Here is a post from RV.net forum on ST vs. LT tire testing. When it comes to replacing tires it gets really confusing.
Posted: 09/22/09 10:22pm Link | Print | Notify Moderator
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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-regul...0163348008f295
LT tire standard may be found at FMCSA Part 571, subsection 139.
http://www.fmcsa.dot.gov/rules-regul...0163348008f2a9
Part 571, subsection 139 references Part 571 subsection 119 which can be found at:
http://www.fmcsa.dot.gov/rules-regul...0163348008f29d
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 05:30pm by SeniorGNC *
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