Bore and stroke are a basic measurement of an internal combustion engine. The ratio of bore to stroke, as well as their nominal value, affect engine performance, speed, horsepower and torque. These are important considerations when designing motorcycles, and in consideration of their use as sport bikes, long-distance cruisers or racing bikes.
Bore refers to the diameter of the cylinder, in which fuel is burnt in a series of rapid explosions to drive a piston. The larger the bore and stroke numbers, the larger and more powerful the engine, and the more horsepower it can develop.
Stroke refers to the distance that the piston travels back and forth within the cylinder. The longer the measure of stroke, the longer the path the piston traces as the engine is running.
The bore/stroke ratio is the number reached by dividing the bore by the stroke. This ratio gives motorcycles their engine characteristic. The higher the proportion of bore to stroke, the more horsepower an engine can develop. The average ratio in motorcycle engines is about 1.2. An engine with a ratio of 1 is said to be "square" in design, while a ratio greater than one is "oversquare" and less than one is "undersquare" or "long stroke."
Ratio and Performance
A bore/stroke ratio of 1 or more means the motorcycle is operated at higher RPMs. The lower the ratio, the more turning power is given to the drive train, allowing the bike to maintain power on hills. Long stroke measurements, however, generally mean higher engine friction, shorter fuel mileage, high operating temperatures and higher maintenance.
Many other features affect engine performance. These include gear ratios, carburettor type, motorcycle weight, the number of cylinders and the brake and suspension systems.
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