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 > U.S. Federal Testing Standards for ST and LT Tires

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SeniorGNC

Houston

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Posted: 09/22/09 09:22pm Link  |  Print  |  Notify Moderator

This post is an open letter to the RV.net community whom have been following the ongoing tire discussions.

I have been following these discussions, but until lately I have been staying on the sidelines until the topic took a turn to a more factual basis. I have personally experienced multiple failures with my 5th wheel and ST tires, both D and E load range and would like to understand why this appears to happen more often with ST (special trailer) vs. LT (light truck) tires. (Based on this forum it must be considered anecdotal and not conclusive evidence.)

To resolve this matter I have investigated the current Federal Motor Carrier Safety Administration (FMCSA) testing standards that ST and LT tires must meet to be certified to be sold in the United States.

First I have some folks that I would like to thank for providing me the desire to delve into this issue:
“Chris” for moving the topic in a more factual direction.
“Tireman9” for helping me find the federal standards.
“FastEagle” for his willingness to challenge group thought and to encourage me to understand the perceived performance discrepancies between these types of tires.
Thanks guys!

For those of you who just wish to get the big picture facts, I have started with a summary section. This hits all the high points and you my quit after reading this. (No need to read all the details if you don’t want to!)

If you wish to understand my research in greater depth (and verify my findings and conclusions), after the summary I have provided the following sections: references, notes, and testing of (1) bead unseating resistance, (2) strength, (3) endurance, and (4) high speed performance.

SUMMARY OF FINDINGS & CONCLUSIONS

I found the testing requirements for both the ST and LT tires at the Federal Motor Carrier Safety Administration (FMCSA) webpage.

The testing for each tire is comprised of (1) bead unseating resistance, (2) strength, (3) endurance, and (4) high speed performance.

The testing for (1) bead unseating resistance and (2) strength were identical for tires representative of moderate to heavy 5th wheels and thus no advantage is given to either tire type.

The testing for (3) endurance was found to be significantly different between the ST and LT tires.

Both the ST and LT are put through the same initial pressure, time and load profile. The total profile lasts 34 hours of continuous run time starting at 85% of rated load and ending at 100% of rated load. To further stress the tires, a load range E tire (nominal 80 psi rating) is tested at a reduced pressure of 60 psi to induce additional load on the tire during testing. (This is reasonable that testing should be conservative.)

But now the endurance testing diverges significantly.

The ST tire is tested at this pressure, time and load profile at 50 mph. After that, the ST test is over.

The LT tire is tested at this pressure, time and load profile at 75 mph. This is a 50% increase over the ST and will induce significant additional load and heating on the tire during testing. After that, the LT test is not complete. Next a “Low Inflation Pressure Performance” test is performed for the LT tire only. The tire pressure is decreased to 46 psi and the tire is immediately run for an additional 2 hours at 75 mph and 100% of rated load.

Thus, the LT tire endurance test is drastically more intense than the ST endurance test.

The testing for (4) high speed performance.

The difference in high speed performance testing between a ST and LT tire is significant. Both tires are tested through a 90 minute speed/time profile.

The ST tire is tested 88% of rated load while the LT tire is tested at 85% of rated load. Thus, the loading is 3% higher based on rated load and this slight advantage goes to the ST tire.

However, the LT tire is tested at significantly higher velocities when compared to a ST tire (99 vs. 85 mph maximum speed). This is a 16% advantage to the LT tire.

Thus, again the overall test for the LT is more rigorous than the ST test.

Conclusion:

It is reasonable to conclude that these test requirements force the tire manufacturer to construct an LT tire more substantially than an ST tire. This is also a reasonable explanation for the same size LT tire is rated at a slightly lower maximum load than a ST tire.

And now, for those of you who need to know all the details, read on!

REFERENCES

The references for my evaluation may be found at the Federal Motor Carrier Safety Administration (FMCSA) webpage:
ST tire standard may be found at FMCSA Part 571, subsection 109.
http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.aspx?chunkKey=090163348008f295
LT tire standard may be found at FMCSA Part 571, subsection 139.
http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.aspx?chunkkey=090163348008f2a9
Part 571, subsection 139 references Part 571 subsection 119 which can be found at:
http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.aspx?chunkKey=090163348008f29d

QUICK NOTES

Each standard for the ST and LT tires has definitions, significant constraints on labeling, etc. that I will not address. There are also tire conditioning (temperature), tire break in, etc. that are the same or similar for ST and LT that I will not address. The details are in the references.

