Do your Tow Vehicle and Trailer match?

Here is some good advice I found on the Interweb and agree with.

Here is a short video from the RV Safety Education Foundation. A longer in-depth video link available at the end of this post.

 You should always do your calculations based on the GVWR of the trailer, not the dry (shipping) weight. The dry weight is a number used by some manufacturers and some dealers to try to sucker people into purchasing a trailer that is too much for their truck

The same goes for tongue weight. The marketing brochure may provide the trailer weight based on the empty trailer (with no propane or batteries, etc.). You should figure your weight based on 15% of the GVWR of the trailer. But the real trailer weighs figure you should know is what the truck scale tells you once the trailer is fully loaded but not hooked to the tow vehicle.

You can not use the "paper" figures found in truck or trailer literature and certainly not the verbal information from the truck or trailer salesperson to calculate your towing capacity. You MUST have real numbers which mean actual scale weights.

Next, take the cargo capacity of your truck (from the payload sticker from the door jamb of the driver's door on the truck). It will say something like "cargo must not exceed ... lbs.". From this payload capacity, you need to be able to deduct all of the following:
- People in the truck
- Cargo in the truck
- Weight of the WD trailer hitch or of the hitch for the 5th wheel
- Total actual weight of the trailer


Then you should also consider the maximum or gross combined weight rating (GCWR) - this is the maximum weight you're allowed to have on the road, which combines the full weight of the truck and trailer combined. To get these, you can use the GVWRs of both the truck and trailer, but really you should go to the scales and have them weighed as YOUR truck and YOUR trailer almost certainly do not weigh what is shown in the literature.

When working all these numbers, you need to consider the following:
- Do I have enough truck (engine, transmission, etc.) to pull this load up and over the hills without causing undue stress on the engine/transmission, etc?
- Do I have enough truck (brakes) to stop this whole load in the event the trailer brakes fail without causing undue stress to the truck brakes?
- Do I have enough truck (suspension, tires, etc.) to control the trailer in heavy winds, while passing or being passed by semi trucks - and especially in the event of an emergency maneuver, like dodging a deer which just jumped out on the road in front of me or a tire failure (blow-out).
I hope you're getting the basic information before you purchase. You'd be way ahead of the game.


In-depth video at RVSEF web site on truck - trailer matching HERE.

##RVT897

Why should I weigh my Trailer tires individually?

Many times when I give a seminar or class to people from the RV community on proper application of tires, I get asked the above question by trailer owners.


Motorhome users seem to understand that knowing the actual load on each corner of their RV will allow them to select the tire pressure they want to run as they balance Ride, Tire Durability, Fuel Economy and Steering Response.
Owners of trailers say they have been told that all they need to do is to run the max inflation on the sidewall of their tires. Almost all the time, that is the information on their placard per the trailer manufacturer’s guideline so that recommendation seems reasonable.

Recently I have been having a discussion via email with an owner of a large 5th wheel trailer.
He even believes that because the load changes on every trip and almost every day it could be “dangerous” to get the trailer weighed as that could mislead the trailer owner to think it OK to lower the pressure to just match the minimum needed to carry the measured load. I would like to offer a different view on why it is important to get the trailer weights on each tire at least once.

More than half of the trailers that have been weighed over the years have been found to have a tire or other suspension component exceeding its maximum load capacity. We all know what happens to people who “Assume” something. When you get the trailer weighed, it does need to be with all the stuff you normally carry on your travels. Knowing this load you will have the facts to see how critical it is to re-distribute the load side to side or front to rear. You will also know if your unbalance has overloaded one or more tires even when the total load seems OK. You will also know if you have proper loading on your pin. I have heard that some axle manufacturers consider having one end of an axle loaded to more than 50% of the total axle rating as overloading the axle. This means that even if you have changed wheels & tires to not overload those components you might still be overloading the axle if you are not careful.

Another thing to consider is the “Reserve Load”. That is the “safety factor” between the actual load and the calculated maximum for a component. Most cars have a 12 to 18% reserve load. One of the other benefits of having a good sized Reserve Load is that it will allow you to occasionally bring home that big load of stuff you just had to buy at the flea market and not overload a component. One way to identify a minimum cushion would be to look at the load capacity of your tires when inflated to a pressure of at least 15 psi below the max inflation and to be sure your load does not exceed that level. A 20 psi cushion would be even better. Reserve load helps compensate for the unbalance that occurs due to side loading because of road crown and wind side loading.

You are probably asking yourself why trailers can’t lower their tire pressure the way motorized units can. The reason has to do with the mechanics of axle spacing and tire side loading whenever you turn a corner. Trailers with two or three axles put enormous side loading on the tires, wheels, and axles whenever they turn corners. A quick look at the tire distortion when you see a big twin axle trailer park will confirm that the tire needs to be as stiff as possible when side loaded. Since we all know it is the air that carries the load we also now realize that it is the air that stiffens the tire and as a result will lower the side deflection and loading.

BOTTOM LINE: Next time you are fully loaded please go to the effort of weighing your trailer and go the extra mile to do the multiple weighing and the math needed to calculate the actual load on the individual tires. Remember a blown trailer tire can lead to substantial damage and financial loss.

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.

New Truck with old Trailer, Sway (Stability) issues

I was asked by the moderator of an RV trailer forum to jump in on the topic by one of the members. Here is my response. I thought the info might be of interest to others.

My prime area of expertise is on tires, but I have also done extensive long-distance towing with a C3500 (454 gas) Dually with an 8' slide-in camper in the bed and pulling a 26' enclosed trailer with a  Camaro race car,

 tools, etc inside. I have towed from OH to FL and Calif and even made a trip on the Blue Ridge Pkwy while towing the race car trailer so I think I understand the handling situation of a loaded truck and trailer.

Some observations on info in some of the posts in this thread.

"P" type tires on the TV should be an indication that the TV should probably be limited to single axle "pop-up" or small boat trailers. It is important to remember that P-type tires must be "de-rated" in load capacity by 10% as they are not intended for truck or trailer application. Multi-axle trailers almost always indicate the need for a real truck (not just something that looks like one) This would mean LT type tires and 3/4 or 1-ton vehicle rating.
HERE is a good post on how to match a Truck & Trailer. It is much better to do a little work and learn the ratings of both your truck and trailer than relying on the salesperson telling you that you can pull a 5-ton trailer with a Ford Ranger just so he can make the sale.

