Special Trailer Tires

Casitas with the factory “high rise” option are delivered with either Goodyear Endurance ST205/75R15 load range D tires, or Goodyear Endurance ST225/75R15 load range E tires, depending on supply chain availability. The ST stands for "special trailer" tire. There are differences in the driving requirements between the tires on your trailer and those on the car or light truck you use to tow it. Therefore, there are distinct differences between the way trailer tires and tow vehicle tires are engineered.

Your tow vehicle is a leader, which means traction is a key focus in the design of its tires. Typical passenger and light truck tires are developed with features like flexible sidewalls and long-lasting tread designs. This is because your tires accompany the steering and braking systems on your car and truck and transmit traction power from its drivetrain to the road. Traction allows your tow vehicle to accelerate down the road, turn around the corner and brake to a stop. Another important consideration is tow vehicle tires are designed for ride comfort, which is achieved in part by allowing their sidewalls to flex.

Your trailer is a follower, which often makes tire sidewall flexing a negative. Sidewall flexing on trailers, especially high center of gravity RVs that carry heavy loads, is a primary cause of trailer sway. Typical passenger tires with flexible sidewalls can accentuate trailer sway problems.

Trailers will be more stable and pull better on tires designed specifically for trailer use. Since Special Trailer (ST) tires are constructed with heavier duty materials, they are tougher than typical passenger vehicle tires.

ST tires are designed specifically for use on towed trailers to carry RV loads heavier than are typical on passenger (P) and light truck (LT) tires. The extra strong sidewalls on ST tires better withstand the wear and tear of extended towing and prevents tires from rolling under the rim in sharp turns and cornering. The stiffer sidewalls and higher operating pressures common with ST tires also help reduce trailer sway. Trailer tires are also designed to dissipate heat that builds up because of the weight they carry, which can be much more than a standard passenger tire.

The major difference in construction between ST and either LT or P tires is the polyester cord and steel wire used. ST tires are manufactured with thicker sidewalls using heavier tensile strength polyester cord and steel wire than is used in comparable P or LT tires. Additionally, ST tire rubber compounds contain more chemicals to dissipate heat under higher loads than tires designed for passenger cars. Trailers will pull better and ride smoother on ST tires engineered and manufactured for trailers.

All ST tires manufactured before 2015 have a maximum speed rating of 65 mph at maximum weight load and inflation pressure — they are not marked with a speed rating. ST tires have the 65 mph speed limit because increasing speeds cause tires to heat up from increasing friction on the roadway. Since ST tires have thicker sidewall construction with heavier material, they can build up and retain heat more readily at higher speeds than other tire types, particularly when they are under inflated for a given load weight. That heat build up can lead to premature wear, from the inside out, and failure of the tire.

Tire experts allow that speeds higher than 65 mph are OK when cold tire inflation pressure is increased by 10 psi for a given load weight — without exceeding the maximum PSI stamped on the tire — for every 10 mph over 65 mph, and the load weight for each tire is decreased by 10 percent. (Goodyear’s guidance for speeds greater that 65 mph may differ from other manufacturers’ guidance. Always consult your specific tire manufacturer’s guidance on tire use.)

Special trailer tires manufactured in 2015 or later have a maximum speed rating, just like passenger car and truck tires. (Older ST tires have no special marking to designate a limit. Their maximum speed is 65 mph.) Newer tires manufactured in 2015 and later should have one of the following codes on the sidewall to show their rating:

N - 87 mph
M - 81 mph
L - 75 mph
J - 62 mph

If a trailer tire has any other marking to designate maximum speed rating, follow it.

ST205/75R15 Load Range D (8 ply) tires delivered on new “high rise” Casitas have a maximum load rating of 2,150 lbs at 65 psi, and speed rating of N. ST225/75R15 Load Range E (10 ply) tires have a maximum load rating of 2,830 lbs at 80 psi, and speed rating of N.

The GTW of our loaded for travel 2017 17’ FD is 3,542 lbs, with 3,128 lbs of that weight on the axle and the remaining 414 lbs resting on the hitch. That yields a weight on each tire of about 1,564 lbs, which is 586 lbs under the maximum load range D rating at a cold pressure of 65 psi.

