You can use this measurement to identify the axle. The tube in our video example measures at 2 and 3/8ths of an inch. You should find it on the driver’s side of the trailer.Ī second way to determine the size axle you need is to measure the diameter of your existing axle. If you bought a company-made trailer (as opposed to building one yourself from a kit) there should be a tag with the VIN, weight and capacities all there for you. The first and easiest option is to check the neck and tongue of your trailer. There are multiple ways to determine which axle you need. We want to show you how to obtain and use these measurements. When you’re replacing your axle, you need to know: