4WD MODIFICATIONS - SUSPENSION & BRAKES
After bar work and roof racks the most common modification made to a 4WD is done underneath – to the suspension. We’ve separated different suspension types and listed the ideal changes needed for each type.
Standard 4WD steel-spring suspensions are built to specifications that are a compromise. No-one makes a 4WD specifically for Australia’s unique on and off road conditions; nor do makers build vehicles with steel springs that are designed to provide excellent ride, ground clearance and handling in both lightly laden and fully loaded (or slightly overloaded) conditions.
Air suspension is a different matter, but we’ve found that even variable-rate and variable-height air suspensions can benefit from shock absorber upgrades.
It’s obvious that a lightly loaded 4WD imposes less dynamic load on its suspension than does a heavily loaded one, so factory springing and damping rates usually have to become a ‘best-fit’ compromise: not too firm at light loads, yet not so soft that the springs and shock absorbers bottom-out when loaded.
Different suspension types have different characteristics, so we’ll consider fixes for each type in turn. Note that the prices quoted are budget-only figures and don’t include fitting charges.
Leaf spring suspensions
The original post-WWII recreational 4WDs – Jeeps and Land Rovers – used leaf springs, because that’s what was available. (Had Germany won the War we may well have seen Kubelwagen-style independent suspensions on 4WDs.)
The leaf spring is a marvellous invention, which is why it’s been with us for centuries. A semi-elliptic pack can be made stronger or weaker simply by adding or taking away leaves. A leaf-spring pack has variable rate action, because the shorter, heavier leaves don’t do anything until the spring is compressed. Tight clamping and shaping of the leaves creates inbuilt self-damping as well.
So, why is the leaf spring relegated largely to utes these days? Ride quality and handling are its limitations, mainly because the spring has the job of locating as well as suspending the axle, and coping with loads that vary by up to a tonne. If the spring is made short-travel to improve axle location and handling, and strong enough to handle large payloads, ride quality and off-road travel suffer.
Mod No 1
The standard leaf spring on 4WDs has a compromise rate that gives reasonable ride, handling and off road ability, but it can be improved for individual vehicles. Instead of being saddled (lousy pun) with a spring that can carry more payload than you need, you can opt for a softer spring that rides better and has longer travel. if you need payload capacity at times a pair of air bellows can be fitted and inflated when the load goes on.
Mod No 2
Leaf springs work best in a vertical plane, when the axle is running on level ground. When one wheel drops into a hole or rises over a bump the springs on both sides of the vehicle have to move through two planes – vertical and lateral. This twists the spring eyes in the shackles and hangers and tends to splay the leaves.
More durable, top-quality rubber eye bushes tolerate this movement better than cheap bushes.
Mod No 3
Eye and shackle pins suffer from corrosion, caused by water being trapped inside the bushes. Greasable pins have nipples, hollow centres and radial holes to allow grease to push from inside the pins and exclude water from the pin-bush interface. Pin life and spring alignment are enhanced.
Mod No 4
Standard shock absorbers are usually the cheapest items the maker can buy. You can do better, but it’s vital that the spring and shock absorber rates and travel match.
Shock absorber prices range from around $100 up to $500 each. Competition ones are around a grand a corner.
Mod No 5
Your optimised leaf spring suspension isn’t worth a cracker if it’s not maintained. Leaf springs are the highest maintenance suspensions on the market.
Regularly jack up the chassis, to separate the leaves, or pry them apart with an old screwdriver and use a busted hacksaw blade to work grease between the leaves. Replace spring and shocker bushes at least annually and chuck away rusted hanger and shackle pins. Replace broken leaf clamps or risk the spring pack getting out of alignment.
Coil spring suspensions
Coil springs provide better on-road handling and off-road travel than leaves and, when correctly matched to vehicle weight, a better ride as well. A coil spring formed from mandrel-bent wire doesn’t have inherent variable rate, like a leaf spring, but most have wider spacing of the coils at one end that provides some rate variability. Proper variable rate coils are fitted to the rear end of the Defender 130 and the Mercedes-Benz Unimog – a pair of outer, softer-riding coils and a pair of inner, stiffer ones.
Coils are fitted to different suspension types: live axle, strut, semi-strut and wishbones.
Some coil spring suspensions are difficult to modify to increase ride height. If you intend modifying a coil-sprung 4WD make sure there are kits available.
We’ve noticed a trend in recent years for 4WD wagon makers to fit shorter, stiffer coils than they used to, in the interests of flatter on-road handling, to counter the criticisms of roll-over tendencies. Ride quality and off-road ability suffer.
Mod No 6
A set of after-market coils can be tailored to suit your vehicle’s load and your on- and off-road needs. An increase in ground clearance is possible at the same time.
