4×4 Suspension & Shock Absorbers
A 4×4 suspension system is more than just a series of control arms, axles, and links designed to keep the bottom of the 4WD vehicle off the road surface: while its primary function is to absorb uneven spots in the road surface to limit body roll, a proper suspension system is also required to maintain sufficient ground clearance, prevent wheels losing contact with the ground, and to allow the chassis to flex in extreme off-road conditions.
However, while some modifications can improve the functioning of 4×4 suspension systems to a limited degree, ill-considered modifications to a suspension system can have the undesirable results of limiting some aspects of its operation, for instance, non-standard coil springs on 4WD front suspensions that do not allow enough compression by being too “hard” can reduce rear wheel traction by extending the rear suspension excessively by allowing the entire off-road vehicle to follow the contours of an obstacle, instead of the suspension on the affected front wheel absorbing the obstacle by means of its inherent ability to compress and extend.
The demands of off-road driving in Southern Africa, or the entire continent of Africa for that matter, are such that careful consideration should be given to the suspension configuration of any and all 4WD vehicles that are going to be used in long expeditions; Africa has very few repair facilities and the few that are capable of executing suspension repairs are very often thousands of km’s apart. It therefore makes good sense to pay particular attention to the suspension configurations of 4WD vehicles during the planning stages of an expedition through Africa.
So, what are the pros and cons of various systems and configurations? There are no hard and fast rules but careful consideration of each component will go a long way towards making the right decision: the carrying of spares should be limited to the essentials; perhaps a shock absorber or two, but definitely not an entire suspension replacement, and given the fact that repair facilities are few and far between, suspensions should be kept as standard as possible for trips in Africa. With this in mind, below are the available options:
Independent Front Suspension (IFS)
• Advantages of IFS:
As the name suggests, these systems have the ability to allow the front wheels to compress and/or extend independently of each other. This ability provides a much smoother ride while at the same time reducing body roll significantly, which in the off-road road context, is of somewhat more than academic interest.
Off-road driving can be dangerous in some situations; rollovers are a distinct possibility and many a 4×4 vehicle has been destroyed in this way. Some obstacles, such as large rocks, of which there are a great many in all parts of Africa, or unevenly washed out dirt tracks, of which there are just as many, are best negotiated with 4WD vehicles fitted with IFS, especially heavily laden vehicles carrying roof racks that have the effect of raising a vehicles’ centre of gravity. By being able to compress and extend independently in this kind of situation, an IFS system on a 4WD vehicle can prevent a dangerous rollover situation by limiting body roll.
• Disadvantages of IFS:
A 4WD vehicle suspension’s ability to maintain sufficient ground clearance to miss obstacles such as rocks, tree stumps, and raised centres of two-wheel tracks is of paramount importance, and in this area, IFS systems have a major disadvantage.
In these systems the differential casing is attached to the vehicle chassis and power is transmitted to the wheels via flexible drive shafts. On level surfaces the ground clearance is thus determined by the suspension set-up, with no variation in the distance between the ground and the lowest part of the differential. On uneven surfaces, the differential thus maintains this height on account of the fact that obstacles on either side of the differential are absorbed independently by the front wheels. Inexperienced off-road drivers may not always be aware of the exact position of the differential relative to the driving position and could conceivably destroy a differential by attempting to drive over obstacles instead of going around them.
It is only at the very limits of suspension compression that the differential is lifted further off the ground by lifting the entire vehicle, and then not by much. This situation places undue and extreme loads on the affected suspension components with the destruction of load bearing rubber bushings a distinct possibility and these kinds of loads should be avoided at all costs, or at least, as far as possible.
Beam axles or Solid Axles
• Advantages of a Solid Axle:
Also known as solid or live axles, these systems have the clear advantage that the differential is lifted off the ground as one wheel passes over an obstacle by lifting the entire axle. There are many places in Southern Africa as well as the rest of Africa that consist of fields of rocks of varying sizes, where a solid axle makes it possible to avoid damage to the differential by being continuously lifted clear of dangerous obstacles by the wheels as they pass over different sized rocks or other obstacles at different times.
