RV Tire Safety: Heat, high speed and the “magic” in ST tires

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with RV tire expert Roger Marble

Heat generation is primarily the result of a combination of high speed, high load, and low inflation. Factors that can counter some of the negative effects of heat on tires can include the use of more expensive rubber compounds, reduced thickness tread and other construction features that are out of our control as tire owners. Without access to secret formulas and other proprietary information, I know of no way to learn which of these features is in which tires, if any. There is one exception – the inclusion of full nylon cap ply. This would be identified in the material list molded on the tire sidewall along with the number and material in both the sidewall and under the center of the tire tread.

However, the driver has complete control over the speed/load/inflation factors.

IMO it would be better if all tires had at least a 15% margin on load capacity, with 20 to 25% being better margin. We need to remember that road slant and crown and curves along with significant side-load from wind can easily result in overloading tires that were measured on a level scale. Multi-axle trailers should shoot for a 25% load margin. (Read about Interply Shear to understand why.)

Ever wonder why RV trailers seem to have a lot of tire problems but your daily driver (car or SUV) doesn’t?

Let’s look at Reserve Load Capacity of your car. Many, if not most, have 30% to 40% reserve capacity on the tires.

Now when we look at an RV trailer we see only 6% with many having 0% margin:

If we assume we want to have 25% reserve load capacity on the above RV, then 13,290/(tire max load X 4) = 0.75 and each tire max load shall be 4,430 lb. So the right choice would be an all-steel ST235/85R16 132/127 (14 with a max load of 4,410 lb.).

Previously I indicated that interply shear (IPS) can be the equivalent of adding 24% more load on the tire while in operation. Using the above tire the IPS effect is reduced considerably.

NOTE in 2017 RVIA and the National Fire Protection Association guidelines for tire selection for RVs was updated to require a MINIMUM of a 10% margin in tire load capacity over the GAWR. Some RV companies achieved this margin by upgrading their tire load capacity while others simply lowered the GAWR number on the vehicle certification label. Do you know what your RV company did?

Speed rating. We should think of this like the engine redline. Everyone seems to understand that running faster than redline will shorten engine life, but that doesn’t mean that 5,900 on a 6,000 redline is good to run for hours on end and may contribute to shorter engine life, just as running 62 mph on tires rated, based on their original load calculations, for a max of 65 mph is okay.

The high-speed test is a 30-minute step-speed test of a brand-new tire on a smooth test wheel. No potholes or curb damage. All that is needed to “pass” the test is to meet the target speed and not come apart. The tire is considered scrap after the test. Would you consider your tires to be “scrap” after you run 70 or 80 mph for an hour or so?

Remember damage (internal cracks) done, even at the molecular level, in tires never repairs itself but only increases in size.

While this post is primarily about trailer application, the limit on high speed also applies to 16″ through 22.5″ tires that have a max rating of 75 mph in RV application, which would be their “redline.” But I hear many Class A owners talking about running over 70 mph for miles on end then being surprised when they suffer a tire failure.

LT tires would be my first choice in any trailer application where heavy loading was required. I do not understand why so many think that there is some magic engineering used on ST tires that allow them to support 10% to 20% more load than an LT tire of identical dimensions. The load formulas used today for ST-type tires are the same ones used in 1969 when they were limited to 65 MAX (redline).

I would be very interested to hear an answer from the engineers at the companies that make ST-type tires why they were able to suddenly, almost overnight, increase the speed rating from 65 to 75, 87, or even 99 on their ST-type tires. Do they have some new materials? Why don’t they put those same super materials in LT tires and increase the load capacity of LT tires to be the same as or near to the loads seen claimed for ST-type tires?

Sorry, I just don’t buy the new claimed high-speed capabilities of ST-type tires that exceed the load and speed capabilities of equal size LT tires.

 

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

 ##RVT935

 

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Tommy Molnar

When it came time for new tires on our travel trailer, my local tire guy (I DO have a favorite shop near me, owned by someone I trust) said he had been having good luck with Hercules tires. So I went from “D” rated tires to “E” rated tires (which he said my wheels would easily handle) and I too have had good luck with Hercules. They are ST tires. Like Debra, I never exceed 60 mph, and if conditions permit, I putt along at 55 mph. So far, so good. In 25 years of RV’ing I’ve only had one tire failure, and that was an OEM tire on our 97 trailer. I almost hate to say this for fear of jinxing myself . . .

Debra

My trailer came with Goodyear Marathon ST tires made in China. They only lasted for 6,000 miles, which seems pretty pathetic to me. I wore out one during that time and put on the spare then another had severe tread separation. At that time I purchased 5 Goodyear Endurance ST tires. I had considered getting LT tires but I wasn’t sure how to select a proper tire and with the condition of the tires I needed to replace them ASAP. So I went with the ST tires. If I buy new tires again I would like to get LT tires. Do you have an article about how to select the appropriate LT tire for a trailer? Also, the old tires are rated for 65 mph. I drive 60mph max. And each time before traveling I check the air pressure and inflate as needed. I also have TPMS. I think the new tires have a higher speed rating. But having read your previous articles questioning the justification for that I still drive 60 mph max. I’m usually the slowest one on the road with even other travel trailers blasting past me.

Dave W

I generally use an LT tire on our 12,000 pound 5ers with excellent luck. What bothers me is that the 45 +pound tire is rated at 3042 at 80 psig while the much vaunted China Bomb Tow Max ST which weighs 35 pounds has a 3640# capacity rating and has sidewalls that virtually collapse under their own weight. Am I wrong in thinking that 3640 lbs is the actual TOTAL capacity of that tire while the LT at 3042 pounds is something arbitrary that someone in the Federal bureaucracy decided was a good number and that it’s really 3500 or 4000 or some other number? Our 5er has aged out the current tires – so I’ll either replace in kind or move up to the G rated Sailuns with 3400# capacity at 80psig