What causes washboarding of roads? Good question.
A little googling indicates:
Lack of moisture. - Moisture allows road to even out more readily,
especially if there is clay in it to retain water better.
Heavy Traffic - Including hard acceleration and breaking.
Poor road 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.
---------------
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.
REFERENCES:
Why Moths Hate Thomas Edison: And Other Urgent Inquires into the Odd
Nature of Nature, Hampton Sides, editor (2001)
-------------
<story warning>
When roads get bad here on camp (dirt and gravel roads), the property
manager uses the TR3 and tears up the road as deep as the tines on it
will go, This takes several passes, and pretty much smooths out the
road base and tears up all the bumps. Then they go over it several
more times smoothing and putting a crown on the surface. It lasts
quite a while, but every year or two, they have to redo it.
The road that must be done most often is the entrance road that gets
the most traffic. ... We have lots of cars and minivans come on camp,
but we also have delivery trucks (FedEx, UPS, etc) and occasionally 18
wheelers (grocery delivery, but sometimes steel and lumber yard
trucks), and concrete and gravel trucks. So the road gets heavy use.
Max speed is supposed to be 10 MPH, but I have seen cars do about
40MPH, most do 15 to 25. All that to say the road does get heavy use.
------------
My dad told me about a class on vibrations he had in college. They
went out and surveyed roads to determine the frequency of the bumps
and had a trailer with adjustable springs. Knowing the spring
constant of the trailer springs and frequency of the road, they
calculated the speed to carry a load on the trailer to give it a
totally smooth ride. ... So if you drive the right speed and know your
suspension, you can get a smooth ride over a washboard road! To slow
will be very rough, and the 'wrong speed' can even double the action
of the washboarding on your ride. ... Oh yes, they actually loaded the
trailer with a water tank loaded for the 'right mass' and drove over
the road towing the trailer to prove it worked.
-------------
When I lived in El Paso there was a section of I10 between El Paso
going north to Las Cruces, NM, about 50 miles or so, that has about 10
miles of washboarded concrete near the state border. ... I never could
find the speed to make that a smooth ride. It was that way on all 4
lanes (2 each way). ... First time I had ever seen it on a concrete
road. (For those that don't know, Interstate 10 is a very heavily used
road that runs from Miami FL(I think) to Los Angeles CA through the
southern teer of states, and I think it is the only major road that
goes across the USA that doesn't have a 'high mountain pass' for
trucks to traverse, so it is popular with very heavy and big loads.
I hope that has been fixed by now.
</story warning>
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