Friday, September 24, 2021
Friday, September 24, 2021

RV Tire Safety: Which tire expert is correct about tire inflation?

By Roger Marble
Internet “experts” have Dennis confused. Maybe I can help. Dennis said: “Okay. I’ve been RVing in Class A’s for 40-plus years and thought I had this figured out, but the “experts” have emerged to confuse me, once again. So, what is the definitive answer to the question of tire pressure. Should it be the coach manufacturer’s recommendation on the placard in most every RV, or the tire manufacturer’s inflation recommendation? I know for sure that the ‘cold pressure’ stamped on the tire is NOT the recommended pressure. Please advise.”

Dennis, I completely understand your frustration. You need to remember that there are three entities that are trying to answer three different questions when it comes to tire inflation.

The tire company

First is the tire company. They make tires that must meet various tire industry standards for tire dimensions and load capacity. The tire company must also certify to the Department of Transportation (DOT) that the tires they make are capable of passing a number of different strength and durability standards if they want to sell tires for use on U.S. roads.

These requirements are why you end up with the numbers molded on the tire sidewall. These include the maximum load capacity for both single and dual position. They must also identify the minimum inflation needed to support both the single- and dual-position loads.

We need to remember that the tire company does not know which RV or truck the tire will be mounted on. The tire company also doesn’t know if the tire will be on the front, in dual on the drive, or even if it will be on a tag axle. (A tag axle is a non-drive, third axle located behind the rear drive axle of a motor home.) So they cannot give a single inflation number that meets all these requirements

The RV manufacturer

Second, the RV manufacturer has to meet some different DOT regulations. The primary one is that the Certification Label aka tire placard must specify inflation for the tire that would be sufficient to support the stated Gross Axle Weight Rating (GAWR). Example: If the GAWR for a front axle was 6,000#, then the tire they select must be capable of supporting 3,000# and the RV company must tell you, the owner, the MINIMUM inflation needed for the tire to be able to carry 3,000#. Since each axle probably has a different GAWR, that is why you may have different inflations on the Certification label for each axle.

RV Industry Association (RVIA)

Third, if the RV company wants to be able to meet RVIA standards, the tire must be capable of supporting 110% of the axle load. So, in our example, that means 3,300#. Here things can start to get messy. The inflation needed to support 3,300# is almost certainly different, so the tire placard would need to state the inflation for 3,300#. It is OK for the inflation on the placard to be higher than what DOT requires, but it cannot be any lower.

Owner’s options

Finally, we get to you, the owner.

Option A is to simply inflate to the number on the tire sidewall. This, in some cases, is significantly higher than the tire needs, so you may get a hard ride and in extreme cases more rapid center wear.

Option B is to follow the placard inflation which, on many motorhomes, is lower than the inflation number on the tire. So the owner gets confused.

Option C is what I and other tire engineers recommend. We have solid reasons for saying that the RV owner needs to get on a truck scale, learn the actual load on the tires, and then learn the minimum inflation needed to support the actual load on the tires. We know this is the best for the tire because the data shows that a majority of RVs have a tire or axle in overload. This is a major reason for tire problems in the RV world.

Option D is a better version of Option C but is not as easy as Option C. The data shows that almost no RV has the axle load split 50/50 side to side, so simply taking the truck stop scale reading and dividing by two can be misleading. Some RVs have been found to be as much as 1,000# out of balance side to side. Option D means you need to learn the individual load on each tire position when the RV is loaded to its heaviest. This is almost impossible at regular truck stop scales.

Where to learn individual tire position loads

To learn the individual tire position load you need to get a group like RVSEF or Escapees to use its scales to get the individual weights. It is also possible, with a little work, to find a platform scale at a gravel yard, or some lumber or cement block companies have large scales.

