Along such trails as the Gibb River and Cape Leveque roads, the Gunbarrel Highway, Tanami Track and the trek to Cape York, 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
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 along the same road.
Here are a few theories I have heard about their causes:
- 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; and
- 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.
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!
THE PERFECT SPEED FOR TACKLING CORRUGATIONS
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!
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 the conditions out there, such speeds can be simply dangerous.
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.
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!”
ISSUES TO KEEP IN MIND
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 off.
Also keep an eye on your shock absorber bushes as they can easily be chewed up by prolonged travels on corrugations.
Furthermore, it is crucial to remember that 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.
Tyre pressures for corrugations are another subject of some 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).
The full feature appeared in Caravan World #535 March 2015. Subscribe today for the latest caravan reviews and news every month!