The (3) endurance, and (4) high speed performance tests must not result in tire failure. Tire failure includes visual evidence of tread, sidewall, ply, cord, inner liner, or bead separation, chunking, broken cords, cracking, or open splices, not just a blowout.

TESTING - BEAD UNSEATING RESISTANCE

ST Tire: (reference paragraph S5.2.2)

The tire is mounted horizontally and a vertical load is applied to the tire’s outer sidewall at a rate of 50 mm (2 inches) per minute.

Increase the load until the bead unseats or a specified value is reached.

Repeat the test at least four places equally spaced around the tire circumference.

LT Tire:

Paragraph “S6.6 Tubeless tire bead unseating resistance” references the ST tire procedure noted above.

Conclusion:

The testing for bead unseating resistance is identical for a ST and LT tire.

TESTING - STRENGTH

ST Tire: (reference paragraph S5.3.2.1)

Force a 19 mm (3?4 inch) diameter cylindrical steel plunger with a hemispherical end perpendicularly into the tread rib as near to the centerline as possible, avoiding penetration into the tread groove, at the rate of 50 mm (2 inches) per minute.

Compute the breaking energy for each test point by means of a provided formula.

LT Tire: (reference paragraph S6.5.2)

Each tire shall comply with the requirements of S7.3 of 571.119, which is tires for vehicles weighing 10,000 lb or more. Per S7.3 of 571.119 for our example tire, the testing is the same as the ST tire procedure noted above.

Conclusion:

The testing for strength is identical for a ST and LT tire.

TESTING - ENDURANCE

The following is for a ST or LT tire of less than nominal cross section less than or equal to 295 mm (11.5 inches) which is typical of a 5th wheel application.

ST tire: (reference paragraph S5.4.2)

There are specifications for the contact of the tire mounted on a test axle and steel test wheel after the test that I will not address because they are similar for the ST and LT.

Inflate a load range E to 60 psi. (410 kPa)

Conduct the test at 80 kilometers per hour (km/h)(50 miles per hour) in accordance with the following schedule without pressure adjustment or other interruptions:

The loads for the following periods are the specified percentage of the maximum load rating marked on the tire sidewall:
Time and Percent of rated load
4 hours, 85%
6 hours, 90%
24 hours, 100%

LT Tire: (reference paragraph S6.3.1.2)

“Conduct the test, without interruptions, at the test speed of not less than 120 km/h…” (75 mph)

Inflate a load range E to 60 psi. (410 kPa)

This test uses the same profile as the ST tire.

Immediately following the above sequence perform a Low Inflation Pressure Performance test (reference paragraph S6.4):
This test uses the same tire/wheel as the previous sequence at a reduced pressure.

For a load range E tire the pressure is reduced to 46 psi. (320 kPa)

The same tire/wheel is run an additional 2 hours at the reduced pressure at a speed of 75 mph and 100% of rated load.

Conclusion:

The difference in endurance testing between a ST and LT tire is significant. Both tires are tested through a equivalent loading/time profile. However, the LT tire is tested at this profile at a higher speed (75 vs. 50 mph) and must still endure an additional 2 hour low pressure test without failure. Thus the overall test for the LT is far more rigorous than the ST test.

