Competitions and division according to cc-type
The AMA holds its Motocross Championships starting in May of each year, and the races have been held since 1972. The current championship series is managed by MX Sports Pro Racing. At the beginning, there were only two classes- the 250 and 500cc divisions; a 125cc class began in 1974. As technology improved, the 500cc bikes were deemed too powerful for the riders, and faced with declining competition, the class was discontinued at the end of the 1993 series.
In 1997, amid more stringent emissions rules, the AMA raised the maximum displacement for four-stroke bikes. This encouraged all the major manufacturers to build more environmentally-friendly machines. The 250cc division was renamed the MX class in 2006, and 150-250cc two-strokes and 251-450cc four-strokes can compete here.
The 125cc class was renamed the MX Lites, and any displacement up to 125cc two-strokes or a 150-250cc four stroke can compete. The MX and Lites classes were again renamed in 2009, to the 450 and 250 Classes, respectively. The most recent round of changes served to reflect the universal adoption of four-stroke technology.