Back roads in Australia often have hundreds of metres of gravel corrugations, or ridges. They are always a few centimetres high, spaced about 30 centimetres apart鈥� and annoying for vehicle occupants. What causes them?
鈥� When a vehicle is driven over an unsurfaced road, its weight indents the material under the tyre. The loose particles ahead of the tyre tend to be pushed forwards, creating a small ridge.
Each passing vehicle gradually raises the ridge, until the rate of material removal from the crest due to abrasion and impact stop it getting any taller. The maximum height of the ridge depends on the material used for the road.
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The surface of a newly levelled road generally stays more or less smooth until it rains. After this, it corrugates rapidly, presumably because indentation can occur more easily with wet materials.
Such ridges also occasionally occur on bitumen roads. I used to pass a bus stop on a downhill slope in Sydney that developed impressive corrugations due to heavy buses halting there.
Tony Cooke
Macgregor, ACT, Australia
鈥� Gravel and dirt roads are maintained to try to keep the surface in good condition, for example by scraping them with a wide blade mounted under a heavy vehicle. The blade bounces slightly, creating undulations. Over time, these undulations are exacerbated by bouncing truck and car tyres. Wind and rain remove loose dirt between ridges, accentuating them.
If your vehicle has feeble city tyres, not big, , four-wheel-drive ones, go slowly. With either type, you can minimise the jarring by reducing tyre pressure, especially if the corrugations combine with the angular rocks of the gibber plains 鈥� the desert landscape encountered on the unsurfaced . I know this from personal experience.
Bonita Ely
Marrickville, New South Wales, Australia
鈥淪ome people drive on the wrong side of corrugated roads, as it feels smoother that way鈥�
鈥� I live by a gravel road in rural Western Australia. Corrugations are common and aren鈥檛 restricted to back roads. They start small and increase in size over time, until you are unsure whether to slow down and feel each bump or just go faster and ride across the tops of the ridges.
Corrugations form across the road, and are steeper on the side facing traffic flow. That is why some people drive on the wrong side of the road when they can, as it feels smoother that way.
The material used for the road is an important factor in the corrugations. The road we live on has two sections: the part covered with sand and gravel is corrugated, while the other, made with gravel and some clay, sets hard and is always in excellent condition.
If motorists slow down, the likelihood of ridges forming is cut.
Anna Butcher
Brookton, Western Australia
鈥� The amplitude of the ridges depends on the size of wheels, the speed of vehicles and to some extent vehicle length. Larger wheels yield deeper corrugations. Higher speeds produce longer spaces between corrugations. So small vehicles using roads more usually frequented by larger, slow vehicles are worst affected.
Australia鈥檚 unsealed roads are much used by large vehicles, so they develop long stretches of corrugations. Cars can minimise the bumpiness by going faster and essentially riding along the tops of the corrugations. But this is dangerous because the vehicle isn鈥檛 in contact with the ground for significant periods of time, meaning it is easy to lose control of the steering.
Brian King
Barton on Sea, Hampshire, UK
鈥� The ridging effect is known as washboarding. It occurs on all loose surfaces, including sand and snow. It is caused by a force acting in one direction on a movable surface, much like ripples on a lake caused by the wind. The ridges are essentially very slow waves.
It helps to use a mix of particles of different sizes to make the road surface, as this prevents the material from moving as easily.
Lewis O鈥橲haughnessy
London, UK
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