Mod No 7
Live front axles ( ‘Cruiser 100s and 70s, Wranglers, Patrols, Series I and II Discoverys and Defenders) fitted with taller, replacement coils need either eccentric leading arm bushes or eccentric knuckle bearing kits to compensate for the resulting forward rotation of the axle and the consequent change to inbuilt castor angle.
Mod No 8
The standard shock absorbers fitted to coil-spring suspensions are usually better quality than those fitted to leaf springs, but you can still do better in the after-market. If you opt for replacement coils that provide a suspension lift the standard shockers will be too short anyway.
Mod No 9
Live rear axles with coils (fitted to all older and many new 4WD wagons) can suffer alignment problems when the spring height is raised more than around 50mm. The axle rotates rearward, altering the angle the propeller shaft makes with the rear differential. The cure, to equalise the propshaft angles, is to fit different trailing and upper arms. The standard Panhard rod that locates the axle laterally may also be too short and can be replaced by an adjustable-length one. Another benefit of this change is that after-market arms are usually stronger than standard ones.
Torsion bar suspensions
Older utes and wagons with independent front suspensions used torsion bars. A torsion bar works like a stretched out coil spring, although it doesn’t look like it should. Torsion bars are fixed-rate springs, so they’re almost never seen on the rear ends of vehicles, where loads can vary greatly.
A torsion bar is splined into an adaptor on the front wishbone and at the aft end to a mounting on the chassis.
Mod No 10
A 4WD ute that’s fitted with two heavy duty batteries in its engine bay, plus a steel bar and winch, will be considerably heavier over the front end than a standard vehicle. In the case of medium-sized utes front suspension sag may be cured by ‘tweaking’ the torsion bar preload, but this may be only a short-term solution. A pair of heavier-rated bars is the answer.
Suspension modifications for specific vehicles
By ‘big utes’ we mean the mothers with live axles front and rear: LandCruiser 70 Series; Patrols; Defenders and F250s.
The all-leaf-sprung 75 Series works all right on its standard springs, unless the tray is heavily loaded, at which point the rear springs sag.
Mod No 11
After-market suppliers have stronger springs for 75s.
The standard dampers are weak and ride quality and handling can be greatly improved by fitting after-market shockers.
Standard spring height is OK on ‘Cruiser utes, but going up 50mm is fine.
Mod No 12
The coil/leaf 70s don’t have front end spring issues, but the rear leaves sag, just like their predecessors, so replacement leaves are a good idea. If you want to improve belly clearance a 50mm suspension lift via new front coils and more cambered rear leaves is worth doing. As with the 75 Series the newer, square-rigged Toyotas need better quality dampers
Mod No 13
Patrol utes have too much rear overhang, so a 50mm spring height increase is welcome, via after-market springs and shockers. Good suspension people offer a range of leaves and coils to suit specific constant-load vehicles.
Mod No 14
Land Rovers used to have supple coils, but the later Defender 110 utes were noticeably hard-riding and could be softened by fitting after-market coils and shockers. The standard ride height is ample, avoiding the need to mess around with front caster kits.
You can’t do much about the ride quality in the 130, which has dual rear coils.
Mod No 15
There’s plenty of stuff from the ‘States and some local suspension bits for the Big F. Better shocks and stronger front springs to handle the weight of bar work plus a whopping winch should be your main priorities.
The bulk of popular utes fit into this category and all have independent front suspension and live rear axles with leaf rear springs.
The move has been to coil-sprung front ends, replacing simpler torsion bar types.
Mod No 16
All these utes need ground clearance improvement by way of suspension lifts. Replacement leaves fit the rear ends and the torsion bar models can usually be ‘tweaked’, almost to match the rear height increase. Coil-sprung front ends can be lifted by fitting replacement coil struts.
Mod No 17
Another rear axle suspension change option is to retain the stock leaves and fit high-pressure or low-pressure air springs between the chassis and the springs.
Small and Medium Wagons
All current 4WD wagons have coil-sprung front ends and coil-sprung rear ends; either with live axles or independent designs.
The coil-front/coil-live-axle rear brigade includes the Toyota Prado, and ute-based wagon derivatives: Fortuner, Pajero Sport, Colorado 7, Trailblazer and Isuzu MU-X.
The all-independent suspension mob includes the Grand Cherokee, Grand Vitara, Pajero, Patrol petrol V8, ML ‘Benz and VW Touareg.
Mod No 18
There are front suspension spring and shock absorber kits available for the most popular of these wagons, improving ground clearance and load carrying ability, to allow for the weight of second battery, bar and winch fitments. After-market rear springs and dampers can be specified to raise the back end and eliminate sag caused by full load operation.
Mod No 19
An alternative fix for coil-sprung wagon rear ends is to fit low-pressure air springs inside the coils, to improve load carrying ability and reduce spring sag.
