Machines

Motocross motorcycles are lightweight and powered by highly tuned two-stroke or four-stroke engines, usually geared for quick acceleration rather than very high speeds. They have knobbly tires for traction on loose surfaces, a highly absorbent suspension designed to cope with the shock of heavy landings, and short gearing designed for swift acceleration rather than the ultimate in top speed.

They often feature pedals and levers that are hinged so that they simply fold rather than bend or snap when the machine is inevitably dropped. Also the exhaust may be routed carefully so that a fall will not damage the exhaust, nor bend it so that it would obstruct the rear swing arm's travel - something that happens to road bikes.

The saddle is curiously shaped, like a long banana, to ride up over the front of the fuel tank so that the tank no longer protrudes. This is designed to allow the rider greater protection when they hit bumps or land hard. However the main purpose of sitting up further on the gas tank is to keep the front wheel on the ground and aiming the bike where they want it, especially when exiting a turn. Lowering the rider's centre of gravity greatly increases the ability to go through a turn at higher speeds. Sitting further back and accelerating hard over a series of bumps or ("whoops") keeps the front tire from dropping into any large gaps between them and causing loss of control of the bike.

The gyroscopic effect of the engine along with the wheels makes jumping the "motocross" bike large distances possible. A common technique to change the attitude of the bike in air involves the use of the rear brake. When rear wheel is stopped while the bike is in mid-air, the gyroscopic effect forces the front of the bike to lower, allowing the rider to force the bike to a specific position or attitude relative to the ground.

Unusual for racing machines, motocross bikes can be purchased in a ready-to-race condition at moderate prices from major motorcycle manufacturers. Professional riders, however, modify their machines further, both for outright performance and to have the bike's behaviour more in tune with their own preferences. The highly tuned machines of the professionals are called "factory bikes."

The latest trend in motocross motorcycles is towards four-stroke engines. New four-stroke machines rival the two-strokes in both weight and power and are significantly easier to use due to the smoother power delivery. These engines have been developed due to environmental concerns regarding the increased emissions of two-strokes. Some predict that two-strokes will not be available to buy after 2007, perhaps earlier in states such as California.

Engine sizes ranges from 50cc right up to 550cc, although bikes with sidecars can have up 1000cc engines. Four stroke motocrossers do not compete on a truly level playing field. Currently, 250cc four strokes compete in the 125cc class and 450cc four strokes are used in the 250cc class. They are very competitive in these classes but need twice the displacement to rival a two-stroke.