What is the proper inflation for my tow vehicle and RV trailer tires?

 

I monitor a number of RV forums and when I see someone asking about tires, I make the effort to learn about the problem and offer solutions, if possible.

Recently there was a question on inflation for both the tow vehicle and the travel trailer. The comments seemed to be all over the place, so I decided there was a need for some basic foundation of information. Following is my reply and attempt to provide the basic information needed by RV owners.

Tow vehicle (TV)

Tire size, type and inflation should go by the Certification Sticker on the driver door jamb. This was established by a team of tire and vehicle suspension engineers over a period of two to four years. There were numerous changes and tests run on different specification and performance parameters that were fine-tuned to meet the goals of the vehicle design team. The sticker provides a recommended inflation that normally delivers 25% to 35% Reserve Load, so those numbers should be followed unless you have done a lot of research and understand the trade-offs. Do not forget that few people in the tire store have technical training beyond how to sell tires, so I would be careful with the information they provide.

Travel trailers (TT)

Tires for almost all RV trailers have a single spec—that being Low Cost. I have never heard of any vehicle testing or evaluation that compares different tire constructions or ratings for the application of a tire to the travel trailer.

To achieve low cost, tires are only required to meet a single requirement: that they meet the minimum load capacity specification needed to support the RV. In 2017, the RV Industry Association (RVIA) established a Reserve Load of 10%, but for RVs built before 2017 there was no Reserve Load requirement.

Those of you who have read the owner's manual and reviewed the Certification Label information and the Load and Inflation information molded on the tire sidewall may have noted that for most RV applications, you can only achieve the tire's 10% Reserve Load capacity when inflated to the level required for the  maximum load capacity.

Yes, this basically means you must run the max inflation for the tire's Load Range if you want to have the minimum margin of Reserve Load recommended by the RV Industry Association.

Tire type nomenclature

In the subject post there was some confusion about the Michelin "LTX" line. One poster thought this was some special "Hybrid" type of tire, so I feel the need to clarify tire "TYPE" nomenclature.

The letters before the numbers in the tire size are critical and important but too often people seem to ignore that important info. The P is for Passenger, LT is Light Truck, and ST is Special Trailer. ST should only be used on trailers and are not approved for use on motor vehicles that carry people. if a P type is applied to a trailer, its load capacity MUST be reduced by dividing the load capacity stated on the tire or in the Load & Inflation tables by 1.10. But if an LT tire is applied to a trailer no reduction in load capacity is required.

Eventually, the following information was added to the Forum thread:

------------------ Steer Axle ---Drive Axle --- Trailer
Weight #1 - Truck only/hitch/bars -- 3180 3520
Weight #2 - Truck and Trailer with WDH bars -- 2980 4320 4880

Side note: The owner had previously stated the TV GVWR was 6500, so his scale reading on the initial weighing seems to exceed his GVWR !

I am assuming the "Trailer" scale was for the four tires on the RV. If so, it sounds like the trailer was empty when he did the weighing.

In the thread, I also saw a comment about Michelin tires. The owner said:

It was helpful to learn of both the Michelin "hybrid" LTX and the Cooper XL alternatives to LT tires although when I looked up the Michelin LTX I saw that it comes in A/T and A/S versions ... even more confusion ... is the A/S the "hybrid" and the A/T the real LT??

My response

I'm not sure what you mean by "Hybrid". Michelin has a "line" of tires it calls "LTX"— this includes both passenger type and LT type tires. Think of this as with General Motors, which has a "line" of cars called Chevrolet with many type vehicles from sedans to SUV and pickups.

In my post I covered the three basic families of consumer tires: Passenger, Light Truck and Special Trailer. Each "family" has many different sizes and many different tread patterns, like the A/T, that is a traction tread pattern, and the A/S which is All Season and provides less off road or snow traction.

Load Capacity

There are also within each family different levels of Load Capacity.

P type "family" tires come in "Standard Load", that has no special markings, and also XL for Extra Load.

LT and ST "families" have greater load capacities, so they have letters assigned to the "Load Range", or LR for short, with the different LR having letters starting at C, D, E and on up in some brands. Each letter has a specific maximum load capacity at a specific inflation. The load capacity is published in tables available in some owner's manuals or tire data books, or even on the internet.

