Standard tractors, developed for pulling heavy loads, are characterized by two large rear drive wheels and normally a fixed width between the left and right wheels.
Row-crop tractors were developed in the early 20th century to permit power cultivation of crops, that are planted in rows, such as corn. Row-crops, tractors have two rear drive wheels and one of three front-end types: a single narrow tire, which can operate between rows of vegetables or other closely spaced row crops; two closely spaced, narrow front tires—known as a tricycle model; or a wide front axle that, like the rear wheels, is adjustable to match various row spacings for different crops. Row-crop tractors provide high clearance (21 to 26 inches, or 53-66 cm) under the axles and are the most popular type tractor today.
Four-wheel-drive tractors were in volume production by 1930 but never accounted for a significant portion of the market. However, the limited power that can be transmitted through two traction wheels has not been sufficient to meet the demand for higher powered tractors, and the market for 4-wheel-drive machines for agriculture and industry has greatly expanded. Many 4-wheel-drive tractors exceed 100 horsepower in size.
The two basic types of 4-wheel-drive tractors are characterized by their steering systems. The first is steered by pivoting the front or front and rear wheels at the ends of their axles. Steering of the second type, referred to as articulated steering, is done by pivoting front and rear sections of the tractor about a hinge point in the middle of the frame.
Crawler, or track-type, tractors run on endless steel tracks, which provide traction. Since tractor weight is supported by the entire track surface touching the ground, these tractors are well adapted for soft, wet, or sandy soils where wheel tractors would mire down. Crawler tractors are used primarily for earthmoving and industrial applications, although some are also employed in agricultural operations. Field operating speed is relatively slow compared to rubber-tired tractors, which limits their use to pulling extremely heavy loads or to work on steep slopes.
Tool-carrier tractors have found greatest application in Europe and are used by attaching a variety of implements or transport boxes to the tractor frame.
Implement carriers, although similar to tool-carrier tractors, make use of a common engine, frame, and wheels to power mounted harvesting machines such as combines, forage harvesters, hay balers, and corn pickers.
Front-wheel-drive tractors were introduced during the early transition from horses to tractors, about 1914 to 1920, and permitted the continued use of old-style horsedrawn implements. The driver sat on the implement to drive the tractor. The steering column, steering wheel, and controls were extended back to the operator's seat on the implement. In some cases the driver even used reins as though he were driving horses. These machines generally did not perform satisfactorily and are now principally museum pieces. Walk-behind garden tillers are a version of this tractor type.
Compact tractors, looking much like scaled-down farm tractors, have been popular for lawn and garden use since the early 1960s. These tractors should not be confused with the generally lower powered riding lawn mowers and walk-behind garden tillers. It is estimated that several million lawn and garden tractors were in use in the United States in 1970.
Most units are powered by 7 to 14 horsepower, single-cylinder gasoline engines, with a few smaller and some larger engines for specialized operations. Some have battery-powered electric motors.
Walk-behind garden tractors with two drive wheels and a variety of rotary or fixed tillage attachments have been available since about 1930. These usually have from one to 10 horsepower and are quite common in Japan and developing Asian countries.
