mi - The Honda CRF series is a line of four-stroke motocross and trail motorcycles manufactured and marketed by Honda.
The CRF line was launched in 2002 as a successor to the Honda XR series. The full sized motocross bikes are equipped with liquid-cooled, single-cylinder four-stroke engines that are available from 149 cc to 649 cc. They now have dual-sport motorcycles.
The entry-level CRF's have simple air-cooled engines, and are available from 50 cc to 230 cc. The Honda CRF450R was the first in the series, followed with the CRF250R in 2004. Further down the line, the CRF450X and CRF250X bikes emerged, both designed for mostly off-road use. They are considered among the best motocrossers of their class, and have been a leading seller since their introduction.
Honda employs advanced four-stroke engine technology in their 250cc and 450cc bikes. The engines in these bikes use technology developed in Formula One racing engines. Honda uses an over-square engine design, this means that the diameter of the cylinder is larger than the stroke of the piston. This allows for higher engine speeds and a reduction of reciprocating mass. Another technology that is used is short piston skirts. The "skirt" area of the piston is the portion on the side of the piston which comes into contact with the cylinder wall and aids piston stability. While the introduction of the shorter skirt on the piston helps to reduce reciprocating mass, it also leads to more "rocking" of the piston, or minute unwanted rotation of the piston around the axis of its wrist pin. This leads to more frequent maintenance intervals for the pistons, piston rings, and cylinder walls.
Overall the new high-performance four-stroke engines in Honda's motocross bikes produce much more power per pound than older generation engines and rev more freely; in the quest for more power, longevity is reduced and maintenance is increased.