Washboard Road Ahead: 5 MPH

I used to live a small mountain mining town. Real small. There were 17 permanent residents in the winter. Population doubled in the summer when the snowbirds landed.

As one might imagine, the roads in and out of town weren’t paved. They were dirt. Every spring the highway department scraped and smoothed them, but it was only a matter of weeks before they assumed their customary contour: Washboard.

Anyone who has driven on dirt roads corrugated by time — nearly everybody in the Rockies — has wondered: Why does this happen? What causes it? A lot of people assume — I always did — that it had something to do with the car bouncing up and down on its suspension, or some similar motion.But this is wrong. Research by some British and Canadian physicists published early this month in Physical Review Letters found that washboard roads are an inevitable result of driving across the dirt surface. Unless you drive slow — really slow, under five miles an hour — a washboard surface is the inevitable result of the wheel moving along the road.

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Any bed of dirt or sand has tiny irregularities in it that slightly bump a rolling wheel. When the wheel hits one of these teeny bumps, it pushes the particle forward slightly, increasing the irregular shape in that spot. Then the next wheel to hit it increases it more.

As the wheel goes, up then down over the top of the bump, the descent pushes more dirt forward, creating another bump. Over the course of hundreds of cars, voila!, a washboard surface.

The scientists, Nicolas Taberlet of the University of Cambridge and colleagues, conducted some experiments to study this mechanism.

They set up a simple system: A rotating turntable three feet in diameter; a layer of sand two to four inches thick; and a rubber wheel attached to an armature. The whole set up is like a large record player, with the sand bed the record and the wheel/armature the needle. (For those too young to have ever seen a record player, it’s a device we used before iPods to … oh, never mind.)

Anyway, this system is much simpler and more direct than a car. There’s no suspension, no tire, and no load on the wheel.

The physicists say:

“The fact that our simplified systems produce washboard ripples is important since it shows that neither tires nor suspensions are necessary to obtain washboard roads, although of course, adding a spring, a dashpot, a tire or an engine would affect the size of the bumps. In other words, it is not because of the suspension of cars that washboard roads exist. The ripple wavelength is NOT simply the speed of the wheel times the bounce frequency of the suspension, which seems to be a common  belief.

(A dashpot — I had to look this up — is a damper that resists motion via friction. For instance, the hydraulic cylinder in a shock absorber is a dashpot. Who knew?)

“Another interesting result is that the size of the wheel and the size of the grains has no influence on the pattern. The mass of the wheel does however. If the size of the wheel is doubled while keeping its mass constant, the size and wavelength of the ripples remain unchanged. Similarly, using very fine sand (average size of 200 microns) and coarse grains (3 mm), we obtained an identical pattern.”

The material of which the road bed was made no difference in the washboarding. The researchers used two types of sand with different coarseness. They even replaced the sand with rice. Same result: Washboard.

This is probably not good news for the state and local road maintenance crews who have to deal with these roads. In order to avoid those spine-jarring jolts, either the roads have to be smoothed more often or the speed limit has to be five m.p.h. Somehow, I don’t see either solution coming around very soon.

Buford Road in Rio Blanco County. Photo by Leslie RobinsonPhoto Sharing and Video Hosting at Photobucket

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