RV Tires - Page 4 - Fiberglass RV


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Old 01-03-2013, 10:11 AM   #43
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Ok, minimum pressure as a synonym for pressure threshold or level which supports a certain load I understand. I believe what evolves from this logic is that the minimum pressure which supports the next listed load must be the maximum pressure "necessary" to support the previously-listed. One might cavil with this use of the term "maximum necessary" as nonsense, which it is except that it makes clear the existence of a range of pressure values adequate to carry a certain load. The increment is such that the upper limit is not significantly more pressure than needed to support the load. I do not see how the terms minimum pressure and adequate pressure are distinct in this context.

You may be right about the rubber rods, Brian.

jack
Goodyear and Carlisle both say that the minimum pressure rating is the point below which the tire must be demounted for inspection if used for towing.
I think that illustrates the distinction between minimum and adequate.
That is not to say that something short of maximum isn't adequate for safe towing.
Optimum then becomes the bone of contention as it relates to efficiency.

Subjective observation;
I think it odd that so many folks are aghast at the idea of using a merely adequate TV, but not bothered with the prospect of minimums in this case.
Ahh... but then it all comes down to informed personal responsibility, which is as it should be.
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Old 01-03-2013, 11:35 AM   #44
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"Optimum" tire pressure appears to be the one that's difficult to tabulate. Is there a clue to that in the fine print?

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Old 01-03-2013, 12:12 PM   #45
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It's in the fine print of the vehicle placard- of course, that assumes one's also using the tire recommended thereon.

Lacking that information, one might start with the tire maker's recommended minimum for the load and find a particular rig's "sweet spot" between that number and the maximum allowed for the tire.

Francesca

P.S.

And don't forget to derate if using LT instead of ST tires on a trailer...

F.
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Old 01-03-2013, 12:41 PM   #46
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"Optimum" tire pressure appears to be the one that's difficult to tabulate. Is there a clue to that in the fine print?

jack
With over thirty years without a flat or a tire failure, I have a pretty good handle on what optimum is for my trailers.
Bottom line, this subject is self referential and infinitely iterive....

It is..."The Song That Never Ends"
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Old 01-03-2013, 02:07 PM   #47
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Originally Posted by Francesca Knowles View Post
P.S.

And don't forget to derate if using LT instead of ST tires on a trailer...

F.
Actually, you don't need to derate LT tires used on a trailer, only P-Metrics or R-metrics (ETRTO passenger sizes) used on truck or trailer need be derated.

(Also mentioned by Brian back in post #4)

The 1.1 derating for passenger tires used on light trucks and trailers is also mentioned in the Federal Motor Vehicle standards which means that it's more than just an industry practice.
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Old 01-03-2013, 02:20 PM   #48
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Old 01-03-2013, 02:27 PM   #49
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Actually, you don't need to derate LT tires used on a trailer, only P-Metrics or R-metrics (ETRTO passenger sizes) used on truck or trailer need be derated.

(Also mentioned by Brian back in post #4)

The 1.1 derating for passenger tires used on light trucks and trailers is also mentioned in the Federal Motor Vehicle standards which means that it's more than just an industry practice.
LT tires more closely resemble P-tires than they do ST's, and for that reason
while not required, derating of LT's when used on trailers is considered a "best practice" by many manufacturers and is so stated in their literature.

The below is taken from the same Goodyear bulletin I linked to earlier:

Quote:
Light Truck (“LT”) designated tires do not have the same load carrying capacity as ST tires. If LT tires are used on trailer applications, the inflation pressure and/or tire sizing will need to be adjusted to match the load carrying requirements of the trailer.
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Old 01-03-2013, 06:42 PM   #50
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Originally Posted by Francesca Knowles View Post
The below is taken from the same Goodyear bulletin I linked to earlier:

Quote:
• Light Truck (“LT”) designated tires do not have the same load carrying capacity as ST tires. If LT tires are used on trailer applications, the inflation pressure and/or tire sizing will need to be adjusted to match the load carrying requirements of the trailer.
This does not mean that derating is required or even appropriate; it means that the load/inflation table or other source of capacity rating for an LT tire may show a lower capacity than that of an ST tire in the same size. It means what it says, which is only that appropriate inflation pressure and size must be chosen.

Goodyear example:
ST225/75R15 LR-D 2150 lb @ 50 psi. (Marathon)
LT235/75R15 LR-D 1895 lb @ 50 psi. (Wrangler SilentArmour)
Yes, I know these are not the same size; I didn't find an ST tire and an LT tire in exactly the same size from the same manufacturer.

