CALL THEM WHAT you like, but there are few people who don't shudder when they peer through the windscreen and see some heart-stopping corrugations on the road ahead. They are one of the real curses of the outback.
Along such tracks as the Gibb River Road, Gunbarrel Highway and the trek to Cape York (where there are even signposts counting their number), the corrugations reach almost legendary proportions - some people even claim they are deep enough to lie in.
Outback gravel roads, however, are not the only places that develop these dreaded humps. They can also be found on bitumen and concrete roads (mostly when the surface is starting to break down), or on beaches, sandy inland tracks, gravel roads around town, and in unexpected places like on well-used ski runs and railway tracks -where the sound of travelling over them is often referred to as'roaring rails'.
Like it or not, corrugations are a fact of life, so let's have a look at how they got there and how best to handle them.
HOW THEY FORM
It was Keith B. Mather, from the University of Melbourne, who carried out an experiment in 1962 that shed light on how corrugations are formed. He drove a wheel attached to a variable-speed motor around in circles on varying surfaces,examining the effect of different speeds, surfaces and tyre pressures on the formation of corrugations.
Because there is no such thing as a totally smooth road surface,when a tyre gets up to speed it will bounce slightly over tiny imperfections. When it lands it pushes dirt forward and sideways,leaving a very slight depression behind. The process is repeated and the earth pushed forward starts to pile up when the next wheel comes along, making the depressions more pronounced. A pattern of small ridges and valleys emerges - and full-blown corrugations will eventually develop.
The corrugations will initially be close together, but as they get deeper they tend to move further apart, until the point their height and distance remain in a stable pattern, dependent upon how fast people travel along the road. The faster people travel, the further apart the corrugations will be.
Mather also found hard tyres caused corrugations to develop more quickly than soft, and that a dry road surface deteriorates more quickly than a wet one.
HOW FAST TO TRAVEL
The other point of discussion is the best speed to travel over these outback ripples. Advice often varies widely, with some suggesting very slowly (literally up and down each corrugation)while others recommend flying across the top of them at least 100 km/h.
Travelling slowly, around 30-40 km/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. However, at this speed it is most likely that they are just making the trip uncomfortable for their passengers while also doing a good job of shaking their vehicle to pieces. It is common to find nuts, bolts, screws, mufflers, tailpipes, bumpers, etc. scattered on the side of corrugated roads.
That said, on really severely corrugated roads, travelling at 15-20 km/h might be your only (sane) option.
On the other hand, travelling at 100 km/h or more on heavy corrugations should have the vehicle flying across the top of them,but in most outback conditions such speeds are probably outright dangerous. Your tyres have much less grip on loose surfaces,and even a slight swerve or poorly applied braking can cause you to lose control (particularly with rear-end slides).
Short-wheel-base vehicles tend to be more difficult to handle on corrugations and if you are towing a trailer, whatever your vehicle, a good deal more care is needed to maintain control.
The most sensible advice is to drive to the prevailing conditions.If the track is windy and rocky you will have to drive slowly,whereas you should be able to get up a bit of speed on a bumpy dirt highway.
Travelling at 65-70 km/h tends to be a happy medium for handling most corrugations; it's also a speed that should allow you to slowdown fairly quickly.
But there are no hard and fast rules. Every vehicle and trailer combination will behave differently, and no two corrugated roads are the same. When you hit a poor section of the road you should deliberately experiment with different speeds and settle on one that feels safe while also being relatively comfortable.
You will notice the initial bone-shaking effect of corrugations diminishes as you build-up speed (as you start to roll over the top of the humps) and develops into a much more comfortable, but still quite tiresome, rippling/droning experience.
LOVE YOUR CAR
Corrugations are tough on your tow vehicle, and it's easy to damage it if you push too hard. In particular, your suspension will behaving a pretty rough time.
Your shock absorbers will be working at their limit over corrugations - especially over a long distance, where heat builds up and reduces their effectiveness. When this happens they experience 'fade', and the ride in the cabin will get noticeably worse, at which stage you should try to stop and let them cool off.Also keep an eye on your shock absorber bushes, which can easily be chewed out on prolonged corrugations.
Some outback cattle grids are pretty rough, and if hit at speed they can buckle rims or put out your wheel alignment. Try to travel at a speed that will allow you to slow down for them.
The correct tyre pressures for corrugations are another subject for debate. Some advocate pumping them right up, but Mather's work shows that this will simply make the corrugations worse for others.Others argue that for a more comfortable ride, tyres should be almost 'spongy'. But too much flexing of the rubber on hard surfaces can cause tyres to overheat and break down, so this isn't a good solution, either.
The ideal pressure is somewhere in the middle, around 35-40 psi for hard outback tracks and 25-30 psi (or even a little less) for softer corrugations. As always, the right pressures will depend on the size and type of tyre, the load they are carrying and there commendation of the manufacturers.
Now you know a little bit about corrugations it's time to get out there and experience them for yourself… while enjoying some of our great Aussie countryside.
Source: Camper Trailer Australia #52