CLASSES
Note: All classes are further divided into Novice, Intermediate and Expert rider ability levels.
bullet Historic 125: Certain 125cc and smaller motorcycles built up to and including the 1977 model year as the first-generation of long-travel machines.
bullet Historic 250: Certain 126-250cc motorcycles built up to and including the 1977 model year as the first-generation of long-travel machines.
bullet Historic 500: Certain 325-460cc two-strokes and four-strokes up to 636cc built up to and including the 1977 model year as the first-generation of long-travel machines.
bullet Historic Four-Stroke: Certain four-stroke machines built up to and including the 1978 model year with up to 636cc. Overhead cam machines are limited to 500cc.
bullet Gran Prix 125: Certain post-1977 motorcycles up to 125cc.
bullet Gran Prix 250: Certain post-1977 motorcycles 126-250cc.
bullet Gran Prix 500: Certain post-1977 motorcycles 325-500cc two-stroke and up to 580cc four-stroke.
bullet Gran Prix Four-Stroke: Certain 185-580cc four-stroke motorcycles that came just after the Historic Four-Stroke period.
bullet Ultima 125: Certain motorcycles up to 125cc that do not have disc brakes or power-valve type mechanism built directly into the engine.
bullet Ultima 250: Certain 126-250cc motorcycles that do not have disc brakes or power-valve type mechanism built directly into the engine.
bullet Ultima 500: Certain 325-580cc motorcycles that do not have disc brakes or power-valve type mechanism built directly into the engine.
bullet Ultima Four-Stroke: Certain 200-580cc four-strokes that were produced either with a single-shock rear suspension or a newer engine technology.
bullet Open Age: Any AHRMA-eligible rider on any eligible machine.
bullet +40: Riders over age 40 on any eligible machine.
bullet +50: Riders over age 50 on any eligible machine.
bullet +60: Riders over age 60 on any eligible machine.
bullet Women: Female riders on any eligible machine.
This is where I read Ultima.
http://www.ahrma.org/Post-Vintage_Motocross.htm