P vs LT type tires. No, they do not make tires "between" these two types. If there is no P or LT before the numbers in the size then it is what is known as a "Euro-Metric" type tire and with an inflation of 36, it is definitely a P-type tire.
The P and LT and ST designations are found in tires designed and intended for highways in America. The inflation level molded on the tire sidewall is the give-away.

35 or 36 is standard Passenger type tire
50, 65, or 80 or higher is an LT type tire or a trailer only ST type.
In Europe, they use a "C" after the size to identify "Commercial" use i.e. LT. Don't confuse this with Load Range C which is the US Light Truck Load Range (old ply rating)

Tires marked "XL" or "Extra Load" are Passenger type tires (Max infl. 42 or 51 on some) that aren't quite LR-C Light Truck tires. This type was originally intended for "Station Wagons" which have been replaced by SUV but are many times used on light duty trucks as a cost-savings measure. (Passenger are really LR-B or "standard" load except for the XL passenger tires, but none say LR-B)

Vehicle "Sway" is a function of the tire inflation and the suspension design and vehicle loading. When inflated to the sidewall pressure there can be different sidewall stiffness but the difference should not be significant within the same Type tire and same Load Range.

Weights. Empty weights are really of little value in determining what will run down the highway safely. Fully loaded and hooked up individual axle loads are what is needed. With trailers (and motorhomes) it is even better to know the scale weight on each individual tire position when the vehicles have been loaded to their heaviest, as many RVs have been found to be 500 to 1,000# out of balance side to side. No, you don't have to get on a scale every trip but should have at least one set of readings when fully loaded.

Stability.
Increased tire inflation will almost always result in improved stability.

New Tire "squirm". When you apply any new tire to any vehicle you may notice for the initial 100 to 200 miles less stability than on the tires you just took off. This happens for a few different reasons.
1. All tires have a "release agent" that allows them to be removed from the curing mold. This is a slippery substance that will wear off in a few miles.
2. When you buy new tires you are not making a clean, apples to apples comparison. You, as the driver, are comparing a worn older tire with a new full tread tire. Even if the tires were identical in construction and rubber compound you will get crisper and more solid handling with a worn tire (less tread) than a new (more tread) tire. In car racing, our tires may only have 2/32 or 3/32 tread depth while new tires come with 8/32 to 12/32 or more tread depth. In some classes of racing that require the use of "street" tires new tires have most of their tread "shaved" off with the express purpose of improving handling.

Getting scale weights. While you may not want to make a special trip to a truck scale or gravel pit or feed supply to get your individual tire loads you can at least stop at a truck stop on the Interstate on your next trip to get a reference number for each axle.
HERE is a worksheet to allow you to calculate individual position weights.
and HERE is a Youtube video on how to weigh on a truck scale.

Trailer tire inflation:
To lower the Interply Shear (force trying to create a tread separation) You should run AS A MINIMUM the inflation required to support 110% of the MEASURED load on the most loaded tire. Better yet run the tire sidewall inflation number. You can learn more from THIS Google search or go to my blog and read up more than you ever really want to know about the tire mechanics involved in the tire construction.


Hope this helps.

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.

Problems with small trailer tires.

 Roger, back in March 2024 I bought a Bear Track Aluminum Trailer to tow my new-to-me Harley Trike. The bed is 10', trike fits great, trailer weight 400 lbs and I love it. But not so happy about the tires. For one thing, I just found that when I bought the trailer, the tires were already over 2 years old, date stamp is "0822". Anyway, we did a short trip in Apr 2024, all was fine. Then a big trip in Oct 2024 and about 80 miles from home we had a flat. A puncture, so not the tire's fault. But then in March 2025 I took the trailer back to where we bought it, because one side the wheel wiggled. They said the bearing needed tightened. They did that under warranty, but also told me the tire on the other side had a broken belt. They got that replaced under warranty as well. Then just last week, returning home from a 5 day trip, the other tire a belt broke while on the road and the tire went immediately flat. So now, all 3 tires, only the patched tire (now the spare) is even usable.

So my question is, what brand do people recommend most for trailer tires and where can I get some that were made very recently. The ones that came with the trailer are MasterTrack UN203 Branded, Size: ST175/80R13, other numbering: 6PR 91/87M Load Range C, rated at 1360 lbs (single) I think that means one tire on each side. So the Trike weight 1200 lbs and the trailer 400 lbs. So that should be 1600 lbs total or only 800 lbs exerted onto each tire. Well below the limit indicated.

Also this tire says Plies: Tread Polyester 1 + Steel 2 Sidewall Polyester 1

I am willing to spend more for a better brand, higher Load Range, whatever it takes.

 

My answer..............

Based on the numbers in your post, I'm not sure why you are having "broken belt" problems. That term is kind of a catch-all, so without seeing the tire and lacking a few pictures of the "Broken Belts", I am doing a little "shooting in the dark, but here goes.

 

An 0822 date tire should not be having belt problems at that light a load. However if the dealer was willing and able to change one tire under warranty at 3 years old and the second tire also having some issue at 3-1/2 years, it is not unreasonable for you to "shop around".

IMO an  ST175/80R13 LR-C when only loaded to 50% of its "rated load" in your type of service should be good for 4+ years assuming it is always fully inflated to 50 psi.

I would suggest you check with etrailer.com as they have both radial and bias tires in LR-D in your size. Even though the etrailer tires are LR-D you can get 1,360# load capacity at 50 psi as long as you keep them inflated to that level.

 

Interply Shear and am I spreading Fear?

On an RV Forum I saw some posts about tire failures:

There were some posts that mentioned the RV total weight capacity or Gross Vehicle Weight Rating (GVWR). Others were mentioning the total of the Tow vehicle plus the RV Trailer or Gross Combined Weight Rating (GCWR). While others were talking about tire load capacity.

