The first thing one needs to know when choosing a
trailer axle is the axle capacity required for your particular trailer.
The capacity is basically determined by the smallest capacity of the
components that make up the trailer, in other words, the weakest link in
the chain. This particular chain includes the frame, axle, hubs,
spindles, bearings, tires, wheels, springs and all hardware (among other
things). Here are the instructions for measuring some of the various
The distance from the outside of one hub mounting flange to the outside of the other flange, measured at the half-way point between top and bottom of hub is called the Hub Face (although it is also sometimes referred to as the axle track or the mounting surface where the wheel studs are). Rather than calling the measured distance "Hub face to Hub face" some refer to it as "the centerline of the tire to the centerline of the tire."
The Drop is
measured from the centerline of spindle to the bottom of the main axle
tube. It is important to note that in a straight axle there is no drop.
The distance from the center of one spring to the center of the other spring is called the Spring Center. The Spring Center will have Spring Center Pads, which are basically locating or centering pads for the leaf springs to be mounted to. They might be left loose or welded on and they may be found at the bottom or top of the axle’s main beam. It is vitally important to use Center Spring Pads rather than drilling holes in beam—drilling holes would create stress concentration points which would weaken the axle beam and create an entry point for water.
Knowing the axle type is important for proper tire clearance. Also, keep your axle type in mind if you’re reusing old mounting hardware. If you’re reusing existing hubs or drums, also consider spindle size.
Straight Axles are simply a straight square or round bar.
Drop Axles have offset spindles and hang a little lower than a straight axle.
Torsion Axles look a little like a square or rectangular straight axle, but they have moving parts within the bar.Mounting the Axle:
The most important thing to do when replacing a trailer axle is to examine how the old trailer was mounted. If it has the spring mounted below the axle, it is called an Under Slung Mount. If the spring is on top of the axle, it is an Over Slung Mount (also known as a Reverse Axle). The centering pads or spring seats can be mounted on either side of the axle (over or under). Ther are some axles with a road set or "camber", giving the axle a bow in the center until weight is applied. These cambered axles should have the center bow on top when they are mounted.