Selecting and Measuring Your Trailer Axle
When selecting an axle it is important to determine the axle capacity required for your trailer. Trailer capacity is based on the lowest capacity of the trailer components. This includes but is not limited to frame, axle, spindles, hubs, bearings, wheels, tires, springs and all hardware. It is important to know what axle type you have when reusing old mounting hardware and for proper tire clearance. This includes straight axles (round or square), drop axles with offset spindles, and torsion axles. Also, consider spindle size when reusing existing hubs or drums.
Trailer Axle Hub Face
Trailer axle hub face is the distance from the outside of hub mounting flange to outside of hub mounting flange, measured one-half way between top and bottom of the hub. The hub face is also known as the mounting surface where the wheel studs are located. Hub face to hub face can also be called the axle track or the center line of the tire to the center line of the tire.
Trailer Axle Drop
Trailer axle drop is the distance from the centerline of spindle to bottom of the main axle tube.
NOTE: In straight axles, you measure from the spindle centerline and axle main beam center line. Therefore, there is no drop.
Trailer Axle Spring Center
Trailer axle spring center is the distance between the center of one spring to the center of the other.
Trailer Axle Spring Center Pads
Trailer axle spring center pads are locating or centering pads that can be welded or left loose and are meant for the leaf springs to mount onto. Springs pads can be located at the top or bottom of the axle main beam. DO NOT drill holes in the beam in lieu of using spring pads. This would set up stress concentration points, which will weaken the axle beam and allow water to enter.
Mounting your Axle
When replacing your trailer axle, determine how it was previously mounted. Under slung mounts have the spring mounted below the axle and over slung mounts have the spring mounted on top (also called reverse axle). Spring seats or centering pads can be mounted over or under the axle. Some axles have "camber" or road set, so there will be a bow in the axle until weight is applied. Cambered axles should be mounted with the center bow on top.