RV Tire Safety: Why are ST tires better than LT type?

10

with RV tire expert Roger Marble

I’m following some posts on an Airstream forum of tire inflation and type tires. Some are complaining about rivets “popping” when they increase tire pressure. There are many questions and some confusing replies. I thought some might find this information interesting.

Some general observations and comments:

On this RV forum, there are two tire engineers with significant forensic tire inspection knowledge. Our focus is on getting a better tire life and seeing fewer structural tire failures.

Out there, I would guess there is a pop rivet expert who would ask why the size, number or type of rivet being used is failing at a high rate. Airplanes are riveted together but I don’t hear about “popped” rivets in that application.

Very few RV owners know the actual load on their tires. Simply dividing the scale weight by two or 4 doesn’t provide the correct answer as it is easy to have one tire position to be hundreds of pounds heavier than another and they only respond to the actual load on that tire and not some mathematical average.

Also, I don’t know the loaded weight of each year/size/model TT so can’t provide an informed estimate on the inflation that TT should run.

I do know that having a MINIMUM of 10% Reserve Load (tire capacity minus actual load) is a new requirement (2017) from RVIA. DOT has no margin requirement (zero%). “Capri” and I are on record of suggesting at least 15% margin. The Interply Shear numbers would suggest that TT need to have a reserve closer to 25% if you want tire life more like a motorhome gets (5 – 7 years).

Since it is the air pressure that supports the load and not the tire, simply going up in Load Range but not changing the inflation will in all probability not gain you anything in actual load capacity.

The pressure on the tire sidewall isn’t really the “Maximum” you can safely use but is the MINIMUM inflation needed to support the MAXIMUM load that is marked on your tire sidewall. Personally, I do wish the wording on tires was more technically accurate but I can only guess that the lawyers and bureaucrats decided to use “max” in relation to the pressure for some reason.

Here is a question I wish someone could answer. Why are ST tires rated to support 10% to 20% more load than the same size LT tire? In the past, the reason was that ST tires were only rated to 65 mph to compensate for the increased load. Now ST and LT tires have almost the same speed rating but ST still carry the extra load capacity. If tire companies could all simply improve their ST tires almost overnight, why don’t they use the same “magic” tire construction to increase the load rating of their LT type tires?

Anyone?

 

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

 ##RVT911

 

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stan twerion
9 months ago

I am sorry but this article means nothing in my opinion. Number one going up in tire rating usually increases weight handling and the air in the tire does not support the weight without the tire which the article makes it sound like. No explanation of which is better trailer or light truck tires.

Max tire pressure is a given amount of air pressure but nothing is said where that max is good, what I mean is it at sea level? Is it at 5,000ft? from sea level to 5,000ft the tires will gain air pressure as it expands I do believe. Also how about air temperature nothing said about it, during winter or cold climate you may have 80lbs then you hit the road and go to where temperature is 40 degrees warmer do you stop and let air as you are now exciding max air. No explanations. I have replaced tires on my 9 year old trailer, all 4, 3 times because of blow outs on the road. Tire pressure checked all year round and tires covered when trailer is not used. it get to 120 plus where I live during the summer if cold temp is 80 outside when I start out with proper pressure what happened on the highway at 65 mph and now 120 outside temperature? Would like some tire engineer to explain why tires are blowing so soon and it isn’t me alone there are hundreds of Chinese bombs, Tires on the highway and lots of trailer damage, $2500 damage this past summer for mine when a good looking tire blew, ended up replacing all series E tires again had another ready to blow that day and 1000 miles later 2 more were ballooning out of shape.

Joe
9 months ago

I know I’m not knowledgeable enough about tires, but after reading this article (and reading some of the paragraphs several times), all I can think is “Huh, what did it say”? Please, talk down to us like we are dummies.

Charles Hampton
10 months ago

Was the title of the article a rhetorical question or was it intended to be ironic? Please tell us if ST tires are better made than LT tires, or should we be buying LT tires for our travel trailers. Thanks.

Roger Marble
9 months ago

I have no info that would indicate LT are “better made” than ST. IMO there are two reasons for the better performance with LT type, those being that ST tires are forced to carry 10 to 25% more load than LT tires, and LT tires need to pass newer DOT test requirements.

Sybil Kenney
11 months ago

Could you explain the difference between LT and ST tires for those of us with no tire knowledge
Thank you

Roger Marble
11 months ago
Reply to  Sybil Kenney

LT stands for Light Truck aka pick up trucks and ST stands for Special Trailer. The physical sizes may be similar or the numbers may be the same, like 235/85R16 and both have Load Range aka “ply-rating” letters such as C or D or E etc, but the intended use is different and the performance requirements from DOT are different. More importantly the load capacity is not the same, even if the numbers are the same. Also ST tire rules state that they are not intended to be used on passenger carrying vehicles.
While it may be OK to use LT tires on a trailer as long as the load capacity is matched, it is never appropriate to use ST tire on a truck or car.
Switching from ST to LT requires some research and measurements to ensure you are selecting LT tires with enough load capacity.

bisonwings
11 months ago
Reply to  Roger Marble

I don’t make any claims of being a tire expert but your statement that it is “OK”, to use an LT tire on a trailer as long as the load capacity is matched should be rejected outright. Perhaps, an emergency to limp in for a few miles, yes, but to use in place of is asking for trouble and wasting your money. Trailer tires are made to different design specifications due to the different force vectors that are intrinsic to the application. Trailer tire sidewalls are designed to handle the sideways twisting that occurs when a tandem or triple axle trailer makes a corner. Light truck tires don’t encounter these twisting forces so their sidewalls and body are not reinforced to handle them as are trailer tires.
A number of years ago I bought a travel trailer 750 miles from my home. The person I bought it from told me the tires were fine and it had a new spare. Those tire started failing in less than a hundred miles. I pulled into a tire dealer just off the Interstate that didn’t have trailer tires in my size but refused to mount LT tires on the trailer stating liability issues.

Roger Marble
11 months ago
Reply to  bisonwings

Well, I have inspected the sidewall of thousands of passenger and light truck and a few “top of the line” ST tires and can assure you that unless there is an air leak the failures in passenger, truck and trailer tires is in the belt area. This is primarily due to the Inter ply shear in the belts and has almost nothing to do with the sidewall stiffness. You can Google inter ply shear on tires if you want to learn the facts.

Rick OHara
11 months ago
Reply to  bisonwings

ST tires have a very different and substantially more lenient test that determines their weight rating. My personal experience with a 3 axle toy hauler that came with ST tires was a substantial and dangerous failure rate with the original tires and with the highly touted Maxxis replacements that I went with. The Maxxis tires in my size weighed 38 lbs. I replaced them all with comparable tires that weighed 56 lbs each. The difference in performance was night and day. I went from multiple low mileage failures to exactly zero failures. ST tires are appropriate for farm wagons that run at 20 miles per hour on country roads. They have no business on any interstate or freeway, even well below their 65 mph speed limit. They have no advantage in heavy RV usage whatsoever. Please don’t drink the Kool-aid. LT tires can’t take the ‘special’ loads imposed by trailers? Please!!!!

Roger Marble
9 months ago
Reply to  Sybil Kenney

Just different “type” Special Trailer and Light Truck.