Eric Gorr, who runs the highly respected motorcycle-modification shop Forward Motion and its companion website, is Dirt Rider rsquo;s Dr. Dirt and the author of Motocross and Off-Road Performance Handbook, 3rd Edition , also from MBI Publishing Company. He has served as a tuner for American and European off-road racing teams, and he resides near Chicago , Illinois . ( www.forwardmotion.com.mx )