Here is a worksheet to help with the numbers. Once you learn the actual load on the heavy end of each axle you would use that weight to consult the tire Load and inflation tables to learn the MINIMUM inflation for all the tires on that axle. We also recommend that if possible you add 10% to the table inflation to give you a nice margin. Just do not exceed the max inflation rating for your wheels.

I hope this helps clear up your confusion.

 

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

 ##RVT997

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Joseph
4 months ago

Roger, thanks for another great article. I have a question about your sentence Just do not exceed the max inflation rating for your wheels”. What is the best way to determine the maximum pressure for your wheels. Is it stamped on the wheel? I have the original 2011 F-53 stripped chassis sheet from Ford with my VIN on it and one of the specs is the rims – PSI COLD: 90. I don’t know if the aluminum wheels on it came from Ford or if Tiffin installed them. (2011 Tiffin Allegro) Thanks, Joe

Roger Marble
4 months ago
Reply to  Joseph

Some wheels are marked with the max cold inflation. If not you will need to contact the seller / mfg.

Phil Atterbery
4 months ago

As far as the “who makes what” list, great idea. Did he here that Dometic bought Valterra? Sounds like Dometic management is following the “Gander World” business model, trying to corner a market.

Bob M
4 months ago

This article on tires is to complicate. When you read this and the poor quality that RV’s and equipment are made. It’s not worth buying a RV.

Lil John
4 months ago

Great article Roger! I worked and taught auto repair for 50 years. Tire pressure was always an issue. We worked overtime when the manufacturer’s started building tires with a 44 psi rating instead of the old 32 or 35 psi rating. Folks would get new tires and then inflate them to the door stickers inflation. They would put 35 psi in a 44 psi tire, and if it was on the front of a front wheel drive vehicle, it almost always was too little air. It would wear out the tire prematurely, and steer terrible. I go with inflating to sidewall pressure and then adjusting to load. Weighing your axles and reading a tire chart is the only way to do it right. The plus side is you can let enough air out sometimes to make the rig ride a whole lot better without sacrificing safety.
Thanks again for the great article.

Mike Schwab
4 months ago

Tires are built for a large variety of vehicles. The minimum inflation is what is needed to avoid too much sidewall flex that will destroy the sidewalls. The maximum inflation is what the tire will hold without blowing off the rim and will hold the maximum weight rating.
The vehicle recommended tires and pressures are what that particular vehicle needs. Those are the pressures you need to set. And best time to check tire pressure is first thing in the morning before you move. Note if you needed to add any pressure at that time. When you do add air, add that amount of pressure increase over what you have when you start airing up.
Also, tire pressure changes in relation to outdoor temperatures. In a desert you can go from freezing 32F 0C 273K to 104F 40C 313K in one day, and your pressure goes up by 15% 313K / 273K. Topping off tires in winter at -20F -30C 243K can add another 15% of air pressure once summer comes. Driving can increase the temperature and pressure even more.

J J
4 months ago

While this person’s question was about a motorhome and your answer seems to address that class of vehicles (no pun intended), I believe you’ve also recommended that non-motorhome tires be inflated to the sidewall stamped pressure.

If that statement is accurate, perhaps a follow-up article could be about why the tires on the two types of RV’s (motorhome and non-motorhome) might have different inflation practices.

Roger Marble
4 months ago
Reply to  J J

JJ, I covered this difference, in detail in my tire blog http://www.RVTireSafety.net
You can read the specific post HERE. The post gets technical but you don’t have to understand all the technical stuff if you are willing to accept the analysis and summary from a tire Design Engineer. If you wish, you can GOOGLE The topic of “Interply Shear” to learn more

David S
4 months ago

Finding someone to weigh each axle end can be difficult. Be advised that the Freightliner Custom Chassis Factory Service Center in Gaffney, SC can do this for motorhomes built on their chassis for a reasonable charge, and will then recommend tire pressures for each axle. I don’t know about other Freightliner service centers.

