450cc motocross bike
In the late 1990s, to try to bring motorcycles with four-stroke engines into racing, the American Motorcyclist Association allowed four-stroke engines with a displacement up to and including 550 cc to compete in the 250cc class. The reasoning behind this rule was that a motorcycle with a four-stroke engine would need at least 550 cc to compete with the water-cooled, high-tech two-stroke engines of the time.
Soon after the rule announcement, Yamaha began developing a high-performance four-stroke machine to compete in the 250cc class. In 1999, it released the YZ400F. This machine featured a dual-overhead-cam, five-valve, water-cooled engine. This engine was mated to a steel chassis that was derived from the YZ250, modified to allow for the larger engine and increased power and torque. Also in 1999, Canondale released the MX400. In 2000, KTM entered competing products 400SX and 520SX.
In 2001, Yamaha bored the engine to 426cc and made some other adjustments to the bike design. The new bike was called the YZ426F, and it was in production from 2001 to 2002. Also in 2002 Honda launched its answer to the 400/426. It was called the CRF450R. The CRF was a very similar motorcycle to 400/426, but it featured an aluminum frame and an eccentric UniCam engine. In 2003, Yamaha introduced the YZ450F. This bike was essentially a larger-bore version of the 426 with even more tweaks to the chassis, suspension, and transmission.
Until 2005, Yamaha and Honda were the only "big four" companies with 450cc motorcycle offerings. That year, Suzuki introduced the RMZ450. In the following year, Kawasaki began selling the KX450F. Since then, many other manufacturers have begun competing in the 450cc motocross market.
Aluminum frames as of 2009 are standard equipment on all of these bikes except KTM models, they use 41xx steel, referred to sometimes as chromoly steel. Some have added fuel injection(Suzuki was the first in 2007) others have added trick steering dampers or titanium shock springs as original equipment. As of October 2011[update], the KTM 450SXF is the only mass production 450 class bike with electric start. 450cc motocross bikes have become so popular that the 250cc racing class was renamed to the 450cc racing class. Recently, some rules have been passed to try to bring the two-stroke motorcycle back into pro and amateur racing, but for now the 450cc bike continues to dominate.
Up until 2004, most all-terrain vehicle (ATV) riders who wanted to race their vehicles would build expensive quads by modifying a Honda TRX250R or a Suzuki LT250R. Some began replacing the ATV's 250cc two-stroke engines with the big four-stroke engines out of MX bikes and stuffing. Others simply juiced up a Honda TRX400EX or a Suzuki Z400.
In 2004, low-slung four-wheel ATV's with modified versions of 450cc MX engines were launched by Yamaha (YFZ450) and by Honda (TRX450R). These quads were a huge success taking over almost all forms of racing to include Grand National Cross Country as well as motocross.
Suzuki added its fuel-injected version, the LTR 450, in 2006. Kawasaki didn't join the club with its aluminum-framed and fuel-injected version until 2008 with the KFX450. Since then Yamaha has released a new version of the YFZ named the YFZ450R. This version is fuel injected and also features an aluminum frame. Honda's TRX450R is the only steel-framed and carburated 450 quad left on the market today.
Since the introduction of the 450 motocross quad there have also been other adaptations to the quad from other manufacturers, to include KTM and Can Am.