Mod No 20
High-pressure air springs can be used to replace the rear coils completely. A pair of airbags and lines for the rear end of a wagon is around $1400, plus the cost of shock absorbers. Fitting is time consuming: around four hours.
The Big Guys are the LandCruiser 200 Series, Troop Carrier, discontinued Patrol diesel and Discovery. For suspension mods to the Troopy, see under the Big Utes heading.
Mod No 21
The 200 Series has coils front and rear and the after-market has stronger, taller coils and matching shocks to replace them and provide a much-needed ground clearance increase.
The 200 Series is a heavy wagon, so opting for a gross mass rating increase at the time of suspension upgrade, to gain some payload increase, is a good idea, but watch out for warranty issues.
Mod No 22
The all-coil, live-axled Patrol diesel was fitted with harder-riding springs in recent years, to improve its on-road handling and roll-over stability. Softer-riding, taller coils and matching shock absorbers are available.
Mod No 23
It’s also possible to fit high-pressure air springs to the Patrol diesel, in place of its four coil springs. A full-air kit, with shock absorbers, for the Patrol is $3700, plus fitting, which takes around eight hours.
Mod No 24
The Disco 3 ‘S’ came with coil springs and had very poor ground clearance that could be easily corrected by fitting taller coils and matching shock absorbers. The suspension upgrade is a $560 ask, for the coil springs. Bilstein and Koni make replacement front and rear strut/damper units that start at $350 each.
Mod No 25
Air-suspended 4WD wagons don’t need any air-spring changes, but the standard dampers can be replaced by after-market monotube types that have better bump control.
Monotube struts to fit inside the air springs are around $350 each and you’ll need an air spring test kit to fit them. R&R time is around four hours.
Not too high
Most after-market suspensions incorporate a spring lift of around 50mm. This lift increases ground clearance under the vehicle body and under the diffs of vehicles with independent suspensions. However, a spring lift doesn’t increase the ground clearance under live axles, which receive a clearance boost only when the rolling radius of the wheel and tyre combination increases.
Many people achieve extra ground clearance under older ute front ends by twisting the torsion bars up to the point where there’s no ‘droop’ left. That’s a great way to break suspension components and CV joints. Don’t overdo it. The same goes for bump stop removal: you’ll bust your shock absorbers, so don’t remove them.
A combined suspension lift and tyre size increase that raises the vehicle more than 75mm can cause gearing, braking, handling and stability problems and may well be illegal in some jurisdictions.
Any change to front or rear suspension ride height alters the alignment of the axles and suspension with the chassis and body, but well engineered suspension kits compensate for the lift.
Once a suspension-only lift exceeds 75mm or so you’re likely to encounter myriad problems, including suspension and wheel alignment difficulties, universal joint failures, driveline vibration, steering vibration and handling irregularities. Lave big lifts to the competition boys.
DIY suspension jobs
Suspension components are safety items, so don’t do anything you’re not confident about. Even if you’re confident, be careful.
Inspection is probably the most useful DIY job. Crawl under the vehicle, or better still, take each wheel off in turn and have a good look at all the suspension bits.
Shock absorbers sometimes have a very light haze of fluid on their outsides, but a seriously wet shocker tube means it’s a dud. If you know the exact model and valving specification, you may be able to buy a new one and replace a free-mounted shocker yourself. However, if the shocker is part of a coil/strut, or is captive inside a coil spring, don’t even think about it. Have you ever seen a coil spring fly out of a spring compressor?
Shock absorber and leaf spring suspension bushes wear at varying rates. If you can safely take the load off the shocks and springs you may be able to replace the bushes. Some gas-pressurised shocks will need a floor jack to compress them so they’ll fit back over their mounts.
Greasing the interleaf spaces of a leaf spring pack is useful maintenance (see Mod No 5).
Buying a replacement suspension
You need to be sure of what you intend to do with your vehicle before you fiddle around with its suspension. Let’s look at some typical scenarios.
Many 4WD wagons are bought only for on-road towing, with little or no off-road driving planned. You may be able to fit a set of low-pressure helper air springs to the back end, to help keep the vehicle level when towing and you have the option of deflating them when you’re not towing.
If you intend to tow an off-road camper behind your 4WD, the vehicle will probably benefit from a suspension height boost, in addition to some rear spring rate increase, so a full suspension kit may be in order.
The bulk of 4WDs that head bush are fully loaded – or more so. Typically the load includes a bar and winch, at least one additional battery, 60 litres of water, 120 litres of fuel, a chock-a-block interior and a loaded roof rack.
Given that powerful engines and higher levels of standard equipment are standard fare these days, the ‘dry’ weight of wagons has gone up, but their payloads haven’t increased: in some cases, payload ratings have actually decreased.
