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Monday, December 19, 2022

Can I change from 65 psi trailer tires to 80 psi GY Endurance tires?

 Denise asked:

My TT came with the cheap tires out of China. I did a few short trips and one long, NJ to FL. I had no issues but decided to change to the Goodyear Endurance. The old tires had a psi of 65 but the Goodyears have a psi of 80. I did use the TT with the tire psi at 80 but I have seen that even though it says 80psi I should have deflated them to 65psi. Which is correct?

The answer is... It depends.  Ya, this sounds like a cop-out but a key bit of information is missing. 
You can skip to the bottom and just read "The Bottom Line"  but for those that want to understand how I got there here are the details.
That being How much load are you actually placing on your tires? The proper way to learn that is to get the RV on a truck scale with the RV fully loaded. From your comment, I am guessing you might be in NJ or spending the winter in FL so it might not be easy to get the RV fully loaded (all your clothes, books, tools, food, water, and propane as it might be when you are starting a trip). Our goal is to learn the maximum load you might ever place on the tires. Also, it is best if you get the weight for each axle separately by parking with one axle on one scale platform and the other axle on a different pad. If the scale you use only has a single pad you would need to get a weight reading with only one axle on the scale and then move the trailer so the other axle is on the pad.
Once you learn the load on the heavier axle you should assume the load on each tire on that axle is not exactly half the axle load as there is a lot of data from thousands of RV that suggests one end is always heavier than the other but without individual tire load readings, we can not know which end is heavier. So our option is to multiply the heavy axle weight by 0.51. With that number as our estimated max tire load, you would go to the Goodyear Load & Inflation tables.  Select your size and Load range tire. Then find the box with a load number that is equal to or greater than the 51% number you just calculated. The inflation shown for that box is the MINIMUM cold inflation pressure you should ever have in any of your 4 tires when you start to travel. I recommend that people running TPMS set the low-pressure warning level to that level of inflation.
I also recommend you add 10% to that minimum inflation to learn what pressure you should be setting your pressure too. The 10% is a small margin that allows for day-to-day temperature changes that would affect your inflation number as tire pressure changes by about 2% for every change in temperature and I want to save you the effort of having to adjust tire inflation every day.

You may ask yourself why all this work and calculation. What we are trying to do is to ensure that no tire is ever run in overload or underinflated. The 65 psi inflation on your RV Certification sticker is based on the GAWR (max axle load) your RV is designed to carry. In theory, tires inflated to 65 psi should be able to support the load but actual measurements of thousands of RVs have found that a majority of RVs have a tire or axle in overload and this is one reason so many RVs have tire failures. Another reason for the high failure rate gets technical but it has to do with the fact that trailer tires are being dragged around every turn and corner. This side loading results in extreme forces on the tire that is trying to tear the tire apart. I have covered the Science of this force in my RV Tire Safety blog.

Having given you more information than you wanted, and knowing that you may not be able to get your RV on a scale in the near future, I will recommend that until you can get the loads measured that you run your Goodyear Endurance tires at 70 psi. Once you do the calculations you may be able to run lower than 70 or you may need to run higher.

  That's a good start. the 6060# would be both axles so we need to be conservative.
I start by assuming one axle has 51%  .51x 6060 =  3,091 (always round up)
and .51x 3091 = 1,577# 
I need to know the tire size for the next step
Tire size: ST 225/75 R15 so the GY chart says 30 psi can support 1,600#
As I covered in my post on "Reserve Load" you can see that normal motor vehicles run about 30%. Since you have such a light weight RV I suggest you shoot for at least that level, so that gets us to 50 psi minimum cold inflation.  Your Certification sticker says 65 psi so I would run 65 psi and set your TPMS Low Pressure warning level to no lower than 50 psi. You will need to review your TPMS info as some systems can be set to a specific level while others automatically "warn" after a certain percent pressure lost.

Bottom Line
Inflate the GY Endurance to 65 psi cold. ensure the low pressure warning level of your TPMS is no lower than 50 psi.

Saturday, December 10, 2022

Six blowouts! Help! "I'm at my wit's end"


Here is a series of posts on tire blowouts from an RV forum. I am inclined to think that the experiences are not that unusual. This is a long post, but I thought that giving the background, and my recommendations, might help others that have tire problems know that they are not alone.

