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Monday, March 2, 2015

Much confusion on some RV forums

 Much confusion on some RV forums on de-rating tire load, Passenger tires and Michelin LTX tires

As some of you know I make an effort to follow posts on a number of different RV forums. Some forums are RV brand specific, some cater to the members of a specific "club" others cover the gamut of Class-A motorhomes to pop-up trailers and even slide-in truck campers. One thing they all have in common are posts from RV owners with questions about tires. Sometimes the questions receive correct and informative answers. BUT I am sorry to say that all to often incorrect information spreads like a virus with posts from some well meaning but uninformed RV owners .

I seem to be posting similar corrections over an over on a couple of topics so felt it better use of my time to cover a couple of points of confusion here in the hopes that if you find this helpful you can provide links to these facts when you come across confusion and incorrect information on other forums.

First off lets cover the use of PASSENGER tires on SUVs, Light Trucks, Multi-purpose vehicles and trailers.  "Passenger type" tires have a size designation that starts with the letter P such as P235/75R15 105T. There are also "Metric" size tires designed primarily for passenger car application made in other countries and sometimes imported into the USA. This tire might have a size 205/60R15 but to help you confirm that this is a standard tire intended for passenger car application you should be able to find the tire has an inflation level of 240Kpa or 35 or 36 psi.
If you apply a passenger type tire to something other than a passenger car (this means an SUV, Light Truck, Multi-purpose vehicle or trailer) you MUST de-rate the load capacity by dividing by 1.10.

Example a P235/75R15 105T would show Max of 2,028# at 35 psi on the tire sidewall. If used on a trailer etc the load capacity is really 2028/1.1 or  1,844# MAX when inflated to 35 psi cold.

The De-rating only applies to Passenger type tires even though many people are incorrectly posting that this De-rating also applies to LT tires.  I believe I have found the reason for this confusion and will cover that topic in a moment.

LIGHT TRUCK type tires have a size designation such as LT235/75R15  LR-E
Note the "LT before the numbers and the "LR" or words "Load Range" followed by letters such as "C", "D" or "E". The Load Range letters replaced "Ply Rating" numbers 6, 8 and 10 decades ago.

In Europe and other countries, they have "Commercial" tires but do not have "LT" as the first part of their size designation. Some may have LT at the end. The inflation may range upwards to the 85 psi level or 600 Kpa.

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One recent series of posts involve converting from ST type tires to LT type of even P type tires. All to often only partially correct information is passed on so confusion gets compounded.

Here is an example
Michelin lists 42 part numbers for the line called "LTX® M/S2". This line of tires is specifically aimed for "SUV/Crossover, Light Truck application" and that's where the problems start. With "LT" as part of the design name some think these tires are just like Light Truck tires. Others have managed to read the fine print that says "* Passenger sizes used in Light Truck/SUV applications have reduced load capacity. This will differ from the maximum load branded on the tire sidewall." and incorrectly assumed that all the tires in the LXT line are LT tires so these folks start passing around information that LT tires should be de-rated.


The majority of the 42 items in the Michelin list are P type tires with a few LT type and even a few Euro-Metric passenger type tires mixed in. The listing even includes two "Xl or Xtra Load passenger type tires.

The LT tires have a speed symbol of "R" and all appear to be Load Range "E" or 10 ply rating for those that prefer to use the out of  date nomenclature.


"LTX MS2" is a design name much as the Bridgestone Dueler H/T 684 or Goodyear Wranger SA and I would never accuse Michelin of intentionally misleading people into thinking of this line of tires to be LT tires but the reality is that even when I engage some individuals about the incorrect and confusing information they are posting I get responses such as "I think my tires are Michelin LTX M/S2 P235/75R15 Load Range D (2150lbs)"  Here we clearly have a passenger tire but the writer has applied Light Truck "Load Range D" and the load capacity of 2,150 is for a Light Truck tire in Dual application at 65 psi. I have to wonder what inflation he is actually running.


The confusion is not limited to Michelin LXT. Earlier today I had a poster tell me that he checked the load capacity of a Mastercraft HSX which is a European passenger tire with American "XL" designation (don't ask, I have no idea how that is legal. Probably just a confused Customer Service rep I talked with)
and a Michelin Primacy of "the same size" and he claimed the load capacity was different. I pointed out that the Primacy is a Standard Load (35 psi rated) passenger tire so it would of course have a lower rating that an Xtra Load tire at 50 psi.

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Much of this confusion can be avoided if people would simply take the minute or so to read the tire size information on their tires and write it down.




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Sunday, March 1, 2015

Why are tires such a hot topic on RV Forums? - Quick question; Quick answer

Here is part of a post I just read...
" I do not understand why there is such an issue on this forum on tires? Yes many RVs come with junk tires and you need to replace them with a better tire as I did.
A few people post the facts but as the saying goes you can lead a horse to water, but you can't make it take a drink."

My reply:
I think the reason tires are a hot topic on each of the RV forums I try and monitor is that most people only have their experience with their personal car as a background. With their car they basically have learned, incorrectly, that they can usually get away with doing zero maintenance i.e. checking load or inflation, and they still never seem to have tire problems.

The reality is that car companies have teams of engineers working on just tires & wheels. The tires have dozens of performance requirements that the tire company must meet before they can sell tires to the car company. In the RV world I think the only requirement from the RV assembler is low cost.

Another thing is that cars specify inflation that gives 15% to 25% safety margin for load (with a few notable exceptions such as Ford Explorer of the 90's with what I think was a 1 psi margin.) This means people can go from oil change to oil change and not have to check their tires and just trusting the service station will adjust the air every few months.

When someone purchases their first RV there is lots to learn and tires are low on their list since they never had problems before so since everyone knows tires are just round black things that cost too much why bother to learn how to make them last?

Then they have a failure or see someone with an RV have a failure and suddenly they learn they need to pay attention. What they get is "Campfire Experts" providing partially correct to completely wrong information.

Then they discover RV forums. So they ask the same questions and with a few notable exceptions they get the same answers they got around the campfire.

I know of only two actual tire engineers lurking on various RV forums. There are few others with what appears to be solid engineering background and sadly a lot of self proclaimed "Experts" who base their answers on their personal experience rather than the Science of tire mechanics.





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