One person responded with: "You need to pay more attention to just what acronyms are telling you. GCWR has nothing to do with tire inflation pressures. The tow vehicle and towed vehicle are individual vehicles and answer to their own standards and regulations.

Maybe you should ask Roger why, during his working years with tires, the interplay sheer problem was never corrected. (Interjecting a fear factor for attention purposes might be Roger's goal).

Tire tread separations are a fact that trailer haulers must respect. Fooling around with RV trailer tire inflation pressures by using less than what the vehicle manufacturer has recommended is, IMO, counter productive".

 

Since I was mentioned if the forum post along with the suggestion I might be spreading fear, I felt the need to reply.

Sorry, but Fear is not and never has been my intent. I covered interply shear and how I discovered that it was different for multi-axle trailers than it is in motor vehicles in my blog post of Nov 20, 2013 when I did the first of multiple posts in Interply Shear as it pertains to multi-axle trailers. The fact is that Interply Shear is well known in the tire industry but not the unique forces imparted on multi axle trailers. It was pure chance that I noticed the irregular path taken by a trailer doing a 180° U turn on freshly smoothed gravel (picture in the Feb 22 2018 post). That was my Ah-Ha moment. 

 

While I was retired by the time I first saw the gravel path, I still had friends that worked with the high power Finite Element and vehicle simulation software and called in a couple favors. I asked them to run a simulation of a truck pulling a tandem axle trailer through a series of "S" curves. All 8 tires were identical with the same load and inflation so we could end up with a comparison of trailer effects on tire belts vs the normal interply forces found in all radial tires in motor vehicles applications. What we discovered after the overnight computer run, was that in  simply S turns (similar to seen in the video) the belts on a multi axle trailer were developing 24% higher internal shear forces than the tires on the truck pulling the trailer.

 

This goes a long way in explaining why tires in multi-axle application have much shorter life and significantly higher failure rate than do tires in car or truck applications.

 

So you see, Science and facts can be used to point out why blindly following 50 year old design decisions and regulations that are the foundation for the current ST tire regulations can be significantly improved upon.

##RVT1054

Q&A on 5th wheel trailer and tow vehicle, and an announcement

 Dear Roger,
"I pull a fifth wheel trailer. When I am towing, I inflate all tires on the truck and fifth wheel to their max load pressure. The truck tires are load E, Max psi is 80. The fifth wheel tires are load G, full steel belted with max pressure 110psi. Weighing the rig at Cat scales shows me that the weights on the axles are not heavy enough to warrant using the maximum pressures. I would like to inflate the tires so that the psi matches the actual load. Is there a way to calculate this? I have scoured the internet and cannot seem to find any tables that would give me a clear answer. Thank you for your advice."

My answer:
Hi. There is much information in this blog that can answer your question and provide good background. But I'll provide some info here so you can get on the road with proper inflation. You can then review the info on the blog as time permits.

First we need to separate the tow vehicle from the trailer as the tires are different and the forces on the tires are also different.

Trailer.  Due to the fact that trailer tires are basically being dragged around corners rather than steered, there are forces called "Interply Shear" that place much higher forces on the belts of the radial tires when in trailer application as opposed to when on a motorized vehicle such as a motorhome or tow vehicle. You did not offer the actual measured loads on the trailer tires or the tire size so I can only offer general guides.  Be sure the load capacity of the tires at the inflation you are running is at least 10% greater than the measured load with 15% better. Were the LR-G tires the OE size and load range or did you upgrade to larger or higher?

Tow vehicle:
If you are running the OE size and load range your owners manual should give recommended minimum inflation levels for a loaded vehicle. Many pickup trucks offer "full load and light load" condition inflation recommendations. I recently (Feb. 15) did a post on tow vehicle tire inflation. I suggest you review it and let me know if you still have questions.

You also asked about load and inflation tables. Here are 5 posts on that topic.

* * *

Now, some of you may feel that a more detailed reply would have been in order, and I agree. However, it's very difficult to provide a more specific answer without more information. This brings me to the "Announcement" portion of this post.

With almost 260 posts in this blog, I've covered most features and topics but after reviewing the posts some people still have questions. So I'll do my best to try and provide specific answers. There are a few specifics that I will need in order to provide a useful reply.

With thousands of possible tire/vehicle combinations there is no way I can know the specifics of them all. So I will need your help. When you send a question please provide as many of the following details as possible:

•Tire size, including the letters in front of the numbers if any.
•Load range or the tire max inflation.
•Measured load on each tire position or each axle. If scale weights are not available then GAWR from the tire sticker/placard
•Specific application, i.e., trailer, motorhome, car, pickup
•Tire DOT if the question is about tire age or country of origin

Send your questions to me, Roger, at Tireman9(at)gmail.com


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What inflation should I run on my Tow Vehicle and is my RV trailer different? draft

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

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

TV  Tire size, type and inflation should go by the Certification Sticker on the driver door jam. This was established by a team of tire and vehicle suspension engineers over a period of two to 4 years with 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 30 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.

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 TT.

 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 RVIA established a Reserve Load of 10% but for RVs built before 2017 there was no Reserve Load requirement.

 

Those of you that 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.

RELATED INFO. The words "Max Inflation" on the tire sidewall is NOT the highest inflation the tire can tolerate, but is the highest inflation that will provide the greatest load capacity of that tire. The published Load & Inflation tables show the direct relationship between inflation and load capacity. If you want to increase load capacity you MUST increase the inflation. BUT once you reach the stated "Max Cold Inflation" there is no further increase in load capacity available for that tire, even if you increase the inflation above the number molded on the tire sidewall.

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

The letters before the numbers in the tire size is 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 passenger carrying motor vehicles. if a P type is applied to a trailer its load capacity is to 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.

A few days later the following was added to the Forum thread

"------------------ Steer Axle ---Drive Axle --- Trailer
Weight #1 - Truck only/hitch/bars -- 3180 3520 (exceeds 6500 GVWR)
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 so the initial scale reading seems to exceed his GVWR !

My response

I am assuming the "Trailer" scale was for the 4 tires on RV. If so it sounds like the trailer was empty when you did the weighing.