That 586 lbs of margin gives me comfort to travel at sustained highway speeds of 65 to 70 mph, depending on road conditions. I rarely drive faster than 70 mph, except for very short distances to move out of dangerous traffic situations. To stay safe, I regularly check tire pressure and look for unusual tire wear before and during our road trips.

ST verses LT tires for RV trailers tends to be a heated topic of conversation on RV forums. Some RVers have abandoned ST tires in favor of LT tires for their trailers primarily because LT tires have higher speed ratings. (Some old-timer RVers using their own anecdotal experience of tires manufactured before 2015, that had a 65 mph speed rating, claim on RV forums the ST design itself is prone to tread separations and other failures. When those ST tire failures occurred, back in the day, the fault was more likely due to under inflation or excess speed, or sun rot, or a combination of all three, rather than tire quality. Always do your own research on information you read online - there are a lot of wild and crazy guys and gals out there.)

Light truck tires have heavier sidewall construction and higher load capacity than passenger tires, but they are not constructed with as heavy materials and thick sidewalls as are ST tires, and ST tire rubber compounds contain more chemicals to dissipate heat under higher loads than tires designed for trucks and passenger cars. Trailers will pull better and ride smoother on ST tires engineered and manufactured for trailers. LT tires can be used for trailer applications, but may not perform as well as ST tires, particularly for trailer sway control.

Most camper trailers are parked unused in storage for extended periods of time. 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. The compounds (oils) used in tire rubber harden and begin to break down when the rubber isn’t regularly flexed by rolling down the road. Additionally, according to rubber industry research, oxidation of tire rubber also causes "rot" deterioration from the inside. The oxygen from pressurized air in tires creates unseen oxidation damage deep inside. Dry rot weathering/cracking may or mat not be visually evident on the outside of tire. Rather rot could be (and usually is) hidden within the ply layers inside the tire.

Do not judge the health of your camper trailer tires just by mileage or the depth of remaining tire tread. Hidden dry rot can cause a weak spot in your tire sidewall or tread ply, even when the tire tread looks almost new. Dry rot is a common cause of tire failure blowouts on the road.

The Department of Transportation (DOT) recommends replacing all tires at least every six (6) years. However, for RV tires on trailers parked for long periods of time it is commonly advised to replace tires every three (3) years from their manufacture date.

To find your tires’ age, you need to locate the DOT Code on the tire. This symbol indicates the tire manufacturer’s compliance with the U.S. Department of Transportation (DOT) National Highway Traffic Safety Administration (NHTSA) safety standards. It tells you who manufactured the tire, where it was made, and other tracking information. Plus, it indicates that the tire has passed the Department of Transportation’s testing and NHTSA manufacturer requirements.

Most tires on the road today have a twelve-character Tire Identification Number or TIN stamped on the tire sidewall immediately following the letters “DOT.” The last four digits of the code (often within an oval) are the date the tires were manufactured. The first two digits are the week of the year the tires were made, and the last two are the year they were made.

In the example shown here, M64BIC1R3317, the tire was made the 33rd week of 2017. Be aware that some tires are manufactured with the date code portion of the TIN only on one side of the tire, so if you do not see the TIN ending in four digits on one side, you need to look at the other sidewall of the tire to determine the date of manufacture. Be sure to check all four tires, as they may not have been manufactured or mounted on the trailer at the same time.

The general rule of thumb for mileage you can expect for your trailer tires is about 25,000 miles. However, typically, trailer tires need to be replaced due to dry rot long before the tire tread is exhausted. You need to replace your tires when the tread gets down to less than 1/8 of an inch or when cracks due to dry rot are more than 1/16 of an inch deep.

Updated June 14, 2021

The Goodyear Endurance ST205/75R15 tire in the photos taken on June 13, 2021 was manufactured on 0417, 4 years old in January 2021, and it has been on the road since July 2017. The other tire on the ground and spare tire were also manufactured during early 2017. The tires have rolled about 22,000 miles. All the tires on our Casita were over three and one-half years from their manufacture when I winterized it and parked it for the winter on November 1, 2020. Our Casita set in its storage place unused through most of the COVID-19 pandemic year of 2020, so the tires did not roll to keep the tire rubber compounds vital for the entire year and prior winter storage season.

