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Monday, April 23, 2012

How do I know what my hot pressure and temperature should be?

Bill P. asked
I just read your article on tires and speed. Seems that most RVers would stay around 60 mph here in the USA since it is the best speed for gas economics along with tire wear. That being said I would love to hear your thoughts on tire pressure/temperature monitoring systems. I have a class C 28' Four Winds and as a new person to the RV world purchased a monitoring system.

In my travels I get a variety of readings from tires and sometimes am concerned as to the pressure max and temp max. All six tires are exactly the same but in all my reading I find it impossible to get opinions or facts on what to expect in pressure and temperature from cold temps to normal driving pressures and temps.

I understand there are a lot of factors such as weight of vehicle to the temperature in the area and highway...but there should be something on what the absolute limits above the max cold pressure and possible the same comparison with temperature. I find nothing. Example: I run the duals at between 75 psi to 80 psi and 80 is the max at cold. Does this tire allow the pressure to rise up to 90 or even 100? If you need the tire size and weight of the RV let me know. If there is some sort of industrial chart concerning these issues that would also be great

Great question Bill and welcome to the world of RV ownership. I know you would like an answer like “the Max temperature is 149.6°F and pressure increase of 12.8 psi” but as you are learning with tires nothing is that simple.

First off I strongly support having a TPMS. Even if you check your pressure with a good digital gauge every morning, how will you know if you drive over a nail as you pull out of the campground at the start of your day’s travels?

Next you need to get your real individual tire loads with the RV fully loaded to be sure you are not overloading any individual tire. You have a Tire Placard that indicates the recommended cold tire inflation you should follow until you know your real loads and calculate your personal minimum inflation levels.

 As you work through my earlier posts you will see the general guideline for pressure increase of about 2 percent for every 10°F, so if you set your cold pressure to 80 psi in Oklahoma City in November and it's 45°F you may see your cold pressure has increased to 86 psi in Phoenix the next morning when it is 85°F. If you watched your tire pressure during your trip you might see the tires run from five psi to 20 psi above your cold pressure as the tire temperature warmed up to 20° to 50°F above ambient. This increase in tire temperature is affected by many variables such as load, speed, air movement around the tires (inner duals will usually run hotter) and moisture content of the inflation air and actual tire inflation. Also if it starts to rain you will see a very quick drop in tire temperature and an attendant drop in pressure as the water draws away the heat being generated in the tire.

Be sure to check out “Quick post on Max tire temperatures” and Tire Temperature & Pressure - A Hot Topic and Are tire pressure monitoring systems “TPMS” worthwhile?

Wednesday, April 11, 2012

How fast is safe to drive on your tires?

I found and responded to a forum thread on tire speed ratings and thought I would share what I learned with you here.

I think it best to think of a tire's "maximum speed" a bit like the engine red line. Can you exceed the red line on your engine? Yes, occasionally, but is it good for the long term durability of your engine? I think you will agree that it's not a good policy unless you like blowing up engines (or tires).

A tire has a finite life. Heat and cycles consume that life. Heat comes internally from under inflation, overloading and fast speed and externally from spending all your time in the South where temperatures are higher than in snow country. Cycles are simply miles.

Heat generation occurs inside the tire structure at the molecular level. It is not linear so one mile at 75 mph “uses up” more of that limited life than two miles at 40 mph. It’s even possible that a mile at 80 mph is worse than two miles at 60mph.

The maximum speed rating is also based on the assumption that the tire has NEVER been operated while 'flat." Note that a loss of 20 percent of the minimum inflation needed to carry the load is considered flat.

Also if the tire has ever been repaired, the manufacturer may no longer support the speed rating marked on the tire. You need to consult the manufacturer's publications to confirm its policy. This would apply to tires with a speed symbol letter as found on most Class-B and Class-C motorhomes.

An example might be LT235/85R16 LR-E 120/116Q. The 120/116Q is the service description with the 120 and 116 being the load index for single and dual position. The “Q” is the speed symbol for 99 mph max. A Class A motorhome might display 255/70R22.5 LR-G 138/134L. The “L” is the speed symbol for 75 mph.

As I cover the topic of speed rating in my tire seminars, there are different requirements for tires in Europe than the USA and Canada. In Europe vehicles are capable of higher speeds (100+ mph). Roadways exist to run them, and it’s legal. The tire description system was created to assure replacement tires were also capable of those higher speeds.

However in North America we have speed limits that do not allow driving at excessive speeds. So the emphasis is placed on handling, not "all out" speed.

The same construction features required to achieve higher speed ratings also improve handling. So, in the Americas a speed rating also tends to be a performance or handling rating.

I know of no highway tire that can run its entire life of 40,000 to 90,000 miles at its maximum speed. The tests to establish the speed rating only require an hour or so to complete and most of the test time is well below the maximum. This test is used to measure the rate of heat energy generated with tires failing due to too high an operating temperature.

If you tow a trailer that has ST type tires you need to remember that they are rated at 65 mph maximum. A few trailers come with commercial tires that are only rated to 62 mph. So your maximum speed is based on the lowest rating of all your tires.

Saturday, April 7, 2012

Looking for FACTS on Mission brand tires.

I'm doing research for an article on Mission brand tires. I believe the design of interest is Radial ST. When I do a search I find a lot of second hand information such as "A friend had a failure," or "I heard about Mission tire failing," or “I read about these China tires failing."

What I do not find very often is a clear statement from a user who personally experienced a failure. The few times there's statement from an owner there's no mention of the design or tire size or the actual loads on the tires, or even how many tires they had and their age. I also do not recall ever finding the DOT provided.

With all the repeated stories on this brand I would have thought it would be easy to get the facts. But there is only a single complaint filed with NHTSA and it contains no DOT info or tire size. The design is an "FT" and the person filing the complaint said the tires were made by General Tire Co. So that single complaint is very suspect and of little help in trying to determine facts.

If you owned Mission brand tires and had good luck or bad please send email to Tireman9(at) with whatever data you can. I really do not want to try and write an article based on rumor and hearsay information.