Roger Marble
4 months ago
Reply to  David S

Yes axle end weighing can be difficult to find. You might check local building supply stores that sell sand & gravel. Untll you can get the end weights I suggest you assume that one end is supporting 53% of the axle load.

Steve
4 months ago

This article refers to Motorhomes. Travel trailers have different requirements. That’s why Goodyear Tire Company recommends inflating trailer tires to maximum pressure as stamped on the sidewall. A lot of inflation confusion comes when articles don’t differentiate between motorhomes and travel trailers.

Roger Marble
4 months ago
Reply to  Steve

Yes, this article was in response to a question from a Motorhome owner. I do cover the difference and reasons behind the slightly different recommendation for trailers in my blog http://www.RVTireSafety.net

Tommy Molnar
4 months ago

My issue has always been about tire pressure increasing with driving, especially when the temperature outside rises by 20-30 degrees as the day progresses. I set the rear tires of my F-350 at 70 lbs (per Ford) even though my tires say 80 lbs for max load. So, at 70 lbs at the start of the trip, it’s not unusual to see well over 80 lbs or more when the outside temps rise significantly, driving at 62 mph. My trailer tires say 80lbs for max load but I’ve decided that 75lbs is a good starting point given my experience with the tow vehicle’s changing pressure. The truck has a TPMS system I can monitor with my Banks iDash gauge.

Roger Marble
4 months ago
Reply to  Tommy Molnar

Tom, Tires have always increased in temperature and pressure. The reason this seems to be new to many drivers is that before TPMS the only people that knew about the temperature rise were tire Engineers. I believe that Ford is suggesting 70psi “cold” is because they know that as long as you are not exceeding the F-350 weight limits you will not need more than 70 psi. RE your trailer, I trust that you have confirmed with actual scale reading, that none of your TT tires are loaded higher than 85% of the tire load capacity at 75 psi.

Joe
4 months ago

Stupid question. If front axle GAWR rating is 6000#, you divide by 2 so each tire has to carry 3000#. If the rear axle GAWR rating is 15000#, you divide by 4, so each tire has to carry 3750#, correct?

Bob P
4 months ago
Reply to  Joe

You didn’t read the article, it said some motorhomes have been known to weight as much as 1000 lbs heavier on one side than the other. Most of the time manufacturers do a fair job at balancing the weight of an empty unit, but how you load it can be a drastic difference. I know in our situation all the stuff we used on a regular basis was on the right (passenger) side and everything else was on the left side in the storage compartments under the big slide where I had to get down on hands and knees to get to it. Lol

Steve
4 months ago

Your last statement about wheel rating has me wondering. Mine are stamped 110 psi. My tires are also rated at 110. As they heat up they get well over 110 psi. Are my wheels in danger at this higher pressure?

Roger Marble
4 months ago
Reply to  Steve

All the tire and wheel ratings are based on “cold” readings. That doesn’t mean you have to park inside a refrigerator as “cold” really means not warmed up from either operation or from being in direct sunlight or artificially warmed by any other manner. The general guideline is to wait 2 hours after the heat source (sun or driving) has been removed or stopped, for at least two hours.

Tim
4 months ago

Great article! Not sure why or where all the misinformation comes from!

Often what we are told is just not true. For example I just bought a used class A and the seller tells me he was told by the dealer to use 110 psi., but he found that a harsh ride so he dropped it to 100 psi.

Later that day I found the manufacturers label on the left wall by the drivers seat. All 6 wheels listed as 80 psi.

Now, I ask myself, is this RV builder’s math based on final veh weight or just Fords numbers for an unloaded F53 chassis?

I will use the scale method to verify the actual weight to verify if this label is accurate and then go with that.

Roger Marble
4 months ago
Reply to  Tim

Some RVs have two tire placards. One for the original chassis (Ford or Chevy or MB) and one from the RV manufacturer. In a small number of cases, there may be one added by the dealer if there was significant weight added with “optional” equipment. Look for the highest GAWR number and that is the placard to follow.

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