All of the above applies to tires made to U.S. Tire & Rim Association standards. Europe has some similar but different standards with different codes, as does Asia. I will not go into those standards, as I probably have already overloaded you with the above.

Clearly, the owner of the truck and trailer needs to learn more facts about tires and proper loading of his TV truck.

Are RV trailer tires exempt of the physical laws of the universe?

In an Internet forum for owners of a well known RV trailer company, there was a discussion on what the correct or proper inflation and load capacity was for tires on the companies trailers. I jumped in with the following observations.

Tires list the Maximum load they are rated to support and tires also list the minimum cold inflation needed to achieve that load capacity. I have covered this "The Maximum is the Minimum" dichotomy in this blog on RV Tire Safety.

I really do not understand why people seem so afraid of running more than the minimum cold inflation needed to support the actual load.

I am in the process of working through the "Rule Making" documents from DOT when they set the minimum margins on inflation for cars, SUVs and trucks equipped with TPMS. I note that RVs were specifically excluded from this rule making. Could that be because the RV companies didn't want to see any increase in costs even if it meant the product would have fewer failures?

No, that couldn't be. No corporation would ever shave costs if the safety of the product might be compromised would they?

The MINIMUM cold inflation a tire should have would be the level needed to support the actual tire load. They also established that the normal cold inflation should be at least 25% higher than the MINIMUM. Their objective was to minimize tire failures that might result in damage or injury. The DOT knows that tire pressure increases with temperature (2% per 10F) and tire engineers know and design and even depend on this physical fact.

For some reason people with RV trailers feel it's ok to have zero margin. It wasn't till 2017 that RVIA started to specify a small 10% margin and some people argue that the RVIA is not a real requirement as it isn't a legal requirement. For RVs built before 2017 many RV trailers have certification stickers that specify ZERO margin or essentially zero margin as the tire capacity that was to be considered acceptable.

With the above as guidelines I have to wonder why people continue to complain about having tire failures. You are making the conscious decision to ignore established engineering recommendations and safety margin guidelines. What is so special about RV trailers that would make you think they are exempt from scientific principles and physical realities?

 

##RVT983

When you "upgrade" the Load Range, what inflation to run on trailer?

Another question from RV trailer owner "I am replacing the tires on my small (12-14 ft, depending on how you measure) travel trailer. I plan to buy Good Year Endurance ST205/75R14, load range D to replace the brand X Chinese ST205/75R14, load range C.

Vehicle manufacturer says to inflate to 50 psi. Sidewall on existing tires have 50 psi listed as max.

The replacement Good Year tires have max of 65 psi on the sidewall.

I see that the opening post on this thread says to use the sidewall pressure. But I have always gone with the vehicle maker's suggestions which are often lower than the max tire rating. For example, my F-250 wants 65 psi in front and 80 in the rear.

Do I use the 65 max on the tire or with the vehicle manufacturer's 50 psi?"

So I said:   Simple question but I can make the answer complex.

The MINIMUM inflation you need to run is what the Load Inflation tables show for the actual measured load on your tires.

Lacking the actual scale reading, we need to look at your RV certification label that identifies the MINIMUM inflation your trailer mfg recommends. This is based on the DOT requirement that the tires be capable of supporting the GAWR load as shown on the label.

RVIA (Recriational Vehicle Industry Association) in 2017 started to require a 10% Reserve Load (Load capability more than the GAWR minimum).

As a tire engineer, I come at the topic of trying to provide the best conditions to allow the tires to run the longest. As I cover in my this blog post, trailers place extreme shear forces that are 24% greater than similar forces experienced on motorhomes. This, IMO is the primary reason for tire life in trailer application being less than on motorhomes.

Also, you need to remember and understand that it is the air pressure, not the tire sidewall that supports the load. If you run the inflation used in LR-C tire in a LR-D tire, you will have no increase in load capacity so will gain nothing.

Having said all this I would suggest you run at least the certification label +10%.  If you can run label +20% that should result in better durability.

See that wasn't too painful.

##RVT912

Why inflate Motorhome tires differently than Trailer tires?

Found the following in a thread on an RV owner's forum. This came after there were comments about the advantages of inflating trailer tires to the tire sidewall inflation number but inflating Motorhome tires based on the measured load on the tires.

 "Such a hard concept for most to understand."

