Found this great article that explains how “washboard” patterns are formed on dirt roads.
YOU’VE SEEN THEM AND HATE THEM, BUT HOW ARE CORRUGATIONS MADE?
THERE HAVE BEEN several scientific studies into exactly how corrugations are formed, and the answer is definitive and simple. You get corrugations any time large numbers of wheeled vehicles travel over a soft surface at speed.
Exactly how they are formed is easy to explain too. When a vehicle travels at speed on a soft surface there will inevitably be a little imperfection, an undulation in the smoothest of surfaces, maybe just a stone on the road, a variance in softness, or even braking hard to dig the wheels in. When the wheel travels over the undulation it will bounce slightly, reducing weight on the wheel for a moment, then landing a moment later which compresses the surface. That creates a tiny crest and dip, so guess what the next wheel over does…same again, but worsening the problem. Every subsequent wheel makes the problem worse and worse and then you have corrugations. It doesn’t matter how perfectly smooth the surface is to begin with, corrugations form anyway given enough time and speedy traffic.
Once established, corrugations run deep. A roadcrew told me they reckon they corrugations go down for 4 to 5 metres, so all they’re doing is a short-term fix to grade them over. There’s no real fix to corrugations other than slowing down, running lower tyre pressures, or making the road so hard corrugations can’t form, which is the bitumen solution.
This explanation also covers why you don’t see corrugations on bitumen (too hard), or on low-speed 4WD tracks (too slow). There are exceptions, such as some sandy tracks, especially uphill where wheelspin plays a part. Extremely soft, fine sand doesn’t form corrugations as it just fills in the gaps, and the same is true of most squelchy mud. The lower vehicle speeds in these terrains also help. Very hard dirt roads tend not for form them either, the sort of hardness where you can leave rubber behind if you brake hard.
Corrugations also tend to form on the inside of uphill corners where drivers accelerate, such as the title photo above.
We have all seen scalloped sand dunes or muddy hills where diagonal wheels dig out ruts, and each vehicle makes it slightly worse. Those aren’t corrugations per-se, but it’s the same principle of wheeled vehicles developing ruts over time.
Basically, it’s the combination of terrain softness, vehicle speed, number of vehicles and speed of vehicles that creates corrugations, and there’s no fixed formula.
One of the earliest experiments to determine the cause of corrugations was carried out by Keith Mather in the early ’60s. He created a test rig of a wheel attached to an metre-long rod which he ran in a circle, powered by an electric motor. He found corrugations formed no matter what material he used so long as it was relatively easy to shift around. Since then, several other experiments have replicated the results, and if you want to read a detailed scientific report you can find one here. Corrugations are also known overseas as washboarding.
The studies raised a few interesting points:
- corrugations are unavoidable if there is wheeled traffic at speed on soft roads
- lighter vehicles create more corrugations than heavy vehicles (there is some evidence that heavy logging trucks with tyres at low pressures and driving slowly have a positive effect on corrugations)
- it doesn’t matter whether the wheels are driven or not, corrugations still form
- the exact nature of the surface doesn’t matter so long as it is soft, and the particles of the surface are tiny compared to the wheel (corrugations don’t form with large rocks, for example)
- the exact wheel diameter appears to be unimportant.
So what can be done? Well, the study linked above concluded with the gloomy words that “washboard roads will no doubt continue to annoy drivers as long as there are unpaved roads and wheels to roll over them”, so there’s no fix for the problem other than bitumen. But you, as a 4X4 driver can take steps to prepare for and drive on corrugations – read about that here.