an oil-filled cylinder. As seal technology improved the telescopic shock absorber took over and is still with us today.
Essentially, shock absorbers are oil-filled tubes, with a piston and rod sliding inside. Valves in the piston and also in the base of twin-tube shockers restrict oil flow and that action slows down the natural action of the suspension, otherwise the bouncing action would go on and on.
Shockers usually have more resistance to extension than compression and that makes sense when you think about it. When your 4WD hits a bump, the springs compress, then they extend as it passes over the bump. Making the shockers too stiff in ‘bump’ action would prevent the springs compressing properly.
However, some ‘bump’ damping is necessary, to prevent the springs from ‘bottoming out’ prematurely.
‘Rebound’ damping slows the natural extension of the spring after bump action, limiting the tendency to ‘hobby-horse’.
However, too much rebound damping stops the spring from extending properly. If the vehicle encounters a series of bumps, excessive rebound damping progressively ‘buttons down’ the suspension and ride quality suffers.
‘Bump’ and ‘rebound’ damping are necessary and dialling in the right amount of each is a science that surprisingly few suspension specialists completely understand.
It’s obvious that shockers need to be matched to the springs. Firstly, the length of travel of the shocker piston must exceed the suspension travel and, secondly, the internal valving must ensure that the shock absorbers aren’t fighting what the springs are doing.
It should be obvious from the foregoing that normal shock absorbers cannot increase the ride height of a vehicle, but some air-charged race shockers can. The only effect that common shockers can have on suspension travel is a ‘damping’ or slowing action. Those with powerful bump damping can reduce body sway by resisting initial weight transfer as the vehicle enters a corner, but this is only a temporary phase.
Check out this video that explains why many aftermarket shock absorbers that have excessive rebound damping don’t work optimally:
Twin and mono tubes
There are two types of commonly used shockers: twin-wall-tubed types and monotube types.
Twin-tube shock absorbers have an outer wall which acts as an oil reservoir for fluid that is displaced by the shocker shaft moving up and down inside the inner tube. Twin tube shockers are used by nearly all 4WD makers and most are gas-charged or foam-cell filled.
Monotube shock absorbers have no foot valve, so damping is achieved by a two-way piston valve. A pressurised gas charge is separated from the oil by a free-floating piston and displacement of the shaft is absorbed by gas compression. Very few standard 4WDs use monotube shocks.
Some monotubes are mounted ‘upside down’ – tube end upwards – to reduce the ‘target’ for stone damage.
Twin tube shock absorbers are preferred by most 4WD makers because they’re generally less expensive and they have inherent stone damage protection, because stone denting of the outer tube doesn’t affect the integrity of the inner tube, in which the piston works.
The traditional shock absorber for 4WDs is the twin-tube that consists of two concentric tubes.
In this design the rod and piston move in an oil-filled inner tube, while oil displaced by the volume of the rod flows in and out of an external tube via an internal foot valve.
Bump (compression) action and rebound (extension) action are controlled by oil flow resistance though the piston and foot valves.
Some quality shock absorbers have internal ‘bypass’ tubes that allow oil flow between both ends of the inner tube, without passing through the piston and foot valves. This design helps restrict oil-flow speed.
Most adjustable twin-tubes work by changing the bump and rebound damping in the foot valve, using an external dial at the base fo the shocker, but some types have adjustment in the piston as well, performed by turning the piston rod from the top of the shocker.
A mono-tube shock absorber has its rod and piston operating inside a single tube, with the gas charge separated from the oil by a free-floating piston. Bump and rebound resistance are controlled by oil flow through the piston valves and displacement of oil caused by the rod movement is absorbed by gas compression.
Much has been done by shocker makers to reduce ‘fade’. Fade happens when the oil inside the shocker turns to froth and there’s no resistance to piston movement and, hence, no damping action.
‘Gas’ shockers are still oil-filled, but they also have pressurised gas inside them, to act like a radiator cap does in preventing gas bubbles coming out of solution in the coolant (boiling). ‘Foam cell’ shockers have plastic foam that expands and contracts in much the same way that a trapped gas charge does.
Some designs have a separate oil reservoir connected to the shocker body, thus greatly increasing the amount of oil available and providing more oil-cooling surface area.
Mono-tube shocks have higher gas pressures and greater oil capacity than similarly-sized twin tubes, and have shown better performance and fade resistance in our suspension testing than all but top-quality adjustable twin-tubes.
The problems we’ve seen with 4WD shockers relate to four factors: shocker quality, track conditions, speed and shock absorber suitability.
Many after-market shock absorbers look great, but are cheaply made and, unfortunately, there’s no way of telling from the outside.
Road and trail conditions are often severe – our corrugations amaze overseas visitors – and very few shock absorbers can withstand prolonged use at high speeds under these conditions.
If you look underneath off-road rally and race vehicles, you’ll notice that none use standard shockers and most have multiple shockers or custom-made, re-buildable types at each corner of the vehicle.
Many race-oriented shock absorbers have visible ‘external bypass’ tubes that are able to regulate oil flow between both sides of the rod piston.
The rule of thumb with race vehicle shockers is: one to two grand a corner.
Suitability of a shock absorber for your vehicle is difficult to judge, because it’s impossible to tell from looking at a shocker what its internal valving is and how it will behave.
