How Tires and Wheels Work

tires, tires everywhere
Despite basic similarities, automotive tires and trailer tires are not interchangeable.
Doug Menuez/Photodisc/Getty Images

In case you hadn't noticed, tires and wheels are vital components of any trailer. Without them, you're not even towing -- you're just dragging stuff behind your truck. The concept of the wheel is as straightforward as they come, but that doesn't mean all tires are created equal. Trailer and automotive tires aren't interchangeable. They're designed to meet different needs and require a whole set of separate considerations.

Automotive tires go on steering axles and drive axles. As such, they're made to bear the load and provide the traction that motorized rotation and steering require. Trailers, by their very nature, don't have powered wheels, and most don't have steering or brakes. Trailer wheels, which mount on trailer axles, simply have to bear the load of the trailer and its intended cargo.


Properly inflated vehicle tires operate at relatively low pressures, generally around 32 pounds per square inch, so as to cushion the passengers while also supporting the weight of the vehicle. Granted, you can follow Hunter S. Thompson's example from "Fear and Loathing in Las Vegas" and inflate them to around 90 psi, but you'll feel every pebble in the road and risk tire failure.

Trailer tires, however, frequently call for even higher air pressure than automotive tires do. Fill them to a mere 32 psi, and the tires walls will flex too much and overheat. While automotive tires have flexible sidewalls, trailer tires have stiffer sidewalls to accommodate higher air pressure. Trailer suspension systems also are generally stiffer than vehicle suspensions and inflict more abuse on tires.

In this article, we'll discuss what factors to take into account when choosing trailer tires and how to install them.



Choosing Tires and Wheels

trailer tires
No matter how much work you plan to throw at your trailer, you're going to want to make sure you have the right wheels.
David Oliver/Taxi/Getty Images

You know that trailer axles require trailer tires, but do you know what kind to buy when choosing tires and wheels? When you walk into the tire store, make sure you have all the essential information on hand so you can get rolling on your new towing venture.

Tires fall into three categories: passenger (P) tires, light truck (LT) tires and special trailer (ST) tires. You'll want to go with ST tires. Your trailer should have each axle's gross vehicle weight (GVW) listed on the body or in the owner's manual. This figure is the total amount of weight the axles can withstand, so you'll need tire capacity that meets or exceeds the GVW. If you know how much weight you plan to tow, you can be a little more exact. First, determine the weight of the unloaded trailer. Then add the weight of your payload. The combined capacity for all of the trailer's tires should exceed the total loaded weight by at least 20 percent.


­Once you've found tires that fit your weight requirements, save yourself a lot of trouble and make sure they're attached to wheels that are going to fit on your wheel hub. To avoid any frustrating revelations, know the bolt pattern for your trailer's wheel hubs. It may have four, five or even eight bolts, and you'll want your trailer tire to match up. The distances between the bolt holes, called the bolt circle, also has to be exact.

The bolt circle is the center-to-center diameter of the imaginary circle that the bolts or bolt holes outline. With even bolt numbers, simply measure the distance between two opposing holes or bolts. With odd numbers, measure the distance between a bolt or hole and the halfway point between the two opposing it. If your wheel hub has five bolt holes and the bolt circle is 4.5 inches (114.3 mm), then your wheel's bolt pattern would be 5 on 4.5.

In addition, bear in mind what kind of roads you plan to tow your trailer across. If you plan to traverse rough roads or open terrain, you might want to invest in all-terrain tires. Finally, whatever variety you wind up getting, make sure all the tires on the trailer are identical in size. If you don't, the trailer weight won't distribute evenly.

Ready to install those tires? Go to the next page to find out how.



Installing Tires and Wheels

change the tire
Installing tires and wheels on a trailer is much like changing an automobile tire.
© Daniel Loiselle

If you've ever changed a tire on an automobile, then you're well prepared for the challenges of installing trailer tires and wheels. If your trailer is wheelless, make sure its frame (not the suspension) is securely elevated. If you're replacing or installing wheels and tires manually, you'll need to check the opposite side wheel, secure a jack to support the trailer's weight and loosen the lug nuts with a lug wrench before raising the wheel off the ground. Then it's just a matter of removing the nuts and sliding the old towing wheel off.

Most trailer tire failures occur due to underinflation [source: Discount Tire]. Always inflate trailer tires to the maximum inflation level to prevent a failure from happening. You can find this level, in psi, listed on the tire's sidewall. If your tires are hot due to sunlight or day-to-day operations, the expanded air will give you a false pressure reading, so check tire pressure under cool conditions if possible. If they're hot, add 3 psi to the maximum inflation to account for the interior air's eventual contraction. To ensure safety and proper performance, tire makers recommend replacing trailer tires every three to five years.


Explore the links on the next page to learn even more about towing.

Lots More Information

Related HowStuffWorks Articles

More Great Links

  • Cook, Miles. "Trailer Towing Q&A." 2008. (Oct. 17, 2008)
  • "Frequently Asked Questions." KMT Service Parts. 2008. (Oct. 17, 2008)
  • "Special Trailer Tires Vs Passenger Tires." Taskmaster Trailer Tires. (Oct. 17, 2008)
  • "Trailer Tires." Natasha's Camping Site. April 16, 2006. (Oct. 17, 2008)
  • "Trailer Tire Facts." Discount Tire Company. (Oct. 17, 2008)