Rocky Ground and Gravel Roads

Rocky ground

This type of terrain is one where vehicle type is very important. Ground clearance and suspension travel are two of the main criteria. High ground clearance allows the larger rocks to pass underneath without contacting vulnerable vehicle parts while good suspension travel allows the wheels to remain in contact with the ground.

Independent suspension usually provides a smoother ride in this type of terrain, but its design limits the wheel travel and ground clearance available. It goes without saying that rocky ground should be attempted at low speeds to minimise vehicle contact with the terrain.

While independent front suspension generally provides a smoother ride, it has a variable ground clearance to the fronf diff. As the front wheels hit a bump, the wheels rise up to absorb the bump. However the diff remains in the same relative position, effectively reducing the ground clearance under the diff. This can result in the diff hitting the ground, even though the obstacle under the diff was only half the height of the static ground clearance.

Live axle front suspension generally does not provide as smoothe a ride as independent front suspension. However many live axle suspensions are now fitted with coils rather than leaf suspension, which   significantly improves ride comfort. The advantage of live axles when off road is that when the wheels hit a bump, the whole axle rises with the wheels to absorp the bump. This maintains the same clearance from the diff to the ground.

While this is an advantage off-road, the fact the weight of the entire axle and diff is constantly moving with any  bumps leads to a rougher ride, compared to independant front suspension.

When large drop-offs or ledges are encountered, they can be tackled at an angle to allow one wheel at a time to mount the obstacle. Bear in mind how this will affect the position of the vehicle as turning at an angle to a down hill ledge may result in the side rollover angle being reached.

 Gravel roads

Travelling on long straight stretches of gravel roads can lull the driver into a false sense of security. Modern 4WD's can make a relatively rough road seem smooth with their long travel suspension and sound deadened interiors. Speed creep can occur in these circumstances and when a bend is encountered the high centre of gravity in most 4WD's may cause you to cross to the other side of the road or it can lead to a high risk of rollover.

If a sudden obstacle e.g. a kangaroo, appears it is very important not to swerve while braking. This goes for any road surface and not just gravel roads. However it is more critical on gravel roads as once a skid or slide starts, it is very difficult, if not impossible, to regain control. There are exceptions to this rule such as a truck heading straight for you at high speed. It is better to avoid a collision with the truck and take your chances hitting a roadside object than to have a head-on collision. However, neither of these options are particularly attractive and it is for better to be travelling at a safe speed for the conditions.

In part-time 4WD's it is best to use 4WD on gravel roads to gain the better traction and road-handling of 4WD. Gravel roads provide enough slip not to cause any transmission windup problems. It is not necessary to use the centre diff lock of permanent 4WD's on gravel roads.

 Corrugated gravel roads can cause severe vibrations that con cause vehicle damage as nuts rattle off and vibrations damage electrical components. It is quite common for glass food containers to shatter even when stored in eskies. The key to minimise these problems is to find the right combination of vehicle speed and tyre pressures.

Increasing vehicle speed until it 'planes' over the corrugations con reduce vibration significantly, but the planing speed may be too high for the driving conditions. Depending on the corrugations, planing speed is around 60-80 km/h. Lowering tyre pressures reduces vibration as the tyre sidewalls act as shock absorbers. However this causes the tyre to heat up and can lower tyre life or even cause tyre failure. It is sometimes better to sacrifice tyre life and lower tyre pressure to reduce the damage being caused to the vehicle, as well as the comfort of the vehicle’s occupants.

After driving long distances on poor quality roads, it is a good idea to check all nuts and bolts to see they haven't vibrated loose. It is especially important to check all suspension components. The easiest way is to use an adjustable spanner and when a loose nut or bolt is found, use the proper size spanner to tighten it rather than risk burring the head.

Dust Control

When driving on dusty roads, its a good idea to have all your windows closed and the ventilation control set to outside air with the fan on high. This pressurises the vehicle interior slightly and helps reduce the amount of dust sucked into the vehicle. When another vehicle approaches, move the ventilation control to "recirculate" to stop dusty air coming in. Remember to move the ventilation back to outside air once you have passed through its dust trail or else the pressurisation effect will be lost. In hot weather, vehicles without air-conditioning have to make a choice between minimising dust intake or winding the windows down.

The biggest problem area for dust intake is from the rear tailgate. Check that the rubber seals are in good condition and that they seal when closed. (c) 4WD Encounter 1998

 These pages were last updated on 16th September 1998.
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