Tires are more than just Round & Black things

A question came up about the Max speed recommendation as published by Goodyear, Michelin, and Bridgestone for tires in RV use.

To help clarify:

Many times the "application" or use of an item results in it additional or different limitations or ratings.

Example

 If I have a tire that is a 225/75R15 and inflate it to 35 psi, How much load can it carry?

If the intended use is on a passenger car 1,874#

If as a single on an LT it is rated for 1,445#

If as a dual on a LT 1,315#

If a Single on a trailer where having passengers in the trailer is actually prohibited 1,760#

and if in a dual application on a trailer 1,570#

Part of the decision process for a tire's capacity in each of these different applications includes such things as:

Will the vehicle carry passengers?  Is the "Reserve Load" of the tire in the application normally close to zero or maybe 10% or normally closer to 25%

An obvious example where the application affects the load capacity is well established in the use of "P" type tires on a truck, trailer or multi-use vehicle such as an SUV. In these applications the load capacity is reduced by Industry standards by dividing by 1.10 so the 1,874# capacity becomes 1,703#

For heavy truck there are published guidelines that allow an increase in load capacity if the max speed is significantly reduced.  As the speed is lowered the load capacity can be increased up to 16% with no increase in inflation.

Similar to above if the tire is made for a specific market a tire company my have more demanding requirements on the tire during the development process. An example might be it the tire was being made for a market where the speeds and Ambient temperature was both very high the tire might require a more robust construction. Another example might be providing a special tread rubber if the tire was to be used at extremely low temperatures where the tread rubber might even crack if a "High Speed" rated tire tread were used.

Tires are much more complex than many people suspect. Ideally, an owner would be more informed and knowledgeable about tires and their limits and capabilities when making a purchase. They are much more than just "Round Black" things that almost no one "wants" to buy.

 

##RVT964

Does your tire dealer know what they are doing?

Just got this message from a reader.

"I just replaced 6 235/80R 22.5 XRV Michelin tires on my 2013 Monaco Monarch Class A Motor home. The dealer installed, balancing bead bags and new valve stems. I have good access to all valve stems now and have an after market TPMS. A couple of the rears were below the 95 Pound alarm point when cold and I decided to add air. When I removed the TPMS sensors, the valve stem seats leaked on three of six tires. I replaced the faulty valve stem inserts, completed pressure checks and all is well. I heard from one of my friends that the bead bags are causing the valve stem seat to fail and I understand there are redesigned valve stem seats that solves this issue. Can you comment?"

I responded:

The dealer that installed the "balance" stuff should have also installed the special valve stem/core air filters. Powder from beads can get into both the core and the TPM sensor and make them malfunction.

Bolt-in valve stems have a rubber insert on the air chamber side and a large metal washer on the outside. There is a torque spec on the bolt in valves. This blog post, covers valve stems of different types and has the torque spec for the bolt in nuts.
 
By "stem inserts" do you mean the valve core? Every valve stem I have ever seen comes with a new core. The only reason for them to leak is some "stuff" got into the new core and most likely was from the balance stuff.

This makes me wonder if the dealer was just interested in selling you the balance beads but didn't want to kill the sale with an increased cost for the valve air filters.  OR He didn't know that filters were needed when you put "stuff" inside a tire.

Also did the dealer give you a written tire warrantee that covered the use of the balance stuff? 

##RVT961

Another question on "Cold Inflation" vs your "Set Pressure"

Got this question from a reader of an RV Forum:

Thank you for all of your informed comments regarding proper tire care. I need one clarification. I have always considered the cold psi on the side of my 22.5 RV tires to be the minimum to carry the maximum rated load, but have assumed that psi was also the maximum COLD psi the tire should see. From your recent post, am I to understand that unless the tire states that it is the maximum cold pressure, I can exceed it by 5-10 psi?
Thank you for your time, Doug

My Answer:

The wording on tire sidewall IMO was written by lawyers, not engineers or users. Info on the sidewall is the inflation needed to support the Max load.  The difficulty is that few understand that the pressure changes with temperature and the only meaningful pressure, measurement is when the tie is "cold". This still confuses some because some want to apply Chemistry Lab practice of adjusting to theoretical 72.5°F when what "cold" really means for tires is at "Ambient Temperature" and does not include any pressure build-up"   In real life terms this means "Not warmed by being driven on or in direct sunlight for the previous 2 hours"

Now we need to address what is meant by "Cold Inflation" vs the psi to set your tires to or what I like to call your "Set pressure".

I like to suggest the "Set Pressure" for motorhomes to be the minimum needed to support the maximum load on the tires PLUS 10% inflation. 

RV trailers are different because of their Interply Shear problem.

For RV Trailers,  I would like to see a minimum of +15% load capacity over the measured heaviest loaded tire, with +20% to +25% Reserve Load capacity being better. Sadly most RV trailers come with tires that provide +0% to +10% load capacity vs GAWR
NOTE: I am not even addressing the tendency for most RV owners to overload their tires.
So for trailers I try and simplify:
- To lower, but not eliminate the Interply Shear problem I suggest the "Set Pressure" when the tires are "Cold" to be the pressure on the tire sidewall. BUT I still want trailer owners to confirm they have at least 15% "Reserve Load" over their measured scale reading.

##RVT962