07 July 2011

The physics of a "washboard road"

When I visit northern Minnesota, I always encounter a road that in midsummer becomes classically "washboard."  The county sends out a road grader to smooth it and give it a new crown, and a few weeks later it's a washboard again.  I have never understood the physics behind this phenomenon, but tonight I found a good review at this link:
The fact that our simplified systems produce washboard ripples is important since it shows that neither tyres nor suspensions 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...

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 speed of the wheel appears to be crucial. Indeed, there exists a critical velocity below which the road always remains flat and above which washboard bumps appear. Typically, for a car this critical velocity is around 5 mph or 8 km/h...
The phenomenon is shown experimentally in this video:

An experiment on the formation of "washboard road" ripples. Here there is no wheel or suspension, just a flat plow blade that is dragged over the surface of dry sand. The sand "road" is on the circumference of a rotating table, and the plow support is fixed in the lab frame. This is a stroboscopic movie made of images taken once every table rotation, so the table appears fixed. The ripples move "down the road" (i..e. in the driving direction) which is opposite to the rotation of the table. 
(It took me a while to understand that the table is rotating widdershins, and the strobe is making it look like it's going clockwise.)


  1. "widdershins" only means counterclockwise in the northern hemisphere. It's the opposite of deosil, which describes motion in the same direction as the sun when facing the equator.

  2. I learned to drive on washboard roads.
    Always thought it was related to the car engine not having true circular motion but a series of "pushes".


  3. Very interesting. Nice detail at the link. Around here we call them corrugated roads and they are quite common. Once formed, there definitely seems to be an optimal speed at which to travel on them so you sort of ride across the peaks without rattling your car to pieces.

  4. I had to look up "widdershins", so even though I didn't watch the video and don't have a whole lot of interest in the washboard road phenomenom, I still learned something!

  5. Thank you, Sandy, for your comment. That's the kind of thing I like to hear.

  6. I've driven over paved roads the caues my car to bounce repeatedly like washboard roads.

    How does that form on asphalt roads?

    Is it the gravel underneath the asphalt, or is it the asphalt is buckling as it slides down hill?


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