When and Why some need to "De-Rate" their tires

Had a question from a TT owner about De-rating the tire load. He wanted to use P-Type tires on his trailer.

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 environment 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 the tire is 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 or so every few hours. The test track however has no sharp turns or pot holes.

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. 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% or less of the time the tire is asked to run anywhere 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 and occasionally 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. 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. This lower capacity is one reason there is no De-Rating for LT tires when placed on trailers.

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

Hope this helps you understand why the De-Rating formula for P Type tires when placed in "mult-purpose or trailer service is  Load Max New = (Load Max on tire)/1.10


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Real life inflation calculations for Michelin XZA2 Energy on 3 axle Class-A RV

Had a post from the owner with the following facts presented.

Michelin XZA2 Energy 295/80R x 22.5 Load range H
Steer axle scale weight is 14200 (tire placard shows 110 PSI closest)
Drive axle scale weight is 17360 (tire placard shows 75 PSI dual closest)
Tag axle scale weight is 7800 (tire placard shows 75 PSI single closest)
Total gross weight is 39360 per scale

MFG front GAWR is 14600# max
Intermediate GAWR is 20000# max
Tag GAWR is 10000# max
GVWR is 44600# max


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OK as an Engineer I always feel Facts & Data are our friends.
Using info on Michelin web site, and assuming perfect 50/50 side to side balance (unrealistic)  I come up with F@110 psi which gives you a 2.8% Margin on load capacity
Drive & Tag 75 psi  for a 8.9% and 27% margin

However if we assume a 45/55% split which is more reasonable
F @ 120 we get a 0.3% margin
Drive at 80 psi gives a 4.8% margin
Tag @ 75 gives a 20% margin

Note I suggest that in a perfect world that once you know your actual individual axle end load, you use the tables to find the minimum cold inflation and then add 10% to that number for your morning cold "set" pressure.

For those smart enough to run TPMS, I would set the warning level to be the minimum cold inflation and not just the standard 20% below the morning "set" pressure. With the proper TPM system that gives not just the low pressure warning but also a rapid air loss warning you have a good chance of coming to a safe stop before damaging the tire or its mate from driving when officially flat i.e. having lost 20% from the minimum needed based on actual tire loading.

Hope this helps.


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Repeat failures on Airstream trailer. Why?

Saw this question of an RV forum on trailers

"We just acquired 16" Sendel wheels and a set of XPS Ribs for our 2009 28' International to help deal with our tendency to have a front left wheel blowout. We have been through a set of Goodyear Marathons and a set of Towmarks (e-rated, 80 psi cold max). I have not had access to individual scales, but I have reason to believe that the left front is carrying more weight because we have 130 lbs of extra battery forward in the coach on the left side. We will be addressing that by replacing all 260 lbs of battery with 84 lbs of lithiums. This will result in an overall tongue weight reduction of close to 100+, but most importantly, it will take at least 50 pounds off the left front, too.

The question is proper inflation. Our Airstream weighs 5960 on the axles when the WD hitch is engaged. Our tire dealer divided by four, which of course, is not accurate, as we can guess that there is much more weight on that particular wheel. But I'm getting input to inflate to less than 80psi when checking on a cool or cold early morning based on the reasoning that the 80psi recommendation is for cold pressure at around 68F. The theory is that as the ambient pressure rises, that same air will increase in pressure, with the coach just sitting there. Therefore, if one inflated to the max 80psi at 68F, that would be way over a "cold" tire pressure taken later in the day when the temp hits 100F. This is what they say all there 18 wheeler customers do when running on a day that they know will be considerably hotter later--and I've had the same advice from three different tire dealers.

Would our resident tire expert care to "weigh in" on this subject? It would be most appreciated.

You are correct that simply dividing by 4 does not give you the actual load on each tire.

The worksheet on THIS page will show you how to do the math.

I have a blog post  with an example how to get your weights for free. Some folks have had success in contacting their State Police and asking if they could go to a location where they are checking trucks.

But setting aside the need to confirm your load balance.
I suggest that to lower the internal structural forces that are trying to tear the belts apart, you need to set your "cold" inflation to the max on the tire sidewall. The "why" of this was covered in this post. My position was developed based on the work done by Turner & Ford as seen in the book "Mechanics of Pneumatic tires" pub by U.S. Department of Transportation, National Highway Traffic Safety Administration, 1981 .

Now to your specific question on how to set the "cold" pressure.

I covered pressure vs temperature in THIS post  and when you review that post you will see that the pressure in a tire will change by about 2% for each 10°F change so in tour example of 68 vs 100F I would expect about 6% change. The intention of the "Set the pressure in the morning" is to not use a warm (higher than ambient) tire pressure when it has been in the Sun.

I think the reality is that if you check your tires in early AM before you start your travels and before the tire is in full Sunlight, you will be fine for the rest of that day, no matter what the temperature is over the rest of the day.
Tires generate heat internally (not on their surface as many think). This heat is transferred to the moving air around the tire and wheel until a steady state is reached where the heat being generated equals the heat flowing away from the tire.



Hope this helps



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Understanding Tire Failure

Yes, when a tire fails there is almost always evidence left behind that points to the Root Cause. I have shown this in tire "autopsy" posts on my blog.

Some "causes" are easy to see or understand:
Punctures, cuts and external damage from hitting potholes or other "road trash" can lead to leaking air (fast or slow) or in some cases broken body cord (see my blog post on sidewall bulges).

"Blowouts" with sidewall flex failures are most likely the result of air leak, not just low air pressure due to poor maintenance. The leak could be caused by puncture, cut, leaking valve, cracked wheel or the like.


