My FIRST recommendation for a brand & size tire to buy

 If you review my blog posts and posts on various RV forums or have even attended one of my RV tire Seminars, you know I have tried to avoid making specific tire recommendations. Well this changes today.

If you use tires in a 225/75R16 dimension tire in  ST or LT or Metric type you might want to consider the Michelin Agilis Cross-Climate C-Metric tire.

The reason I am making this recommendation is based on the load capacity of their "C-Metric" tire.

Based on the data published by Michelin  HERE, Tire dimensions are the same.  What is different is in the "Fine Print" of the stated Maximum load capacity for their C-Metric when inflated above 80 psi.

The LT Michelin has a stated Maximum Load Capacity Single of 2,680# and Max Dual load capacity of 2,470 per tire when inflated to 80 psi   (LR-E).  This max load applies to the Michelin LT225/75R16 XPS-Rib and the regular LT225/75R16 Agilis Cross-Climate. The same max load is stated for both Bridgestone, Conti and Goodyear and Toyo tires and others. (If you find another company making the use of the C-Metric load capacity please let me know.

The Agilis C-Metric however has a maximum load capacity of 3,195# Single and 3,000# in Dual application when inflated to 83 psi.

The "C-Metric" can give you an extra 515# load capacity in single or 530# per tire in dual position.

For a Class-C Motorhome this works out to an additional load capacity of 3,150#. 

Now I would certainly hope that people not add another 3,000# of "Stuff" into their RV as in my opinion, you would be better off using the Michelin Agilis to give you a nice increase in the "Reserve Load" capacity of your tires to get you closer to a suggested 20% figure.

For you folks with RV Trailers your current ST225/75R16 LR-E tires are giving you a total load capacity of 11.320# for 4 tire applications. Switching to the Agilis could increase your capacity to 12,780# or better yet increase your reserve load from your current 10% to 24% if you simply resist the temptation to pack more "stuff".

It appears that Michelin has made some construction changes in their C-Metric line to provide more or better reinforcing materials to support the extra level of inflation and that is where you get the extra load capacity over LT type tires as it is the inflation that supports the load not the tire itself.

I can only assume that other tire companies have chosen to make the marketing decision to not build similar "C-Metric" tires. This option is available to all tire companies as there is nothing Magic involved here, just a marketing decision.

 

##RVT1015

How cold does it have to be to set tire pressure correctly?

With thousands of new RV owners out there, the answer to the question of "How cold does it need to be for me to check tire inflation" continues to come up. So this will be new information for some and a refresher for others.

 
Some people want to refer to a Temperature compensation chart and go through some calculations to learn the "correct" tire inflation when it is 82F or 62F outside. This is not what you should do.

Tire pressure is not based on any laboratory standard temperature (some claim 70 F or 68F) but is based on the tire not being warmed from either use, i.e. being driven in previous two hours, or from being in the Sun for previous two hours. Even partial sun can affect the reading.

Classical "Temperature in the Shade" is the "Ambient" we tire engineers are talking about. Not temperature in a theoretical laboratory.

It is correct to say, "The BEST time to check cold inflation pressure or CIP is the FIRST thing in the morning BEFORE the day's temp has had a chance to increase and BEFORE the sun has had a chance to shine on the tires and BEFORE you have used the vehicle."

With many people installing TPMS that provides a temperature reading to the driver for the first time, some are surprised when they see that both temperature and pressure increase as they drive. Please remember it is NOT correct to bleed pressure from a "hot or warm" tire after you have started that days travels.

As I have covered in other posts, the tire pressure will change by about 2% for each change of 10F. There are some pages on the internet that say 1psi for 10F but those are talking about standard 36psi passenger tires not 80 or 100 psi RV tires.

Now if you are driving from the campground on top of Pike's Peak and stop for lunch for two hours in the shade in Flagstaff where it is 90 and check your air, you might find a change of a few psi. You could adjust your pressure before continuing to Phoenix, where it is 120 F, but I don't bother to adjust inflation by the 1 to 3 psi variation I observe day to day. In my mind that is too much work. This is one reason why I suggest a +10% "cushion" on the required inflation as that eliminates the need to chase inflation with every change in ambient  of 15 to 20F. We tire engineers know that tire temperature and pressure will increase and we have taken that into consideration when we design and test tires.

NOTE: My personal CIP is  75/80  F/R on my Class-C MH.  Both of these pressures are more than 15% above the minimum pressure needed to support the measured load on each tire so I have a  nice "cushion".

I usually wait till I am home and am getting ready for the next trip before I adjust my inflation to my personal CIP, so I simply monitor the running inflation pressure which goes up and down as ambient temperature, driving and Sun exposure changes the inflation. My TPMS will warn me of air loss, so all is good as I motor down the highway.

I hope this info helps some of the first time RV folks out there. If you are an "ol' timer" you might direct the "newbies" to this post when they ask about setting tire pressure.

 

##RVT1014