There are few caravanners who don’t shudder as they peer through the front windscreen and see corrugations on the road ahead. They are one of the real curses of the outback. Along such trails as the Gibb River and Cape Leveque roads, the Gunbarrel Highway, Tanami Track and the trek to Cape York, the corrugations reach almost legendary proportions – some even claim these roadway ‘ripples’ are more like furrows deep enough to lie in! Outback gravel roads, however, are not the only places that develop these dreaded humps. They can, in fact, be found on bitumen and concrete roads, on sandy beaches, inland tracks and gravel roads around town. Like it or not, corrugations are a fact of life, so let’s have a look at these strange humps and bumps (or ‘washboards’, as some people call them) that cause us drivers so much anguish, and how best to handle them.
CAUSES OF CORRUGATIONS
When we talk about causes of corrugations, there are about as many theories on how they got there as there are travellers out there cursing them. Surprisingly, after all this time, there is still debate on the reason for their existence, why they are so uniform, and why we get them in some sections of the road and not in other sections of the same road.
Here are a few theories I have heard about their causes around happy hours and campfires:
• Axle-hop from vehicles travelling too fast with high tyre pressures;
• The road expands and contracts with temperature change, especially shrinkage as the track dries out after rain;
• They result from air movement over the surface – either natural wind or, more likely, the force of wind on the road surface caused by fast moving vehicles;
• They are caused by the vibration/resonance of engines as they pass over the top, breaking down the once-smooth road surface;
• They are influenced by wheel mass, the action of its suspension and tyre pressures at generally similar speeds;
• Corrugations, it seems, are also made worse by hard braking and acceleration, as often seen around corners and near cattle grids where braking and acceleration is required;
• While there are a variety of tyre sizes travelling the tracks out there, the differences are not significant enough to affect the result – it’s more to do with how most vehicles bounce from one bump to another, landing at the same point in the valley;
• Corrugations are caused by water run-off across the road surface (this theory, however, doesn’t seem to ‘hold water’ as many of the nation’s worst corrugations appear in the dry season traffic);
• It is the constant pushing of a small wave of dirt in front of car tyres; and
• Other theories involve the type of surface, how well it has been initially compacted, as well as the effect of vehicle breaking and acceleration. Another common belief is that they are mostly caused by ‘big trucks’ using the road.
Whichever explanation is actually correct, or whether it is a combination of several, or something else entirely, they are certainly a popular (or depressing) talking point around the campfire at the end of yet another teeth-chattering, bone-shaking day of travel!
SLOW OR SUPER FAST?
The other point of discussion is the best speed to travel over these outback ripples. Again, there are differing ideas, varying from going very slowly to flying across the top at 100km/h. Travelling slowly, say up to 20 or 30km/h over corrugations, is an option sometimes taken by inexperienced outback drivers who probably think they will do less damage to their vehicles by crawling along than they will at higher speeds.
Generally, however, at this slow speed, the main things they are doing is making the trip much longer, more uncomfortable for the vehicle’s occupants, and probably also doing a pretty good job of shaking the vehicle (and anything they are towing) to pieces! It is not uncommon to find an assortment of nuts, bolts, screws, mufflers, tail pipes, bumpers and the like, scattered along the side of the road. Having said that, on some particularly severe sections of corrugations, travelling for short distances at 15 to 20km/h might be your only safe option.
On the other hand, travelling at 100km/h or more on heavy corrugations might well have the vehicle ‘flying’ across the top of them, but generally in those conditions such speeds can be simply dangerous. Remember, your tyres have much less grip than normal on these surfaces so it is all too easy to lose control. And trying to regain it on these surfaces can be extremely difficult and certainly not pretty to watch. It seems also that short wheelbase vehicles are often more difficult to handle on corrugations and, if you are towing, a good deal more care is needed. The main thing to remember is to always drive to the prevailing conditions – or, as an experienced bush traveller once told me: “Drive to the conditions, not an itinerary or timetable.”
This could mean that sometimes, despite the heavy corrugations, you will need to drive at slow, bone-shaking speeds, especially in conditions where there are tight corners, rocks, washaways, other road hazards, or where you can’t see clearly ahead. Another thing to watch out for and always approach with caution is cattle grids – some of them (with surrounding soil washed or blown away) are real doozies! If you are travelling too fast and then have to brake suddenly on corrugations, the result could shatter more than your driving confidence and self-control, but also your vehicle’s suspension! Hit at speed, some of these grids, often in poor condition, can buckle rims and do wonders for your wheel alignment.
Where conditions allow, I have found travelling at a constant speed of around 65 to 70km/h to be a happy medium for handling most corrugations on clear, open, straight sections of roadway. It is also a speed from which I can slow down much more easily (without heavy braking) if I have to. Bear in mind that there will be differences between vehicles and also the particular corrugation pattern you’re travelling over. When towing a caravan, it is usually better to err on the slower side as, obviously, your caravan will react somewhat differently, particularly if wheel/tyre sizes are different from the towing vehicle.
You must also be conscious of doing damage to your tow vehicle if you push things too hard. When towing, remember that you do not have as much control as you do without a van behind. The more experience you gain with corrugations, the more opportunity you will have to see what works best for you and your van.
As you build up speed from a standing start on corrugations, you will notice the initial bone-shaking effect progressively diminish (as you start to roll over the top of the humps) as it develop into a much more comfortable, but still quite tiresome, rippling/droning experience. Another thing to remember, now that you’re achieving as comfortable a ride as possible for you and your passengers, is that underneath, your vehicle’s suspension system is having a pretty stressful time.
Your shock absorbers, in particular, work hard over corrugations – especially during sustained sections where heat can build up and reduce their effectiveness. If they do begin to ‘fade’, their ability to control your vehicle’s springs will diminish and the ride in the cabin will noticeably worsen. In these conditions, it is best to stop and let them cool. Also keep an eye on your shock absorber bushes as they can easily be chewed up by prolonged travels on corrugations.
Tyre pressures for corrugations are another subject of debate. Some people advocate pumping them right up (probably the same group advocating the 100km/h-plus approach), while others argue that for a more comfortable ride, tyres should be almost spongy.
The general consensus, however, seems to suggest an approach somewhere in the middle – around 30psi for hard outback tracks and perhaps lowered to 25-28psi (or even a little less) for softer, sandier corrugated conditions. Reducing tyre pressures on your caravan or camper trailer on gravel roads and corrugations is also desirable (also perhaps in the range of 25-28psi).
Be aware, however, that tyre pressures will depend on the size and type of tyre and the load you are carrying and should be as recommended by the manufacturer. You must remember to reinflate the tyres when you get back to highway driving conditions.
A final point about corrugations that does seem to be based on fact is that the faster the average speed travelled by vehicles on a particular section of road, the further apart each corrugation will be – often extending out to around 0.75m, sometimes more than 1m on straight outback stretches, particularly when the road is frequently used by heavy vehicles.
But the only real way to find out more about corrugations is to get out there and experience them for yourself and, at the same time, enjoy our great Aussie countryside and outback!
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