Maximum Operational Tire Speed

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

Back in the 70's there was a National Motor Vehicle Speed limit of 55 MPH, and the RV market wasn't as big as it is today. Plus RV's were much smaller vehicles and many used Passenger Vehicle 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 of 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 and they did not wan to.pay for more expensive LT type tires so the RV companies asked for "Special Trailer" 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 publish Industry standards as seen here.

 

 Goodyear even issues 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. 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 guidelines.

In 2017 the US 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 with Interply Shear being a major physical cause for this degradation of the belt strength which can eventually lead to a "Belt Detachment"  AKA "Blowout".

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 speeds 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.

You should note that the Loads and inflations published in the Industry Standards book AKA TRA manual are based on a maximum operation speed of 65 MPH. The "Speed Symbol" is a sales and marketing tool and 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.

- https://www.rvtiresafety.net/2013/11/interply-shear-and-other-techno-babble.html

- https://www.rvtiresafety.net/2018/09/warning-super-technical-post-tire.html

- https://www.rvtiresafety.net/2018/07/question-on-radial-tire-belt-interply.html

- This link has a video showing the effects of tire cornering and the resultant Interply Shear

       https://www.rvtiresafety.net/2018/02/tire-inflation-not-same-for-all-trailers.html

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.

 

 

Reader's questions on inflation and type tire load capacity.

 Questions from Readers

"Is there a way to verify the tires on my trailer have been inflated with nitrogen? My tire dealer said they did but I am not sure if I can trust them. The pressures stay even tire to tire and not a lot of change while driving which I think tells me that the tires are inflated with nitrogen."

My answer: As a Tire Design Engineer, I see no reason to worry about the claimed use of Nitrogen, The air we breathe is 78% Nitrogen. While using N2 to inflate tires is not bad it just isn't worth spending any money on it.

About your question on inflation:    Are you using your TPMS to monitor tire pressure or are you using a hand gauge? TPMS are generally accurate to +/- 2% and they can warn of puncture while driving down the road. I have seen a tire fail because the owner used his hand gauge but the valve core stuck slightly open and the tire failed 50 miles down the road.

Here is a post of my inspection of that tire. I believe that if he had been running a TPMS he would have avoided that failure.
https://www.rvtiresafety.net/2020/11/claims-of-defective-tires-causing.html

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A question on "LT 225/75/16E. vs. 225/75/16 Commercial tires"

"I’m looking to replace tires on E-450 chassis motorhome. The OEM tire is a LT 225/75/16E. Reading the forums, a number of people are replacing the OEM tires with 225/75/16 Commercial rated tires. They are doing this because the OEM Load Capacity is very close to the sum of the load of the four E rate tires. Four Commercial adds appx. 500 lbs. per tire load capacity or 2000 lbs. for four tires."

My research has turned up seven tires that have both an E rated and Commercial in the same model tire. What I found and surprised me was the as the weight difference between the two tires. With the exception of one brand, all show that the Commercial rated tire weighed less than the LT E rated tire. As a lay person, it seems counter intuitive to me. While there are other tires, here are two tires I am considering:

Michelin Ailis CrossClimate
Commercial: 37 lbs.
LT E rated: 39 lbs.

Toyo Celsius Cargo
Commercial: 36 lbs.
LT E rated: 38 lbs.

I would have thought the commercial tire would have been heavier given that the commercial tire has appx. 18% more load capacity. Can you explain and shed some light on this?

Thanks

My answer:

ED We need to be careful when comparing tires as the actual "TYPE" and Load Range can result in our selection.The Michelin Agilis is a good example. This is a tire developed in Europe and does not carry the "LT" designation as in Europe they call some lines "Commercial" and other tires of the same size are not considered "Commercial".  The Load Index number is the tip off.   You can consider this a bit like the "Load Range" AKA "Ply rating" in the US market.
The additional load capacity differences come about because of the conversion of Metric to "English" dimensions. I think that if you look at the actual specifications for the tires you are looking at, you will find the stated inflation numbers are slightly different. Close but different.  The expected usage can also result in some tires being considered "Commercial"  vs "RV".
I do see many people asking about or posting information on tires but they do not include the "Complete" designation in their Internet posts. Some include the ST or the LT on US sizes. European tire sizes do not have "ST" type tires or "LT" as leading letters but have a "C" after the rim diameter standing for "Commercial". (Not to be confused with Load Range C) To further confuse the issue some tires are available with two different Maximum Inflations and different Maximum load capacities.
For your question I would suggest you look at the Maximum Load Capacity and the inflation necessary to support that load. If the inflation is higher than that shown on your Certification Label, which I think might show an LT type tire in Load Range E and 80 psi. Your "Commercial grade" European tire may specify inflation a bit higher than 80 psi. If so you need to confirm the max inflation rating for the new tires.

I would not be concerned with the weight of the tires you are considering as design and wear goals can add or subtract a couple of pounds per tire
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I got this request:

How about a recommendation for a quality tire pressure gauge? I no longer trust the typical stick gauge.

I have found the Accutire MS-4021R Digital Tire Pressure Gauge from Amazon to be very accurate.

I have 3 and they always read +/- 0.5 psi from the ISO Laboratory Master I used to check my gauges.

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This question from a Class-A owner:

We have a Class A diesel MH. Our maximum cold pressure on the front tires is 120 psi. After weighing our coach we need to run 113 psi. We add an extra 5 psi for safety, so our cold pressure is 118 psi.

We live along the Texas coast, so our summer morning temps can be in the high 70's or low 80's. Before leaving for a trip I set my cold pressure to 118 psi first thing in the mornings.

During the summer we often travel to Colorado or the mountains of New Mexico, chasing the cooler temperatures. A few weeks ago while in New Mexico the morning temperature was in the high 40's and my tire pressure had dropped to 106. We were leaving that morning for Amarillo TX. The high that day in Amarillo was to be 101. My dilemma: add air to increase my pressure to 118 and chance once getting to Amarillo of having my pressures into the high 130's or low 140's; or drive several miles on the lower pressure tires knowing that they would increase in pressure in several miles to a safe traveling pressure.

What I did was to add about 5 psi to each of my front tires. Upon reaching Amarillo my pressures were in the mid to high 130's. The next morning before starting our day I checked the pressures and released about 5 psi in each front tire to get me to my normal pressures of 118 psi.

My answer:

Yes, you set the COLD Inflation Pressure in the morning before you start to travel and before the tires have warmed up from travel or being in the direct Sun. You do not have to worry about the increase as you travel into warmer parts of the country or as the tire warms up from travel.
You never want to travel on underinflated or low-pressure tires. Your 130 or 140 psi is not a problem.
You can adjust to the cooler temperature and lower PSI the next morning after the tires have cooled down.

Your +5 psi is a good idea. That is about what I use on my RV.

Side question: Where did you get the suggestion to inflate to 113 psi cold? The Load Inflation tables are usually arranged in increments of 5 or 10 psi. You should not interpret the inflation between the individual boxes found in the tables. If you need more then jump up to the next level.

 

Roger Marble

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 ##RVT1173