While waiting for a response I saw a comment about Michelin tires

"- 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 who has a "line" of cars called Chevrolet with many type vehicles from sedans to SUV and pickups.

In my post I covered the 3 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.

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 of 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 and the load capacity is published in tables available in some owner's manuals or tire data books or even on the Internet as seen HERE.

All the above applies to tires made to US Tire & Rim Association standards. Europe has some similar but different standards with different codes. So do tire made to the standards in Asia. I will not go into those standards as I probably have 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.

I am wondering how many readers would be able to offer advice to this fellow RV owner.

How would I set inflation on a smaller single axle trailer?

Got this question from a reader. 

Our Jayco Hummingbird came from the factory with P235/75R15SL tires. The TT GVWR is 3,750 lbs and the GAWR for the axle is 3,500 lbs. This is a single axle TT. The P-rated tires were like a pogo stick at max inflation.

We changed from the factory tires to Maxxis 8008's in ST225/75R15 size. The factory aluminum wheels are good for a max of 80 psig according to the stamp inside.

Also converted to metal valves stems for running our TPMS....because I'm an engineer who tends to overdo everything I touch .

The heaviest CAT scale weight has been 3,320 lbs on the axle and 3,780 lb GVWR. We've since removed a few items to stay within the 3,750 lb. GVWR.

I've always kept the tire pressure at the minimum sidewall stamp of 65 psig (Max load of 2,540 lbs at 65 psi cold). After reading some of your blogs and looking at the Maxxis load chart, if I assume an equal split weight on each wheel we would have a worse case of 1,660 lb load. Of course a perfectly balanced load isn't likely to ever happen. But even with adding 10% it would put us at 1,826 lbs per wheel. Maxxis says that for our particular tire 40 psig would give us 1,880 lbs capacity.

I can't say that I'm comfortable going all the way down to 40 psig, but I feel ok with 50 psig, even though this is grossly over-pressurized for the given load. I know that at 65 psig the TT rides like a log wagon and we recently bent a spindle on the axle without even knowing it, I wonder it the limited travel of the torsion axle combined with the tire pressure came into play because we were under the GAWR of the axle and never even felt anything out of the ordinary during the trip, of course we are pulling a 3,750 lb trailer with a '17 GMC 2500HD w/ Duramax so we don't feel much anyways.

So if it were yours what pressure would you choose? I've been running at 65 psig and I think that's too much, 50 psig sounds good to me, but it's still too much pressure according to the weight charts.....

http://www.maxxis.com/trailer/trailer-tire-loadinflation-chart

Thanks!"
Here is the answer I gave him.

My approach
OE tires P235/75R15 are  rated for 2,280#@35 psi but on a trailer we need to De-Rate the load capacity so 2028/1.1 = 1844# load capacity.

Your measured axle load was 3,320#
If we assums a nominal 53/47 side to side split we get 1,760# for heavy end  and a 60/40 split gives 1,982# for the possible heavy end of the axle.

An ST225/75R15 LR-C is rated 2,150# at 50 psi. Since we are looking at a single axle trailer we can check the tables and find 40 psi is rated to support 1,880# and 45  psi can support 2,020#.

Since we always select the pressure needed that can support the heaviest end of an axle and we always inflate all tires on any one axle to the same inflation we could select  40  to 45 psi for our CIP.
I would set my TPMS Low Pressure warning level to 40 psi and my CIP to 45psi.


If this was a multi-axle trailer we would want to lower the special belt shear forces and run a higher inflation. Maybe 50 psi minimum

Strange wear on trailer tires - Camber Wear Not proof of a "Tire Defect"

 If you have one shoulder of a trailer tire showing more wear than the other as seen here,

 

 

 

 

 

 

 

You probably have Camber Wear. Unlike the front end of your car or truck few RV trailers have  alignment that can be adjusted with hand tools. BUT the first thing you need to do is confirm you are out of alignment

Assuming that the axles are still firmly bolted to the trailer frame and there are no obvious bent or damaged spring shackles you might need the help of a professional RV Trailer Alignment shop as seen in THIS video.

But you might want to first confirm that you even need to have the axle replaced or "bent".

 You can measure your Camber and the Toe-in.

Low cost Camber Tool....

 How to measure TOE at home. This video is not of an RV trailer but it does a good job of showing how to be sure the axle is square. While this is on a car you should be able to see that we are looking at having the front and rear of the two tires on the same axle being the same distance apart.

https://youtu.be/Wab8IcOQvQw

Here is another trailer having the axles measured

 https://youtu.be/5GnL6rWZhw4

 I tried contacting Lippert to learn the alignment specs for Camber and Toe but they would not give me the information without providing the VIN and other info on the trailer. I don't understand this as logically Camber should be near 0° and Typical toe-in specs vary from one-thirty-second to one-eighth-inch with closer to 1/32" being better for tire wear.

I did find these Reference Documents for Lippert Axles

https://www.carry-ontrailer.com/wp-content/uploads/2015/03/Lippert-Axle-Manual.pdf

 ##RVT1065

Tow vehicle tire question

Dear Roger,

"Last week I purchased new E rated tires for my F150. Went with the BF Goodrich KO2 (275/65/18) and so far I'm very happy with them. These are larger than the tires that came from the factory (265/60/18). I have a quick question.

"The tire shop originally put the PSI at 35 based on the door sticker, but after reaching out to BF Goodrich they suggested 45 PSI for everyday driving. They didn't give any recommendations for tire pressure while pulling a trailer since there are so many variables like trailer size, weight and cargo. The tire shop suggested going to 50 pounds and then deflating back down after the trip. I'm just curious if others actually bump up their PSI while towing, and have you noticed a difference in doing this? Just trying to figure out if there is a benefit to doing this.

Some important information is missing, so let's identify our assumptions:

1. We are talking about the tow vehicle tires - the F150.

2. The original poster didn't identify the OE (original equipment) tires as being P type or LT type. This makes a major difference and providing that information up front will eliminate guesswork for those trying to help with the answer. With a 35 psi door jam sticker number, I suspect the OE tires are Passenger type and the replacement tires are LT type.

