[podcast]https://edwardwillett.com/wp-content/upLoads//2009/07/The-Washboard-Effect.mp3[/podcast]
Saskatchewan, as has oft been noted, has a lot of roads: more than 190,000 kilometres in all, in fact, giving it one of the most extensive road systems in Canada.
Not all of those roads are paved, however. In fact, most aren’t. And as anyone who has had occasion to drive extensively on the rural road system can tell you, while gravel roads are better than mud roads, they have their own…interesting…characteristics, of which one of the most annoying is the “washboard effect.”
Washboards are fine if you’re a 19th century pioneer woman trying to clean the clothes or the abs of a 21st century male bodybuilder, but washboard-like ridges on a road are downright dangerous, reducing traction, causing extreme wear and tear on the vehicles vibrating over them, and threatening to extract the fillings of the unfortunate drivers.
Nor is this washboard effect limited to gravel roads: those annoying ripples appear on just about any road with a loose surface, from sand to snow.
Nobody has been entirely sure how this annoying phenomenon arises—until now.
A team consisting of Anne-Florence Bitbol and Nicolas Taberlet of Ecole Normale Superieure in Lyon, Jim McElwaine of the University of Cambridge, and Stephen Morris of the University of Toronto, working in England and France, have managed to create washboard surfaces in the laboratory—and just as importantly, model them mathematically.
Seeking to find the simplest possible instance of the effect, they created an experimental setup consisting of a rotating table a meter in diameter, covered with a layer of sand between five and 10 centimetres thick. A hard rubber wheel attached to an arm was free to roll on this granular “road,” which moved beneath it at a velocity ranging from 0 to 10 km/h.
Unlike there would be a with a car driving over a road, there was no suspension involved, no torque from the engine, and no bouncy inflatable tire—but even so, the ripples formed, rapidly, after just a few passes of the wheel.
As the researchers put it on the experiment’s website, “The fact that our simplified systems produce washboard ripples is important since it shows that neither tyres nor suspension are necessary to obtain washboard roads, although of course, adding a spring, a dashpot, a tyre 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.
Also surprising: neither the size of the wheel nor the size of the grains covering the road influenced the pattern (although the mass of the wheel did). In fact, the wheel didn’t even have to spin: even when it just plowed along the surface, the washboard developed.
In fact, ripples formed in sand both wet and dry, with both fine and coarse grains, and even in long-grain rice; with or without an added spring on the wheel; for various weights of the wheel; and at a large range of speeds. In scientific terms, the phenomenon is “very robust.
It’s not just seen on roads, either. You see it on ski hills (as moguls), steel railway tracks (as tiny bumps), and even in computer hard disks, where the hopping of the read head sometimes creates a washboard pattern
So if the fault lies not in our automobiles’ suspensions, where does it lie
According to the U. of T.’s Morris, the effect is related to the physics of stone skipping. A skipping stone creates a ripple that the stone then launches off of into the air, landing and creating another ripple, and so on. This carries on until the stone’s speed falls below a certain threshold
Similarly, a car’s wheel, travelling over a granular surface, creates ripples that it launches off of, landing and creating another ripple, and so on, and so on. The difference is that, unlike water, a granular surface “remembers” the ripples, which grow larger with each subsequent pass of a wheel
Fortunately, there’s a simple way to avoid a washboard effect: you just have to keep the cars travelling on the road below a certain critical velocity. For cars that’s around 8 km/h, which means there is a strong scientific argument for the government setting the rural speed limit to, oh, say, 5 km/h
I’m sure that would be acceptable to all concerned.
1 comments
Oh yeah, you’ve got your hands on a winner of an idea.