Researchers examining the behavior of beds of granular material find that vehicles moving at more than about three miles per hour will inevitably cause ripples in an unpaved road. Steve Mirsky reports.
If you’ve ever driven on a dirt road, you’ve encountered the ridges and ripples that make a conversation in the car sound like this. Well, a study soon to be published in Physical Review Letters finds that so-called washboard roads are virtually inevitable. Wheels rolling over dirt will almost always cause the rippling pattern that rattles your teeth as you drive.
The researchers made miniature road beds made of various sizes of grains of sand and even some long-grain rice to learn about the physics involved. And found that wheels rolling over loose particles just naturally causes washboarding. Doubling the bed thickness and halving the grain size had almost no effect, and wheel size didn’t matter. But the mass of the wheels did--heavier wheels produce larger amplitude ripples with shorter wavelengths—making for a really rough ride. Experiments showed that you could avoid generating the ripples—by keeping traffic at less than three miles an hour. Therefore, they conclude, "Washboard road will no doubt continue to annoy drivers for as long as there are unpaved roads and wheels to roll over them."