A ST/LT question

[jlb27537]

I read thru all this, and after looking at the Bridgestone Truck Tire link, it is interesting the ability of your load carrying increases as the speed lowers. So using that thinking, if a LT tire is rated @ 3042 lbs and it is speed rated @ 101 mph, then at 65 mph it's carrying capacity would be much higher than the posted 3042 lbs?

I realize the 3042 lbs is fact and I respect that, but....?

Another thing I notice, some owners are so concerned about their ST tires are at or below capacity, and MUST have a G or heavier tire on their trailer, but haul a heavy trailer with a SRW truck and think nothing of having the rear axle of their truck at close to 100%.

Is it because of the reserve in a LT tire and the fact that a ST tire has no reserve?

Just wondering?

Jim

[sambam]

That's how I understand the LT works Jim, but it's still unclear to me as to just how much reserve is there at 65 mph. I have a somewhat heavy trailer with 7k lb. axles. That means I need at least 3500 lbs. a tire. I'm still not sure if, at 60-65 mph, there is the reserve I need. Lots of guys on this forum have luck with LT tires, and they'll probably work ok, but a G rated tire, at least in my case, seems like more of a sure thing.

[pineranch]

Factor the 20% lawyer's reserve and you are well within OEM limits.
JMHO
Mike

[Art-n-Marge]

And then from the lawyer's respective you've priced a pretty cheap lawyer at 20%. My guess is that they are more like 50%... pineranch, that was a clever analogy.

As for tires, I'd save calculating the reserve for the engineers. As a consumer, I can only go with the posted numbers for tires. After all, I wonder if the 110mph rating goes for a trailer application. I'll bet any LT tire manufacturer would say no and that the speed rating would be for light truck applications and not so that an owner can tow their huge, heavy trailer at 110 mph.

[TLightning]

quote:Originally posted by Art-n-Marge

... I'd save calculating the reserve for the engineers. As a consumer, I can only go with the posted numbers for tires. After all, I wonder if the 110mph rating goes for a trailer application. I'll bet any LT tire manufacturer would say no and that the speed rating would be for light truck applications and not so that an owner can tow their huge, heavy trailer at 110 mph...

Same here. IMHO, some folks over think the tire issue.

[Art-n-Marge]

LOL, Tom, I almost used those same words. Just get tires that meet or exceed the ratings of your axles, drive under 65 mph when towing and you'll probably satisfy all the concern for trailer towing and tire ratings. Anything other thinking means you are trying too hard and are only seeking to go beyond and asking for trouble.

[rohrmann]

The G rated tires are actually ST tires, but due to their design are rated LT and speed rated 75 mph. Note, the LT rating and only 75 mph, not the 101 mph like the referenced LT tire. The other reason we have gone to G rated tires is there are no E rated tires capable of safely hauling these heavier rigs and proven to not blow out at the frequency the E rated tires do. And yes, they do cost quite a bit more, but that is just like paying insurance.

[jhudson]

quote:Originally posted by jlb27537

I read thru all this, and after looking at the Bridgestone Truck Tire link, it is interesting the ability of your load carrying increases as the speed lowers. So using that thinking, if a LT tire is rated @ 3042 lbs and it is speed rated @ 101 mph, then at 65 mph it's carrying capacity would be much higher than the posted 3042 lbs?

I realize the 3042 lbs is fact and I respect that, but....?

Another thing I notice, some owners are so concerned about their ST tires are at or below capacity, and MUST have a G or heavier tire on their trailer, but haul a heavy trailer with a SRW truck and think nothing of having the rear axle of their truck at close to 100%.
Is it because of the reserve in a LT tire and the fact that a ST tire has no reserve?

Just wondering?

Jim

Well I couldn't resist commenting on this since I am one of those SRW guys....
First I do not plan on replacing my Marathons yet. It most likely will be 2-3 years out. I plan on getting some mileage out of them and according to my figures under their load ratings. I travel at 60 and monitor my air pressure. Am I worried about having Marathons on? No, not at all. I ran 55000 miles on 2 sets of Marathons with my SOB over a 9 year span with little issues. Last set from China, one tire lost pressure(nail), TPMS reported and we had to replace because nail damaged the sidewall. So where am I going with this....reason I am looking...I am one who likes to do his homework. I like to know what my options are and I feel a "G" rated tire would be a better choice down the road.

