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4x4 Driving Hills & Rocky Surfaces

4x4 Driving Hills & Rocky Surfaces

Hill Ascent

It is usually best to tackle a hill climb that’s any sort of challenge in low range. Although you may be able to just reach the top of a hill climb in 1st high, it doesn't leave you with any lower gears if the need arises. By using say 2nd or 3rd low range instead of 1st high, you have lower gears available if needed. If you run out of gears on a hill climb, you may have to back down to the bottom of the hill and start again. Most 5-speed gearboxes allow vehicles to travel up to 6Okm/h in low range, so you don't have to be continually swapping between low and high range as you travel between hills.

Diesel vehicles are generally better for hill ascents and descents as they usually have better low rpm torque for climbing the hill and higher engine compression for descending. However, in long sandy hill ascents, where traction is low and a run-up is required, the greater power of gas engines can be an advantage.


Hill Descent

When descending hills, you should always be in low range and have selected the gear required before descending. You should always avoid braking on downhill sections as you run the risk of locking up your wheels, causing a slide. Engine braking slows the vehicle without causing wheel lockup. However it is better to use the brakes than allow the vehicle to 'run away' and pick up speed. Always apply the brakes gently, rather than applying them abruptly and risking a wheel lockup.

Often hills get steeper the further you go down, so its best to be in 1st low from the beginning rather than having to try and change gear halfway down. When changing down a gear while descending, it is best to be feathering the brakes as you depress the clutch, as otherwise the vehicle freewheels while the clutch is depressed and picks up speed. It is far better to be in too low a gear and travel down slowly rather than be in too high a gear and have a potentially dangerous situation arise.


Hill Stall Recovery

A very common occurrence on hill climbs is the stalling of the vehicle when the hill becomes too steep. When this occurs, you will be left in gear with the engine switched on but stalled. To recover from this situation you should perform a Hill Stall Recovery, rather than just fire up the engine. This is accomplished by:

  • Switch the (stalled) engine off and place your foot firmly on the brake. 
  • Depress the clutch and select reverse gear in low range. 
  • Take your foot off the clutch then slowly take your foot off the brake. The vehicle is now in reverse gear with the stalled engine stopping it from rolling down the hill. 
  • After checking the track behind is clear, start the engine and keep your feet off all pedals. 
  • The starter motor will start to drive the vehicle backwards as the engine begins to fire.

This will result in a smooth downhill progression while starting the engine and ensuring you have been in gear at all times. If you simply start the engine and then select reverse gear, it will result in a fast jerky takeoff and being out of gear during some stage of the takeoff.

The Hill Stall Recovery does not apply to automatic vehicles as they should never stall, just lose forward drive when the hill becomes too steep for the gear its in. When this occurs, simply place your foot on the brake while restarting the engine, select the appropriate gear and gently take your foot off the brake.

A lesser-used method after an uphill stall can be done in situations where the terrain is not extremely steep and you wish to continue forward up the hill but you do not wish to attempt to let the clutch out while trying due to tire spin.  This method only applies to manual transmissions and drive trains that have very low forward gears.  By leaving the vehicle in gear and starting the engine without pressing the clutch in the vehicle will begin to move forward while the engine fires.  Once the engine fires you can begin forward movement.  This method is hard on the starter and electrical system and should not be attempted on very steep terrain due to the possible overload on the electrical system and starter.  Some vehicles will not allow the starter to be engaged if the vehicle's clutch is not depressed.  In some cases, this feature can be overridden by use of a clutch safety cancel switch.


Vehicle Rollover

Hills should always be tackled straight up or down and sideway slopes avoided like the plague. A vehicle has to be at an extreme angle for it to roll head to tail, whereas a sideways roll can occur at much more modest angles. The way your luggage is packed has a pronounced affect on the angle a vehicle will roll sideways. You should avoid placing containers of extra fuel and water on the roof of your vehicle, as this will significantly decrease the angle at which a rollover is possible. This occurs because the extra weight on the top of a vehicle increases the 'center of gravity'. Heavy gear should always be packed low inside the vehicle and as centered as possible while only lightweight gear should be placed on roof racks.

The type of terrain will also affect the sideways rollover angle. In sand, mud or other soft terrain the weight transfer to the downhill wheels will cause them to sink, increasing the actual slope angle. So what may appear an acceptable angle on firm terrain may result in a rollover on soft terrain.


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 minimize vehicle contact with the terrain.

While independent front suspension generally provides a smoother ride, it has a variable ground clearance to the front 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 smooth 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 absorb 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 independent 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

Traveling 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 minimize these problems is to find the right combination of vehicle speed and tire 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 tire pressures reduces vibration as the tire sidewalls act as shock absorbers. However this causes the tire to heat up and can lower tire life or even cause tire failure. It is sometimes better to sacrifice tire life and lower tire 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 and when a loose nut or bolt is found, use the proper size wrench or socket to tighten it rather than risk rounding 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 pressurizes 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 pressurization effect will be lost. In hot weather, vehicles without air-conditioning have to make a choice between minimizing dust intake or winding the windows down for the (cough) fresh air.

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.

Information from:
Andre Botha
Member of SONOTA (South African National Offroad Trainers Association)

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