Because inquiring minds want to know...
Just about any road with a loose surface — sand or gravel or snow — develops ripples that make driving a very shaky experience. A team of physicists from Canada, France and the United Kingdom have recreated this "washboard" phenomenon in the lab with surprising results: ripples appear even when the springy suspension of the car and the rolling shape of the wheel are eliminated. The discovery may smooth the way to designing improved suspension systems that eliminate the bumpy ride.
LINK: http://www.sciencedaily.com/releases/2009/07/090707131834.htm