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On the gravel roads around here it always occurs at the entrance to a turn or before a stop sign. I think it's a matter of the gravel resisting a bit when the brakes are applied so the gravel slightly slides, when a few stones pile up in front of the tire resistance to the slide increases and the tire rolls over the "hump" created, then the cycle starts again. Running this cycle over and over a few hundred times and pretty soon you have a washboard.
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