washboard road effects

There is a review on a PRL [PRL 99, 068003 (2007)] paper investigating the so-called washboard road effects. Washboard road features ridges that might cause bumps, making driving on such road quite annoying and uncomfortable. What do you think could be the relevant factors ? The rotation of the wheel ? The size of the sand grains ? No. What appears involved are the wheel velocity, the wheel density and the gravity, in addition to the density of the road bed. This study looks mundane, but may have wide ranging applications and complex physics behind. As Zz said in his blog, "This is another one of those "mundane" stuff that perks up my interest and what got me into physics in the first place. Of course, these things APPEAR to be mundane, but the physics of these things have wide-ranging impact and application. It is just that the phenomena that manifest the principles looks so benign. Still these are the stuff that I find most fascinating. You can go solve the mysteries of dark matter and CP-violation. Just give me rippled roads and grapes that bounce up and down in sodas!"

Experimentation and analysis lead the physicists to conclude that the washboard effect is not, in fact, due to the suspension of the vehicles driving over it. As well, the size of the wheel and the size of the sand grains are irrelevant. Most surprising of all, the rotation of the wheel was also irrelevant, since the effect could be reproduced with a fixed, non-rotating object! In the end, all that mattered was the mass and velocity of the wheel, density of the road bed, and the acceleration of gravity. The fact that the velocity of the wheel was important also explains why the effect is worse on certain sections of road:

The speed of the wheel appears to be crucial. Indeed, there exists a critical velocity below which the road always remains flat and above which washboard bumps appear. Typically, for a car this critical velocity is around 5 mph or 8 km/h.