• Disadvantages of a Beam Axle:
One major disadvantage of beam axles is the fact that the entire vehicle responds to axle movements during suspension compression, giving a sometimes very uncomfortable ride. Another, more important drawback is the fact that these systems cannot absorb angles in the way that IFS systems can: badly eroded or washed out tracks can sometimes have dangerous angles, which are transmitted to the entire off-road vehicle through the beam axles. An IFS system can absorb or lessen these angles to some degree, limiting body roll, or the angle at which the vehicle is leaning.
Of course, axles and suspension systems need springs of some sort to absorb the sudden movements imparted by obstacles of all kinds, and while some might be thought to work better than others, in standard form they all work equally well with the only real drawback or advantage to each type being the degree of difficulty in replacing each should they fail or break during an overland expedition through Africa. While some types of springs are used in combination with others, some can only be used with a particular set-up or design, all are used in conjunction with shock absorbers, so, exactly how do springs and shock absorbers work, and what are the types in use in modern 4×4 vehicles?
• Coil Springs:
The effectiveness of coils springs depends on the overall diameter of the coil, the diameter of the rod used to make the coil, the separation, or distance between each successive coil, and its angle of installation.
Coils are most often used in IFS systems but not always. Design specifics of IFS systems vary but coils, where they are fitted, are designed to regulate the suspension travel, with the top end of the coil spring usually attached to or supported by a fixed point on the vehicle structure. Both control arms being free to move up or down, the weight of the vehicle acts as a counter balance to the force of the compressed spring which pushes downwards on the bottom control arm. This balancing act between the force of the coil and the weight of the vehicle is what determines ride height and to a large degree the hardness of the ride.
Standard suspension systems are designed to accommodate the forces and counter-forces generated by the coils so it stands to reason that any modification to the coil will have adverse effects on the durability of all the components in a given system. For example, ball joints are more than just the pivot points around which the steering hub turns; they are also the means by which the outward pushing force of the coils is contained. Thus, by using less compressible coils, the load on ball joints and rubber bushings are increased in direct proportion to the reduced compressibility of the coils.
• Progressive coil Springs:
The only really safe way to increase the effectiveness of coil springs is to replace them with progressive coils: a standard coil with equidistantly spaced coils requires a constant load to compress it to full compression. A progressive coil on the other hand, has coils that are wound closer to each other nearer one end, which means that the force needed to compress it has to be progressively increased as it nears full compression, for the reason that once the coils spaced furthest apart are compressed, the coils that are closer to each other needs more force to be compressed because there are more of them.
In practice this means that during “normal” conditions, these springs do not compress at the same rate over their full length, but rather in a graduated fashion according to the load applied to them. This ability reduces the chances of damage to ball joints and rubber bushings although the overall load capacity of the off-road vehicle is not increased. The only advantage of these springs is their ability to better absorb the varying loads imparted by extreme suspension movements, without placing undue stresses and strains on other suspension components.
• Leaf springs:
Also known as semi-elliptical springs, these are the familiar bow shaped springs found on beam axles on 4WD vehicles. Consisting of several leaves, or blades, these springs are attached to the vehicle via “eyes”, with a solid fixture at the front and the back end attached to the vehicle via a moveable shackle, to accommodate the flattening of the spring during compression, which effectively increases the overall length of the spring.
While these springs are not true ellipses because they do not have constant radii, their effectiveness is determined by the number of leaves, their individual thicknesses, overall length, width, and the position of the attachment point of the axle onto the spring, which is always somewhat before the effective length of the spring. Not having a constant radius, the spring does not compress at the same rate along its length, making it a progressive spring to some extent.
