Trailer Tires and Wheels Info

Different Tires for Different Jobs

Trailer tires aren't like auto or light truck tires. They don't steer, transmit power from an engine to the road or swerve to avoid obstacles. Automobile tires perform all of these functions, so they need flexible sidewalls to keep their tread anchored to the pavement. Car tires are mated to sophisticated suspension, steering and braking systems, while free–spinning trailer tires sit underneath stiff suspensions, and smaller trailers don't have brakes at all.

Special Trailer Tires

Special Trailer (ST) tires are designed for your trailer's requirements. Their stiff sidewalls help prevent your rig from swaying. Their heavy–duty load capacity is critical; ST tires have 10% more load capacity than equivalent light truck (LT) tires, and 40% more than passenger car (P) tires. They're designed for trailer wheels, typically narrower than those on a car, and have shallower treads, so they wiggle less and run cooler.

Oxidation not Treadwear

According to rubber industry research, trailer tires need to be replaced after 3–5 years of use, even though they usually appear to have plenty of tread left. Unlike your car's tires, it isn't miles of driving but oxidation of the rubber that wears out the tire. Mostly, a trailer tire sits in one spot for days or weeks at a time, and may travel only 10,000 miles (or sometimes a lot less) per year. UV radiation from sunlight and ozone from exhaust cause exterior damage to the sidewalls, and oxygen from pressurized air creates unseen damage deep inside.

Underinflation and Overloading

Underinflation is the prime cause of tire problems, and an underinflated trailer tire won't sag like your car's tires, because of its stiff sidewall construction. We recently pulled into a gas station with our 21' boat. Both tires looked perfectly normal, but our pressure gauge showed that one tire had only 15psi (instead of the rated 50psi), which would have quickly destroyed the tire. Don't eyeball it! Check your tire pressure with a quality gauge.

Maximum Loads for Radial Tires

Radial or Bias Ply Tires?

Radial tires have plies that run perpendicularly across the tire and belts (often made of steel) running below the tread around the tire's circumference. Bias ply tires have their plies running at 30° angles (like the stripes on a candy cane). Most motorists believe Radial are better (and they are for your auto). Should you buy them for your trailer?

The answer depends on your trailer's handling. Bias ply tires have stiffer sidewalls, so if your rig tends to sway, they may help reduce this problem. They also have advantages for carrying heavy loads. Radial, as most of us know, are vastly superior for tread wear. They'll last an average of 40,000 miles vs. 12,000 for bias plies. However, oxidation, not treadwear, wears out trailer tires. Bottom line: consider your trailer's handling and the manufacturer's recommended tire type. If you travel long distances and/or put lots of miles on your tires every year, Radials are worth the extra cost.
All your trailer tires should be the same type, size, and construction–do not mix bias–belted and radial tires. In selecting tires for your trailer, buy the size, type, and load range found on the trailer's certification label or in the owner's manual.

Keep in mind that tires have a load rating that indicates the amount of weight they can carry safely. That includes toys, Igloo coolers and camping gear. Overloading can lead to a large heat buildup, causing accelerated wear or a blowout.

Understanding Tire Sizes

Trailer tires use one of three different marking conventions to indicate tire size: numeric, alpha numeric and metric.

Numeric identification is probably the most commonly used convention for indicating the size of small trailer tires. 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.

4.80 x 8B
Section Width: 4.80"
Rim Diamieter: 8"
Load Range: B

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. 

20.5 x 8.0 x 10C
Overall Diameter: 20.5"
Section Width: 8.0"
Rim Diameter: 10"
Load Range: C

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. 

B78 x 13C
Air Chamber Size: B
Aspect Ratio: 78%
Rim Diameter: 13"
Load Range: C


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. 


Application/Type: ST (Special Trailer)
Section Width: 175mm
Aspect Ratio: 80%
Construction: R (Radial)
Rim Diameter: 13"
Load Range: C

How To Measure Your Bolt Pattern


Bolt Pattern Guide

Don't Forget the Spare Tire

Lock It Up
Worried about someone taking off with that lovely spare? Several spare tire carriers and locks are available, allowing you to keep that externally mounted tire under lock and key.


Wheel locks are another great ideal to prevent your trailer and cargo from being stolen.  They may well be the answer to the theft problem consumers have had with all types of vehicles and equipment. Within as little as five seconds, a wheel lock is installed directly onto the wheel/tire, immobilizing the wheel and making it impossible to move the trailer.


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