Other failure modes are harder to detect, such as:

Belt separations come from long-term heating of the tire belt/body interface. This heating comes from a combination of insufficient air pressure for the load and excessive speed.

Now while separations may be harder to detect, many can be found if a little effort is made by the person doing the tire inspection.

I previously published a video showing one way to "measure" a defect.

Sometimes you don't need to "spin" or rotate the tire but just get a good look at it and roll it across the floor for a full revolution as one part may look OK while another portion clearly would only look straight to a drunk.

If any of your tires look like the above, I would bet $$ that they had a belt separation similar to what is seen here.

Driving on a tire in this condition is simply asking for trouble as it could come apart when you least expect it as you drive down the road.

As we move out of Winter and you "DE-Winterize" your RV, especially trailers, Please take the time to do a thorough tire inspection.  Spin or off-the-vehicle inspection is much better than a stationary on-the-vehicle "Look At".




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Minimum or Maximum inflation or Load...I'm confused

Read a post of an RV forum from a motorhome owner who was a bit confused by the wording on his tires about Max Inflation and max Load when some people are telling him to learn the minimum inflation. He said
"Uhh.... So why does it say MAX PSI in front of it???  MAX means minimum???"

My answer:
Yes this is confusing.  The intent of the information is to let you know that when you inflate the tire to the pressure associated with the highest level for the Load Range it is rated to carry the stated load and no more.
Remember that "Load Range" is a replacement for the outmoded "Ply Rating"

When you look at a Load/Inflation table you will see a series of inflations in 5 psi increments. You will also see a series of load capacities.

I'm going to use a LT size but the same process would be used by other Motorhome owners.

Size LT225/75R16
Single  1790#@45  1940@50   2060@55  2190@60  2335@65 
2440@70  2560@75  2680@ 80  3000@95

Now the other bit of important information in the table is the Load Range limits for this size tire.  LR-C is 50psi  LR-D is 65  LR-E is 80 and LR-F is 95

If you had this size tire and it was a LR-E tire it would say 2680# at 80 psi Max or some such

Note some tire companies use the wording like 2680# Max at 80 psi Cold or some variation thereof.


The inflations in the table can be considered a Minimum when you start by looking at your measured load.  In the above example if you measured 2500# on the heavier front tire you would select the inflation from the table associated with at least 2500#  which means 2560# and you would then know that the Minimum cold inflation you should ever run was 75 psi.

Finally we know that pressure varies as the ambient temperature changes. This change is about 2% for every 10°F increment so to avoid having to chase around looking for a service station every time the temperature drops 10°F, I suggest you add 10% to the required inflation level which in the case of this example would be 7.5 psi

I wouldn't worry about getting the inflation to the exact 1/2 psi just know that if you add 7 or 8 psi to your minimum of 75psi you are good to go. You also don't have to worry about adding air if it gets cold and the inflation drops to 79psi as you have a nice margin built in.

Over time all tires loose air pressure so eventually your pressure will drop to the point that your day to day pressure variation will result in you having only 75 psi in the tire. I would not wait till your margin has been used up but would have added a bit of air once I got down to 77psi or so and bumped it back up to the 82/83 psi level.


Long explanation but I wanted to give you the background so you would understand why I say

1. Weigh the RV
2. Select the minimum cold inflation for your tires based on the table minimum inflation
3. Add 10% margin to that inflation to learn your "cold set pressure"
4. Inflate all tires to the inflation for the heaviest loaded tire on that axle. (all tires on each axle should have the same inflation)
5. Check your inflation in the morning before you travel more than 1/2 mile and before the tires are warmed by sunlight or monitor the inflation all the time with a TPMS.


Hope this helps.


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New Speed info for trailer tires

Learned about some interesting new developments that involve speed limit ratings for ST type tires as used on a majority of RV trailers.

On Nov 24 and updated Dec 30 the US Dept of Commerce issued preliminary guidelines as they pertain to imported tires. This involves what are called "countervailing duties" (CVD) These anti-subsidy duties, are trade import duties imposed under World Trade Organization (WTO) rules to neutralize the negative effects of subsidies from foreign governments that benefit the production of goods from foreign companies.

Now without going into all the legal details this means that some import tires will have a fee added to their price unless they receive an exemption. The fee can be in the 12 to 81% range, so these are meaningful.

This new fee will apply to P and LT type tires primarily made in China. ST type tires will receive an exemption but in addition to the ST as part of their size designation the tires must have either
“For Trailer Service Only” or “For Trailer Use Only” molded on the tire sidewall.

Now the Nov statement indicated that ST type tires must also have a Max Speed in MPH or a Speed Symbol letter also molded on the tires. I thought this was great news as it would then become obvious to RV owners that they needed to limit their towing speed.

However the Dec "clarification" excluded the speed limit requirement.

BUT

I have also learned that a few tire importers have gone ahead and will probably be marking their ST type tires with either "Max Speed xx MPH" or with the Speed Rating letter symbol. The information I have is that the symbol would probably be "M" (81 MPH)  or possibly an "N" (87 MPH) and that tires might be available by early this Summer with these speed ratings.

Of course if a tire company is moving from a tire rated for 65 mph as current ST type tires are, to a higher speed they will need to make some changes in the actual tire construction.

While the final rule has not been published, I wanted to let people know that there may be some short term confusion as both tires, literature and web pages need to be updated to reflect what the tire capabilities really are.

In my opinion the information actually on the tire takes presidence. So unless you can get something in writing about your specific set of tires that states otherwise, the lowest speed you find on your tires is what your tires are rated for. For normal ST type tires this means 65 mph Max even if the number is not on your tires..  Not occasionally but ever.

I will publish an update when I learn more.


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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.

======
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.

======================

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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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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