John wrote:

"I'm at my wit's end on tires. 2000 Winnebago Journey, I have blown out 6 tires since I have had this unit, over 10 years, and done over $30,000 of damage to it. Just got back from a 2,000-mile trip to Colorado and back, after blowing two tires in two days. I have tire sensors, the unit is always kept garaged, no sun on tires, always check pressures, have moved up to 255/70R22.5, which is slightly larger than stock but a lot easier to find. I run 110 psi cold, which is less than the listed 120 PSI. Maybe the age of the tires is getting me, but none are older than 6 years. I am going to start buying two tires every 2 years, to keep the age down. Any other suggestions?"

John continues in another post:

"This is really getting annoying to find tires on the road and then repair all the damage. Thank goodness for insurance. The original tires were 235/80. I upgraded to 255/70, and still have over an inch between tire sidewalls. The tires are not rubbing. It would appear that I have overlooked a very important detail in my quest, and that would be the tire pressure chart many have referenced. I have been running pressure at 110 or slightly below, thinking that might give me a softer ride, when in fact, at that pressure I do not have much "free space" between the load capacity of tires, and the weight of the motorhome.

My coach has a GAWR on the rear of 15,500, and with my tires at 110 psi, the load capacity is uncomfortably close to capacity. I never really understood pressure charts, but do now and will abide by them. In addition, I am going to put my coach in a weight reduction program by removing things I do not need to carry all the time. That, and a program to ensure my tires are not over 6 to 8 years old, and spinning my tires instead of sitting on my concrete slab for months at a time should cure my ills. Thanks to each for your suggestions and help."

There were some replies and suggestions from myself and other RV owners concerning the RV weight.

John posted in reply:

Okay, I finally got my rig weighed, and here are the results. My GVWR on the rig is 24,850 lbs. The weight of my rig with just me was 7,760 lbs. front axle and 13,460 lbs. rear axle for a total of 21,220 lbs.

I figure loaded with people and stuff add 3,000 lbs. with 75 % on rear axle, so that would be 15,710 rear and 8,510 front axle, for a total of 24,220 lbs. weight when loaded.

This is pretty close to GVWR, but the front axle rating is 9,350 and the rear axle rating is 15,500, for a total of 24,850.

Tires are rated 4,670 lbs. @ 110 psi X 4 = 18,688 lbs. rear and front 5,205 lbs. @ 110 psi X 2 = 10,410 lbs. front for a total capacity of 29,098 lbs. for the coach.

So I am close to GVWR but slightly under, closer to limits on axle capacity, but axles aren't breaking, tires are. Seems that I am within limits, and would be even safer if I air tires up to 120 psi, which would give me 5,070 X 4 =20,280 on the rear and 11,020 on the front. Tires will carry far more than the coach is rated for, so my problem must be just old tires. Thanks for all the suggestions on this.

Another RV owner, Mike, offered:

John, do yourself a favor and don't guess what your loaded weight is. On your next trip, at your 1st gas up, go to a station with a CAT Scale, and immediately get your MH weighed at it's full travel weight. You're at 6 blown tires, and counting, with oversized tires and super high pressures. You need more than a back-of-the-envelope estimate of your real travel weight. Somewhere in that combination is a problem unique to your setup.

At this point, I added another and hopefully a final post:

John, it's good that you got some info on the weight, but I think Mike's suggestion to get actual weights and not guess, especially since you are close to the limit, is spot on.
Another point is that GVWR is not what you need to be concerned with. Ideally, you should try and learn the load on each tire position aka "4-Corner Weights". The reason for this is that tires on one end of an axle do not share the load on the other axle end. The load is almost never split 50/50. One end of an axle could have 200# to 500# more than the other end of that axle. I have seen an extreme case with one end supporting 1,000# more than the other end.

To learn the load on individual tire positions you can check with building supply or gravel sellers. But until you can get "4-corners" I suggest you confirm your GAWR from your Certification Sticker and go to a truck scale. But be sure to get individual axle weights as there will be a significant difference Front vs. Rear. I have a few posts on "4-Corner Weights" on my blog you can read HERE to help you understand how and why this is the best system of weighing.


1. For each axle weight, I suggest you assume there is a side-to-side load split of at least 51/49% and use the 51% number for inflation selection until you get actual 4-Corner weights.

2. Look on the Load Inflation Tables to learn the MINIMUM inflation for all the tires on that axle based on the 51% number. Yes, all tires on an axle should be inflated to the same level.

3. For your "cold" inflation, I recommend you run at least 10%, with +15% more than the inflation shown in the tables better, if possible. This will provide a larger "Reserve Load". BUT DO NOT EXCEED the max inflation number for the wheels which might be 120 psi for your coach. So do a bit of research.

