What determines the spacing of the ripples? Is it the radius of the wheels rolling over the road, or the composition of the dirt/gravel?
I’m thinking the resonance frequency response of typical cars springs. There has to be a bump or pothole to get the effect started, then waves are compressed into the dirt when it’s soft enough to be compressed.
I’m sure someone will point out that various cars are of different weights and suspension design but I suspect that the consumer’s expected degree of springiness results in similar effective spring rates in most passengers cars.
This is an interesting question. Washboarding also occurs to snow. Flat ski runs frequently experience washboarding exactly like dirt roads.
My vague recollection is that vehicle speed is the dominant determinant of the pitch of the ripples. Which is somewhat counter intuitive. The point being that the wheel is skipping with a ballistic path that is not controlled by periodic components of the vehicle. I found one recent study that seems to support this here.
Not too many wheeled vehicles on ski runs, so that rules out that it’s an effect specific to wheels.
Link bad, but maybe this one? [1209.5560] Modelling Washboard Road: from experimental measurements to linear stability analysis
The wheel is skipping because of the bounciness of the the tire and spring system becomes periodic at some rotational frequency (the speed).
A wheel at a slow speed will not bounce. A high speed wheel will jump over small bumps because the spring system cannot follow the surface variations. At just the right speed - and especially with bad shocks 
- there will be periodic bouncing.
Ski runs may get washboarded because of the springing of the skiers “suspension” - their muscles and tendons flexed at the knees and hips.
From this; “…light vehicles have a bigger effect on producing washboarding than heavy trucks.” Light vehicles CAN bounce and have soft, comfortable suspensions; heavy trucks have big slow wheels and stiff, high frequency springs.
The truck’s wheels never roll fast enough to excite their spring resonant frequency.
There was a Scientific American article on the topic, ever so long ago. Decades. Might have been in the 1960s. They made an experimental rig where a wheel rolled over the ground, in a big circle, and they varied things like speed, weight, tread, and so on, to see what changes this made in the washboards.
I know from personal experience that if you drive at just the right speed, to sync yourself up with the washboards, you can get an eerie “frictionless” sense to your driving, where your car feels like it’s skating over ice. You lose a measure of steering control.
There was a colon missing from my link (Odd - it was copy/paste on my iPhone).
This is it.
The important first bit is this:
And if you hit it at just the right speed, you can “slingshot” yourself into a change of direction in a manner that cannot be done any other way. I’ve done that on tight curves, for fun.
Their test setup HAS suspension! The “plow” has weight and is dragged from a pivot point. They have the “spring” of gravity.
So, essentially, if you drag a ski tip across sand at some speed it begins to skip and bounce. As it repeatedly bounces over the same surface the divots get bigger. I agree they quantified their test setup but it’s pretty far from real world wheels.
Nitpick - gravity does not act as a spring - increased displacement of a mass on a constant gravitational field does not increase the force. (ie it does not obey Hooke’s Law.) The force remains the same. Nor can gravity be repulsive - unlike a spring. A mass travelling in only a vertical direction does not have a resonant frequency of any given period. This is essential to the point they make.
A mass on a spring has a natural frequency that is dependant only upon the mass and the spring constant. A mass in a gravitational field has no such natural frequency.
Thus the paper essentially says that a mass skipping along the ruts does not vibrate at a frequency given by a mass on a spring system. It skips along on a ballistic trajectory, one that is independent of mass, and only depends upon forward speed.
Missing, so far, is the human control that comes to bear. Once a road begins to washboard, the velocity of vehicles driven over it decreases, and braking may even occur on the developing washboard. How does that affect the evolution of later-stage washboarding?
13 posts, several cites, and no answers.
I knew he’d written on it at some time.
Not to question Cecil or anything…
But his answer seems to be contradicted by the previous cites, which aren’t able to answer the question either.
Argh. And it wasn’t - again. :eek:
This isn’t the typical washboard road we are talking about, but I have seen simple road construction in swampy areas where logs were laid down side-by-side, perpendicular to the direction of travel, then layered with dirt, sand, or gravel. The logs tend to not sink in the swamp, which is the reason for this kind of construction, and the gravel evens them out, but they eventually get pretty washboarded over time as the gravel settles.
Parts of the road near my house were made that way when the road was first built ca. 1920-30’s. Logs were cheap, plentiful and available, and had to be cut down anyway to clear a path for the road.
You’re not. It was a staff report so all bets are off.
In the Civil War those were called “corduroy roads”. Sherman’s army famously advanced through swampy territory in South Carolina that had been considered impassible using those.