cor 1There were lots of dirt roads where I grew up and I learned to drive on them, my instructor, who was also my dad, holding to the philosophy that whatever happens with a car’s handling happens earlier and slower on a dirt road than on a bitumen one. This means that when learner drivers get it all sideways, and that happened a lot with me, they learn to recover prior to the scenery rushing up to meet them. This is a truth I still hold dear.

Get used to dirt roads (they take a lot getting used to because they vary a lot) and they can be immense fun, but there’s one big problem; corrugations. Corrugations are rough, wearing, depressing and no fun at all. If you’re ever planning a big trip into the great Australian vastness you’re going to meet lots of corrugations and you’re not going to like a single one of them.

You hear lots of myths about them. A mate of mine was once told, and has believed faithfully ever since, that if you drive them in increments of 56 kmh they set up a harmonic in the car that keeps the body even and smooth even though the wheels are pumping up and down furiously. This has never worked for me so must be complete nonsense.

The one question about corrugations I could never get answered was what causes them. I’ve heard all sorts of theories, the most interesting and plausible of which is that they are formed by the breezes generated by cars passing over the road, much as sand dunes are formed by desert winds. When I eventually met a road engineer who specialises in unsealed roads this theory brought him a great deal of mirth.

cor 2He explained that while sealed roads have a fixed surface, the surface of dirt roads is what’s known to engineers as mobile. And as a car moves along a mobile surface a small ridge of dirt, dust and detritus builds up in front of the rolling tyre, and the tyre pushes it along until it becomes too big to be pushed any further. Then the tyre runs it over, but some of it remains and the rear wheel adds a bit more substance to that.

And so does the next car and the one after it and especially the trucks following them, until after a decent bit of traffic you have a bone-fide corrugation, or more accurately a solid line of them. This explains why dirt roads become corrugated so soon after the council grader has been along.

This guy had been in the job 35 years and said I was the first person who had asked him about how corrugations are formed. Does this make me a truly deep thinker among motoring writers or am I the world’s most boring person?

Published June 2015