4. I also suggest you set your TPMS low pressure to the psi in #2 above.

Hopefully, this confirmation of load and a little bit of math and adjusting your inflation will solve your tire problems.

Friday, December 2, 2022

Short post on Who makes that tire?


I am not aware of a similar list for trailer tires. The best you can do is use Google and post the Question " Who makes TowMax tires" for example. You will get the reply that "Towmax brand is owned by TBC brands. If you have a tire read the DOT serial. The first 3 characters identify the location and name of the tire plant where the tires were made. Here is the link  

Complete List of Tire DOT Plant Codes - With All Old and New ...

Wednesday, November 23, 2022

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.

Thursday, November 10, 2022

Have a broken wheel stud? Here is what I would do...

 Read a post about someone discovering a broken wheel stud on their RV. The wheel had been changed after there was a tire problem and the reported labor done at the tire shop is suspect.

In my experience studs bet broken from car crashes or over tightening of the lug nuts.

Here is a picture of the wheel with the broken stud at the 

8 Oclock position

Click image for larger version

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From my 40 years Endurance Sports Car racing


and mounting hundreds of tires while doing evaluations as a tire design engineer, I STRONGLY recommend that all studs on that tire position be replaced and a HAND torque wrench be used to fully seat the studs unless a stud hydraulic press was used to install a full set of studs.

Also you should check all the studs at 50, 100 and EACH 50 miles thereafter till ALL the new studs have been confirmed to NOT need any additional turning of the torque wrench for two torque checks.

 I believe so strongly in the importance of lug nut torque I required our lug-nuts be torqued whenever we changed tires. In thousands of miles racing, including winning six consecutive 24 hour endurance races my cars NEVER lost a wheel due to broken stud or loose lug nut.

Saturday, October 29, 2022

Tire Autopsies - Run Low Flex Failure and Impact break

 I thought it might help to show a few examples of what was observed while doing a "Tire Autopsy".

A tire would be received with a complaint letter. The letter might say something like " I was driving down Route 66 at 55 mph when suddenly I had a blowout. I wasn't speeding and had checked the inflation just that morning. What is wrong with your tires? There must have been a defect.

The tire looked like this. 







Another view 

On close inspection I found what looked like a hole in the upper sidewall where the

rubber had been worn away.






 Using a bent paper clip,



I was able to confirm the hole


and looking inside we can see where the puncturing item


Note a bent paper clip will fold at the bend if you try

 and push it through a tire sidewall, but if there is a

 hole the paper clip can follow the hole.

For this case I provided the above information and pictures.


Another complaint involve a bulge in the tire sidewall.

 The location was marked by the tire dealer.

On the inside we see a lump

We can also see a dark curved line where the tire

had extra bending.

By cutting away the rubber from the inside of the tire,

down to the body cord we can now see the broken

cords from the impact.

I trust that after seeing these examples you have learned that

sometimes additional examination is required to find and 

confirm the actual failure.

After doing hundreds of similar examinations it allows the

experienced examiner to quickly identify that the reason for

 a tire failure was not some design or manufacturing "Defect",

but the result of external causes.

More like thin in future posts.

Friday, October 21, 2022

"Optimal Inflation" or not?

 Many think the RV sticker (aka tire placard)








or in tire Load & Inflation tables







PSI number is an "Optimum" pressure when it is in fact, it is the MINIMUM pressure needed to support the GAWR which assumes a perfect 50/50 side to side split which is unrealistic.

We tire engineers want to ALWAYS protect the tire from overload. We know that pressure will change with changes in the Ambient (air temperature in the shade). We also know that tires will always get warmer when they are driven.

The pressures found in the Load & Inflation tables are almost universal across all tire companies (a handful of Michelin sizes differ by 5 psi or so, so not significantly different) and they are not playing games with tire engineering science.

Unless there is a specific reason, which should be mentioned in a post, we are ALWAYS talking about "cold" inflation pressure as we know that tire pressure changes by about 2% for each change in temperature of the tire as I covered in these posts.

The Short answer to all tire inflation questions.
1. Get the weight on each RV axle from a truck scale (or similar)

2. If you do not get individual tire position weights (aka 4 Corner weights) assume the heavy end of each axle is supporting 51 to 53% of the axle total.

3. Consult tire Load & inflation tables to learn the MINIMUM inflation to run based on the heavy end of each axle.

4. I suggest that your cold inflation on all tires on that axle, be _at least_ 110% of the MINIMUM psi found in the table in #3 above, but do not exceed the max inflation rating for your wheel. (number may be on the wheel or you may need to consult RV company).

