Thoughtwax

Driving in New Zealand takes a bit of getting used to. For a start, other than the sparse network of state highways that link the main towns, there are no real roads. Sometimes it’s possible you’ll have to drive a long, roundabout route to get to a destination that’s geographically quite nearby (Milford Sound and Queenstown, for example, are about 70km apart as the crow flies, which translates into a 286km drive).

Along with this, after a couple of days driving you realise that it’s useful to double the amount of time that you might have naturally alloted to get through that long drive. A road may look like a straight line of the map, but the more remote stretches are probably going to be creeping up and down the side of a mountain range, or winding along a narrow coast road or cliff face. All of which is great, that’s why you’re there after all, and you can also take for granted that the views are going to be astonishing. But vast – vast – areas of the country are unserviced, so if there’s somewhere in particular that you really want to go, you might have no choice but to go offroad, or at least onto dirt roads.

Head north from Wairoa towards Lake Waikaremoana and the sealed asphalt quickly gives way to a dirt road. The road is generally clear and dry, and wide enough for two cars to pass comfortably, and the surface is covered with a packed gravel dirt. The thing that makes the roads so difficult to drive on though, is the corrugated surface. Across the width of the road, perpendicular to the direction the road is traveling, are small waves in the surface, like tiny regular speedbumps about an inch tall, right next to each other. The pattern doesn’t seem to change at different stages, remaining consistent for dozens of kilometers of dirt road. And apparently this happens everywhere.

Image: Washboard road study, University of Cambridge.

Although it’s a commonly known phenomenon, nobody in our van seemed to know what causes the corrugation. We ventured a few guesses (it was a long journey), but none seemed entirely plausible:

• The force of the tyres on the road causes a pattern to emerge. At first this seemed like the most likely explanation: the vehicles that drive on the road create the corrugation. The forward motion of the tyres, according to this theory, caused dirt to be propelled backwards from underneath them, and this dirt somehow settled into tiny rows that were compounded the more they were driven on and packed into hard ridges. Although this at first seemed to be the simplest explanation, it didn’t hold up well under scrutiny: How did the dirt begin to settle in that pattern, and remain regular despite all of the different sized vehicles that drove on it at different speeds? If it was caused by the tyres alone, why was the corrugation uniformly deep across the width of the road and not just under the main tyre tracks? And although certainly not impossible, this hypothesis seemed to rely quite heavily on some sort of magical emergent behaviour in the dirt. Particle physics was mentioned. It didn’t add up.

• Wind erosion somehow leaves ripples on the road surface. Much like the above scenario, except that the force acting on the road surface was the wind, not tyres, thus sidestepping the problems of tyre tracks and variation of force. The most compelling argument made in favour of this was the comparison to the ripples that you might see on the surface of a sandy desert or dune, which are probably made by the wind. This pattern also seems to be made by the water’s edge on a beach, although the shoreline ebbs back and forth and probably exerts a lot more force than the wind, so I’m not really sure if this is comparable. One problem with this was that the grooves were always perpendicular to the road, which was not at all straight, so the variable direction of the wind would need to have no bearing on the pattern (which is obviously influenced by the direction of the road) for this to work. All in all, though, this somehow seemed like the most plausible explanation to me.

• The way that the rough road was originally created was imperfect. Only mentioned here to complete the set – I don’t think anyone really believed this to be the explanation. But anyone who has seen tar being spread behind a truck before it has been steamrolled flat can imagine the gloopy way that the base material might have settled and hardened into grooves.

Consensus eluded us. Vague guesswork and wild conjecture had failed to deliver the goods yet again. Research time!

Luckily most of the legwork on this one had already been done by one Keith B. Mather of the University of Melbourne, as published in the January 1963 issue of Scientific American. Mather created a controlled laboratory apparatus that allowed him to test the effects of a tyre on a dust road in a number of simulated environments (here’s a video of a similar experiment carried out just last year in the University of Toronto), and cracked it:

It’s based on the fact that you can never make a road perfectly smooth. There will always be tiny little bumps. Once his wheel got up to about 6-7 kph, it would bounce up when it hit a tiny bump. As the wheel came down and hit the sand, it would spray sand both forwards and sideways off the track, leaving behind a little crater. This crater would then be the valley of a corrugation. As the wheel came up out of the valley, it would jump into the air again, and so the pattern of valley-and-mountain would repeat itself.

Making corrugations is a two-stage process - first the corrugations establish a stable pattern, and then they spread along the road.

Mather saw that the first few corrugations to appear on the “smooth” road were quite shallow, and very close to each other. But as the corrugations got deeper, they gradually moved away from each other, until their height and their distance apart had settled into a stable pattern. Once this stable pattern of corrugations was set up, then the entire pattern of corrugation would migrate down the road in the direction of travel of the wheel. In the Australian Outback, engineers have seen corrugations heading in opposite directions on each side of the road from (say) a cattle grid, with each set heading in the direction of travel of the cars.

Despite this apparent resolution, the details of the matter remains robustly debated and discussed. At the very least though, corrugation is in fact caused by tyres, and and is indeed a result of some mad particle-physics-level emergent property of the dirt. Who knew?