RV Tire Safety: How fast can you drive safely on ST-type tires?

There are still a number of folks asking about tire operational speed and specifically with regard to ST-type tires. I hope this post will help.

Back in the ’70s there was a National Motor Vehicle Speed Limit of 55 mph. The RV market wasn’t as big as it is today. Plus, RVs were much smaller and many used passenger vehicle-type (P-type) tires.

Then the RV market started to grow and RVs became larger and heavier, so tires offered on RVs needed to increase in capacity.

Based on info from someone in the tire industry at the time, we know that some RV companies did not want to use P-type tires because of the loss of 10% of the load capacity. Also, they did not want to pay for more expensive light truck-type (LT-type) tires. So the RV companies asked for “special trailer” (ST-type) tires.

Some tire companies do not make ST-type tires

Not all tire companies agreed to the proposal, and even today many companies do not make ST-type tires.

The formula to calculate tire load was adjusted and included a statement on 65 mph max speed. This showed up in the published Industry Standards as seen here.

Goodyear even issued a Bulletin, PSB #2011-13, in 2011, stating that “Industry standards dictate that tires with the ST designation are speed rated at 65 mph (104 km/h) under normal inflation and load conditions.” Adjustments for higher speed up to 75 mph would be allowed if there were either a decrease in load capacity or increase in inflation limit or both.

Tire industry standards

The wording is convoluted but it specifies:

Based on these industry standards, if tires with the ST designation are used at speeds between 66 and 75 mph (106 km/h and 121 km/h), it is necessary to increase the cold inflation pressure by 10 psi (69 kPa) above the recommended pressure for the rated maximum load.

– Increasing the inflation pressure by 10 psi (69 kPa) does not provide any additional load carrying capacity.
– Do not exceed the maximum pressure for the wheel.
– If the maximum pressure for the wheel prohibits the increase of air pressure, then the maximum speed must be restricted to 65 mph (104 km/h).
– The cold inflation pressure must not exceed 10 psi (69 kPa) beyond the inflation specified for the maximum load of the tire.

Please note that the Goodyear bulletin does not completely match the publisher TRA (Tire & Rim Association) guidelines.

Tariff on “non-speed rated” tires

In 2017, the U.S. Federal Trade Commission placed a tariff on all “non-speed rated” tires. So the tire companies in Asia, where the low-cost ST tires were made, quickly adopted the use of a “Speed Symbol” based on the use of the SAE passenger car speed test to justify the use of higher speed rating.

This “Speed Test” only requires that brand-new tires be capable of running 10 minutes at a stated speed. So, in reality, the SAE test is just a high temperature resistance test. After running the test, tires are not subjected to any additional testing and are considered scrap.

Through all of this there has been no adjustment to the original load calculations that specified a 65 mph maximum operational speed.

We have all learned or read of the general degradation of tires in trailer application. Interply shear is a major physical cause for this degradation of the belt strength, which can eventually lead to a “belt detachment” aka “blowout”.

What does this mean?

So what does this all mean?  In my expert opinion:

1. ST-type tires should not be run for any length of time above 65 mph at the published max load for the given load range of the subject tire unless adjustments to load and inflation are made.

2. If you want to run your ST-type tire at a speed above 65 but no greater than 70 mph, you MUST increase the cold inflation by 10PSI. This inflation increase DOES NOT provide any increase in load capacity

3. If you want to run your ST type tires at speeds above 70 but not greater than 75 MPH you MUST increase the cold inflation pressure by 10 psi PLUS you must reduce the tire load MAX capacity by 10%.

4. At no time should you ever increase the cold inflation pressure above the maximum PSI rating of the wheel as identified by the wheel manufacturer.

Maximum operation speed

You should note that the loads and inflations published in the Industry Standards book aka the TRA manual are based on a maximum operation speed of 65 mph. The “speed symbol” is a sales and marketing tool. I find no information in the TRA manual that recognizes the SAE speed symbol as superseding the existing TRA standards.

If or when such approval is issued by TRA, I will adjust my statements above accordingly.

If you want to understand Interply Shear I suggest you review the following posts.

• “Interply Shear” and other Techno Babble
• The science of trailer tire inflation and failures
• Question on radial tire belt “Interply Shear” or IPS
• Tire inflation not the same for all trailers – This link has a video showing the effects of tire cornering and the resultant interply shear.

I hope this clarifies the question of why ST tires in RV service seem to fail at a higher rate than regular radials when used on cars and light trucks.

Roger Marble

*****

If you have tire questions, check out Roger’s Blog as well as his posts on RVtravel.com. There are hundreds of posts covering everything to do with tires.

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