Any ST tire of this age and mileage are at the end of life, particularly when our Casita (and tires) had been virtually unused since the fall of 2019. I intented to buy a new set of tires in April 2021, while dewinterizing out Casita and getting it ready for the 2021 travel season. But I procrastinated on making the trip to the tire store in April before our planned local camping trips in May 2021.

I almost pushed our luck too far by not buying a set of new tires in April 2021 - as I felt I should - and running them for our short local trips in May. In June 2021 as I preparing to finally make that trip to the tire store I discovered the curbside tire in the photos had developed a pronounced bulge where the sidewall plys started to separate.

The tread separation is clearly pretty extensive. The big bulge developed where the tire was on ground, in the position the Casita was berthed when we returned from our last short trip in May. That tire was ready to go — I’m thankful it waited to let go while the Casita was berthed, not on the road during our short trips in May. 

I replaced the Goodyear Endurance ST205/75R15 Load Range D (8 ply) tires delivered on our new 2017 FD  “high rise” Casita with Goodyear Endurance ST225/75R15 load range E (10 ply) tires. Our Casita has the OMW / Perfect Casita Shock Absorber Kit and the ST225 tires, which are 20mm wider than ST205 tires, fit perfectly OK.

End of June 14, 2021 Update.

Do not overload the trailer beyond its maximum weight. Extra weight means extra heat, extra wear and could lead to tire failure. Maintain correct air pressure. Underinflated tires may fail under load. Underinflated tires wear out the tread near the sidewalls prematurely while overinflated tires wear out the center of the tread prematurely. Remember, temperature impacts air pressure. For each 10 degrees in ambient temperature, tire pressure increases by 2-5%. Check tire pressure when the weather changes and when travelling to warmer or cooler climates. Inspect tires regularly. Check tires regularly for proper inflation pressure, sidewall cracks, irregular tread wear patterns or any other signs of damage. Under-inflation is the prime cause of tire problems, and an under-inflated trailer tire won’t sag like your car’s tires because of its stiff sidewall construction. Don’t eyeball it! Check your tire pressure with a quality pressure gauge.

In my opinion, it is better to stay with ST tires. For those who want to increase their travel speed over 65 mph, it makes more sense to run with the maximum tire pressure of 65 psi with the weight load on each ST205/75R15/D tire well under the maximum load rating of 2,150 lbs.

When it’s time to replace original equipment tires, some Casita owners, including myself, have upgraded from ST205 tires to a higher load rated ST225 tires. For example: Goodyear Endurance ST225/75R15 load range E (10 ply) tires have a load rating of 2,830 lbs at 80 psi. With load range E tires on our Casita, each tire has a maximum excess carrying capacity of 1,266 lbs OVER our loaded trailer weight, limited by the maximum load rating of 3,500 lbs of Casita’s Dexter axle, divide between each tire. While probably overkill, load range E tires provide an extra margin of safety for traveling over 65 mph. 

(Tire Width is the width of the tire measured in millimeters from sidewall to sidewall. (The first three-digit number in the tire size refers to the tire width. For instance, for a ST205/75R15 tire, the width is 205 millimeters, and for a ST225/75R15 tire, the width is 225 millimeters, an increase in width of 20 millimeters — just over three-quarters of inch.)

Review

Visually inspect your tires before and during any trip. It’s best to check when the tires are cooler.

Trailer tires are limited by time and use, instead of being designed for use until the tread is completely worn out.

After three years in service, approximately one third of the tires strength is gone.

It is best to replace your trailer tires every 3-4 years even if the remaining tread looks good.

Prior to each use, inspect the tires for any cuts, snags, punctures, or bulges, as well as ensuring each tire is properly inflated.

When trailer tires are not in use or are in storage try to store in a cool, dark garage at maximum tire inflation with material between the tire and pavement (like a thin piece of plywood). Tire covers can help protect the tires if stored in direct sunlight.

Reference

• Endurance Tire Sizes Specs
• Endurance Tire Inflation / Load Weight Specs By Tire Size
• Rim Weight Capacity Specifications
• Dexter 600-8k axle complete service manual pdf

This content is offered for informational purposes only and is not intended to replace manuals or instructions provided by the manufacturer or the advice of a qualified professional. No warranty or appropriateness for a specific purpose is expressed or implied.