Nicolas Taberlet, Stephen W. Morris (also of the Department of Physics, University of Toronto, Ontario, Canada), and Jim N. McElwaine, all of the University of Cambridge (United Kingdom), researched the problem and found the answer. They wrote their results in the article “Washboard Road: The Dynamics of Granular Ripples Formed by Rolling Wheels” for Physical Review Letters (99, 068003 [2007]).

Their abstract states: “Granular surfaces tend to develop lateral ripples under the action of surface forces exerted by rolling wheels, an effect known as washboard or corrugated road. We report the results of both laboratory experiments and soft-particle direct numerical simulations. Above a critical speed, the ripple pattern appears as small patches of traveling waves which eventually spread to the entire circumference. The ripples drift slowly in the driving direction. Interesting secondary dynamics of the saturated ripples were observed, as well as various ripple creation and destruction events. All of these effects are captured qualitatively by 2D soft particle simulations in which a disk rolls over a bed of poly-disperse particles in a periodic box. These simulations show that compaction and segregation are inessential to the ripple phenomenon. We also discuss a simplified scaling model which gives some insight into the mechanism of the instability.”

To research the problem the three researchers built a circular turntable and covered it with dirt and sand. They took a hard rubber wheel and placed it on top of the dirt-covered turntable. They let the wheel turn on the surface. Even though they changed the tire size and weight, changed the texture and size of the dirt and sand, and altered other such physical variations, the road surface always resulted in the washboard effect, whether it be with large or small ripples or other similar patterns.

They found that any dirt or sand road has tiny irregularities in it (even after a bulldozer has just smoothed out the ripples) that slightly or roughly jar a moving vehicle going over it. Their computer modeling program simulated the actual motion.

When a wheel contacts a bump, the force of the downward motion on the road pushes the dirt or sand forward into the next bump. After numerous contacts with the road, the wheels cause the ripples that are called the washboard effect.

The scientists contend that the only way they know to get rid of the washboard effect is to travel slowly over such roads. Slow motoring—five miles per hour or less—seems to prevent the ripples on dirt and sand roads from coming back.

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