I have the 1972, 75 & 76 AMDRA rule books. From the 75 rulebook: Min disc brake size is 7 inch dia x 1/8 inch. Handle bars must be mounted in stock location, no part of grip may be below bottom edge of top fork crown. Must use stock frame, only mods are removal of unused mounting tabs ad removal of outer top tubes on Honda for easy engine access. I was suprised that raking was not allowed yet. Aftermartet forks allowed, 34mm min tube dia, and within 2 inches of stock length. Stock type head light and mounts must be retained. Aftermarket Swing arm allowed, max is 28 inches from center of counter shaft sprocket to center of rear axle. They gave the 2 strokes 30 inches. Struts and wheelie bars permitted. Looks like Bruso won the 74 PRO championship without wheelie bars. 2 inch min seat padding. No ground clearance rules or wheelie bar length in PS or general rules.