DRIVING/TOWING - 4WD DRIVING SKILLS
The soft, powdery dust known as ‘bulldust’ is a common hazard on bush roads and there are many horror stories about vehicles disappearing in huge bulldust holes. Fortunately, most bulldust encounters are far less dramatic.
You’re humming along a gravel road Back‘o’Bourke when you notice the road surface up ahead is different from what you’re used to: there are noticeable, soft folds in a slightly depressed section and the tyre tracks through it lose definition and become V-shaped troughs.
When you drive into this stuff, you notice that your 4WD moves around a lot more than it has been doing and you need to press harder on the accelerator to preserve road speed. All the while, great clouds of fine dust billow up around the vehicle. After you’re through the bulldust section you can see a pillar of dust behind you.
You’ve had a close encounter of the first kind: a shallow bulldust ‘hole’ that’s only a few centimetres deep and with soft surrounding edges.
Your next encounter may not be so pleasant, if the bulldust section has rock-hard edges and the dust bowl is a foot or more deep. Your arrival into this dust trap is accompanied by a noticeable ‘drop’ and the front end suddenly loses ‘feel’. Despite losing forward vision as the dust pours around the front of the vehicle you think you’ve got it back under control until you hit the sharp edge at the exit from the hole. Hopefully the front tyres and suspension cope with the sudden bang, but the back end now tries to come around to join the front. What you do in the next half second determines whether you emerge safely, but shaken, or roll the thing over.
Do it right
The main problem with driving through bulldust road sections is determining how long, how rutted and how deep they are. If you’re in a convoy the CB is a handy tool for relaying dust conditions to the mob behind.
With a clear view of the road ahead you can see the beginning and the end of the dust section, because the surface changes noticeably. A clear view is essential, so if you’re following other vehicles, keep sufficient gap to let the dust columns blow off the road: sometimes that can take minutes.
Dust holes with shallow tyre marks are generally low-risk hazards, but those with deep, soft-edged ruts that undulate are trickier.
OK, what to do?
The safest procedure on bulldust-prone roads is to switch your headlights on to low beam, so you can be seen more easily from in front and from behind. If you haven’t already lowered pressures for dirt running, stop and drop your tyre pressures by around 10-15 percent. The pressure change enlarges the tyres’ contact patches, for better flotation in the soft stuff, and improves tyre flexibility and ‘spring’ effect.
After the pressure drop, keep your top speed to no more than 80km/h.
When you see a soft patch ahead, reduce speed fifty metres before the bulldust hole, select 4WD high range and a gear ratio that gives strong engine response, with the engine operating in the rev band between its peak torque point and its maximum power, and pick an exit point that doesn’t look like it’s hard-edged.
When you’re in the hole, don’t try to correct every little change in direction, but keep aiming for your exit point. If the bulldust section is on a bend, don’t make radical steering wheel movements and expect some sideways ‘drifting’.
Provided there’s no oncoming or overtaking traffic it’s quite in order to use both sides of the road when seeking a good line through bulldust sections.
Beware of bulldust holes that have obvious debris in them: torn-off mudflaps, bits of tyre or lumps of wood. You should stop and make a visual inspection of ‘polluted’ holes.
If for any reason you get stuck in a bulldust hole, put the hazard lights on immediately and, if you’re in a convoy, warn following vehicles of your predicament. When you can see ahead and behind, get everyone out of the vehicle and well off the road.
Passing in bulldust
Stopping or overtaking in bulldust sections are big no-nos. These actions need to be taken on hard sections of the road, where you can be sure of vision and traction.
If the bulldust is severe it may well be impossible to overtake anyone safely for quite some time. What’s the hurry: you’re supposed to be on holiday.
It’s not uncommon to come across a heavy vehicle on bulldust roads. A triple stock crate road train kicks up a huge cloud of dust. If the truck is headed the way you’re going and you want to pass, try talking to the driver on UHF Channel 40. However, if the wind is blowing the dust across your overtaking path, forget passing the truck until either the road or the wind direction changes.
Beware of flying stones and stock poo when overtaking trucks.
If you see an oncoming truck, pull off the road where it’s safe to do so and stop, with your hazards on. If you’re in a convoy warn those behind that you’ve stopped and get feedback that they’ve heard your message.
Bulldust is a fact of bush driving life and it has no terrors for those who take it easy. Too much weight – especially in roof racks – too much speed and too much tyre pressure create the main dangers.
What is bulldust
Bulldust forms when the road surface loses cohesion and breaks up into individual dust particles. It’s most likely to occur in dry areas, or after long, dry spells in areas that normally have regular rainfall.
Bulldust-prone road surfaces are clay-like, fine-particle soils.
Bulldust most commonly occurs in depressions in the road, where the finest particles get washed by rain and also in spots where vehicles ‘pound’ the road surface, because of uneven alignment.
Heavy vehicles hammer the road surface harder than 4WDs, so if you see truck dual tyre marks on a dirt road, keep an eye out for bulldust sections.
It’s possible to come across bulldust patches on gravel or sandy soil roads that wouldn’t normally have the problem, because it’s not unusual for such surfaces to be ‘clayed’ for improved traction and surface durability.
The classic example of a clayed sand road is the Simpson Desert Rig Road that was built for oil and gas exploration. The sand ridges were coated with clay, dug from the numerous claypans along the track. Now that the Rig Road has no maintenance, many of the clay sections have broken up into bulldust holes.