Trailer axle hubs attach the trailer wheel to the axle of the trailer. These hubs come in varying sizes to accommodate the type of trailer you are hauling as you might wish to put different wheels on the axle than what originally came with the trailer. If you are travelling in varying weather conditions or if you have another trailer and wish to be able to swap the wheels, you can choose a hub that works for you. The hub's weight is listed to coincide with the axle rating of the trailer.
Four-bolt trailer axle hubs are usually only rated to 907kg. This means you should not try to use this hub on any trailer that will weigh more than that when loaded. You might choose to change out your axle hub based on how much weight you intend to put in a trailer; remember to add this weight to the standard weight of the trailer. Depending on where you purchase your trailer, a smaller hub might have been used to keep costs down.
A five-bolt trailer axle hub is weighted to 15.9kg. These patterns are more difficult to mount as finding the centre can be complicated. You must fully triangulate between the five bolts to find the true centre of the hub when placing on the axle. This will ensure tire wear and tear is equal and that your trailer is not off balance. Even bolted hubs are easier to centre.
Six-bolt hubs can either work with a 15.9kg.-rated axle or a 23.6kg.-rated axle. The hubs themselves are rated differently so you need to be sure you are getting the six-bolt hub for your axle. The six-bolt 15.9kg.-rated hub was designed to eliminate the issues with the five-bolt pattern standard for that axle. This eliminates the triangulation process and the off balance issues of the other hub. You can replace your five-bolt hub with a six-bolt hub, but you will also have to replace the mounting on the tire.
The eight-bolt patter is specifically for the 3.18kg.-rated axle. This configuration is meant to equally distribute the weight on the significantly larger tires required for the increased capacity. These trailers are often times harder wearing on the tires due to the increased cargo load.
In each of the weight classes, you can have different sized hubs in diameter as well as the bearing dimensions. This will affect the rest of the mechanics of the wheel. When you are replacing any kind of trailer hub, you should be sure your plates are of similar size and that you have confirmed the bearing and seal dimensions.