One major drawback of leaf springs is their sensitivity to overloading: a severely overloaded off-road vehicle means that the entire weight of the vehicle is supported only at the very end sections of the topmost one or two leaves. Because the spring can no longer flex freely, being nearly fully flattened, or compressed, a sudden impact from a hidden rock or other obstacle could cause the spring to break close to either end, with potentially disastrous results.
There are various gadgets available by which the carrying capacity of leaf springs can supposedly be increased, most often by preventing springs to flex.
Two major concerns regarding these gadgets are firstly that by preventing a spring to flex, the load must be absorbed elsewhere, and this is at the ends of the spring where it is at its thinnest, and most likely to break. Secondly, the manufacturers of these gadgets invariably fail to mention the fact that off-road tyres cannot bear unlimited loads; all off-road tyres are limited to a specific load range and the use of a gadget to prevent spring flex could cause an inexperienced off-road driver to seriously overload a vehicle, because “the springs still look OK, they can still take some more weight.” This extra weight is borne by the tyres directly, which could lead to multiple catastrophic tyre failures, not to mention unpredictable 4WD handling characteristics.
• Torsion bars:
Torsion bars are long, rod like springs that are attached to the lower control arm at one end, and to an adjustable pivot point under the cabin at the other. Mostly used where space is limited, and then for the most part on soft roaders, they work well in conditions that do not require repeated maximum suspension compressions. Since these springs deform along their entire length, instead of compressing like a coil, they are more susceptible to failure and breakage than any other type of spring. Any loss of tension in these springs also directly affects wheel camber, which in the off-road context, can cause the loss of control of a vehicle in difficult 4×4 driving conditions.
Replacing a torsion bar in the bush is also difficult; adjustment controls ride height, camber, and spring effectiveness, with under adjustment just as likely to cause another failure as is over adjustment. While these springs have a place in the off-road universe, their use should be balanced against the very high likelihood of failure and breakage.
Shock absorbers have the critical function of dampening sudden suspension movements, by the passing of fluid through a series of orifices. All shock absorbers work on the principle of oil, in this case hydraulic fluid, being forced through tiny holes in a piston attached to a rod that is in turn attached to a fixed point on the vehicle, and all modern shocks use a combination of oil and gas to improve shock damping.
However, by forcing oil through tiny holes repeatedly, the oil eventually becomes so hot that it starts to foam as the piston moves through it. This has the effect of creating air bubbles and air being highly compressible, the overall effectiveness of the shock absorber is reduced to the point of uselessness. To prevent this, manufacturers have taken to the idea of adding a high-pressure charge of nitrogen, an inert gas, to the pressure chamber to prevent the oil cavitating, or foaming, as it is forced through the tiny holes in the piston, much like the pressure inside an engine cooling system prevents the coolant from boiling.
During the compression cycle, when the piston is forced against the oil contained in the pressure tube, oil passes through the orifices in the piston at a rate controlled by the size and number of orifices. This action slows down the movement of the suspension in its upward motion, thus preventing wheel bounce and body roll. The opposite happens during the extension cycle, when the piston is forced upwards through the oil during the suspension’s downward movement. By arranging the orifices in such a manner that the extension cycle happens with less resistance than the compression cycle, the wheel movement is faster in its downward movement than its upward movement, thus keeping the wheel firmly on the ground.
All suspension movements are dampened in this way, and while design specifics vary, off-road vehicles probably sport the largest number of designs, brands, and “innovations” in shock absorber technology than any other class of vehicle. So, what are the main types of shock absorber available to the 4×4 driver today and how do they work?
• Twin Tube Shocks:
Shocks of this type consist of two tubes, one inside the other. The inner tube is generally made of lighter material than the outer, which in the off-road context adds protection to the inner tube from impacts by flying stones and other common hazards. In a mono tube shock absorber, the slightest dent or deformation of the tube prevents movement of the piston and the shock absorber becomes useless.
