I finally stopped to look more closely. These ripples are about 10 inches wide ands look to be almost exactly uniform through long stretches of road. Where does this precision come from?
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Rear wheel drive cars accelerating in loose material. Wheel spins a little, skips over a bump, spins again.
Wheels bounce a bit when when the vehicle they belong to drives over a bump. While suspension is meant to dampen the bounces felt in the passenger cabin, the wheels themselves still bounce once or twice. Most vehicles wheels bounce at roughly same rate as those on other vehicles. Ruts form beyond a prominent bump due to the aggregate effect successive cars’ bouncy wheels.
I have suspected that they come from ABS brakes since I don’t remember seeing them when I was young. And they show up in snow near red light
Years ago, Scientific American had an Amateur Scientist column where someone tested this with a home made rig of a bicycle wheel on a long arm and using split peas for dirt in his driveway.
He came to the conclusion that the bumps were formed by the “dirt” being pushed in front of the wheel like a bow wave until it was driven over. Then the process would repeat. The corrugations would grow over time and even propagate if you watched them over time.
Tl:dr the tire pushes a bit of dirt into a mound before driving over it. They grow as the process repeats.
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There are several mechanisms at play, all simultaneously.
First is the angle of stacking for the local sand/dirt. For most sands/dirts the angle is just in a sweet range to allow dunes to form (which is all washboard is, really small dunes), and so the very wind itself will help enhance existing washboard. Flowing water with particulates will do this too, it’s a function of ripples, vortexes, and hydro/aerodynamics. This includes the moving air caused by fast moving vehicles going down the road, so to slow the formation of washboard GO SLOW. Won’t stop it, but it will slow the formation.
A second mechanism was mentioned elsewhere, the “bow wave” effect. Underinflated tires will push loose material forward until it reaches a particular density, at which point the tire compresses slightly and rolls over the material. No matter how inflated the tire is, this will still occur over time, even completely solid “tires” will do this, they simply just move up and over the newly formed ridge instead of compressing slightly.
The third mechanism is a compression/bounce effect. A tiny, unnoticeable to you, bump forms in the surface, could be as simple as a pebble in the road, and forces your wheel up ever so slightly. This compresses the ridge, making it more dense and more likely to resist the NEXT tire that comes across it, but then the tire slams back down on the far side of the ridge, pushing material out of the way both forward and back, creating a tiny new ridge and reinforcing the first one. Repeat this process hundreds of times and the ridges cease being unnoticeable, you really end up feeling the washboard.
There are other mechanisms happening too, but those are the biggest ones, it’s all about the same things that create dunes and ripples, just on a smaller scale.
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“According to the Federal Highway Administration (FHWA), the four primary causes of washboarding are driving habits, lack of moisture, poor gravel quality, and lack of crown on the road surface.”
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For a really simple explanation. Imagine a flat road with no ripples. Then it rains and the water runs down the road. It picks up some of the dirt and goes over small bumps like maybe a pebble. The water gets pushed up by some of the small bumps enough to slow down and the dirt in it settles. Then the water drops off the small bump and picks up more dirt. But then it makes a small hole behind the bump and has to go back up and out of the hole. Which slows it down and it releases the dirt. This continues until you have a continuous up down ripple in the road. The length, height, and shape of these bumps and holes depends on the dirt, speed and ammount of water. You will see this effect anywhere you have water running over sand or dirt.
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