TESTING - HIGH SPEED PERFORMANCE

ST tire: (reference paragraph S5.5.4)

Load the tire to 88 percent of the tire’s maximum load rating as marked on the tire sidewall. Inflate to 72 psi (500 kPa). Run the test sequentially without interruption at:
75 mph (121 km/h) for 30 minutes
80 mph (129 km/h) for 30 minutes
85 mph (137 km/h) for 30 minutes

LT Tire: (reference paragraph S6.2.1.2.7)

Load the tire to 85 percent of the tire’s maximum load rating as marked on the tire sidewall. Inflate to 72 psi (500 kPa). Run the test sequentially without interruption at:
87 mph (140 km/h) for 30 minutes
93 mph (150 km/h) for 30 minutes
99 mph (160 km/h) for 30 minutes

Conclusion:

The difference in high speed performance testing between a ST and LT tire is significant. Both tires are tested through a speed/time profile. The ST tire is tested 88% of rated load while the LT tire is tested at 85% of rated load. Thus, the loading is 3% higher based on rated load and this slight advantage goes to the ST tire. However, the LT tire is tested at significantly higher velocities (nearly 100 mph!) when compared to a ST tire. This is a 16% advantage to the LT tire. Thus, again the overall test for the LT is more rigorous than the ST test.

* This post was edited 09/23/09 04:30pm by SeniorGNC *


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BiggerBen

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Posted: 09/22/09 10:07pm Link  |  Print  |  Notify Moderator

Wow, thanks for the great post.

To quote from your summary:


"However, the LT tire is tested at significantly higher velocities (nearly 100 mph!) when compared to a ST tire."

This would seem to be consistent with the speed ratings of ST tires vs LT tires and why the are not rated to exceed 65mph?


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time2roll

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Posted: 09/22/09 10:34pm Link  |  Print  |  Notify Moderator

Where is the test where the 80 psi tire sits in the sun motionless for six months at 50 psi under full load and then is aired up and run out at 70 mph in 110F heat and the pavement is way hotter. [emoticon]


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Posted: 09/22/09 11:05pm Link  |  Print  |  Notify Moderator

Thanks so much for the time involved to get this information

Chris

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Posted: 09/22/09 11:08pm Link  |  Print  |  Notify Moderator

Kevin, thanks for taking the time to research and post this information. I think this supports the ongoing thought that many of us have, that LT tires are built to much higher standards, and that is why we see a lot less failures in similar service as the ST tires. It also plays into the tire weight differences issue that I have referenced so many times. Now if there was just a few more LRG 16" LTs available, those with axles above 6K and up to 7K would have more choices.

Thanks Again

Chris

* This post was edited 09/22/09 11:24pm by Chris *


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ol Bombero-JC

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Posted: 09/22/09 11:29pm Link  |  Print  |  Notify Moderator

SeniorGNC: Excellent post & info.

Few probably never bothered to go to this link which was provided here on RV Net, but -

Your conclusion is previously affirmed by "Beginner" on the Airstream Forum/s.
(Or you affirmed his, LOL!)

He called the NHTSA and spoke with the Engineer responsible for the Federal Acquisition Regulation (FAR)- and Speed testing.

The link will take you to the forum page.
You will need to *scroll down* (3 posts) to Post #31.
Also see the following question, and the reply (Post #34).

ST Tires & NHTSA (FAR) Speed Tests

~
JC

* This post was edited 09/22/09 11:43pm by ol Bombero-JC *

gitane59

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Posted: 09/23/09 06:49am Link  |  Print  |  Notify Moderator

WOW SeniorGNC!!!! Thank you for analyzing and quantifying the differences in testing and standards. You have added greatly to this debate with clear thought and consideration. You have pulled the answers out of the government speak better than anyone yet to the question of the differences in ST and LT tire testing and standards.
Kudos to you!!!!

This thread should become a sticky!!!!


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FastEagle

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Posted: 09/22/09 11:53pm Link  |  Print  |  Notify Moderator

Although the testing procedures are a nice thing to know I've not referred to them much because they will not answer any of the most important questions between the two considerably different designed tires. The LT tire is designed for the extra demands placed on them by the drive and steer axles in all conditions and can be ordered accordantly whereas the ST tire is pointed at only the demands of a static trailer axle. The superior suspension systems in the auto, light truck and SUV industry can be designed to dampen the effects of a very stiffly designed tire whereas the RV trailer has a very primitive suspension design when compared to the others above. For the high mileage 5th wheel trailer hauler a stiff tire will take it's toll on the unit and it's contents as the miles increase.

It is well known at the highest levels that there is an overweight problem with the RV trailers over 10K. New measures have been taken by the DOT to insure that the manufacturers and dealers provide the new owners with the correct tire and trailer weight information at the first sale. Below is a web site that contains the new (12/2007) “final rule”.

