Hydraulically actuated disk brakes were introduced in 1970, and they have since become standard on front wheels on all but low-weight, low-performance models. Many motorcycles retain drum-type brakes for the rear wheels, but there is increasing use of disk brakes for the rear wheel as well. Another trend is from wire-spoked wheels toward one-piece cast wheels.
Motorcycles have a great diversity of engines. Present-day motorcycle engines may have up to six cylinders, arranged in line, or horizontally opposed, or in a V configuration. The total piston displacement may be as low as 50cc or as high as 1,200cc. Both water-cooled and air-cooled engines are mass-produced.
The popularity of the two-stroke-cycle engine is waning among highway-machine builders, but it is very widely used where sheer performance is the primary consideration. Automotive engines rarely exceed a specific power output of 1 horsepower per cubic inch (16cc) of displacement, but the two-stroke engines used in motorcycles commonly produce 2 horsepower per cubic inch of displacement. Four-stroke-cycle engines in motorcycles operate at about 1.3 to 1.4 horsepower per cubic inch (16cc) of displacement, which is very high for unsupercharged, mass-produced engines. These high specific outputs are in large measure explained by the design features of the typical motorcycle engine. They include the use of overhead camshafts, individual carburetors for each cylinder, and hemispherical or pent-roof combustion chamber shapes containing two, three, or four valves.
In all, the specifications for a typical motorcycle engine closely resemble those for an automotive racing engine -with much the same narrowing of the useful engine-speed range. For this last reason, motorcycles almost invariably have five-ratio transmissions, and many have six forward speeds. Multiplate oil-bath clutches are almost universally featured, with clutch engagement controlled by a hand lever. A few small motorcycles have had automatic clutches.
