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Jun 16, 2016

I recently was called in to examine a vessel which had suffered some malady while in transport. The trailer tires in the incident appear to be the culprit.  It reminded of an article I saw in USBoat a few years ago.

What To Know About Trailer Tires

If there is a common question to be found in the Q&A trailering section of, it's about a boat trailer tire. While there may be many reasons for this, there's also this, as explained by Legendary Trailer Repairs owner Dustin Hoover, one of more than 10,400 service providers for BoatUS Trailer Assist: "Boat trailers are always behind the driver so they are forgotten." So it stands to reason that a majority (44%) of the calls to BoatUS Trailer Assist come from Members having problems with their tires.

BoatUS Trailering spoke with experts at two trailer tire manufacturers: Goodyear (maker of Marathon radials) and Kenda (maker of Karrier ST Radials and Load Star bias ply tires). As you will see, there are differing opinions with some issues, although there is much helpful agreement.

Passenger Tire or (ST) Special Trailer Tire?

There are too many Internet sites with "experts" telling readers that a passenger car tire works fine on a boat trailer. But despite the stories about "always using passenger tires with no problems at all," a single fact remains: Boat trailer tires (identified with ST on the sidewall) are designed to handle the load of carrying a boat around turns and corners at highway speeds. Passenger car tires aren't built to do that.

Trailer tires are designed for heavy-duty, free-rolling applications with emphasis on tread wear, rolling resistance, stability and ease of towing. They normally have a heavier construction than passenger tires in order to meet the additional load-carrying requirements of trailer applications. Passenger tires are designed for passenger car applications and may not meet all the service requirements of trailer tires. — Goodyear Tire Company

Passenger car tires and boat trailer tires are designed differently because they are used for different purposes. It becomes a safety issue. We do not recommend the use of passenger tires on trailer applications. — Kenda Tire Company

Bias Ply or Radial?

There are two kinds of tires: bias ply, made with layers of nylon textile cords placed on top of each other at 30-degree angles, and radials, composed of a single layer of rubber-coated steel cables with more steel belts placed at the crown of the tire (the part that touches the road). Bias ply are less expensive than radials but are more vulnerable to developing "flat areas" after sitting in one place for a long period of time. Radials are preferred by many for use on long highway trips as they run cooler than bias ply.

Radial tires offer many advantages for trailer applications including less heat buildup, better ability to carry loads, less rolling resistance (better fuel mileage), longer wear and softer ride. Bias tires offer stiffer sidewalls, which may be beneficial in some applications, but usually not in "trailer" (free rolling) service. — Goodyear Tire Company

Radial tires cost more and some tire makers push for it because they don't have bias ply to offer, but the fact remains that bias ply tires have stiffer sidewalls that will be more stable on the highway and give better comfort performance on off-road condition. Bottom line: Either radial or bias ply tires are fine as long as they are designed for trailer applications. — Kenda Tire Company

Inflate Tow Vehicle Using PSI on the Driver's Side Door or the PSI on the Sidewall?

This can be confusing since many times the psi (pounds per square inch) recommendations will differ from what is listed on the door to what is listed on tire. That said, use the driver's side door psi (if it isn't on the door, some models will have it on the fuel door). The psi on a sidewall indicates the maximum inflation for that particular tire.

One common mistake is thinking the psi for the tow vehicle tires will be the same as the psi for the boat trailer tires. Very rarely is this ever the case. In a very general sense, tow vehicle tires will have a lower psi than trailer tires.

For trailer tires, inflating near the max pressure indicated on the tire sidewall is a good option for cooler running, lower rolling resistance and load-carrying capacity. If the load the tire is carrying isn't near its capacity, lower-than-maximum psi can be used (see load and inflation tables for proper inflation) and this may give a slightly better ride, but never allow the tires to be under-inflated (lower than what the trailer manufacturer recommends). — Goodyear Tire Company

Single Axle vs. Tandem Axle and Tire Maintenance?

The psi should be checked before the trailer (single, double or tandem axle) is used. During long trips, check it every morning. The rest of the time, inspect the psi once a month.

All tires should be balanced to ensure proper tire wear and to reduce vibrations.

Rotation is normally not required on single-axle trailers. On tandem-axle trailers, rotation is also not normally required unless fast wear is noticed on the front or back tires during normal inspection. In that case, tires should be rotated front to back. — Goodyear Tire Company

We recommend using what is called an "X" type rotation. — Kenda Tire Company


Used in the tires of bikes in the Tour de France, NASCAR, the Indianapolis 500 and Formula One racers as well as aircraft and the military, nitrogen is being used more and more by the public for tire inflation. Unlike air, psi from nitrogen isn't affected by air temperature so the tire actually runs cooler. Nitrogen is dry whereas air contains moisture. As a result, nitrogen has found its way to boat trailer tires. Any tire filled with nitrogen is marked with a green valve stem. So, should it be used for boat trailer tires? Tire manufacturers aren't saying "yes" or "no."

Goodyear does not offer nitrogen filling of tires at its 750 company-owned stores, although some independent Goodyear dealers offer it. Proponents of nitrogen filling of tires claim there is better retention of tire inflation. Goodyear suggests consumers check their tires- both the physical condition and the inflation pressure — at least monthly or before a long trip, whether the tires are filled with nitrogen or air. Consumers should practice good tire care and keep their tires inflated to vehicle manufacturer recommendations. — Goodyear Tire Company

There is lots of research about nitrogen and certainly it has more benefits than just using regular air, the major one being that less is lost over a period of time than air (one study shows that in one month, air loses 1.5 pounds per square inch while a tire filled with nitrogen loses that amount in six months). This is why nitrogen is a hot topic today because 85% of people don't check air pressure regularly.

As for the purity level of nitrogen, most of the research to date shows all the benefits begin when the tire is filled with 93 ~ 98% of nitrogen. The air used to inflate your tire at a service station already contains 78% of nitrogen. But not all the pumps can get the purity level needed and it's still a service option at most tire dealers. Regardless of whether air or nitrogen is used, check the tire pressure regularly. — Kenda Tire Company

How Fast?

Industry standards dictate that ST tires are restricted to a speed of 65 mph unless a different speed restriction is indicated on the tire sidewall. If speeds from 66 to 75 mph are used, the tire cold inflation pressure can be increased by 10 psi without any increase in load. — Goodyear Tire Company

Like a passenger car tire, which has a speed rating, trailer tires are also designed with a certain speed rating. However, the trailer tire is used to sustain the higher loading weight and will generate higher heat than the tire going at the same speed on a car. Since heat build up will potentially degrade a tire's structure and cause either a blow out or separation, trailer tires are usually designed at lower speed rating. — Kenda Tire Company

How to Understand "Load Rating"?

Every tire, be it for a car, light truck, motorcycle or boat trailer, has a load rating, with the lightest being "A." Most boat trailer tires have load ratings of B, C or D. For example, a small single-axle boat trailer may use tires with a B load rating which may have a load capacity of 590 pounds. Since there are two tires on the trailer, the total capacity of the tires being used is 1,180 pounds so the boat, trailer engine, fuel and equipment can't weigh more than 1,180 pounds.

For single-axle trailers, tires can handle 100% of their load rating. For dual-axle trailers, loads must be reduced by 12%. — Goodyear Tire Company