Words Dean Mellor
The crunching sound made when the underside of your 4x4 comes into contact with the ground is never a pleasant one, especially if you haven’t equipped your vehicle with adequate underbody protection. There are several vulnerable components underneath any 4x4 that can be damaged when driving off-road, some of which can affect the way the vehicle drives and others that can render you stranded.
The best way to protect the underside of your 4x4 is to ensure you have adequate ground clearance for the terrain you’re going to tackle and by making sure vulnerable components are tucked up and out of the way, and that they are protected by appropriate guards and bash plates.
Just about all 4x4s can use a bit more ground clearance when it comes to off-road driving, and there are a few ways to achieve this, including fitment of a body lift, fitment of an aftermarket suspension system and fitment of larger diameter tyres.
A body lift consists blocks placed between the chassis and body of a vehicle, so can only be fitted to vehicles with a separate chassis construction. As there are legal constraints on how much a vehicle can be lifted, body lifts have fallen out of favour in recent year, and while they offer increased clearance under the vehicle itself, they do not offer increased clearance under the vehicle’s chassis, nor under components directly attached to the chassis including the engine, transmission and transfer case, nor components such as bull bars, side rails and rear bars. Body lifts will sometimes also require the fitment of other components such as extended brake lines, fuel lines and diff breather extensions, as well as the fitment of extended gear levers and relocation of other components.
Suspension lifts are the most common method for achieving increased ground clearance. Raised, longer travel springs not only provide more ground clearance, but also increased wheel travel, which will improve off-road capability. It should be noted, however, that with live-axle vehicles, overall ground clearance beneath the diff centres and axles will not increase with a suspension lift, but there will be more clearance under suspended components, which is essentially everything else including engine, transmission, transfer case, bull bar, side rails, rear bar and the vehicle body itself.
The fitment of larger diameter tyres results in all vehicle components receiving an increase in ground clearance. Go too big with tyres, however, and they’ll rub on the wheel arches or inner wheel guards, especially at full suspension travel or when the steering is at full lock. The solution is to fit raised suspension.
A combination of larger diameter tyres and raised suspension is the best way to increase ground clearance, but there are limits. Most states and territories apply the rules outlined in Vehicle Standards Bulletin 14, National Code of Practice for Light Vehicle Modification and Construction (VSB 14), which states that (without the need for assessment or certification) “Modifications to the ride height up to 75mm that incorporate a maximum change in the suspension of 50mm, and/or an increase in the diameter of the wheel and tyre combination of up to 50mm.”
In layman’s terms, the maximum allowable suspension height increase is 50mm and the maximum allowable height increase from the fitment of larger diameter tyres is 50mm, and combined these two modifications must not exceed 75mm.
While an increase in ride height of 75mm might not sound like much, it can make and incredible difference out on the tracks, preventing underbody components from coming into contact with the ground on all but the most extreme terrain.
Poke your head under any new four-wheel drive and you’re likely to find a number of components that are susceptible to damage when driving off road. Sure, some of these might be afforded the protection of something that looks like a bash plate, but this will often be little more than a plastic splash guard or a piece of mild steel that will bend as soon as it settles on a rock.
While it might look as though there’s plenty of ground clearance under your vehicle when it’s parked in the driveway, when it’s out on the tracks it can be a different story. Vehicles with live-axles provide a constant ground clearance, but those with independent suspension lose ground clearance as the springs compress. Nearly all modern 4x4s have at least independent front suspension, which means components at the front of the vehicle are susceptible to grounding when driving along tracks with a high crown or when crawling through rocky terrain. Components that need protection include the engine sump, the transmission and the transfer case, and in some cases the suspension system itself and things like steering rods and linkages.
Well-designed and properly engineered bash plates are manufactured from much heavier material than OE protection systems, and they will cover a much greater area under the vehicle. When choosing the right bash plates for your vehicle there are several things to consider – you should ensure the plate has been specifically designed to suit your vehicle, that it is manufactured from thick enough steel or alloty to handle severe punishment and that it covers all of the components that need protection. It should also be compatible with other vehicle protection systems such as bull bars.
Most of the big 4x4 aftermarket companies produce under-vehicle protection systems that are designed for specific vehicle compatibility and will work with other
systems, providing protection all the way from the front of the vehicle to under the transfer case.
Depending on your vehicle make and model, you should also be able to source bash plates to protect components such as the fuel tank and the differentials.
Bull bars, rock rails and rear bars
The lower sections of your vehicle’s body also need protection, and for that you’ll need to flick the OE bumpers and side steps and fit quality aftermarket gear including a bull bar, rock rails and rear bar.
Steel provides the ultimate protection and a well-designed bull bar will not only offer a defence against animal strikes, but it will also provide improved approach angles, especially at each side of the front of the vehicle. Bull bars manufactures from steel will also offer the ultimate in protection if the front of the vehicle comes into contact with the ground.
The standard side steps fitted to most 4x4s are manufactured from lightweight aluminium or plastic, and while they might make it easier to step in and out of the vehicle, they don’t provide adequate sill protection when driving off-road. A number of aftermarket options are available, the strongest of which will be designed to offer increased clearance under the vehicle sills as well as enough strength to fully support the weight of the vehicle if it grounds on a rock. Some will even incorporate a step to aid vehicle entry and egress, while others will simply replace (or cover) the vehicle’s OE sill.
The back of vehicles can also be vulnerable to grounding, particularly the current crop of 4x4 utes which tend to have longer rear overhangs than their wagon siblings. When looking at a replacement steel rear bar for your vehicle, ensure that it doesn’t adversely affect departure angle, and that it provides protection for the rear quarter panels behind the wheels as well as at the rear of the vehicle. Also make sure it’s compatible with existing vehicle equipment such as a tow bar, or that it incorporates a tow bar and any other features you might want, such as hi-lift jacking points.
Under vehicle checks
Even the most well protected vehicles can still suffer from underbody damage when driven off-road, so it’s important to have a regular squiz beneath your 4x4 to make sure that everything is as it should be.
As well as obvious problems such as big dings and scrapes, have a look for any leaking fluids (coolant, oil and fuel etc.) and look for anything hanging down where it shouldn’t be (fuel lines, brake lines, electrical cables etc.). Also have a poke around under the wheel arches to make sure inner guards are in place and mudflaps are secured, and while you’re there inspect your tyres for damage.