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Vehicle axles

Axles are an important structural component of a wheeled vehicle. The axles maintain the position of the wheels relative to each other and to the vehicle body. Since for most vehicles the wheels are the only part touching the ground, the axles must bear the weight of the vehicle plus any cargo, and also any acceleration forces between the vehicle and the ground. In addition to the structural purpose, axles may serve one or more of the following purposes depending on the design of the vehicle:

Drive

One or more axles may be an integral part of the drive train. An engine exerts a rotational force on the axle, which is transmitted to the wheel or wheels to accelerate the vehicle.

Braking

Conversely a vehicle may be slowed by applying force to slow the rotation of the axle. Most vehicle brakes are part of the wheel assembly and therefore exert friction on the wheels directly, but engine braking or gearing down still exerts rotational forces on the axles.

Steering

The front axle of most automobiles is a steering axle. The vehicle is maneuvered by controlling the direction of the front wheels' rotational axis relative to the body and rear wheels.

Structural features

A straight axle is a single rigid shaft connecting a wheel on the left side of the vehicle to a wheel on the right side. The wheels rotate in unison, and about the same axis. Such a design can keep the wheel positions steady under heavy stress, and can therefore support heavy loads. Straight axles are used on trains, for the rear axles of commercial trucks, and on heavy duty off-road vehicles. The axle can be protected and further reinforced by enclosing the length of the axle in a housing.

In a split axle design, the wheel on each side is attached to a separate shaft. Modern passenger cars generally have split front and rear axles. This allows independent suspension of the left and right wheels, and therefore a smoother ride. It also permits the left and right wheels to rotate at different speeds as the automobile turns, improving traction and extending tire life.

A tandem axle is a group of two or more axles situated close together. Trucks designs will use such a configuration to provide a greater weight capacity than a single axle. Semi trailers usually have a tandem axle at the rear.

Drive axles

An axle that is driven by the engine is called a drive axle.

Modern front wheel drive cars typically combine the transmission and front axle into a single unit called a transaxle. The drive axle is a split axle with a differential and universal joints between the two half axles. Each half axle connects to the wheel by use of a constant velocity (CV) joint which allows the wheel assembly to move freely vertically as well as to pivot when making turns.

In rear wheel drive cars and trucks, the engine turns a driveshaft which transmits rotational force to a drive axle at the rear of the vehicle. The drive axle may be a live axle, but modern automobiles generally use a split axle with a differential.

Some simple vehicle designs, such as go-karts, may have a single drive wheel. The drive axle is a split axle with only one of the two shafts driven by the engine.

Constant Velocity Joints

Constant Velocity Joints or CV joints allow a rotating shaft to transmit power through a variable angle, at constant rotational speed, without an appreciable increase in friction or play. They are mainly used in front wheel drive and all wheel drive cars. However, rear wheel drive cars with independent rear suspensions typically use CV joints at the ends of the rear axle half-shafts.