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Old 03-02-2015, 04:30 PM   #1
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Much confusion on de-rating tire load

There is much confusion on some RV forums on de-rating tire load for Passenger tires and Michelin LTX tires and even LT type tires so I did an entire Blog post on the topic. Here is part of the information. If this topic is of interest to you then you might want to read the whole post. If not then you can move on to another thread. Have a nice day and safe travels.

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.

More examples and info on the blog if you are interested.
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Old 03-02-2015, 08:00 PM   #2
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Can you explain why anyone would need to de-rate the weight rating just because a tire is used on a trailer? I don't understand why if the rating is 2510 lbs on one vehicle why wouldn't it be the same on another at the same pressure?
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Old 03-04-2015, 07:51 PM   #3
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Quote:
Originally Posted by MnTom View Post
Can you explain why anyone would need to de-rate the weight rating just because a tire is used on a trailer? I don't understand why if the rating is 2510 lbs on one vehicle why wouldn't it be the same on another at the same pressure?
The quick and simple answer is that the service or environment seen by cars is not the same as the service seen by trailers. You don't design a home for northern Canada the same as a home in Mexico as the enviroment is significantly different.

Now if you don't like the simple answer here is a longer approximation.

When tires are designed there are certain things that can be expected. First off they are expected to pass all the regulatory tests. Some of these tests will be at or near the tire's max rated load. None of these tests are for 60,000 miles but may only last 2,000 miles at the max when running in 100F temperature on a curved drum (which is much harder than running on flat surface) and never be allowed to cool down as it is also run at say 50 mph. Outdoor testing may be at 100% load and 30,000 miles but the test car does come to a stop for 30 min every few hours. The test track however has no sharp turns or pot holed.

If a tire can survive the above in all probability it can run 60,000 on regular highways but occasionally at higher speeds and with occasional pot holes etc but we know that there is a "reserve load" of say 10% to 20% between the tire's load capacity at the placard inflation in normal car operation and there may only be 10% of the time the tire is asked to run at or near it's max load.

Now if that same tire is put on a small pick up or SUV the % of the time it may be required to run at or near it's max load may be 50 % or more.
It was learned many years ago with the introduction of station wagons that if we were to avoid premature tire failures something needed to be done. At the time there were only passenger and true truck tires so the only option was to reduce the load allowed when the passenger tires were placed in station wagon service. Later on, when SUVs were invented they replaced station wagons in the market place so the definition of "Multi-purpose" vehicles was coined and since pick up trucks were "civilized" to be more like cars and less like trucks passenger type tires were applied to these trucks. Again the percentage of time the tire was asked to operate at its max load capability was much greater than when the same tire was placed on a car.
The "de-rating" was established at a factor of 1.10

Now in reality early RV trailers were single axle but would spend an even greater portion of the time at their max load but to counter that their service life was much less so the 1.10 factor was applied to passenger tires in trailer service.

If you look at the rated load capacity of a true LT type tire vs a P type at the same inflation you will see that the LT is rated at about 25% lower load capacity. This is because vehicles that are expected to be used as trucks ie a major part of their life fully loaded the reality of the physics involved dictates that if you want to get 40 to 60,000 mile life you must reduce the load.

It may help if you think of tire life as a formula in the form of

LIFE = 100,000 - (A x %of time at 100%load) -(B x % of time at 90% load) - (C x % of time at 80% of max load)-(D x %of time at speed greater than 50 mph)- (E x % of time at speed greater than 65 mph)- (F x number of 4" deep pot holes) etc

If you think of A, B C etc as large numbers such as 50,000 or 20,000 I think you can see that it is easy to "consume" the finite tire life of 100,000.

Now there is no such formula in reality as it would have hundreds for factors, each of which is almost impossible to establish without hundreds of thousands of dollars research.

Hopes this helps
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Old 03-05-2015, 11:18 AM   #4
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Yep, that does help. Thanks for the explanation.
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Old 03-06-2015, 09:16 AM   #5
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Lot of effort put in, thanks.
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