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Fascinating stuff

The phenomenon you refer to is known as "washboarding," a wave-like pattern on unpaved roads that might more aptly be called speedbump hell. As you've observed, the ruts occur with striking regularity, belying a chaotic event like erosion.

According to Tom Pettigrew, a Forest Service engineer, the cause is an unlikely source: your car's suspension. (Well, maybe not yours specifically, but it's not innocent in this matter, either.) A vehicle's suspension system distributes the shock and energy of road irregularities with a bouncing rhythm called harmonic oscillation. At each downstroke, the wheels exert extra force on the road, causing the particles in the road to either pack or displace at regular intervals. Once a pattern of ruts starts to establish itself, it becomes self-reinforcing due to what engineers call forced oscillation. The next car hits the same irregularities in the road and bounces at the same rate, causing the pattern to become more and more defined. Forced oscillation overcomes minor variations in oscillation rate that might otherwise arise due to differences in car weight.

Wouldn't variations in speed affect the washboard pattern? Sure, which brings us to another critical part of the feedback loop: you, the driver. Drive too fast on a washboard road and the downstroke exerted by the car wheels may meet the road at a point where a bump is ramping upwards. You know what that means: You bounce off the ceiling. Instinctively most drivers slow to a speed at which the downstrokes coincide with the troughs between bumps, reinforcing the pattern.

Washboarding is inevitable in any unpaved road that sees fairly heavy traffic. The only way to avoid it is to: (a) radically redesign how automotive suspensions are made, (b) give up suspensions altogether, or (c) keep off those dirt roads.

joe bartels wrote:I'm doubting it for two reasons

The first I'm not so sure about. I've only had one Tacoma with a rear locker. Really couldn't drive at speed with the locker or you'd studder off the road. Granted it studders but I can't image hordes of people driving like that. Perhaps over use of 4x4 but that shouldn't affect it.

The second is more of the clincher. We had private irrigation roads through the corn fields. They were among the worst washboard around. No lockers on the work trucks, no one else around for fifty miles. Believe they were graded yearly.

I'm only talking about the curves, the straightaways are obviously not due to to locked diffs. I'm imagining the roads along those cornfields are pretty straight!

Maybe the wheat fields, the corn fields are mainly circles. Anyhow, I really don't think that's it. Turning corners with your differential locked just doesn't seem like a typical thing... perhaps I'm dead wrong

Ok, no mechanic here, but I notice my old '78 Blazer, which is what I mostly drive on FS roads, is terrible going around a washboarded corner. I have learned the hard way to slow way down. It just has no traction at all if there is any washboarding, especially on a corner.

^^^^^^Yep our 82 Bronco lived up to it's name.
We now have a FJ and I've finally learned to air down to at least the mid 20's psi.

Here is Wikipedia's definition!

Washboarding is an instability that occurs when vehicles move above a critical speed, that depends on the properties of the vehicles and the road surface. If all the vehicles move below their critical speed the road will remain flat, but if they move faster, ripples will slowly grow and move in the direction of the vehicles. It has been argued that the vehicle's suspension is important, but this can not explain why washboard road forms when vehicles' suspensions vary so much. Many have argued that suspension is irrelevant and recent experiments confirm that washboard roads form from vehicles without a suspension system.[1] A similar instability also occurs on railroad tracks, where it is known as roaring rails, and between rollers in machinery such as printing presses.