It takes an informed consumer to make good buying decisions, but when it comes to boat trailer tires, being an informed buyer means more than spending wisely. The safety and, indeed, the lifespan of your boat trailer rest on the integrity of your boat trailer tires.
The easiest way to ensure you’re purchasing the right set of replacement tires for your boat trailer is to simply write down your current size designation (located on the sidewall of your tire) and bring this information to your local tire distributor.
If you’re the do-it-yourself type, the shopkeeper will grab exactly what you need, and you can be on your way to replace those old worn-out tires. If you bring your trailer into the shop, the dealer will probably even install them for you.
Boat Trailer Tires vs. Automobile Tires
Trailer tire requirements differ greatly from automotive tires. Automotive tires must maintain traction during all driving conditions: pulling, stopping, turning, or swerving. Because of this, they must have more flexible sidewalls to maintain tread to road contact. Since boat trailers have no driving torque applied to their axles, the only time trailer tires must have traction is during the application of trailer brakes.
Boat trailers with heavy loads, high vertical side loads (like camper trailers), or trailers with inadequate tongue weight can be affected by trailer sway problems. Automotive bias or radial tires with their more flexible sidewalls can accentuate trailer sway problems, whereas the stiffer sidewalls of the ST (special trailer) bias ply tires help to control and reduce sway problems. For this reason it is not recommended that (P) Passenger or (LT) Light Truck tires be used on trailers. Best trailer control will be achieved with (ST) Special Trailer tires.
Size and Weight Matters
One of the most important elements in purchasing trailer tires is buying tires that can support the weight of the loaded trailer and those that are the correct size for the trailer: A tire that is incorrectly sized and/or unable to carry the weight of the trailer could fail, and the consequences of a blown tire range from merely inconvenient to catastrophic.
First, find out the maximum loaded weight your trailer can carry. That information (as well as recommended inflation pressure for the tires) is provided by the trailer manufacturer on a placard or sticker placed on the trailer, and it is sometimes covered in the trailer manual. Look for the "GVWR" (gross vehicle weight rating) or "GAWRs" (gross axle weight ratings).
The GVWR is the value specified by the manufacturer as the maximum loaded weight that that particular vehicle can carry safely. The weight of the trailer plus everything in it—shipment, spare tires, and any other equipment—should never exceed the manufacturer’s gross vehicle weight rating. On the other hand, the GAWR is the maximum weight that each axle can support. Since most boat trailers have one axle, the GVWR will most always be the sum total of each individual axle rating.
Second, see what kind of shape the tires are in and look for uneven wear patterns, which can indicate alignment problems or a bent axle. Same goes for the bunks and rollers. You can easily put some new material over a worn-out bunk, but rollers are another story. Worn rollers will need to be replaced at a cost ranging anywhere from $6–$30 apiece.
Boat Trailer Tire Size Identification
Numeric: Numeric identification is probably the most commonly used convention for indicating the size of small trailer tires (the ones usually used by boat trailers). Most numeric identification systems indicate tire Section Width in inches, Rim Diameter in inches and Load Range. Load range is the weight carrying capacity of an individual tire represented by an alphabetic symbol.
Some larger trailer tires utilize a numeric size designation that also includes the tires Overall Diameter. This number is placed at the beginning of the tire’s size description, which also includes Section Width, Rim Diameter and Load Range.
Alpha Numeric: More descriptive than the Numeric system, Alpha Numeric tire size designations indicate Air Chamber Size, Aspect Ratio, Rim Diameter and Load Range. Aspect Ratio is the ratio between tire height and width expressed as a percentage.
Aspect Ratio is determined by dividing the tire’s section height by its section width, and then multiplying the result by 100 (section height/section width x 100 = aspect ratio). Just remember that the higher the percentage, the taller the section height of the tire; the lower the percentage, the shorter the tire’s section height.
Metric: Of the three, metric designations cram the most information into the tire size description. This includes the addition of alphabetical codes that indicate tire application/type, section width in millimeters, aspect ratio, tire construction, rim diameter (inches) and load range.
A word of warning: Light Truck (LT) and Passenger (P) designated tires should never be used as trailer tires. Special Trailer (ST) tires are built tougher than P and LT tires to withstand the abuse that causes tire bruising and blowouts. The ST tire’s stronger sidewall also helps alleviate trailer sway problems.
This section should help you make an informed decision next time you’re in need of a new set of trailer tires. Should some sly tire shop owner try and stick you with a set of LT tires instead of the appropriate ST type, you’ll know the difference. Heck, you might even take your newfound knowledge to the web and save a few bucks.
Boat Trailer Tire Maintenance
Remember, in boat tire maintenance, check your tire pressure. Check the tire sidewall for correct pressure (usually 50-65 psi.) Also check your tire tread. Use the penny test by inserting a penny into the tread. The tread should touch the top of Abe Lincoln’s head.
Also, check for cracking and general deterioration on a regular basis. There are only two reasons you see a trailer broken down. One’s a tire and the other is a wheel bearing. You’ve got to treat your tires like the truckers you see on the side of the road—they get out there and kick them to make sure they’re okay.