Trailer and Semi-Trailer Rear Axle

Dead axle (lazy axle)[ edit ] A dead axle, also called a lazy axle , is not part of the drivetrain, but is instead free-rotating. The rear axle of a front-wheel drive car is usually a dead axle. Many trucks and trailers use dead axles for strictly load-bearing purposes. A dead axle located...

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Dead axle (lazy axle)

A dead axle, also called a lazy axle, is not part of the drivetrain, but is instead free-rotating. The rear axle of a front-wheel drive car is usually a dead axle. Many trucks and trailers use  dead axles for strictly load-bearing purposes. A dead axle located immediately in front of a drive axle is called a pusher axle. A tag axle is a dead axle situated behind a drive axle. Dead  axles are also found on semi trailers, farm equipment, and certain heavy construction machinery serving the same function. On some vehicles (such as motorcoaches), the tag axle may be steerable. In some designs the wheels  on a lazy axle only come into contact with ground when the load is significant, thus saving unnecessary tire wear.


Vehicle axles

Axles are an integral component of most practical wheeled vehicles. In a live-axle suspension system, the axles serve to transmit driving torque to the wheel, as well as to maintain the position of the wheels relative to each other and to the vehicle body. The axles in this system must also bear the  weight of the vehicle plus any cargo. A non-driving axle, such as the front beam axle in heavy duty trucks and some two-wheel drive light trucks and vans, will have no shaft, and serves only as a suspension and steering component.  Conversely, many front-wheel drive cars have a solid rear beam axle.

In other types of suspension systems, the axles serve only to transmit driving torque to the wheels; the position and angle of the wheel hubs  is an independent function of the suspension system. This is typical of the independent suspensions found on most newer cars and SUVs, and on the front of many light trucks. These systems still have differentials, but will not have attached axle housing tubes. They may be attached to the vehicle frame or body, or integral in a transaxle. The axle shafts (usually constant-velocity type) then transmit driving torque to the wheels. Like a full floating  axle system, the drive shafts in a front-wheel drive independent suspension system do not support any vehicle weight.


Lift axle

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Dump truck with an airlift pusher axle, shown in the raised position

See also: Federal Bridge Gross Weight Formula

Some dump trucks and trailers may be configured with a lift axle (also known as an airlift axle or drop axle), which may be mechanically raised or lowered. The axle is lowered  to increase the weight capacity, or to distribute the weight of the cargo over more wheels, for example to cross a weight restricted bridge. When not needed, the axle is lifted off the ground to save wear on the tires and  axle, and to increase traction in the remaining wheels. Lifting an axle also alleviates lateral scrubbing of the additional axle in very tight turns, allowing the vehicle to turn more readily. In some situations removal of  pressure from the additional axle is necessary for the vehicle to complete a turn at all.

Several manufacturers offer computer-controlled airlift, so that the dead axles are automatically lowered when the main axle reaches its weight  limit. The dead axles can still be lifted by the press of a button if needed, for better maneuverability.

Lift axles were in use in the early 1940s. Initially, the axle was lifted by a mechanical device. Soon hydraulics replaced the mechanical lift  system. One of the early manufacturers was Zetterbergs, located in Östervåla, Sweden. Their brand was Zeta-lyften.

The liftable tandem drive axle was invented in 1957 by the Finnish truck manufacturer Vanajan Autotehdas, a company sharing history with Sisu Auto.



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