So you're ready to buy a trailer ball. But how do you go about choosing the right one for your vehicle? There are four factors to consider here: trailer weight, the diameter of the hole on the ball mount, the thickness of the ball mount platform on the receiver, and the size of the trailer coupler [source: etrailer.com].
First, let's get to know a little more about weight - specifically, gross trailer weight and tongue weight. Gross trailer weight is the fully loaded weight of the trailer you're towing, including any and all gear placed on it. Tongue weight is the downward pressure that the trailer exerts on the ball [source: Truck Stuff USA].
The most important factor in choosing a trailer ball is its gross trailer weight rating. Each ball lists its own weight capacity. Be careful here: Your trailer receiver may be rated to carry up to 10,000 pounds, but if your ball is rated at 5,000 pounds, that's the maximum you can tow. In addition, always make sure the hitch ball's weight rating is greater than the gross trailer weight so you don't overload the towing equipment.
Trailer balls are classified by the dimensions on the different parts of the unit. There's the diameter of the ball itself (the distance around the center of the ball), the shank's diameter, the shank's length, and the size of the circular piece of metal between the ball and shank. The most common ball size is two inches in diameter, but heavy-duty industrial trailer balls can go up to 2 1/4". The diameter of your ball mount's hole must be no more than 1/16" greater than the ball shank diameter in order to fit [source: etrailer.com]. The necessary shank length is determined by the thickness of your ball mount platform - you don't want to get a ball whose shank is so small you can't lock the nut in place. You'll also need to buy a ball to fit into your coupler.
In short, you need to buy a ball that's 1/16" smaller than your ball mount's hole and that can support an amount of weight greater than your trailer, has a long enough shank to fit into your ball mount platform, and whose diameter also fits into your coupler.
In this next section, we'll learn how to properly install hitch accessories like trailer balls and switch balls so you can start towing.