Most 4WD owners who buy after-market suspensions opt for load-capacity and ground clearance improvements: taller, stiffer-rate coil springs, thicker torsion bars or more-cambered leaves that lift the chassis, matched with the correct-length shock absorbers.
These components achieve an increased ride height at full load, but there are compromises when the vehicle is lightly loaded. Stiffer springs and larger shock absorbers often give a firmer ride than standard ‘softies’. At the other end of the ride spectrum, replacing firm standard coils with taller, softer ones can affect roll stability.
When you do take the suspension kit plunge it’s important that you buy from a company that has done considerable research and development, and warrants its products.
Our suspension testing over the years has shown that some have done more homework than others.
Shock absorbers look much the same from the outside, but there are major differences in the design and quality of the internals.
The first shocking fact is that all shock absorbers are oil-filled. ‘Gas’ shocks additionally have a pressurising gas that’s there to help reduce oil foaming, or ‘fade’, but they’re not ‘gas shocks’ as such – gas/oil is a more accurate description.
There are two basic types: single tube (monotube) and twin-tube. In the case of a monotube the tube has a single wall, inside which the active piston moves up and down. Monotubes have a second, floating ‘piston’ that separates the pressurising gas from the oil and that piston thickness limits the mono-tube’s range of travel, when compared with a twin-tube shock of the same open length.
Twin-tube shocks are the most popular 4WD shocks, because the concentric outer tube acts as a stone-damage shield for the internal one, in which the piston slides. The outer tube holds displaced oil and the pressurising gas, so any stone denting doesn’t affect the inner, piston tube. Twin-tubes have only one piston and longer travel than an equivalent-length mono-tube.
The action of both types looks similar, but in a monotube the piston valving controls bump and rebound actions. In a twin-tube a foot valve in the base of the inner tube regulates bump and the piston valving regulates bump and rebound.
Our bench and on-road testing has shown that the most capable shockers are mono-tube types, but they can be damaged by flying stones.
Adjustable shockers can theoretically be set for different load and road conditions, but our testing has shown that there are wide quality variations between them and the adjustability isn’t always consistent.
The problem we’ve come across most often with modified vehicles is mismatched shock absorber and suspension travel. It should be easy enough to match replacement shock absorbers and springs, but some makers, who have a limited availability of shock absorber tube and shaft lengths, settle for a ‘best fit’ arrangement.
Also, some of them seem to misunderstand the relationship between spring and shock absorber travel. This ‘relative motion’ is the difference between the spring movement arc and the shock absorber movement arc, which is only zero in the case of shock absorbers that are fitted inside springs. With shock absorbers mounted beside springs the travel distances of the spring and the shocker are slightly different as both move from full compression to full extension.
If the calculations aren’t done correctly it’s possible to combine shockers with extended and closed lengths that don’t match the travel limits of the springs. Shockers that are too short can be torn apart by extreme suspension travel, while those that are too long can bottom out and break the shocker or the chassis and axle mountings.
More and more 4WD wagons are leaving the factory with air suspension. Although this route may be viewed by some as radical, it’s well to remember that trucks and buses have been using air suspensions for the past 30 years.
Air suspension offers many advantages over steel-springs: quieter operation, almost no friction, constant body ride height, variable spring rate and variable suspension height.
As designers look for ways to reduce the centre of gravity of large 4WDs in the interests of on-road safety the air spring allows automatic or driver controlled height selection: road hugging for fast bitumen and up on tippy toes for off road.
In addition, opposite-side air bags can be linked, so that as the vehicle tries to roll when cornering the outside suspension bags can be pumped up and the inside ones deflated slightly. Computer control lets this happen almost instantly. In-line air bags can also be computer-linked, to reduce pitching
There’s no need to fiddle with standard air springs, but the shock absorbers fitted to some air-suspended 4WDs can do with improvement. Replacement shock-struts for air springs are now available in the after-market.
Retro-fitting air suspension to some steel-spring 4WDs can be done. Helper air springs can be fitted between the top of a leaf spring and the vehicle chassis – kits are available for popular 4WD utes. Air springs can replace coils in suspensions where the shock absorber is fitted adjacent to the spring, not inside it. Patrol and 100 Series kits are available.
The bump stops here
Many 4WD bump stops are simply dense rubber blocks, designed to prevent spring overload when the suspension ‘bottoms out’ and to prevent the shock absorbers being slammed shorter than their design closed lengths.
However, some coil- and air-sprung vehicles have more progressive bump stops that come into play before suspension bottoming and work to increase the suspension spring rate.
After-market bump stops such as Aeons are designed to replace standard bump stops and provide more progressive action.
Our testing shows they can convert a fixed-rate suspension into variable-rate and enhance ride quality.
Our thanks to Sydney Shock Absorbers, ARB Elizabeth SA, Suspension Technology Australia and Snake Racing for their help in preparing this article.