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Why Washboard Sucks, A Technical Explanation

February 22, 2023 7 min read

Why Washboard Sucks, A Technical Explanation

Washboard—the regular corrugation developed on unpaved roads—is one of the most punishing surfaces for both drivers, and their vehicles. It’s also hell on campers, and is one of the most significant challenges GFC has tried to solve for by making our products radically simple and immensely strong. Learning to understand washboard may help you understand why GFCs are made the way they are. 

What’s Washboard?

“In the U.S. today the motorist can still find his way off the 2.5 million miles of paved highway onto the earth and gravel byways that total a million miles in length. On these roads, even in a well-sprung, shock-absorbered 1963 automobile, he will sooner or later experi­ence shuddering vibrations generated by stretches of corrugated surface often called ‘washboard’ or ‘corduroy.' Over the smooth stretches his passage will, in dry weather, inevitably start the process that imposes on the surface a transverse pattern of parallel crests and valleys that may ripple along for a few yards or for miles on end.”

That’s a quote from Keith B. Mather, an Australian scientist who set out to understand the causes of washboarding in the early 1960s. His study was published in the January, 1963 edition of Scientific American, and remains the seminal work on the topic. 

Mather deemed washboard an appropriate topic for scientific research because it represents such a huge challenge for both passenger and commercial transportation in remote areas. Not only will it eventually destroy vehicles that drive over it regularly, but it’s also notorious for causing crashes, and even rollovers. 


(Illustration:Scientific American)

How Does Washboard Form? 

Before Mather’s research, various theories posited answers like erosion caused by rain, some unknown peculiarity in soil materials, a natural sorting process between fine and coarse soil grains, the vibrations produced by internal combustion engines, the wind created by moving vehicles or exhaust gasses, or simply wheel spin caused by an over enthusiastic application of the gas pedal. 

Mather began his study by first watching traffic as it passed down dirt roads in Australia. 

“On our Australian roads we could ob­serve at will the development of corru­gations in an initially smooth surface,” he writes in Scientific American. “Starting with a road surface of a given degree of rough­ness, the rolling wheels of traffic did not make it smoother by wiping off the high spots and filling in the low. On the con­trary, every time we watched we saw a smooth, freshly graded road trans­formed into a washboard. The con­clusion was plain: smooth roads are unstable under the action of wheels; the stable surface is the corrugated one.”

“Most instruc­tive was the rapid bouncing of the wheels and the way this raised the dust,” Mather continues. “Even on uncorrugated roads the dust comes off in little spurts—not uniformly, as one would expect. Each wheel is sub­jected to a random succession of small impacts, which are due to stones and pebbles of diverse sizes. The resulting semiregular oscillations raise the dust more at some points than at others” 

To eliminate environmental factors and create a repeatable experiment, Mather filled a large pan with sand, and used a spring to fix a wheel to an arm that rotated under the power of an electric motor. The speed of that wheel, along with its diameter and mass, whether it was able to rotate or blocked, the load placed upon it, and the rate of the spring could be altered, in order to study the effects those variables might bring to the equation. 

Starting slowly at speeds under 4 MPH, Mather observed that the wheel would displace the sand, but didn’t create corrugations. Increasing the speed of the wheel above that point, however, almost immediately created washboard. None of the other variables fundamentally altered that result. Repeating the experiment in granulated sugar, rice, and dried peas—representing soil granules of different sizes and shapes— produced the same results. 

Mather also observed that the corrugations would begin at natural irregularities in the surface, and propagate from there. 

"When the wheel reaches the critical speed, it begins to move in short hops, bouncing on the random irregularities of the surface,” describes Mather. “The wheel is projected into the air at a definite angle and strikes the surface again at a definite point farther along the track. Where the wheel strikes after each ‘flight’ it sprays sand forward along the track or sideways off the track. Hence it tends to form craters, which become the valleys of the corrugation pattern. Each time it digs itself in at a crater it has to ride out again. In the course of doing so it is again projected into the air. Thus the pattern tends to repeat.”

And there you have it: washboard. It’s created by objects bouncing across a loose surface above a critical speed. In Mather’s experiments, that speed was found to be 4 MPH. Mather goes on to explain that the pitch of the corrugations—the distance between their peaks—involves a complicated interplay of speed and vehicle dynamics, while amplitude—the height of the peaks and depth of the valleys—is determined by the surface material. 

Mather also concluded that, despite the vagaries of tire pressures, spring rates, and wheel and vehicle masses, the single most important variable is simply speed. And, because vehicle speeds along a given stretch of road tend to cluster into a narrow average, this contributes to a regular periodicity of corrugations along any given stretch. 