3. Since it is the air pressure and not the tire construction that supports the load on a tire, buying LR-E (80 psi) tires when you never run higher than 65 (LR-D) would be a waste of money. Also if you are changing from P type tires with a max inflation of 36 to 41 you need to confirm the wheels are rated for higher load and inflation of LT tires.

4. The door jam pressures are based on the car company making some estimates on how much and how often you have the vehicle empty or fully loaded. You might check the owners manual and see if they give an empty and loaded tire inflation suggestion. The inflation in the OE tires is what is needed to support the GAWR but few people run that heavy all the time with their pickup.

5. I suggest you get the F150 on some truck scales when empty and again when fully loaded with the trailer also fully loaded, and learn the real facts of the various axle loads under both conditions.

6. Knowing the real loads, you can use the tire Load & Inflation tables to learn the MINIMUM cold inflation pressure for the F150 for the two situations. I suggest you use a margin of +10% of the table inflation to establish your CIP (cold inflation pressure). See other posts on weight and inflation if you need more details.

7. Trailer tires are a completely different situation and should always be inflated to the tire sidewall inflation. You should also run no more than 85% of the trailer tire max load rating when on a scale. Cornering, sway and side wind loads have been shown to shift loads side to side by 10% or more. Also the Interply Shear on tires in trailer application needs to be considered as this is a major contributor to trailer tire belt separations.

Hope this info helps clarify.

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##RVT781

Changing tires on a trailer - NEW load capacity requirement

On many of the RV forums I monitor that focus on trailer application, there is a recurring question about changing tires. Some wonder about going up in Load Range (Ply Rating) some wonder about changing the "Type" tire P > ST, or P> LT, or ST > LT. Others want to change the tire dimensions. While there are many reply posts, I do note that not everyone offering answers has worked as a tire design engineer. It takes years of working with the engineering and scientific knowledge before you can be given the responsiblity to develop a new tire capable of passing various company and DOT regulations and be produced for sale in the tens or hundreds of thousands.

While I have tried to provide answers, I seem to end up saying the same things over and over so this post is intended to be a go-to post for those asking tire change questions.

First, it is important that the owner know the ORIGINAL tire size including the Type and Load Range and the recommended inflation from the RV company along with the GAWR for their specific Trailer. Finally if considering a change we need to ensure that the new tires can properly support the ACTUAL load on the new tires.

I will start off assuming the owner is keeping the tire dimensions the same i.e. 225/75R15 > 225/75R15. Note I said "dimensions" not the "size" as a tire engineer "size" to me includes the Type + Dimensions + Load Range

For P > ST or P > LT you need to remember that application of a P-type tire on trailer required that the RV company "De-rate" the load marked on the tire sidewall   Sidewall/1.10 = load capacity of a P-type tire on a trailer.
For ST > LT you will probably need to increase the dimensions and or Load Range to achieve sufficient load capacity.

The general rule of thumb "Any replacement tire MUST be capable of supporting equal or greater load than the original tire".
Another rule: You need to ensure that any tire you use is capable of supporting your actual MEASURED load, Not the load your neighbor said he has and not an estimate or the measured load someone posted on a forum. The load measurement ideally should be obtained with your trailer at its heaviest i.e. fuel, water, propane, clothes etc  If you can't get individual one side weights DO NOT assume a 50/50% side to side load split. While some trailers may be balanced at 49/51% but some have been found as much as 10% off balance i.e. 40/60% As a rule of thumb I suggest you use at least an assumed 47/53% splits you would use the 53% figure.
If making an investment in new size tires & wheels you really need to learn the real loads before making the change or you may discover you bought tires you should not be using.


You will need to consult the published Load & Inflation tables for your old and new tire to confirm load capacity numbers. I have THIS post with links to many different tire companies. Be sure you understand how to read the tables as while most provide load capacity per tire, some load figures are per axle. DO NOT use the "Dual" load numbers as these only apply when there are two tires mounted as a pair on each end of an axle.

Comment on valves. I always recommend that whenever changing tires, even if you are just replacing with same size and type, that rubber valves be replaced with bolt on metal valves and if you already have metal bolt in valves that you get the various rubber gaskets and "O" rings replaced as these rubber parts age out just as tires age out and it's awful to read about a $2 valve failing which can result in hundreds or thousands of dollars in damage and costs.

Finally, some new info that all RV owners should consider:
Late last year RVIA updated the tire type and load spec such that "based on the rating of the axle the tires have to be 10% greater than the axle rating,"  You will note that RVIA decided to ignore the reality of load unbalance.
Clearly, if you are getting new tires it makes sense to incorporate this new safety margin in your calculations.

I want to thank my fellow RV owner and tire design engineer CapriRacer for doing a bit of technical editing on this post. He also has a blog on tires. 

Next week I will do a post on trailer tire inflation.

If you find this post helpful and happen to see someone posting questions about changing tires please consider posting a link to this post as I don't see every tire question posted by all RV owners.

How do I set inflation on my TV and RV trailer?

Got this PM from someone who reads RV forums.

"Good evening, I'm looking for some professional advice. I have a 2500HD pulling a fifth wheel, my steer axle is 4,000 lbs, drive axle is 5,380 lbs, and our trailer is 7,700 lbs. Tires on the TV are rated for 3,100 lbs a piece at 80 psi. The RV tires are rated for 2,800 lbs at 80 psi. Do you think I am ok to run 65psi in the steer axle, 70psi in the drive axle? What I've been doing is 75psi steer, 75-80psi drive, and 80psi trailer. Our TV tires are Michelin MS2 LT265/70-17 the load range chart says I can run much less psi. Also, our trailer tire's are Maxxis 8008 225/75-15 LR E, also the load range chart shows I can run less psi. Any thoughts on what you would do would greatly be appreciated, thank you. New Crusader owner"


To start with it is important to remember that we should never operate tires with inflation lower than what is needed to support the actual load on the tires.
As I have pointed out in my blog we first need to confirm the actual load on our tires. For TV we will find the side to side loading is usually 49/51 to 50/50 split so we can take the measured axle loads divide by 2 and then use the resulting figure when consulting the Load Inflation tables for our size tire.
I have links to many load/inflation tables and related info HERE. Once you find the MINIMUM inflation, I suggest you add at least 10% to that number to allow for day to day variations. Having said this, it is important to remember that the car company spent a lot of time and money evaluating many different tires at different pressures to arrive at the recommendation that is on the Tire Placard on the driver door jam. You should note that the Placard inflation may be higher than my above-suggested calculation as my calculation is only addressing load and not handling, braking or the fuel economy.
The scale weight for the TV for the above needs to be the reading with the RV & TV fully loaded as heavy as you ever expect to tow.