Now to the SRW, the Axle is rated in my case at 7050 pounds. This is because of the tires. Now I will say that I am close to my LT tire rating of 7050 pounds, I am 430 pounds under and just like my trailer with the Mararhons, both truck and trailer have almost the same margin. I am perfectly comfortable with both for now.

I did not want a DRW at this time and thereby limited to 7050 axle rating. There is nothing I can do about that. However, the tires on the trailer can be upgraded and will be when the time is right.

Anyway, this is my thinking, It would be silly to dump perfectly good tires at this time. When I do, they will be upgraded to a "G" rated one. I am not sure at this time what brand they will be as am I still doing my homework.

[TLightning]

To me, the speed ratings are irrelevant...nobody should be cruising these rigs above 65 mph...JMHO, of course.

[DQDick]

Agreed. I never tow over 63. But have been passed by many a 5th wheel going over 70 pulling a boat or other trailer behind the 5er.

[steelpony5555]

Here is the info a guy compiled from the government site on tire testing....

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.

[bncinwv]

And, with all that said, when you get to the point where you experience tire failure (been there, done that) anything you read and supposedly believe(??) will be irrelevant. We have been on 614's on the last two rigs (Montana's heaviest rigs), for over three years now, without a problem (knock on wood and pray that I have not jinxed myself). Therefore, based on this personal experience, we will stay with them, until we can comfortably come up with an alternative (note to self: the G-rated Sailuns are intriguing!). It amazes me that we as owners contemplate a couple of hundred dollar savings on tires when they are carrying a major investment that typically is in the $50,000 - $100,000 range. I am now resigned to the fact that I have posted too much tonight regarding tires and will graciously step down off of the cardboard soapbox which is not rated to support my weight anyway!!!
Bingo

[Bigsky3625RE]

We upgraded to 17.5" wheels and GY G114 tires.

Why? The increased safety margin. Yes, we sold our 614's to a MOC member with rims at a very reasonable price.

Overkill? Maybe. Do I feel better as we travel? Absolutely!!!

[Irlpguy]

quote:Originally posted by jlb27537

I read thru all this, and after looking at the Bridgestone Truck Tire link, it is interesting the ability of your load carrying increases as the speed lowers. So using that thinking, if a LT tire is rated @ 3042 lbs and it is speed rated @ 101 mph, then at 65 mph it's carrying capacity would be much higher than the posted 3042 lbs?

I realize the 3042 lbs is fact and I respect that, but....?

Another thing I notice, some owners are so concerned about their ST tires are at or below capacity, and MUST have a G or heavier tire on their trailer, but haul a heavy trailer with a SRW truck and think nothing of having the rear axle of their truck at close to 100%.

Is it because of the reserve in a LT tire and the fact that a ST tire has no reserve?

Just wondering?

Jim

Here is what I think for what it's worth. The load capacity of a tire is not determined by it's speed rating. Ply rating, type of ply's and other tire characteristics are what are used to determine the load capacity. The load rating always refers to tire pressure there is never a reference to speed in that rating. Does that tell us something. While we can carry a heavier load at a very slow speed on any given tire, the DOT capacity does not increase or decrease at different speeds. Tire life will certainly be affected because of the slow compared to rapid changes in shape of the tire which creates heat the deadliest contributor to tire failure.

If you notice the load capacities of your truck are not based on the load capacity stamped on the tire, they are based on a percentage of that capacity at a pressure as posted on the door post. That is what "reserve capacity" is all about and is required on all passenger cars and light trucks.
So if you take the load rating of the tire at it's maximum capacity as stamped on the tire and determine the load rating from that you will seldom be over that limit. If you are under the GVWR of the truck you will not be putting undue load on either front or rear axle unless towing a RV with a very heavy pin weight and exceed the GVWR. Some do this and think all is fine because it "pulls fine". We all make choices.

There is NO reserve capacity built into an LT tire, the load ratings of passenger cars and light trucks are by law required to be no more than 94% of the load rating/pressure of the tire as posted on the door frame. They cannot use the DOT rating on the tire to determine the load of the vehicle, including passengers and cargo, if they did there would be no reserve capacity. I don't see why that concept alludes most folks and they continue to make the claim that the tire itself has reserve.

