Solo machines have engines ranging from 50cc to 550cc. The most common distinction between motocross machines is whether they have two-stroke or four-stroke engines. Originally all bikes were four-stroke (in the Pre-65 era), but as the two-stroke engine improved they became more popular with their domination of the sport in the 1970s, 80s and 90s. The two-stroke engine produced more power than a four-stroke engine of the same capacity (cc), however fuel had to be mixed with oil. Four-stroke continued to be raced in specialist classes throughout this period, partially in the MX3. Due to environmental legislation a new generation of four-stroke was introduced in the late 90s with the Yamaha YZ400. With a change in the class rules to allow four-strokes of one capacity to compete with a two-stroke of a lower capacity (see table below), and the increased power of this new generation of four-strokes (they run at a much higher RPM, using shorter pistons when compared to previous "Long Stroke" four-strokes) the sport had largely adopted the four-stroke in place of two strokes, although they are still used extensively and still constructed by manufacturers. This struggle between two- and four-stroke has aroused tensions with many MX fans and riders.
Motocross machines have extensive suspension to enable riders to tackle large jumps and ride at speed over the rough terrain. The advent of improved suspension transformed motocross from its scrambling origins with natural track to the current form today, with multiple jumps that would have previously been impossible.
To bring the weight down to an acceptable level (usually 200 to 250 lbs for full-size machines), things such as speedometers, lights, kickstands, electric starters, etc are all omitted and lightweight materials such as aluminium and magnesium are commonly used right from the factory or sold in the after-market.
Because of the physical nature of motocross, riders spend much of their time standing to absorb shock with their knees. The long, flat seat is designed to allow riders to shift their weight quickly and easily in corners, rather than for comfort.
Unusually for racing machines, motocross bikes can be purchased in a ready-to-race or nearly ready-to-race condition at moderate prices from major motorcycle manufacturers. Many riders, however, modify their machines further, both for outright performance and to have the bike's behaviour more in tune with their own preferences or just for enjoyment.
The above five are the major five manufactures in most markets, the manufactures below command little market share (currently - 2005).
The object of the contest is to complete either a defined number of laps (usually three to seven for amateurs, more for professionals) or fixed time period (anything up to 40 minutes) first. A race typically made up of two or three motos (or legs) with the scores combined to determine the overall result. There is a mass start
Motocross racing is one of the most visually appealing forms of motorsport, with riders performing seemingly death-defying leaps, turns visibly at the edge of traction (as indicated by a sliding, spinning rear tire throwing dirt at all behind it), and the effort of riders clearly visible as they move their bodies around their motorcycles to balance the bikes for maximum speed.
Recently the sport has evolved with sub-disciplines like Supercross and Arenacross (both are indoor motocross), Freestyle (or FMX) (a display of jumping skill rather than racing), Supermoto (Motocross machines racing on both tarmac and off road) and vintage motocross (competitions with bikes usually pre-dating 1975).
Motocross can be an entry sport for motorsport in general. Classes for children as young as 4 years old exist for competition on 50cc machines.
The world is dominated by two main Motocross series; one in America - the AMA Championship, also known as "the nationals" or the "outdoors". The other series is World Motocross Championship, also known as Grand Prixs.
The AMA Motocross Championship season begins in early May and continues until mid-August, consists of 12 rounds are 12 major tracks all over the continental United States. Each event is held on consecutive Sundays. There are two classes; "Motocross" and "Motocross Lites". Each round has two 30 minute-plus-two-lap motos, per class, held approximately two hours apart.
The World Motocross Championship is predominately held in Europe with some "flyaway" rounds, recently in South Africa and Japan, but over its history it has visited numerous countries including America. The format is similar to America with two classes; MX1 and MX2 and two motos per class. The World series is longer, generally incorporating around 16 or more rounds.
Both series are united with the annual Motocross Des Nations or MXDN (generally left untranslated from French, but occasionally called the Motocross of Nations). The format sees rider represent their nations in teams of three, each representing a class (MX1, MX2 and "Open"), there are three motos with two classes challenging the other - thus addressing the disadvantage the smaller MX2 bikes would face against bigger bikes. The location of the event changes from year to year. Some of the most successful teams are America, Belgium and Great Britain.
Physical demands
One of the least understood aspects of motocross racing by non-participants is the extreme level of physical fitness required of competitors. Those unfamiliar with the sport often assume that the rider is doing nothing more strenuous than steering a motorized vehicle around a field, no more demanding than driving the family car around the block. Motocross is very physically demanding on the arms, shoulder, and glutes. Observing in detail a rider's actions while at speed on the track reveals why. He or she must maintain ultra-precise control of a machine traversing terrain that most people would have difficulty walking across while maintaining as high of a rate of speed as possible. The rider is astride a machine weighing at least two hundred pounds and, at the most elite professional level, has an engine that produces at least fifty horsepower. A rider's arms and legs are in constant motion during a race, fighting for control of the motorcycle and absorbing the energy produced by high-speed landings from heights that often exceed twenty feet or from two-foot high stutter bumps (called whoops) that jackhammer the motorcycle and the rider. The G forces produced test the ---- limits of a rider's strength and endurance. Finally, a typical professional moto (heat race) lasts at least thirty minutes. That represents half of an hour in which the faster the rider goes, the more violently and frequently he or she is put to the test. And there are no pauses, breaks or pit stops. At least not if a rider expects to win.
The National Sport Health Institute in Englewood, California tested several professional motocross racers in the early 1980s as part of a comparative study of the cardio-vascular fitness of athletes from various disciplines. Athletes from track, American football and soccer were tested, among others. The cardiac stress and strength test results compiled there revealed that the motocross subjects had as high of a fitness level as than any other discipline tested. (original article appeared in DirtBike magazine in 1980. Interview with Brad Lackey, World Motocross Champion and one of the test's participants appeared in Racer X Illustrated in 2004 and is recounted here