The RV tire inflation is different. Due to SIDE LOADING in trailer application, I strongly recommend running the tire sidewall inflation as your CIP. I think that if you review the RV Placard info from the RV company you will also find that they say to use the inflation number on the tire sidewall.

I know you were hoping I would just tell you to run xx psi here and yy there and zz on the trailer but without knowing tire sizes and Load Range for all your tires, I don't have enough info to provide an informed answer BUT I feel you now have enough info that after reading the sidewalls of your tires - Be sure to check each as they might be different on TV and RV and possibly even different F & R on the TV - You now know how to establish the correct inflation for your current vehicles and for any vehicles you own in the future.

I hope this information and instructions help. If not you can always contact me directly using the email under my video picture to the right.

Post a complaint with NHTSA - Part 3

 Lets finish up this thread.

How to write a useful Description of tire failure for NHTSA

Lets take a short break from all those screen shots and review how to write a meaningful "Description" of the failure as seen in Step 1 of my post of Jan 8.

You need to remember that NHTSA needs facts if they are to take any further steps as they consider if an investigation is warranted.

Just ranting "My tire exploded, tore up my RV and I want new tires and compensation for the repairs and my expenses." is not useful to NHTSA. If you can't be bothered to provide critical information such as tire DOT serial why should NHTSA make an effort to investigate? Other information such as location of the incident, your cold inflation and info on what warning you received from your TPMS, if you still have the tire and can provide pictures or contact information on who has the tire, would be bonus information and potentially very helpful to the engineers at NHTSA. I previously outlined the MINIMUM information needed in my post on Dec 31 2012.

Here is an example of an actual complaint #10262837    "I  OWN A 2004 GMC SIERRA 3500. THE VEHICLE HAS POWER KING TOWMAX TIRES, SIZE LT235/80R16E. WHILE PULLING THE TRAILER AT 65 MPH, THE SIDEWALL ON THE PASSENGER TIRE BLEW OUT."

Looking at the Power King catalog on pg 10, I do find a Towmax STR in ST235/80R16 LRE size but ST type tires are not approved for use on passenger carrying vehicles such as a GMC 3500 pickup. Moving on to LT tires on pg 14 & 15 I find no metric sizes such as LT235/80R16.

So we are confronted with a few questions.   Did the owner provide the correct tire name but wrong size?
Did the owner provide the correct size but wrong tire name?    Did the owner mean the tire on the passenger side or that the tire was actually a passenger type tire?   Was the tire that failed on the GMC?
No mention of an RV or trailer brand is made by the owner. I also note that the tire manufacturer is entered as Cooper Tire and Rubber Co but according to my contacts at Cooper, they do not have a Power King or Towmax tire line and have not made ST type tires for many years.

If you were NHTSA would you consider this person a reliable witness to what actually happened  with a tire?

Now you are wondering what a good report looks like. Lets look at #10471651
" THIS COMPLIANT IS FOR A TANDEM AXLE TRAILER TIRE FAILURE USED ON TRAVEL TRAILER CAMPER. THERE ARE THREE TIRES FAILURES WITH THE SET OF FOUR ON MY TRAVEL TRAILER. THIS REPORT IS FOR THE FIRST TIRE. THE TIRE IN QUESTION IS A DENMAN ST225/75R15 LOAD RANGE D. TYPE: ST EXPRESS RADIAL DOT CODE: 4FEH M5M 1908 TIRE INSTALLED: 4-5-2009 FIRST TIRE FAILURE OBSERVED ON 4-21-2012 MILES USED UNTIL FAILURE NOTICED, 10,164 MILES TOWING SPEED: 55 TO MAX OF 60 MPH AS STANDARD TOWING SPEED ON HIGHWAY. AIR PRESSURE OF 65 PSI CHECKED AT THE START OF EACH CAMPING TRIP. TIRE FAILURE OCCURRED WHEN THE TIRE WAS MOUNTED IN THE LEFT REAR POSITION. I HAVE CERTIFIED SCALED WEIGHT SLIP THAT CAN BE PROVIDED OF THE CAMPER AFTER THE FAILURE.
THE REAR AXLE LOAD IS 4,040# ON BOTH REAR TIRES. THE TOTAL RATED CAPACITY OF THE REAR TIRES ARE: 2,540 X 2 = 5,080# AT 65 PSI COLD PRESSURE. CAMPER VIN [XXX] CAMPER MANUFACTURE: SUNLINE COACH COMPANY, DENVER, PA MODEL TRAVEL TRAILER: SOLARIS SR, T310SR, GVWR: 10,000# GAWR: 10,000# I AM THE ORIGINAL PURCHASER OF THESE TIRES WHICH REPLACED THE ORIGINAL TIRES ON THIS CAMPER. THE SIZE, LOAD RANGE AND INFLATION PRESSURE MATCH THE OEM TIRE PLACARD. I HAVE PICTURES AND CAN SEND IF NEEDED. THE TIRE HAD A TREAD BULGE INCREASING THE OD OF THE TIRE IN AN APPROX. 180 DEGREES OF THE TIRE AND WAS FOUND DURING CAMPER AXLE MAINTENANCE. THE TIRES WAS REMOVED FROM SERVICE UPON FINDING ON 4-21-2012 BEFORE TOTAL FAILURE OCCURRED DURING TRAVEL AND TO PREVENT CAMPER DAMAGE. THE TIRE WAS SENT TO A TIRE ENGINEER AND SECTIONED FOR ANALYSIS. NAME CAN BE PROVIDED IF NEEDED. I HAVE PICTURES THAT CAN BE PROVIDED TO MATCH WITH THE ENGINEERS REPORT ALONG WITH HIS
NAME AND ADDRESS. THE REPORT DECLARED THE TIRE FAILED DUE TO DETACHMENT. THE FAILED TIRE WAS REPLACED WITH A NEW TIRE OF THE SAME SIZE AND LOAD RANGE, HOWEVER A DIFFERENT BRAND. I HAVE ALL PICTURES AND REMAINS OF THE TIRE IF NEEDED FOR FURTHER INVESTIGATION."