I also noticed this:
P235/75R15 XL 2183 lb (1985 lb after derating by 1.1) @ 50 psi (Wrangler SilentArmour)
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Old 01-03-2013, 09:57 PM   #51
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Originally Posted by floyd View Post
With over thirty years without a flat or a tire failure, I have a pretty good handle on what optimum is for my trailers.
Bottom line, this subject is self referential and infinitely iterive....

It is..."The Song That Never Ends"
and to think it all started with a post by Jim what was I suspect meant to be a simple guideline as to tire inflation.
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Old 01-04-2013, 07:32 AM   #52
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I've posted a variation of the attached chart but I redid it this morning for a 205/75R15 size since I could find all three types of tires in the Tire and Rim tables.

Each tire's load is calculated based on different formulas. ST and LT tires carry less load per PSI but larger loads because they can be inflated to higher pressures (than a P-Metric).

LT and ST tires have stronger carcasses because it's basically the inflation pressure that is carrying the load and the tire strength is largely determined by the maximum pressure it could be inflated to. (When I started out I was taught to use steam boiler formulas to calaculate the stress in the sidewall, break it down to the tension in the individual sidewall cords then pick the size, material and cord density that would carry the tensile load. TMI, I know...)

Beyond the carcass strength, the tires have different performance expectations, particularly steering. Trailer tires don't do much steering but P-Metric and LT tires are expected to steer so in that sense lateral stiffnesses are similar between them (mostly a belt/tread area issue).

The Goodyear resource sited, nor any of the further references I followed, don't suggest industry recommendation is to derate LT tires for trailer use.
Attached Files
File Type: pdf P vs LT vs ST Load Capacity.pdf (33.2 KB, 13 views)
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Old 01-04-2013, 11:09 AM   #53
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Originally Posted by Steve L. View Post

The Goodyear resource sited, nor any of the further references I followed, don't suggest industry recommendation is to derate LT tires for trailer use.
"Industry recommendation" is to use trailer tires for towing since they're designed mainly to track efficiently rather than to function as drive/steer tires and ride softeners like most LT's/P's must do. I myself think that putting LT's on a trailer makes about as much sense as putting fatslicks on the steer axle of a Formula one car.

I only brought this up since some folks do use LT's on trailers and may be unaware that since that's not what they're designed for some adjustment should be made/allowed for.

Since the argument seems to be about my use of the word "derating" when referring to Goodyear's assertion re. LT's, what word would work for y'all???

Here it is again:
Quote:
• Light Truck (“LT”) designated tires do not have the same load carrying capacity as ST tires. If LT tires are used on trailer applications, the inflation pressure and/or tire sizing will need to be adjusted to match the load carrying requirements of the trailer.
Link to source

Would "upsizing" work???

Francesca
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Old 01-04-2013, 12:02 PM   #54
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Would "upsizing" work???
Francesca
Not for me, anyhow, since upsizing isn't always required if there is enough load capacity left in the LT size. If the goal were some sort of heads-up for the new owner, I suppose that I'd offer that ST and LT tires are not a one-for-one swap in terms of needed inflation pressures and be sure to check the appropriate load tables. I'm not clear that there is one word that describes the concept.
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Old 01-04-2013, 03:13 PM   #55
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Originally Posted by Francesca Knowles View Post
"Industry recommendation" is to use trailer tires for towing since they're designed mainly to track efficiently rather than to function as drive/steer tires and ride softeners like most LT's/P's must do. I myself think that putting LT's on a trailer makes about as much sense as putting fatslicks on the steer axle of a Formula one car.
The steer axle of a Formula one car does have fat slicks - just smaller than in the rear due to the much lower weight, aerodynamic downforce, and dynamically transferred load on the front.

Goodyear apparently disagrees about the use of LT tires on trailers, since they recommend their G614 RST model specifically for "5th Wheel and Travel Trailer" applications.

Quote:
Originally Posted by Francesca Knowles View Post
I only brought this up since some folks do use LT's on trailers and may be unaware that since that's not what they're designed for some adjustment should be made/allowed for.
That's a good point, but rather than decreeing a single - and invalid - solution, why not do exactly what Goodyear does in that quoted statement...

Quote:
Originally Posted by Francesca Knowles View Post
Since the argument seems to be about my use of the word "derating" when referring to Goodyear's assertion re. LT's, what word would work for y'all???
...
Would "upsizing" work???
"Derating" is certainly incorrect in this case - it has a specific meaning which does not apply.
How about "choosing a suitable size and pressure for the tire type?"
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Old 01-04-2013, 03:23 PM   #56
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Originally Posted by Brian B-P View Post

Goodyear apparently disagrees about the use of LT tires on trailers, since they recommend their G614 RST model specifically for "5th Wheel and Travel Trailer" applications.
"RST" and "LT" are not interchangeable terms, and I don't see any LT tires on the page you linked to, so....?

What else you got?

Francesca
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