Exquisitely engineered superhighways allow cars to fly along smoothly at high speeds, but many of the world's roads are surfaced with gravel that develops washboard patterns of periodic bumps.
This car-jarring phenomenon not only makes for an unpleasant ride but can be a safety problem, causing car wheels to lose contact with the road. Now, at long last, scientists have figured out why a smooth gravel road goes bumpy.
A group of scientists from France, England, and Canada performed a battery of tests to determine why any road covered with loose material, such as sand or gravel or snow, develops ripples. They set up an artificial road consisting of a suspended wheel allowed to travel around a rotating road surface.
No matter what the material covering the surface -- the researchers even used rice -- washboard bumps appeared after the test apparatus reached a certain combination of speed and wheel weight.
The scientists eliminated any suspension effects that would mimic the way a car can bounce up and down on its shock absorbers, but the bumps appeared anyway.
They even replaced a rolling wheel with a flat blade, and still a washboard pattern emerged.
University of Toronto scientist Stephen Morris and his colleagues concluded that the development of washboarding is very similar to skipping a stone across the surface of a lake. The wheel feels a combination of forces that cause it to both push off and dig into the surface.
This force combination causes the wheel to jump off the surface when it is going fast enough, creating the ripples. The difference between a skipping stone and a washboard is that the road surface "remembers" its position and does not, like a momentary water ripple, fall back into place.
Country Road, Take Me Home....
Morris says that ripples similar to washboard appear in a number of settings, shaped by wind and water on sandy beaches or by skiers on ski slope moguls.
Washboard ripples even turn up on some solid surfaces, such as on train tracks and on the surface of magnetic hard drives, which develop slight ripples in response to the passage of a read-head, which acts like the "wheel."