MXA’S AMA RULE OF THE WEEK: THERE IS ALWAYS ONE RULE-BREAKER

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rulesbrakesThis rule effectively does three things:

(1) It requires every bike to have front and rear brakes. This rule doesn’t seem to be all that unusual, but at one time AMA dirt track bikes only had rear brakes and, even to this day, speedway bikes have no brakes. The most famous breaker of this rule was ATK, who mounted the rear brake on the countershaft sprocket to lower unsprung weight on the rear wheel. When a rear wheel brake was required by an observant tech inspector, ATK added a rotor and caliper to the rear wheel, but didn’t hook any hydraulics to it.

(2) It stops Ken Roczen from going to the starting line on his Honda CRF450 with drum brakes from a 1980 CR250. Obviously this rule was written when bikes had drum brakes (about 36 years ago) to stop an enterprising team from switching to disc brakes.

(3) It bans carbon fiber, aluminum or titanium brake rotors (and carbon fiber brake carriers). Carbon fiber brake rotors don’t work very well in the low speeds and moist conditions of an motocross track  where it is hard to get enough temperature into the rotors to allow the pads to work). Coated aluminum rotors would work in motocross and there are some riders racing with titanium brake rotors at the local level, but the flexibility of Ti doesn’t make a good rotor material in a world where the temperatures on the rotor face change constantly—which causes the titanium, especially in the thin application of front brake rotors, to warp.

atk406

The ATK 406 was the only production motocross bike to ever be produced without a rear brake. ATK designer Horst Leitner moved the rear brake to the countershaft sprocket to lessen unsprung weight on the rear wheel. Look closely and you will see that Horst mounted the master cylinder on the frame’s front down tube and turned the brake pedal around backwards so that it could not be bent in a berm. ATK raced the 406 at the Hollister USGP, where they put a fake rear brake on the bike, but in the AMA Nationals ATK only raced the ATK 604 four-stroke with Greg Zitterkopf.

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