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Monday, March 30, 2015

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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Tuesday, March 24, 2015

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


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

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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