Experiments conducted since Mather’s have all reached similar conclusions. Because washboard is the stable state for any loose surface over which weight passes above a critical speed (they’ve also found that speed to be approximately 4 MPH), its formation is inevitable. 

Is there any way to prevent the formation of washboard? “Perhaps we should build vehicles that walk instead of roll,” Mather concludes. 

Why Is Washboard So Uniquely Hard On Vehicles?

One word: duration. 

Mather found that the average period for corrugations is between 24 and 36 inches. While one small bump, or even a few in a row may not have much effect on your vehicle, a single mile of washboard may expose you to 2,640 bumps. And all those small bumps, pitching your wheels up and down rapidly for long periods of time, cause vibrations. 

Vibrations propagate through a solid medium in longitudinal waves. It’s easier to show you how that works than describe it, so take a look at the radio aerial on the 200-series Land Cruiser in the above video as it drives over corrugations in slow motion. The vertical movement of the wheels, axles, and suspension is directly translated to a vertical wave passing through that antenna. 

While the effect on more rigid materials—a vehicle’s frame and body, or the structure of your camper—may not move so dramatically, the same forces are acting on them, and exposing them to multiple alternating instances of contraction or expansion every second. As the video’s presenter notes, all the small little bumps can add up to big problems. Vehicle frames can crack, bolts and screws can come loose, bushings are slowly destroyed. 

You also have to consider the effect on safety. Driving along on a smooth surface, your tires will remain in constant contact with that surface. As Mather’s research explains, tires will begin to bounce off washboard above about 4 MPH. A tire that’s not in contact with the ground cannot provide grip.

The common approach to smoothing out ride quality over washboard is to accelerate to a speed where your wheels and tires begin to feel like they’re floating. This means the tires are only contacting the peaks of the crests, and are not in complete contact with uphill or downhill slopes, and may not touch the troughs of the valleys at all. And this leaves very little grip through which your tires can resist vehicle momentum. You will likely feel that in the form of lateral instability—your vehicle will start to sway from side to side—and it may become difficult to change your direction of travel by steering. 

It’s that last problem that can lead to over-application of steering input. If your tires leave their state of reduced grip suddenly—such as contacting a berm on the side of the road, or simply hitting a portion of the roadway not covered in corrugations—a sudden return of grip can combine with a dramatic steering angle to pitch the vehicle away from the direction of steering input. And combined with a high center of gravity and momentum, that’s what results in rollovers on stretches of road that may, from the outside, appear unchallenging. 

Everything described here can also interfere with your antilock braking system’s ability to effectively actuate your brakes. Slam on the slow pedal to avoid a sudden obstacle, and if you’re traveling fast enough, your tires will simply skip off the crests of the corrugation, which ABS will interpret as attempting to brake on a surface with little to no traction—a condition in which it will back off braking power in order to retain the ability for you to steer. Combine this paragraph with the one above, and you’ve discovered yet another potential issue. 

Here, you can see what makes the GFC camper so strong. Billet aluminum joints capture forces, and distribute them through proprietary aluminum extrusions. Two points form a line, three form a plane. This spreads forces across not only the entire frame of the camper, but the full perimeter of your truck bed, too. And, with only four connection points to monitor for tightness, there's just not much that can go wrong. 

So, What Can You Do?

Reducing tire pressures will increase the amount of deflection the sidewalls are able to provide, while lengthening their contact patch. This will improve ride quality and increase traction, while helping to keep the tire’s tread in contact with a larger portion of washboard’s peaks, slopes and valleys at a given speed. If your vehicle is equipped with adjustable damping, backing off the rebound will also help tires remain in contact with the surface for longer periods. 

You’ll also want to take steps to totally secure any loose objects in or on your vehicle. Anything that can come loose, will come loose given enough time driving over this stuff. 

In terms of maintenance, applying thread locker to any and all fasteners is crucial, before you tackle any corrugated surfaces. Then, after any significant exposure to washboard, you should thoroughly inspect your vehicle and its accessories. Crawling under it, look for any signs of leakage, any shiny metal surfaces (which may indicate rubbing or abrasion), and inspect fasteners for tightness. A rubber mallet may be useful here; whacking mechanical components with one can make loose bolts rattle, while wayward movement may indicate worn bushings. Drivers who regularly tackle washboard should plan on inspecting and replacing bushings in their suspension and steering systems, and anything else that may contain moving parts. 

But, the most important step you can take is prevention. By keeping your entire vehicle as simple and light as possible, and affixing anything you add to it with more strength than might seem necessary, you can reduce forces and prevent failure. Or, at least that’s GFC’s method. Until we invent vehicles capable of walking over loose surfaces instead of rolling, that’s as good as you’re going to get. — Wes Siler

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