The ball mount raises or lowers the hitch ball for a level connection between trailer and tow vehicle. A trailer tongue riding significantly above or below horizontal could cause poor handling, load shifting, suspension stress, uneven tire wear and tire failure.
Correct tongue height is especially important for tandem-axle trailers, as uneven axle loading puts potentially dangerous stress on axles, wheel bearings and tires.
The correct ball mount for your tow-hitch system will match the diameter of the receiver tube and will have a hole sized to match the shank of the ball you'll be installing.
When you have selected a hitch, you also need to select the correct ball mount. The right ball mount has a shaft sized to match the receiver tube, and raises or lowers the hitch ball to ensure both tow vehicle and trailer ride level when connected.
To determine ideal towing height, measure the height of a level trailer tongue from the ground to the bottom of the coupler. Then measure the receiver height from the ground to the top of the receiver opening. The difference of these two measurements will put you in the ballpark for determining the correct ball-mount drop or rise.
Most standard ball mounts are reversible, meaning they flip over to compensate for either drop or rise. For example, if the receiver is 3" higher than the trailer coupler, then a 3" drop would be a good choice. If the coupler is higher than the receiver, flipping the ball mount over will compensate for this rise. Please note: Ball mount rise may differ slightly from the measurement given for drop.
Adjustable hitch bars, or ball-mount systems, are also available. They feature ball-size selection and drop/rise adjustment by simply pulling a couple of pins.
Whichever option you choose, be sure to stay within the trailer- and tongue-weight capacity limitations of your vehicle. Safety is the first rule when towing any trailer, and a well-chosen ball mount is key to starting on the path of safe towing.