Here is where I always get into trouble. The owner and hauler of his fifth wheel is the one that has to answer to himself, honestly, what he could have done to prevent any tire problems he has experienced. If he has no answers than he needs to do some serious troubleshooting to discover what's going on. If your unit is less than five years old report all failures to the manufacturer. He has to keep records on your tires for the first five years. Get a certified tire tech to look at any failed tire and give an opinion as to the cause of the failure. If you have a tread separation that has not blown out, an inspection will reveal if it was caused by poor manufacturing processing or poor maintenance practices.

Like the Passenger Tire and Light Truck Tire the Special Trailer Tire must be kept in it's own category. The parameters for it's safe and dependable operation lies in understanding it's needs. It needs max air pressure all the time. It needs to stay at or under a constant 65 MPH. It has a shallow tread making it more prone to damage from road trash so do a walk around on every rest stop. It needs to stay cool. If you can't put your hand on it and keep it there then there is a problem. Even in a 15-20 MPH cross wind the lee ward tires may get hot from overloading caused by the wind. Always check your trailer brakes and get the desired setting from your controller. Severe tire scrubbing will happen whenever you exceed any a turning radius of more than 60 degrees. If done on pavement check for damage or flat spotting. An out of balance trailer tire will get hot in a hurry at highway speeds and with the fifth wheel connection you will never know it unless you spot it in your mirror. Read your owners manual. Most of them have chapter 10 from the DOT safety brochure as a reference.

We will never know how the LT tire holds up to the heavy trailer punishment because there were just not enough of them used as Original Equipment on the 6000# axles and those that have chosen to use them as replacements may be a little reluctant to report them because of the backlash they receive for using a lower load ranged tire as a replacement.

REF: DOT

FastEagle

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Posted: 09/23/09 12:13am Link  |  Print  |  Notify Moderator

ol Bombero-JC wrote:

SeniorGNC: Excellent post & info.

Few probably never bothered to go to this link which was provided here on RV Net, but -

Your conclusion is previously affirmed by "Beginner" on the Airstream Forum/s.
(Or you affirmed his, LOL!)

He called the NHTSA and spoke with the Engineer responsible for the Federal Acquisition Regulation (FAR)- and Speed testing.

The link will take you to the forum page.
You will need to *scroll down* (3 posts) to Post #31.
Also see the following question, and the reply (Post #34).

ST Tires & NHTSA (FAR) Speed Tests

~
JC


So are we now saying that running at 85 percent or so of the speed rating of a tire is more important that running at 85 of a tire load rating?

And have manufactures like Michelin already factored that into tires like the XPS RIB, which carries a Q or 99 speed rating on trucks, but Michelin gave it a speed rating of 75 for trailer use, back when?

Chris

* This post was edited 09/23/09 12:26am by Chris *

NC Hauler

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Posted: 09/23/09 12:34am Link  |  Print  |  Notify Moderator

Again, I believe that the Quality controls that are in place and enforced at most, if not all the LT tire manufacturers makes a big difference in the quality and reliability of the tire. Most of the ST tires that are manufactuered in China, I truly believe that quality isn't something that is stressed to the degree that it needs to be. Thus the reason we seem to read more about these tires failing more than some of the LT tires.
Also somewhat understated in the testing, if any, is how the tires react or wear when mounted on TT's or 5er's, in several different circumstances, such as mentioned earlier, sitting for long periods of time while not in use and being exposed to the elements, towing in an overloaded conditon, towing with under inflated tires, etc. It is a much smaller market that these tires are used in this manner, but I also believe that some of the above mentioned situations also factor into how long a tire is going to last and wear. I see no where this type testing is being done. I guess there may really be no reason for it. I think what really needs to be studied or addressed is the way the Chinese manufacture their ST tires and somehow, make sure that "quality control" has a much larger role in the manufacturing process than it does now, (which can't be much). Reliability and Accountability are two things most consumers come to expect, and should, whether we get it or not, and for some reason, when it comes to ST tires, the Chinese seem to be exempt from these two terms.


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