The research described in this article draws sound conclusions. However, the context suggests the tests were done at constant speeds. I submit there is yet another cause. It has been my observation that washboarding occurs initially and primarily at areas where there is acceleration, such as coming out of curves and starting up inclines.

Don Buelke
Victor, Mont.

It’s unfortunate that researchers don’t look for previous studies before duplicating efforts of others. In January 1963, a Scientific American article describes the controlled experiments done by Dr. Keith B. Mather, using a powered turntable, which resulted in identical conclusions.

David A. Coats
Minneapolis, Minn.

The authors do not seem to have any real-world experience driving on dirt roads. I can attest that it is almost solely the materials that make up the road that determine whether or not it will develop washboards. Loose materials create washboard roads. Too much sand and gravel, without the appropriate amount of clay, or glue, and you get washboards.

David T. Allen
Page, Ariz.