On the flip side a trailer manufacturer uses the full load rating of the tire to determine the load capacity of the trailer. In the case of my 3402RL Keystone has reduced the GAWR of my 7k axles to 6750 lbs because that is all the tire is capable of handling. From the factory if G rated tires were used the GAWR would be the full 7k on the sticker.
Unlike cars and trucks, RV's have no reserve capacity because the manufactures are not bound by the same rules as passenger cars and light trucks and can use the full DOT rating at maximum tire pressure of the tire for their load ratings.

Buy a tire that exceeds your axle capacity and expected load and you will have considerable less to worry about when hauling down the road.

[steelpony5555]

I think peoples are just over thinking this whole thing. If you got 7 axles you need the G rated tires to use your full weight rating. If you got 6k axles then you can do LT tires and be fine. Just don't put Marathons back on it lol lol.......

[mhs4771]

And if you have 8K axles and want to make full use of them, you need to go the next step above the 614s which in most cases means the 17.5 rims with 114s which are H rated (4805 Lbs)
That's the route we went.

[fauch]

The load capacity IS determined at a maximum speed. AND YES they are very relevantly correlated! AT 65 MPH you have used the load capacity of most ST tires (75mph with the 614)
This will ALWAYS be the reason there are SO MANY trailer tire blow-outs compared to vehicle tire failures.

[mlh]

I've read this forum almost from its start and been a member since 2004. I don't recall a single LT tire blowout but there has been dozens and dozens of ST tire blowouts. On my wife's and my last trip I saw a dozen campers with blown tires but not one truck. That tells me something. I try to learn from my and others mistakes. There will be no ST tires on my camper.
Lynwood

[Irlpguy]

quote:Originally posted by fauch

The load capacity IS determined at a maximum speed. AND YES they are very relevantly correlated! AT 65 MPH you have used the load capacity of most ST tires (75mph with the 614)
This will ALWAYS be the reason there are SO MANY trailer tire blow-outs compared to vehicle tire failures.

I have never read anywhere that the "Load Capacity" of either an LT or ST tire is reached at the speed rating of the tire. All tires I have seen are Load rated at their maximum recommended air pressure, with no reference to speed.

At 65 mph you have reached the speed rating of all ST tires that I know of. When these tires are loaded to maximum load capacity or even beyond and the speed is also exceeded then the tires life will be short, you can count on that. My Goodyear G614's are NOT ST tires, they are clearly stamped with LT235/85R16. They cannot be stamped with LT if they are an ST tire. Indeed they have a lower speed rating than the majority of LT tires but not all LT tires are rated at or near 100 mph.

There are many reasons we here on these forums have problems with tires, some who post say they have never been overloaded, under inflated or ever hit anything and still the tires are blowing out, that suggests no matter what you do to manage your tires if they are ST tires they are going to blow and that is simply not true.

If we as RV owners are so convinced there are significant problems with ST tires then we should be reporting all of these incidents to the authorities who can do something about the problem.

Millions of P and LT tires are replaced each year due to failure, the cause of which can be as varied as the tires themselves, but no one should suggest there are no LT or P tires blowing out even in the application for which they were intended. If you loaded your truck the way we load our RV's and subjected them to the same forces as we do on our RV tires we would see just as many problems with them. It is a different beast and different forces are in effect.

As in my other posts, I say do your own research on the tires you are considering buying and if you are comfortable with your choice then fill your boots, I did not want to spend big dollars to replace tires on my RV, however for my own peace of mind I have done that with G rated G614's. I will watch the pressure closely and will monitor them no less than I did my Marathons, these are far superior tires but that does not stop a bolt, steel pin or something penetrating one of my new tires, if not noticed it could cause as much or more damage as any Marathon, they are still just a tire.

[fauch]

MAX speed means just that. The other info links posted here describe the testing and pertinence of speed to load. And that DOES NOT just mean shorter life. Again, the load capacity IS directly affected by speed. look back at some of the info included in previous posts, not just our opinions.

Is your tire in fact a Light truck tire or is it the LT235/85R16RST? which is a trailer tire?
http://www.trailertiresandwheels.com...l-trailer-tire