Axle spindles are components used on the steering axle of a vehicle as well as on a trailer and are designed to fasten the braking system and mount the wheels and tires to the axle. On a steering axle, the axle spindles are fastened to the axle assembly by the use of ball joints or king pins, and the wheel hub is attached to the spindle with a castelated nut. The wheel bearings ride on the axle spindles in both applications, with a trailer's spindles often being mounted to an axle as a welded or bolt-on component.
Unlike a drive axle, the axle spindles are not charged with powering the vehicle. In an automobile application, the axle spindles are used to allow the front tires to turn, so the spindles are required to pivot on the axle. In the common straight-axle configuration, king pins are used as a type of hinge, allowing the spindles to pivot on the ends of the axle. Tie rods, drag links and similar components are attached to points on the axle spindles to push and pull the spindles, creating a turning motion as the steering wheel and the steering box are turned by the vehicle's operator.
When used in a trailer axle configuration, the axle spindles can be mounted in one of three methods. The first is a typical axle assembly configuration with the spindles bolted or welded to the ends of a tubular axle. This application often has the axle suspended by springs beneath the trailer's chassis. The second method of mounting the spindles uses a short, stub-axle attached to a torsion beam. The torsion beam functions as the spring and flexes slightly to smooth a bumpy ride.
This style of axle does not span the entire width of the trailer. The torsion beam axle commonly is affixed to the side frame rail of the trailer, with the short axle spanning only a short distance from the frame rail to the outer edge of the trailer. This leaves the center of the trailer open for lower ground clearance.
The final application of an axle on a trailer is for it to be welded solidly to the trailer's frame rail, which typically allows the lowest trailer of any type of axle configuration. The downside to these types of axle spindles is the lack of suspension on the trailer. These trailers rely on the suspension system of the vehicle being trailered to cushion the ride.