As an actual tire design engineer, not just someone that has used a lot of tires or bought or sold a lot of tires, I feel I might have a slightly better understanding of the science behind why tires fail.

I try and make the information easy to understand but I find that many simply refuse to accept the fact that my 40 years experience that includes thousands of failed tire "autopsies" might qualify me to give sound advice.

If you simply look at the experience of three groups of tire users. Excluding punctures or pothold impact breaks.

1. Regular motor vehicles. People get about 40 to 50,000 miles before the tires "wear out"  less than 1% experience tire failures.

2. Class-A and Class-C motorhome users. Many only drive 5 to 8,000 miles a year. It is recommended that starting at 5 years of age, tires be professionally inspected. This does not mean a simple walk around to look at the tread depth but close inspection with good lighting. Maybe even using a pit to allow the inner sidewalls to be inspected. Annual ispections thereafter are recommended and replacement at 10 year tire age "no matter how good a tire looks" This group also has a low structural failure rate not tracable to air leak or impact.

3. RV Trailer users "Towables". Based on numerous reports of higher structural failures i.e. belt/tread separations, and some strange patterns left in lose gravel where a trailer was turned 180 degrees I had some computer simulations run and the numbers provided an explination for the "why" towables have a mush worse structural failure rate. The forces inside the tire structure are significantly higher (+24%) in trailer application (i.e. towables) than in motor vehicle applications This force is identified as Interply Shear and it shows up as trying to separate the top steel belt from the bottom steel belt in radial tires.

While it would be possible for RV Trailer companies to make design changes to trailer suspension to allow for "passive steering" as seen on large cement trucks with a tag axle, I doubt they would go to the expense simply to extend tire life.

While lowering the actual load on a tire in trailer service can lower the Interply Shear force I doubt that it is possible to lower the load by 40 to 50%. One thing trailer owners can do to lower this force is to increase the cold inflation to the tire sidewall "max" Sorry to say you can not reduce the Interply Shear to zero as this is the nature of radial tires.

It appears Goodyear agrees with me

I have written a number of times on the advisability of running trailer tires at the inflation molded on the tire sidewall.

Some posts dove deep into the Science behind the recommendation. I know this can make your eyes glaze over so how about just following what Goodyear says in their RV Tires information web page:

"Unless trying to resolve poor ride quality problems with an RV trailer, it is recommended that trailer tires be inflated to the pressure indicated on the sidewall of the tire. Trailer tires experience significant lateral (side-to-side) loads due to vehicle sway from uneven roads or passing vehicles. Using the inflation pressure engraved on the sidewall will provide optimum load carrying capacity and minimize heat build-up."

##RVT890

Small Single axle travel trailer tire inflation

I have received questions on tire inflation from folks with small single axle travel trailers, boat trailers, and utility trailers. There is some confusion, which I understand, so let's see if I can clarify my recommendations for inflations.

One common bit of information would be to learn the actual load on each tire position when the vehicle is loaded to the heaviest you ever expect it to be.
- For each axle identify the heaviest end and use that load when consulting tire load & Inflation tables.

Motorhomes  and Trucks
These vehicles would use the heaviest axle end load when consulting the Load & Inflation tables.

- The inflation needed to carry or exceed the measured load would be your MINIMUM inflation.
- I suggest you select inflation that will provide at least 15% "Reserve Load" for your Cold Inflation Pressure. Some find it easier to go with a +10% inflation over the inflation in the tables.
- Just be sure you have some Reserve Load capacity.
- I see no problem with running higher inflation as shown on the "Vehicle Certification Lable"

Trailers with a single axle
- There can follow the same guidelines as seen for Motorhomes.

Trailers with two or three axles
- Run the inflation molded on the tire sidewall that is associated with the tire "Maximum Load" capacity. many RVs show that inflation on their label, but it is still a good idea to ensure you have a reserve load capacity. SOme RV companies provide almost no load margin so it is up to you to make the appropriate adjustments. Running the tire sidewall inflation will give a lower Interply Shear force which, as you know, is a primary cause of belt separations on these RVs and are why tire life is about half what it is of comparable tires and loads on Motorhomes.
- Select the heaviest load as measured on all four or six tire positions and confirm that the tire load capacity at the sidewall inflation provided at least a 15% Reserve load.

For those interested in the science behind Interply Shear you might read this post.