There are a few common spindle sizes found on trailer axles rated at 3,500lbs or less. These are the most common axles found on boat trailers, Small to Medium Sized utility trailers, Travel Trailers, and more. There are 4 main areas of importance on the spindle to consider when purchasing a trailer hub or bearings. They are, Seal Surface, Inner Bearing Surface, Outer Bearing Surface, and Thread Surface.
One of the most common trailer spindles out there is a 3,500lb “Tapered Spindle” (when bearings have different Inside Diameter’s). This spindle is usually found with a 4-hole brake flange welded to the axle. The flange is there for trailer brakes to be mounted but does not have to be used. Here is a diagram showing the 4 important areas on a 3,500lb spindle:
Another commonly found spindle is when both bearing Inside Diameter’s are the same and this is called a “Straight Spindle”. These spindles are commonly found on axles rated at less than 3,500lbs. Most trailer manufactures used and still use what is referred to as a “Standard Spindle” while others used, less commonly, what is referred to as a “Short Spindle”. If the Hub is available to measure, than Hub Length*
can be used to determine the difference. If the hub is not available you can measure the spindle from the seal surface shoulder to the beginning of the thread as shown:
is the distance through the center of the hub from casting edge to casting edge. A “Standard Spindle” hub will measure between 4” and 4-1/16” while a “Short Spindle” hub will measure 3-1/2”.