July 14, 2009

Scientists study ‘washboard’ road ripples

Physicists from Canada, France and Britain studying so-called washboard road ripples say their findings might eliminate bumpy vehicle rides.

The researchers said they discovered ripples appear even when the springy suspension of a car and the rolling shape of a wheel are eliminated.

To understand the washboard road effect, we tried to find the simplest instance of it, University of Toronto physicist, Stephen Morris, a member of the research team, said. "We built lab experiments in which we replaced the wheel with a suspension rolling over a road with a simple inclined plow blade, without any spring or suspension, dragging over a bed of dry sand. Ripples appear when the plow moves above a certain threshold speed.

We analyzed this threshold speed theoretically and found a connection to the physics of stone skipping. A skipping stone needs to go above a specific speed in order to develop enough force to be thrown off the surface of the water. A washboarding plow is quite similar; the main difference is that the sandy surface remembers its shape on later passes of the blade, amplifying the effect.

The study that included Anne-Florence Bitbol and Nicolas Taberlet of National University in Lyon, France, and Jim McElwaine of Britain's University of Cambridge appeared in the June 26 issue of the journal Physical Review E.