What Causes Washboard Roads?

How does your car contribute to washboard roads?
How does your car contribute to washboard roads?
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Have you ever driven down a back road and suddenly felt like you'd just ran over a series of speed bumps? Did you wonder what they're doing in a road that seems to be in the middle of nowhere?

The phenomenon you've experienced is what's called a "washboard road." When a car travels on an unpaved road, a wavy pattern will ultimately develop. At first, tiny ripples form, and then they get larger as more cars pass over them. Whether you're traveling on roads in Australia or Africa, South Dakota or Southern California, washboard roads — with a wavy pattern that resembles an old-fashioned metal washboard — are a common occurrence.

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While washboard roads are familiar to drivers on country roads around the world, the phenomenon occurs elsewhere in science and technology. Whenever a sideways force acts on a malleable surface, ripples occur. You may have seen the same little ripple on wind- or water-driven sand at the beach, and on moguls that develop on ski hills. Motocross bikes and snowmobiles also cause ripples to form in dirt and snow. Small bumps on steel railway tracks are a result of the washboard phenomenon as well.

Washboard roads often are more than a nuisance because they can damage suspension systems and increase road maintenance costs. They also create a safety problem because a car or truck that doesn't experience full contact with the ground might not be able to brake properly. So what causes washboard roads?

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Physics of Washboard Road Formation

The wheels of a vehicle push back dirt, and over time that dirt builds up into small ridges.
The wheels of a vehicle push back dirt, and over time that dirt builds up into small ridges.
Stockbyte/ Getty Images

Some experts who maintain dirt roads say that your car's suspension system causes the problem as it actually tries to smooth out the bumps in the road [source: U.S. Forest Service]. As a wheel moves over a bump, the suspension system absorbs the shock and then pushes back against the road surface. On a soft surface like a dirt road, the push back either packs or displaces the dirt it hits. Over time, as more and more cars go over the bump, the washboard pattern develops.

However, in the summer of 2009, physicists from Canada, France and the United Kingdom published a new study about the physics of washboard road formation. They discovered that ripples will form, even when the springy suspension of a car and the rolling shape of a wheel are eliminated [source: American Physical Society].

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They built an experimental vehicle, replacing the wheel with a suspension rolling over a road with a simple inclined plow blade, without any spring or suspension, dragging over a bed of dry sand. Ripples appear when the plow moves above a certain speed.

After observing the results, they compared this phenomenon to the physics of stone skipping: A stone needs to be thrown above a specific speed in order to have enough force to bounce of the surface. A washboarding plow is similar, except the sandy surface remembers its shape and the effect is amplified.

They concluded that the formation of a washboard pattern is inevitable. The ridges will form, even if the wheel diameter, suspension or surface is changed. The only way to avoid the effects of a washboard road is to stay below a certain speed, but that's impractical: you'd usually have to drive at 3 miles per hour (4.83 kilometers per hour) to eliminate the problem altogether.

Some day, such discoveries may lead to improved suspension systems or improved road surfaces that smooth out a bumpy ride. In the meantime, there's not much you can do about washboard road except brace yourself, watch your speed and hang on for the ride.

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Causes of Washboard Road Formation

Slowing down is one way to avoid the speedbump-like effect of washboard roads.
Slowing down is one way to avoid the speedbump-like effect of washboard roads.
Tyler Stableford/ Getty Images

While washboard roads can occur anywhere, they are most often seen in hot, dry areas and on sandy, dirt or gravel roads. Three main factors cause washboarding to be worse in some locations than others:

  • Lack of moisture: Washboard roads are particularly prevalent when weather conditions are dry. Frequent rainfall reduces the chances of washboarding.
  • Traffic: Hard acceleration or braking can accelerate washboarding, and that helps explain why you'll see it frequently at intersections, before and after sharp curves, near business entrances, and even in driveways. If a car's tires lose a firm grip on the road and begin to spin or skid, some gravel will be displaced. If this happens repeatedly, the gravel will be displaced, often uniformly, and a washboard pattern will form.
  • Poor quality surface: Washboarding occurs most frequently if the surface quality of the road is poor, whether it's sand, gravel or even pavement. When a heavy load passes over the surface repeatedly, it develops irregularities like washboard patterns. Potholes are a common result of poor-quality surfaces, too.

If you travel dirt roads frequently, you know that you can find an ideal speed that will help smooth out the ride. That's the speed at which your car's suspension system is pushing the car down at the same time you experience a dip in the road. The problem is that the more the tires press down, the worse the washboard depression becomes. It's a catch-22: your suspension system is giving you a smooth ride, but it's making the washboard problem worse.

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Lots More Information

Related How Stuff Works Articles

  • Hough, Harold "Dirt roads, washboards, and dust control." Miners News. February/March 2007. (Nov. 28, 2009) www.minersnews.com/FebMar07/200702A5.html
  • "Science News: Scientists study 'washboard' road ripples." UPI. July 14, 2009. (Nov. 28, 2009) http://www.upi.com/Science_News/2009/07/14/Scientists-study-washboard-road-ripples/UPI-30121247584088/
  • ScienceDaily. "Physics Of Bumpy Roads: What Makes Roads Ripple Like A Washboard?" July 9, 2009. http://www.sciencedaily.com/releases/2009/07/090707131834.htm
  • Skorseth, Ken. "Preventing Washboarding." U.S. Roads.com. August/September 1998. (Nov. 28, 2009) http://www.usroads.com/journals/rmej/0006/rm000601.htm
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