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Friday, August 28, 2020

"Cold Inflation Pressure" and clarification of tire terms.

It appears I messed up in my previous post on "Cold inflation and Set Pressure". I failed to properly define the terms I was using, so if you bear with me I will try and clear things up.
"Cold Inflation Pressure" This is the pressure number you see published in tire company "Load & Inflation" charts. It is also the pressure you see on the Certification Label sticker AKA Tire placard that vehicle manufacturers apply to all cars, trucks, and RVs. It appears that some folks have a little difficulty with the word "Cold". This does not mean the tire needs to be refrigerated or that inflation pressure needs to be "adjusted" by calculating the difference between some theoretical laboratory standard and the current air temperature. "COLD," for tires, simply means at Ambient air temperature and not warmed by either being driven on or being in sunlight for the previous two hours.
When I am discussing tire pressure I am always referring to the Cold inflation unless we are specifically discussing the pressure increase due to Sun exposure or due to being driven on and reported by the TPMS or if the driver checked the "Hot" pressure at a rest stop with their hand gauge.

Finally, the pressure number molded on the sidewall of tires is the cold inflation pressure required to support the load that is also molded into the tire sidewall. The load number is the maximum load capacity for the tire and so the cold inflation would be considered the minimum cold inflation required to support that load. The wording on tire sidewalls does vary a bit. If you look at a variety of tire types from different manufacturers you will see some variation in the wording and IMO this contributes to some of the confusion. One fact that many do not think about is that increasing the tire cold pressure above the number on the tire sidewall WILL NOT increase the load capacity number molded on the tire sidewall. It is also important for people to understand that tires can tolerate a significant increase in pressure due to operation under load or at speed. While I can't provide information on the specific design limits used by different tire companies, What I can say is that in my personal experience that many new tires are capable of tolerating inflation increase of upwards of 100% or more over the number molded on the tire sidewall so the idea that an undamaged tire will suddenly explode due to an increase in inflation due to operational heat is not justified.

Tire Pressure increases. In my post of March 3, 2014, I covered the science and math of pressure change due to temperature change. You can read that post HERE, or just accept the rule of thumb that pressure changes by about 2% for each change in tire temperature of 10°F.

While we are talking about pressure change You can review my post of July 8, 2011, where we pointed out that driving from Death Valley to Denver CO will only result in about + 2.5 psi but the change (drop) in Ambient temperature will probably decrease the pressure by more than that increase due to elevation. This is why we tend to ignore tire pressure due to changes in elevation.

Tire Load is important information. You know the GAWR or Gross Axle Weight Rating is on your certification label. The problem is that the actual load is almost never split side to side to give a 50/50 split. While many RVs may have a 48/52 % or similar, side to side split on an axle the actual scale readings have confirmed some RVs have as much as 1,000# unbalance. So without actual scale readings we could only guess which tire is loaded more.

A tire on one end of an axle has no idea about the load on the tire on the other end of the axle so simply dividing the axle load by two is not sufficiently accurate to be confident that you "know" the actual load on your tires. The other problem is that many people simply estimate the load on their tires. The reality is that a majority of RVs (10,000+) that have actually checked the tire loads have been found to have a tire or axle in overload. This data demonstrates the importance of learning the actual load on your tires. While learning the load on each tire position is not easy, at minimum RV owners need to confirm the load on each individual axle and this is easily done with a visit to a local Truck Stop. This needs to be done with the RV loaded with as much "Stuff" as you ever carry. With the axle loading known and until you can get individual tire position weights, I suggest you assume one end had 53% of the axle load.

My final point for this post is Reserve Load and this is where we get to the "Set Pressure".
First, we need to remember that Reserve Load is the load capacity of the tire at its cold inflation pressure that is in excess of the measured or calculated load of the RV on the tire. Some use the term Safety Factor but as an Engineer, this term is not really appropriate.

In general, it is suggested we have at least a 15% Reserve load. Most new cars come with 20% to 30% Reserve load and this is a major reason why we seldom see tire failures on cars. An exception was seen in the 90's when one vehicle MFG provided for less than 10% Reserve Load and a number of tire failures occurred and even made the TV News.

Many Motorhomes may have less than 10% Reserve load even if the inflation pressure and the loading shown on the Certification pressure are followed. IMO this is a major reason for the relatively high failure rate of tires in RV Motorhome application. RV Trailers have it worse. In addition to having 0% to 10% Reserve Load the suspension design contributes to high Interply Shear due to being dragged rather than steered around corners.

So what should an RV Motorhome owner do?
1. Learn your actual loads on your tires by getting on a scale for each tire position.
2. If you can't get individual axle end loads assume the heavy end has 53% of the axle load.
3. Use the tire Load & Inflation tables to learn the minimum cold inflation needed to support your actual (or 53%) load.
4. Consider applying a +15% to the load figure to give yourself a reasonable Reserve Load and consider that your Minimum Cold Inflation. You could also consider adding 10% to the load Table pressure if that is easier for you to calculate.
5. Consider adding 5% to the inflation in #4 and use that as your "Set Pressure. This gives you a cushion for day to day temperature variation which can change inflation pressure 2% to 5%
So what should an RV Trailer owner do?
Do #1, #2 & #3 above
#4 If you want to try and lower the Interply Shear I recommend you increase the inflation to the number on the tire sidewall and use that for your Set Pressure. If you have increased the tire Load Range from say a LR-D to LR-E you can use an inflation number the 65 PSI for LR-D and the 80 PSI used for LR-E tires and use that as your Set Pressure.
#5 Try and learn the wheel max pressure rating and do not exceed that number.


Friday, August 21, 2020

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.

 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.

Friday, August 14, 2020

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? 


Friday, August 7, 2020

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.