The twin tube design also allows a chamber between the two tubes into which the gas and oil can expand as they heat up. This chamber also allows for greater volumes of both oil and gas to be used, which helps to keep internal temperatures down. From a design perspective, the ability to vary the amounts and pressures of the oil and gas makes it possible for engineers to create shock absorbers for any given off-road vehicle to accommodate a wide variety of driving conditions.
Generally speaking, twin tube shock absorbers have lower gas pressures than mono tubes, giving softer ride characteristics at lower speeds but these units rapidly build up resistance to suspension movements as speed increases, thus providing progressively improved vehicle control characteristics.
As the name suggests, this type of shock absorber consists of only one tube; the tube the piston
moves up and down in, therefore this construction method makes these shocks highly susceptible to damage from flying stones and other debris. However, since these shock absorbers consist of only one tube, cooling by passing airflow is superior to twin tube shock absorbers, but this at best marginal advantage should be carefully weighed against the extremely high likelihood of damage to the shock absorber tube occurring by flying stones and other debris.
Shocks of this type are also generally available in only one degree of “hardness” for any given vehicle, which from the off-road perspective, is less than ideal. Driving conditions in Southern Africa and indeed throughout Africa vary greatly and having shock absorbers that are capable of handling only one, or only certain types of terrain, is not the way to approach a long distance overland expedition.
• Shocks with Remote Reservoirs:
The only real advantage of this type of shock absorber is the fact that they generally do not get as hot as other types, because the oil and gas charge are separated; with the oil in the main pressure tube and the gas in a separate, remotely mounted reservoir. This makes it possible for more oil to be used, helping to keep temperatures down in addition to allowing for longer extension/compression strokes.
Notwithstanding this, it is the gas component that presents most problems: in theory, the separate gas reservoir allows for varying gas pressures, which is the main determinant of the effectiveness of a shock absorber. The theory states that by varying the gas pressure, the shock absorber can be adjusted for varying or different driving conditions, as conditions change.
However, the required nitrogen gas is not readily available to the average off-road driver, but that apart, setting up gas pressure-controlled shock absorbers requires skills and equipment that are not available to the average off-road driver, leading to under-or poorly performing shocks that could lead to undesirable, if not dangerous handling characteristics. In addition to this, the valves on the gas reservoirs are similar to those found on bicycle tyres and are known to leak. Loss of gas pressure through defective valves can leave a 4×4 vehicle with no effective shock dampening, and thus no effective suspension.
In practice and from the “normal” off-road driving perspective, all of this means that remote gas reservoir shock absorbers are more trouble than they are worth. Without properly calibrated test equipment it is virtually impossible for the average 4×4 driver to set up these shock absorbers accurately, even if an adequate supply of nitrogen was available. These units may be of use in competition vehicles where skilled and trained technicians are available to change and reset damping characteristics but in the depths of Africa, the fickle nature of these shock absorbers will almost certainly cause delays, breakdowns and cancelled trips.
On long, difficult overland expeditions through any part of Africa where the next repair facility could be several thousand kms away, the reliability of all suspension components are of critical importance, and the use of this type of shock absorber should be avoided, because the ratio of perceived benefits to cost simply do not warrant the risk of total shock absorber failures in the middle of nowhere.
Which 4×4 Suspension then?
Regardless of whether a beam axle or an IFS system is chosen, no vehicle, off-road vehicles included, can be safely operated without a suspension system in good repair or functional and fit-for-the-purpose shock absorbers.
Uncontrolled suspension movements cause dangerous and uncontrollable swaying motions that usually end in a loss of directional control. Even at low speeds, worn or dysfunctional shock absorbers cannot control the excessive body roll that goes with going around corners or the swaying motions caused by negotiating uneven terrain.
All vehicles, 4WD vehicles included, have centres of gravity, points around which vehicles pivot during a rollover, and with 4×4 vehicles generally having very high centres of gravity, it means that without functional shock absorbers and an effective suspension system to limit, if not prevent, body roll, rollover situations will be reached at much lower speeds than would be the case with good shock absorbers.