It’s an ugly encounter: You’re driving along a dirt road, and a seemingly endless series of equally spaced ripples in the road threatens to shake your vehicle to pieces. The more traffic, the bigger they get. It’s called the washboard effect, and it is the bane of drivers and highway maintenance crews alike. (The same phenomenon is sometimes seen on heavily used ski slopes.) No matter how many times they are vanquished by a road grader, smoothed away into oblivion, the washboard ripples reappear with maddening regularity.
What’s going on? Nicolas Taberlet, Stephen Morris and other physicists and mathematicians at the University of Cambridge in England decided to construct a simplified system for determining the mechanisms of washboard formation. The researchers built a rotating bowl three feet in diameter and filled it with several inches of sand. A rubber wheel on the edge of the bowl rolled on the “road” as the sand-filled bowl rotated.
Sure enough, after a few passes of the wheel, waves began to appear in the sand, taking the shape of the familiar washboard series as the number of wheel passes increased. This elegant experiment immediately proved that washboarding does not result from the bounce frequency of a vehicle’s suspension system, as engineers had commonly believed. Moreover, changing the wheel size and the size of the sand grains had no effect on the result. Washboard ripples appeared when the experimenters used wet sand, beach sand, or long-grain rice. It didn’t matter whether the sand was loosely or densely packed. The weight of the wheel didn’t matter, either.
What DID matter, however, was speed: As reported in Physical Review Letters , below a certain critical velocity, the sand surface remained flat. As Nicolas Taberlet told Science News, “The critical velocity below which the surface would remain flat is about 5 miles per hour.” When vehicles are traveling faster than that, the rolling wheels push tiny irregularities in the road surface slightly forward, enlarging them into a minuscule bump. As the wheels travel over the new bump, they whisk a portion of that bump’s sand or dirt forward, creating another tiny bump. After a few hundred passes, the bumps become large, and visible washboarding begins to occur.
Sadly, the results indicate that there is no solution to the problem–washboards on dirt roads are here to stay. The only fix is for users of the road to drive really, really slow—and at 5 miles per hour, you might as well walk.