See the difference?

You do not need to write as detailed a complaint as in the last example but IMO the MINIMUM information you need to provide would be:

1. A brief description of the tire condition, i.e., "Tread and belts came off" or "sidewall disintegrated"  along with a list of damage done to the RV with the associated costs would be desirable.

2. Vehicle VIN - This is on the vehicle label and probably on your registration slip.

3. Tire DOT serial - The complete serial number including the date code at the end.

4. Brief statement letting NHTSA know if you have additional information such as: "I have photographs of the tire" or where the tire is: "Tire is in my possession" or "Tire delivered to Fred's tire in Anytown, USA.

5. If you were using a TPMS that would definitely be information I would include.




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Careful when you do research or before you buy

Recently I was asked to clarify some information an RV owner discovered as he did some research. What we see here is an error that is easy to make. Even for the more experienced RV owner.

The RV owner asked about a Michelin XTA
215/75R17.5 LR-J tire that came on a 5th wheel trailer. He had confirmed the load capacity of the tires at their max rated inflation and said they could handle the load on his 8,000# axles when inflated to the placard inflation of 120 psi. His question was about the max speed rating of the tires.



He consulted the Michelin Truck Tire Data Book and found that the subject tires were intended for Low Platform Trailer application.
In fact the brochure for his tires even identified the maximum speed rating for his tires as 62 mph.


He wanted to know if the maximum speed rating had been waved for RV application, and if it had what was the new speed limit. He found a statement from the Tire & Rim Association that specifically stating that when a tire had a speed rating below 65 mph it could not be manipulated (increased) with adjustments in inflation and load. He also found a Michelin truck tire service manual specifically saying that low bed trailer tires like the XTA, cannot have their speed increased.
Despite all this information he wanted to know how to work out the adjustments needed to allow a higher speed limit for the RV and could I please help him understand what he felt was confusing information given that the RV manufacturer had selected these tires for his trailer.

I had to tell him that in my opinion there was no way the speed limit could be adjusted. I pointed out that his error was in looking for information in data books and manuals of different type tires such as Truck Tires, RV tires and Low Platform trailer tires and assuming that since his tires were sort of like truck trailer tires and looked like tires he has seen on other large 5th wheel trailers, he could pick and choose which information applied to his specific tires.

The lessons to learn here are:
1. Not all RV manufacturers pay a lot of attention to proper tire selection for the application.
2. When doing research on tire type and specific operation limitations you need to look only at the literature for your specific tire if such literature is available from your tire manufacturer.
3. You cannot depend on the RV dealership to warn you about the real quality or limits of some components of your RV. You need to take responsibility for what you purchase. i.e. Let the buyer be aware.

Important Note:
Speed ratings are like the “Red Line” for your car, truck or motorhome engine. While it is possible to exceed the max speed rating of a tire just as it is possible to exceed the rev limit and run your engine in the red zone, I think we all understand that if we do run our engine that fast we are pushing the odds. In the same manner if your tires are rated for 65 mph max you might be able to run faster for a short time but you are consuming the finite limit of the tire structure and simply slowing down does not repair the microscopic damage you have done to the internal structure of the tire. You should not be surprised if you blow your tire just as you would not be surprised if you blow your engine after running in the red zone for engine speed.

Question on Interply Shear on trailer tires vs tow vehicle tires

A question from a reader of a post on a trailer forum

"Tireman, your concerns over shear puzzle me. G614s are LT tires as are the tires on my 2500 pickup. Only difference is G rated vs E rated. The truck mfg recommends 60 front 75 rear with 80 psi max on sidewall. Why is there no concern about shear on the front tires of my truck. It seems to me the frequency of shear forces is much greater on the truck than the trailer."


Ya, I understand your confusion
 But the reality is that when computer analysis is used to look at the internal structural loading, the fact that tow vehicle tires are all operating at very low "slip angle" (difference between travel direction and angle the tires are pointed to) is significantly lower than for tires on a trailer.

The reason for this is that the center line of of tire rotation for the tow vehicle tires points to the center of the radius while the trailer tires, especially on tandem axle trailers, does not.


This translates into a higher slip angle which means higher internal structural twisting forces on the belts. The computer model suggests 24% higher on the TT tires than TV tires even if all tires were the same with identical vertical load and inflation.

TV front tires have "Ackermann" alignment designed into the suspension but TT have no allowance other than bending of tires, springs, spring mounts and bushings but the forces to bend the springs etc have to go through the belts of the tires.

This is a MAJOR reason for travel trailer tire life to be much shorter than motorhome or tow vehicle tire life.


Hope this helps folks understand a bit of what makes tire engineering a challenge.

Editor: Here is an earlier article Roger wrote about "interply shear," if you want more information. 




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Are ST type tires better because they have a higher "Speed rating"? Maybe not.

Saw an RV forum post about better durability of trailer tires. 

The poster said: " Old trailer tires were rated to 65 mph. Newer tires are rated to 81 or 88 mph. That is a big difference, and since i upgraded to Load Range D and Speed Rating M (81 mph), my trailer blowouts have disappeared. "

My response:

Glad you are getting better durability from your trailer tires. I would you not assign all the better durability on the higher speed rating as we are seeing that many/most ST tires manufactured since 2018 to be more durable than the tires of 2000. The increase in Load Range probably gave your tire durability a good boost. One thing you can look at is the material list molded on the tire sidewall. I think you will see that older tires did not list a layer of Nylon or other material on top of the two steel belts in the tire tread, while many of today's ST tires list the Nylon.

