A boat trailer axle is a device used to attach tires and a suspension system onto a boat trailer. Commonly made of a hollow steel tube with bolt-on spindles, the boat trailer axle is typically mounted to a set of leaf-type springs and suspended below the main frame of the trailer. Depending on the size and weight of the boat, a boat trailer axle may have a set of electronic brakes attached to the wheel hubs. It is common for most trailers used to haul boats to include wheel hubs that allow the wheel bearings to be greased without removing the wheel assembly.
The suspension system of a boat trailer is commonly nothing more than a shock absorber attached to a boat trailer axle. The axle is mounted to a set of leaf springs to provide the cushioned ride for the boat as it is pulled on the roadway. By using a simple straight axle, the unit is able to back in and out of the water without damaging exotic or complicated suspension components. Many manufacturers drill a series of small holes into the bottom of the boat trailer axle to allow water to enter the axle, thereby eliminating some of the buoyancy associated with a hollow axle tube being sealed at all ends.
Many boat trailer axle designs utilize a flat plate welded to the end of a steel tube. The plate is drilled to accommodate a bolt-on spindle package, and this allows the wheel spindle, hub and brake assembly, if so equipped, to be easily attached to the boat trailer axle. This type of axle construction and assembly allows a manufacturer to produce several types of spindle assemblies and axle tubes as individual components. Once ordered, the appropriate spindle can be mounted to the proper axle tube and the boat trailer can be assembled.
While the typical and most common method for suspending a boat trailer axle is from a leaf-type spring, some trailers use a torsion bar suspension system in place of the leaf springs. On a torsion bar-type trailer, the boat trailer axle is often a half-axle design with the spindle ends being bolted to a stub or half axle. This type of trailer is often used when a lower profile is desired since the absence of a full-length axle running under the boat can allow the entire system to sit much lower. This can occasionally allow the boat to be launched and recovered in shallower water.