What makes a dirt road on a hill get washboard bumps?

Answer 1

Scientists and engineers have puzzled over this phenomenon for many years, but only recently did a group of scientists (Stephen W. Morris of the University of Toronto and Nicolas Taberlet and Jim N. McElwaine of the University of Cambridge) actually perform controlled studies on how washboarding forms. The result of their studies showed that even with the most regular of dirt/sand surfaces, there are irregularities in the road that cause the wheel to bump. As the wheel rises, it pushes some of the road grains ahead of itself, creating a small ridge. After the bump, the wheel strikes the road surface just a little harder than normal, causing the dirt/gravel to shift forward, creating a little dip and pushing some of the grit into new ridges, which the wheel then has to rise over--which causes the next bump, etc. etc. It doesn't take many passes by vehicles to get a long row of ridges, and your washboard is born. It isn't just hills, but curves, straight sections or any part of any kind of dirt road. There's nothing that can be done about this, except call the graders and smooth the road out again. And keep your shocks in good repair!