What creates the bumps in a “washboard” dirt road

Dear Matt: What creates the bumps in a “washboard” dirt road? And is it really true that if you drive on them fast you can decrease the jarring effect? — P.R., Encinitas

To make your own washboard road, you’ll need a road with an unstable surface — dirt, gr or even some kinds of blacktop. Then put a bump in the road and send over it a stream of vehicles of roughly the same size and weight, traveling at about the same speed. As the front of a car the bump, it leaves the road briefly, then the car’s suspension forces the vehicle back down. When the tires hit the road, they displace a bit of the surface and form another bump. Continue driving cars over the bumps and you create new ones, reinforce the hills and valleys created by the bouncing suspensions, and there you have it. A washboard road. And it is true that driving faster over the ripples will reduce the jarring. There will be some particular speed for the road that will allow you to hit the peaks and bypass the valleys or at least disrupt the periodicity of the washboard, though you may turn your teeth into wind chimes in your trial-and-error attempts to discover the rate for any particular road.