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steelpony555 · 10-27-2015, 09:20 AM

Ok get on your reading glasses folks....here is the testing standard from the FMCSA...Federal Motor Carrier Safety Admin.....I found it once on there but since then they have revamped their site and is very hard to navigate to find anything...Government..go figure....but if you read, the ST testing standard is a joke..you will see that LT tires are tested at higher speeds at heavier weights then the ST tires......Why you ask??? Because LT tires are passenger vehicle tires....think wrongful death law suits.......But everyone questions the reserve built in to LT tires so here it is......Oh and remember some manufacturs do put on LT tires on their trailers straight from the factory and are listed on the yellow tag on the side of the trailer...that is when cost is not and issue....otherwise they put the cheapest bidders tires on......

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.

hone eagle · 10-27-2015, 11:59 AM

This should be a sticky ^^^^^^

Titanguy · 10-27-2015, 12:31 PM

I have been in the tire business all my life and the term reserve capacity is used to describe the difference in the GVWR and the combined load capacity of the tires.

Example: a 2015 Ford Escape has a GVWR of 4,720 lbs.
Tire is a 235/50R18 97S Load Capacity 1608 lbs.
So at the GVWR of 4,720 lbs. this vehicle is 73% loaded or has a 27% reserve capacity.

Reserve capacity is NOT some unknown number.

IC2 · 10-27-2015, 01:38 PM

I sure am happy using General Grabbers Low Rolling Resistance LT - 'E' rated tires on two pretty heavy 5ers with, to date zero problems beyond a junk valve my installer used. These LT tires actually allow me to safely drive over 65mph in 80mph though will never come close to that near warp speed.

(Oh and 'shouting' and excessive use of colors on a post --- really doesn't look nice)

FastEagle · 10-27-2015, 04:47 PM

Quote:

Originally Posted by IC2

I sure am happy using General Grabbers Low Rolling Resistance LT - 'E' rated tires on two pretty heavy 5ers with, to date zero problems beyond a junk valve my installer used. These LT tires actually allow me to safely drive over 65mph in 80mph though will never come close to that near warp speed.

(Oh and 'shouting' and excessive use of colors on a post --- really doesn't look nice)

Useful info about your replacement tires.

Tire Plus-sizing for Passenger and Light Truck Vehicles

jacknife · 10-27-2015, 09:18 PM

It doesn't matter what that company makes or doesn't make . There are good ST tires such as Goodyear 14s and 16s .

The tests he showed are true spending the time to gather the data so people can make a informed decision on tires is hardly being a schill. What is shameful is certain attitudes people have , when all the guy was doing was putting the data out there. So folks can understand the rubber rhey are riding on.

FastEagle · 10-28-2015, 02:06 AM

You know, in the real world, NHTSA seldom, if ever, tests tires. They just write the minimum standards for testing each individually designed tire. The tire manufacturer may test a first run batch (lot) of tires then it’s normally done by a third party testing facility. Almost all tire recalls are initiated by the individual tire manufacturer.

The only way a tire manufacturer can get a DOT certification onto a tire’s sidewall is to certify that the proper testing has been performed. Even then, not all tires are tested. Established brands previously tested only require periodic testing or lot testing.

Tire testing does not pit one design against another design. What would it prove? Tires are tested individually on their design function. That’s why there is P=passenger, LT=light truck & ST= special trailer. All of those designs can be used on trailer axles, at the vehicle manufacturer’s discretion, or, “as appropriate”, as the regulation says it.

kedanie · 10-28-2015, 01:38 PM

So, are you saying that if one has 6,000 pound rated axles and wheels stamped with a 3,042 pound load rating that it is acceptable to install load range E tires with a load rating of 3042 pounds each?

Keith

glennwest · 10-28-2015, 04:26 PM

Really all this is good reading but bottom line is on a heavy camper commercial steel carcass tires are basically trouble free. Keep air in them and they age out. Good use of our money.

steelpony555 · 10-29-2015, 04:58 AM

Unless you want to join the club of torn up trailers from blow outs....then yes switch to LT E rated truck tires

Cumminsfan · 10-29-2015, 11:04 AM

FWIW Airstream puts Michelin LTX MS tires on it's Classic line. Other manufactures also install LT tires on the trailers. It's about the bottom line dollar. RV MFG's need to keep cost down to make their units more attractive as well as increase their profit margins. Only a few MFG's put higher quality ST tires or LT tires on their trailers. The rest just put the cheapest ST they can find. It has nothing to do with ST only. It's all about keeping prices down.
Same with everything else in RV's.
Most RV buyers are weekenders and seasonal. Their RV's sit idle for months on end during the fall to spring. They don't want high payments when it's just sitting there. Thus the low price coupled with 12-15 yr financing. It's all about making them affordable. Bang for the buck scenario.

IC2 · 10-29-2015, 11:50 AM

When we were trailer hunting I asked a dealer how much extra good tires would cost. He ran the numbers, did our price negotiations with this as a non-negotiable adder. They were a $1500 and change adder.

Today, a China Bomb in the normal 235-85x16" size is $94 each at 35 pounds . A Michelin XPS Rib is $238 each at 56 pounds each plus shipping and installation. A Goodyear G614 is in the $350 price range plus shipping and installation each. You can bet that an RV manufacturer is trying to maximize his profit as I can assure you that he buys the Bombs cheaper by the container load then $94/each.

(I really like my General Grabber LRR's at $190 each including shipping and installation plus my TPMS tells me how well they are doing temp and pressurewise)

Tireman9 · 10-29-2015, 10:14 PM

Quote:

Originally Posted by justafordguy

I always thought that the LT tires were rated for a lower weight because they are rated for a higher speed. At 65 MPH I bet they have a higher cap than the ST.

As a tire engineer, I know of no standard that allows for an increase in load capacity of an LT tire by running at 65 mph.

Tireman9 · 10-29-2015, 10:41 PM

Quote:

Originally Posted by kedanie

How 'bout it FastEagle? Don't want to answer a direct question!

Seems that you post a bunch of junk from every source you can find, but never state your real opinion. Sounds very much like the corporate line. Got to talk to the lawyers before making a stand on the subject.

Come on Calvin, tell us what you really think. Not what the lawyers let you say!

Keith

I will let Fast Eagle provide his own answer, but here is mine.

As a tire design engineer I would say that while your example in post $37 may meet the federal regulations and there are probably even some RV sold with that combination IMO that is not a proper or adequate choice of tires as the reserve load of 42 pounds per tire is not really sufficient to allow for load position variation and in all probability will result in one or more tires being in an overload condition even when the tires are inflated to the specification established by the RV company.
IMO the RV company knows this but is betting that the tire will survive long enough to get past the short warranty period, if there even is one, so a failure that is likely will not cost the RV company any money.

That a clear enough answer?