Dirt bike engines come in two varieties: two-stroke and four-stroke. Two-stroke engines are the traditional engine design for dirt bikes. However, four-stroke engines have become more popular as technological advances make them more attractive and durable for dirt bike enthusiasts.
Dirt bikes, unlike road bikes, emphasize light weight design and rugged chassis construction as they are regularly jumped and launched into the air over rough terrain. Dirt bike engines must be light, have good low end power and be simple to service and repair.
The traditional dirt bike engine has been the two-stroke engine. A two-stroke engine is a very simple design that entails the spark plug firing every crankshaft revolution (it fires once every two strokes). Since the motor fires every two strokes, it makes roughly twice the power of a four-stroke, allowing a small engine to produce good power, an important feature for a dirt bike. A two-stroke engine contains only three moving parts. The crankshaft is attached to a connecting rod, which is attached to a piston. There are no engine valves, no lubrication systems, no fuel injection and no timing gear. Fuel and oil are combined in the fuel tank and gravity feeds the engine. Due to the simple hardware, two-stroke engines have short service lives and require frequent rebuilds. In addition, they produce more pollution becaue the oil and fuel are mixed and burned together,
Four stroke engines have the same engine design found in cars and trucks. The spark plug fires every other revolution of the crankshaft (every four strokes). Four-stroke engines were originally considered a poor choice for dirt bikes since they are far more complicated and are heavier due to additional hardware. As the designs improved, however, they have become increasingly attractive, since they offer better engine life, better efficiency and have a lower environmental impact.
Variety of Displacements
Dirt bike engines come in a variety of sizes from 50cc to 400cc and larger in extreme cases. At the very small end of the spectrum, a complete 50cc dirt bike might weight 125 pounds in total.
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