What causes the even pattern of ripples in a “washboard” gravel road?

Question

I finally stopped to look more closely. These ripples are about 10 inches wide ands look to be almost exactly uniform through long stretches of road. Where does this precision come from?

In: 879

Answer 1

Wheels bounce a bit when when the vehicle they belong to drives over a bump. While suspension is meant to dampen the bounces felt in the passenger cabin, the wheels themselves still bounce once or twice. Most vehicles wheels bounce at roughly same rate as those on other vehicles. Ruts form beyond a prominent bump due to the aggregate effect successive cars’ bouncy wheels.