Washboard Effect Dirt Roads

The Physics of Washboard Roads

If you’ve ever driven along a gravel or dirt road in the summer, you’re undoubtedly familiar with the “washboards” that develop. For those of you who have never encountered this phenomenon, gravel and dirt roads tend to develop a series of ridges, much like an old-fashioned washboard. As you can imagine, driving over a deeply ridged surface is annoying and also a little dangerous, due to the reduced contact and control. When the washboard becomes bad, the only solution is to bring in a grader and scrape the road smooth again.

Using our experimental setup, we were able to obtain washboard roads for wide variety of parameters. Ripples were seen for dry or wet sand, with fine or coarse grains, using long-grain rice or beach sand, with an added spring, for various weights of the wheel and for a large range of speeds. This shows that the phenomenon is very robust (i.e. general). In fact, similar phenomena exist on rail tracks and on ski slopes.

The researchers created both a digital model and a real “atom-based” recreation of the system. The computer version consisted of a 2D simulation of 50,000 minute spheres (i.e. grains of sand) on a flat surface over which a wheel was repeatedly rolled. The physical version was very similar, and included a rotating table of sand over which a wheel was suspended (see the video below).

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:

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Boise to Montour — Bike Overnights

The timing couldn’t have been better. It had been a difficult week and the combination of work-a-day events and everyday average life events were beginning to feel oppressive. Funny how halfway through our first day of the trip, the thought patterns in the brain were beginning to be a little bit more objective and coherent. Such is the power of getting out on the bike!

Just after the road turns to dirt and just before it becomes Pearl Road