We need to remember that "Speed Rating" is a bit misleading. ST type tires have their load capacity formula based on a max operating speed of 65 MPH since 1970's, otherwise their load capacity would be similar to LT type tires. The test used to establish a "Speed Rating" is a 30 minute step speed test designed for passenger type tires. To pass a "speed level" a tire only has to be able to run 30 minutes without failing, after which, the tire is scrap. So clearly a tire with a 81 mph "rating" should not be considered acceptable for running many cumulative hours at 80 mph.

We really need to only use the "rating" as a measure of RELATIVE heat resistance. ALSO it can be misleading to try and compare the rating on tires from company "A" with tires from company "B" as each company will use test results from a small number of tires that are actually tested, to establish the "rating" symbol for that group of tires. This is done statistically and the statistical prediction used by company "A" is not going to be the same as used by company "B".

Also there is no DOT test for this rating so I doubt that you can depend on all tires of a specific speed rating to perform the same. With no federal regulation for speed performance, I do not see a reason to expect any ST type tire to be capable of running at the stated speed for more than 30 minutes when brand new on a perfectly smooth surface as used in the test laboratory.

Bottom Line:

When shopping for new tires for your trailer I suggest you "read the fine print". Look at the material list for the tread area molded on the tire sidewall. An ST tire with more than just steel and polyester listed i.e.  Polyester + 2 layer Steel + 1 layer Nylon, will probability be more durable than one without the Nylon when operated at speeds above 65 MPH. IMO a "Speed rating" of L ( 75 mph) should be more than sufficient for RV trailer use.

##RVT998

Why do trailer tires fail more often than tow vehicle tires?

Here is the question posted on a travel trailer forum.
Why so many trailer tire failures and so few, if any, tow vehicle tire failures?

This post could be considered Part 2 of my post of last week. 

The primary reason IMO is staring everyone in the face and the information is molded onto the tire sidewall so you do not need to be an Einstein, but almost everyone chooses to ignore the science so they can save a few bucks.

Fact: Tire load capacity is a function of air volume (tire size) and inflation pressure. This is well established and is confirmed by every Load & Inflation chart printed. Increase size or increase pressure and you increase load.

If we all drove around on tires with LT level sizing and inflation there would be almost no failures other than those caused by road hazard or punctures.

BUT
People want softer ride, better fuel economy and lower cost so alternatives were introduced into the market.

LT tires can be considered the standard for tires designed to carry load at highway speeds.

P type tires have a higher load capacity than identical size LT tire but if you check the actual load on most car tires, you will see they have 15% to 25% or more "Reserve Load" capacity (more capacity than needed). Inflation is set for cars based not on load but ride, handling and fuel economy. You also know that P tires must have their load capacity de-rated when P tires are used in truck, SUV or trailer application. When P type tires are run with zero reserve load you will probably get more failures. (See Ford Explorer crashes.)

ST tires have a higher claimed load capacity than LT tires and TT companies use the smallest (lowest cost) tire that can, most of the time, meet the specification for load.

How then is it that ST tires are rated to carry more load?
The load formula for ST tires is based on a 65 mph Max operation speed and an expected shorter life. The formula was developed back when bias tires that wore out at less than 15,000 miles were the norm

Since 1990 when ST tires were still "Alpha" sized ( H78-15ST) the load formula has not been changes as far as I can determine.

The science indicates that if you want tire durability on your trailer as you get on your truck maybe you need to consider running the same type tires i.e. LT type and of course that means you also need to use LT Load & Inflation limits.

Here is some info I posted Sept 14 2011

"Here are some facts from the Tire & Rim Association industry standards book 

P235/75R15 105S (Standard Load -35 psi @ max load)
2028# 35 psi 112mph on a Passenger car
1844# 35 psi 112mph on a SUV or P/U or trailer (no Dual)

LT235/75R15 LRC
1512#single 1377# Dual 50 psi 85mph

ST235/75R15 LRC
2340#single 2040# Dual 50 psi 65mph"

NOTE I did not mention the actual load on a tire but only the specified max. Weight data indicates many TT have one or more tire in overload.

Another question on why Trailer tires fail more often than tires on motorized vehicles.

On a thread on an RV trailer forum Peter made this statement

"At highway speeds interply shear is not as much of an issue, as I understand things."

I posted this reply:
Speed itself is not the issue that causes Interply Shear in Radial tires. Two things happen to radials as they are driven.
One is the tread (and belts) must flatten out when the tire contacts the road. This results in the steel cords moving relative to each other. This is a shear force.
Second is that when the contact patch or "footprint" is forced to turn a corner there is some slipage between the direction of travel that aligns with the center of rotation and the actual direction of travel.

On motorized vehicles the front tires have a slip angle and the side forces are what actually results in the vehicle turning. But of you were to project the center of rotation toward the center of the driving radius you will find that those centers are close together This is due to the "Ackerman" designed into the front end alignment.This graphic from Wikipedia shows how Ackerman works.

Multi axle trailers however have two axles and 4 tires, with no tire rotating around a centerline that points to the center of the turn radius.

Here is a rough sketch of what happens to trailer tires.

As you can see the center of the turn for the tires on the motor vehicle is the same fo all 4 tires while all 4 of the trailer tires are rotating around different centers. This is why the side loading as seen in the video is significantly higher than it is for tires in motorvehicle application.

These tires are forced to higher than normal "Slip Angle" through any turn, Not just the extreme tight turns when backing into a parking space.
Damage to tire structure is cumulative and while a small turn imparts less shear than t tight ture even small forces can do damage on a molecular level.
Here are some links to various articles on Interply Shear.

Duals on a large truck slip on the pavement when forced through tight turns and with inflations over 100 psi they do not deforn as much as trailer tires with lower levels of inflation.

Increased inflation will lower the interply shear. It will never lower it to the level seen on a motorized vehicle Only Passive Steer axles on trailers similar to wat is seen on the back end of cement trucks can lower the shear too but I don't see any RV trailer company offering that expensive alternative. Afterall they know you can't make a warranty claim on failed tires on the RV company as belt failures from Interply Shear is a long term proposition.