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Under the AMA rule that bans outside assistance, unless it takes place in the designated pit and signal area, no one can touch a rider’s bike—except the rider himself. The exemption to this “no touch” rule are for track personnel and flag marshals who are allowed to push a bike to a safe location. However, the track personnel cannot work on the rider’s bike—only push it out of the way. If a rider has a mechanic problem, he can only get mechanical help in the designated repair and signal zone, which at the AMA Nationals is behind the pit lane tents (and in Supercross it is off to the side of the starting line behind where the mechanics are standing).
If a rider throws a chain or suffers any repairable problem outside of the designated signalling area, he must fix the bike by himself (once insuring that it is off the racetrack). He is allowed to receive advice from fans or mechanics, but they cannot touch the bike.
Fans have been known to jump the fence to help an injured rider or push a bike off the track—and, in most of these cases, the AMA turns a blind eye to this rule violation because the fans acted in the interest of safety (and without the rider’s consent). However, the fans should leave safety to the trained medical and safety officials—since they are endangering themselves and other riders by entering the racing surface.