Lift axles can be defined as non-powered axle which can be moved up when the truck is partially loaded or empty, which when installed ahead of driving tandem axles is called ‘pusher’ and if it’s behind it’s a ‘tag’.
Lift axle suspensions either uses airbags or steel springs to raise the axle if it is not in use. In case of airbags, if two sets are being used, one set will be inflated while the other one will be deflated. Similarly, in case of steel springs raising the axle, the airbags containing the weight will be first deflated so that the springs will be allowed to retract the axle. This raise of the axles can be controlled by the driver, either by using electric switches or by air valves close at hand.
Considering the number of rules and regulations applicable to axle weights and spreads, there is no single lift-axle configuration that works everywhere. No doubt payload capacity can be increased by adding liftable or steerable axles to a truck but it requires great expertise to handle the additional weight and complexity. One needs to consider a variety of factors to get an adequate number of axles in the right place on the frame for a particular application.
One of the few factors that need to be considered before making the choice of single versus dual tires; is the desired suspension style and the desired turning angle of the axle. Choice of your tire may greatly impact the wheel cut on a steerable axle and the placement of components e.g. tie-rods and brake chambers. Tire choice can often be swayed by the measurement of the distance from frame-to-ground to match the ride height of the suspension to the chosen tire.
Other important factors that need to be considered is the air system capacity in case of multiple lift axles, driveline clearance when the suspension is inflated and deflated, and to ensure the adequate clearance for the tires while turning (with steerable axles).
While the majority of the considerations were focused on lift axles centers carrying capacity but one must also consider the weight-bearing capacity of the truck when the lift axles are raised. When the axles are raised, the drive axles and the steer axle supports the full weight of the load which is often defined as “job site rating.” Lastly, one should also consider whether lift-axle control to be placed inside the cab or out on the chassis.
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