Even with the best shock absorber quality and matching, shock absorbers shouldn’t be viewed as long-term suspension components on hard-worked 4WDs. They’ll often need replacement at about the same time as you fit replacement tyres.
Suspension height changes
Japanese 4WDs are notorious for suspension sag, which occurs when the standard springs and torsion bars take a ‘set’ that is lower than the new-vehicle ride height. This can happen to lightly loaded vehicles, but is almost a certainty with those that are used at gross vehicle mass or coupled to heavy trailers with high tow-ball loads.
The most popular cure for suspension sag is a quartet of after-market springs, made to provide the same ride height as the standard spring, or a higher setting. Most buyers opt for the height increase, because if they’re shelling out for new springs, they might as well have the benefit of increased ground clearance and larger approach and departure angles.
However, vehicles with ride height changes beyond 75mm are becoming progressively more difficult to register and need an engineer’s certificate.
Even a 50mm lift may cause some problems. When longer coil springs are fitted to live axles, the diff ‘noses’ tilt, unless eccentric bushes are fitted to the leading and trailing arms. This tilt at the front alters the inbuilt caster angle of the front axle, which can affect steering feel and accuracy.
Height changes to live-axle suspensions alter the standard propeller shaft angles. Ideally, propeller shafts are set up with equal angles where the universal or cardan joints meet the axle and the transmission. Changes to these inbuilt angles can cause vibration in the driveline and premature wear.
Independent semi strut suspensions that are common in modern 4WDs don’t tolerate lifts above around 35mm without redesigned links and wishbones. Just lifting a standard suspension means there’s no suspension ‘droop’ left and there’s often metal to metal contact when the suspension is at full downward travel. Also, caster and camber angles can be adversely affected, resulting in weird handling and irregular tyre wear.
Often, the replacement shock absorbers fitted to modified vehicles get the blame for some of these mechanically-induced problems.
The advantage of after-market components is that they can be tailored to suit individual use, rather than being compromises for a wide market. However, springs and shock absorbers need to complement each other.
Departing from standard 4WD suspension components means discarding a formula that the vehicle maker feels is a reasonable compromise of ride, suspension travel and handling characteristics. It’s therefore incumbent on after-market suspension designers to perform adequate research and development before marketing their products. Our suspension testing has shown that some have done more homework than others.
It’s obvious that shockers need to be matched to the springs, so if you change from standard rating and length springs, you need to match the spring change with new shockers. It’s particularly important that the ‘open’ and ‘closed’ lengths of the shock absorbers match the spring travel.
One of the great dangers in changing suspension components is fitting dampers that have open and closed lengths that don’t match the travel limits of the springs. Dampers that are too short can be torn apart by extreme suspension travel, while those that are too long may bottom out and break shocker mountings.
Many after-market shocker sellers don’t know how to calculate the correct open and closed lengths of a shock absorber and that’s understandable, because it isn’t always easy.
It’s simple enough to measure the ‘block’ – compressed – length of a coil spring by multiplying the number of coils by the diameter of the spring wire. It’s also usually safe to assume that the unloaded height of the coil is its extended length, as it comes out of the box. Basing shocker travel on these two dimensions is approximately correct, for installations where the shocker sits inside a coil.
However, for installations where the shock is mounted away from the coil or for leaf spring fitments, suspension travel alone is not the correct dimension.
An after-market shock absorber seller needs to know the ‘motion ratio’ – the relative movement of the shock absorber and the spring – to find out the correct open and closed lengths.
Where shock absorber open and closed lengths cannot match the actual suspension travel provided by after-market springs the suspension travel needs to limited, by increased-height bump stops or travel-limiting straps, or both.
Under no circumstances should bump stops be removed to increase suspension travel, because this means the suspension will be over-compressed at full-bump. Under these conditions a leaf spring will eventually break, from over-flexing and a coil spring will become ‘coil-bound’ resulting in solid metal-to-metal suspension action that can break spring mountings.
Plastic or rubber bushes
If you look at the shock absorber mountings on 4WDs that have a beam axle, with coil or leaf springs, you’ll notice that the mounts are not really designed for the type of suspension movement that a hard-working 4WD suspension encounters on difficult trails. Nearly all 4WD shocker mounts are ‘linear’, with straight bolts or pins that accept the shocker bushes.
Obviously this design is a compromise, because designers know that most 4WDs spend most of their time experiencing ‘linear’ suspension movement, on formed surfaces. There’s less of a compromise in the case of 4WDs with independent suspension, because there’s far less suspension twist with independent suspension action than there is with a coil-sprung or leaf-sprung live axle.
However, with all 4WD shock absorber installations there are bushes between the shocker and the chassis mount and between the shocker and the axle or suspension arm. The bushes are there to allow for the inevitable twisting action that occurs off road. (Race and rally vehicles have very expensive spherical mounts to absorb the twisting action.)
Shock absorber bushes also absorb some severe-bump shock.
In most cases, shock absorber bushes need to be flexible, which is why most standard bushes are rubber. Many after-market bushes are plastic, which is firmer in action and more durable than rubber, and reputable shock absorber makers who fit plastic bushes ‘tune’ their shocker valves to work with the harder material.
Our bush testing experience favours two-piece, conical rubber bushes. Sure, they don’t last as long, but they’re better at absorbing twist and they’re very easy to replace.