What type of tire should you run

 

Ok, Lots of opinions and partially correct information here.  Lets go with the data and facts.

 

First  RV manufacturers are required by law to apply tire placards that state both the tire size/type and the MINIMUM inflation you should run.  This is the same type information you will find on your Smart car or your F350 dually.

 

Second  There are a number of "type" tires that the RV owner might run into:

 

P-Metric  These are primarily intended for Passenger car applications.  They can be used in other applications but their load capacity needs to be adjusted DOWN by 10%.  P-Metric are capable of sustained 85 mph or greater. P235/75R15 105S   is capable of 2028# @ 35 psi in car application. Special Note The load is reduced to 1843# at 35psi in Light Truck, Station Wagon, Van, Trailer or Bus operation.  The 105 is the Load Index and the "S" is the speed rating, in this case 112mph.  I do not find any information on Dual application of P-metric tires in Industry standards.

 

LT-metric  These are primarily intended for Light Truck application and most have a RADIAL construction as in LT235/75R15 104/101R  LRC.  This tire is rated at a Max of 106 mph and at 50 psi can carry 1985# single or 1820# when mounted in Dual application.  For reference at 35psi it can carry 1530# or 1390#. Special note on speed.  If a LT tire is operated above 65 mph it's cold inflation must be increased by 10psi but never above the tire Max.  If you cannot increase the inflation you Must decrease the load.

 

ST  These are Special Trailer tires and come in a more limited range of sizes.  Many ST tires are Not Radial but carry a "D" in their size indicating Diagonal or Bias construction.  An ST225/75R15 is rated at a Max of 65 mph and 2150# in Single application at 50 psi minimum.  I do not find any information on Dual application of ST tires in Industry standards.

 

High Flotation tires such as 31x10.5R15LT 109R LRC  are sometimes considered for the pickup tow vehicle.  This tire is rated at a Max of 106 mph and at 50 psi can carry 2270#. at 50 psi. It is 30.52"OD  while the LT235/75R15 is listed at 28.9" OD.  I do not find any information on Dual application of High Flotation tires in Industry standards.

 

Class-A RV TBR (Truck Bus Radial) such as 295/80R22/5 is rated at 7830/6940 Lbs Single/Dual at 125 psi.  For this application it is imperative that you check the tire manufacturer's data sheets for the details on your specific application and size.

 

Third and most important - ALL RV applications.  You should visit  http://www.tiresafety.com and look at the tire RV specific info under Size & Classifications on how to properly weigh your FULLY LOADED rig and how to calculate the individual tire loads.  Only with this information can you really know what Minimum size and inflation you MUST run.

Do not do this to your RV tires

 

Do not bleed down your RV tire pressure because you think it will “explode” if you exceed the PSI number molded on the tire sidewall.

Almost every week I see posts on one or more of the various RV forums where people say they limit the PSI in their tires. Sometimes they say why, as in “to prevent a blowout.” Sometimes they do not say why, but their reasoning behind this practice is clear to me.

It appears that many people incorrectly believe that exceeding the “PSI” number molded on the tire sidewall is going to result in tire failure and explosion.

It may help to let people know about some of the testing that tires undergo during the development phase.

First some background

The “Max Inflation” statement on your tire sidewall really is the inflation associated with the Maximum Load capacity. If you actually read your tire sidewall, you will see that is what it says. So the number of PSI is really the minimum needed to support the stated load.

It is important to understand that we are always talking about the “cold” inflation and not the inflation of a tire that is running down the highway or has been in direct sunlight or driven on in the prior couple of hours. “Cold” inflation does not mean the tire needs to be refrigerated. It means the tire is at the prevailing ambient (surrounding) temperature.

Tire pressure monitoring systems

With the introduction of aftermarket tire pressure monitoring systems (TPMS) that report both tire pressure and the temperature of the sensor, people are now being exposed to information they have no previous experience with. They see tire pressure rising as they drive down the highway. Some may see a 10% to 20% rise in pressure from the pressure they set their tires to just a half-hour earlier. This increase is a cause for concern, not because they had any working knowledge of tire operating temperature or pressure, but simply because the pressure was higher than they expected.

What not to do

A few people have decided that they need to bleed down the high pressure because they thought the pressure number of the tire sidewall was the maximum it could tolerate. Of course, the action of bleeding down the hot pressure was exactly the wrong thing to do, as that meant that the tire would no longer be operating at the pressures expected by the tire engineer when they originally designed and tested the tire specification.

Obviously, this raises the question of how much pressure a tire can tolerate before the owner should be concerned. Well, I am going to give you some numbers, but these are only examples. I cannot speak for every tire company’s process or specifications. But if we start with a few guidelines, I think you can get comfortable and, hopefully, you will believe that tire design engineers do have a reasonable idea of what they are doing.

First off, we are only going to discuss regular production street tires that have passed DOT testing. The numbers I will use would basically be seen on new tires. I cannot advise on a tire’s capability after it has been damaged or run for tens of thousands of miles. Damaged tires can fail when reinflated, as seen in this video of a 22.5-size tire.

This video was made to show that welding on a wheel with a tire still mounted can result in a tire failure. Note the pressure at failure.

 

Tell service person if you drove on under-inflated tire

If your tire suffered a puncture and you drove any distance with the tire underinflated, you may have permanently damaged the body cords. That is why you should always tell the tire service person that you drove on the under-inflated tire so they know to use a cage or other restraining safety equipment. Age and miles can reduce the strength of any tire, but we do consider this degradation when approving a design.

Another consideration is the availability of high-pressure air. Most home compressors or those that supply air to hoses made available to consumers have an upper limit of about 150 PSI. Therefore, we don’t expect any consumer to have access to or to use industrial air pressure, which can be 400 PSI, or even special inflation units as used on aircraft tires.

Basically, many tires are tested to 300% to 400% of their rated inflation, or above 200 PSI, whichever is lower. I had a number of tires that could contain over 300 PSI when new. If they had been damaged, however, the same tire might fail at less than 200 PSI. But I also never, in my 40 years as a design engineer, saw a tire fail from normal pressure increase—and I was involved in the testing and evaluation of thousands of tires.

The bottom line

The bottom line is: Please do not bleed down your hot inflation pressure. You should ONLY set and adjust inflation when the tire is cool and at ambient temperature. Doing otherwise MAY result in you having a failure days or weeks later because you were running lower pressure than what was needed for the tire load you were applying.

Also, unless you have damaged your tire, it is unlikely to “explode” simply by adding 10% more cold air pressure.