5. *Always* run a TPMS with the low pressure warning level set to the pressure in #3 above ( this might take some calculation effort as different TPMS have different ways of setting the warning level.



Thursday, October 20, 2022

The "safety factor" for tires


Many people ask about the "safety factor" for tires. The dictionary offers this definition for "safety factor": "The ratio of the maximum stress that a structural part or a piece of material can withstand relative to the maximum stress estimated for it in the use for which it is designed." While that sounds reasonable, it really only works when talking about items that fail from simply increasing the load placed on the component.

Items like tires do not really have a "safety factor," as tires generally do not fail by simply increasing the load too much. In a non-rolling situation, I would not be surprised if we could load tires to 200% or maybe even more than 300% of the load marked on the tire sidewall if they never had to roll. However, as soon as you introduce rolling or time or operating temperature, the maximum load before failure is much closer to the max load number molded on the tire sidewall. The exception to Max speed is affected by temperature, time and load. With zero load, many tires can probably handle 200+ mph, but again, for how long and at what temperature?

So you see that tire durability is affected by a combination of load, temperature, speed, and time.

Since tires are basically a structure made of some steel but mostly "organic" components, time and temperature can have a significant impact on the maximum load capabilities of the tire.

If we think of non-organic items like a steel girder or maybe even a stone block as used in a pyramid, we can see that time and normal atmospheric temperatures have essentially no impact on the long-term maximum strength. The exception would be if we were to allow steel to rust or stone to be exposed to water and freeze/thaw cycles.

Tire engineers prefer to use the term "Reserve Load" when talking about the load capacity of a tire. Here we would find a tire engineering definition as the difference between the tire's maximum capacity when inflated to the stated level for the specific application (the inflation on the tire placard) and the actual load to be placed on the tire.

Here are a few comparisons: First some normal car and truck applications.

As you can see, tires in normal vehicle service have their Reserve Load well in excess of the RVIA suggested 10%. I would suggest that in addition to the special, internal tire structure forces such as Interply Shear, this low level of Reserve Load is a significant contributor to the shorter tire life seen in RV service.

While you, the owner, have little input or control over the materials used in tire construction, you do have significant control over the Reserve Load on your tires as well as operating speed and tire inflation. By decreasing the actual load you place on your tires while ensuring the tire has the highest Load Capacity (the "Max Load" number molded on the tire sidewall), you will increase the "Reserve Load" for your tires.

You can also ensure the Reserve Load stays high by not exceeding the speed for the service. For LT and truck tires (19.5" and 22.5"), the max speed for RV service is 75 mph, except for a few tires that have a lower limit of 65 or 62 mph. Check the "Data Book" publication from your tire manufacturer. Goodyear, Michelin, and Bridgestone state their max speed in RV service in their data book (75 mph max). I see no reason to go faster than that unless you find a data book from your tire manufacturer that states faster. If you have a tire salesman that claims faster is OK, ask to see the speed IN WRITING from that tire manufacturer or ask for a signed letter from the RV dealer on their letterhead paper stating the max allowable speed of operation for the tires they sold you.

Thursday, October 13, 2022

Why you should have road hazard tire insurance

 Are you prepared for the cost of tire failure? When buying new tires for your RV, one thing to consider is road hazard tire insurance. That is especially true for ST-type tires you put on a trailer or fifth wheel.

  ( A bad "Fork" in the road)

If you have a failure, it could be blamed on some impact or cut which would not be covered by any manufacturing warranty. But with road hazard insurance any failure would be covered. Even something unusual, as seen here.

Road hazard tire insurance can be worthwhile in all RV applications

Not all brands and not all dealers offer this insurance for RV applications. But when I recently got new tires for my Class C, the Firestone store offered road hazard insurance. I said yes, as I wanted 100% coverage and some assurance that I would not have any expenses related to tire failure. No, my tires are not ST-type. But my advice to consider road hazard insurance applies to all types of tires when used in RV applications.

All I have to do is keep track of the sales receipt and I'm good to go if there ever is a tire problem. The receipt has the full DOT serial along with the notation that I purchased the insurance. So that makes keeping the invoice information doubly important as I do not have to go crawling around under my RV trying to read the DOT identification number aka "tire serial".

Speaking of keeping important papers, I keep copies of papers like car insurance, registration, truck scale weights, roadside assistance insurance, and tire invoices, etc., on my phone in a folder that is backed up "in the cloud" so the information can't get lost.



Sunday, September 25, 2022

Do we need to be smarter than the tire salesperson?

 While some know all of this I continue to see posts on various RV Forums where it appears some still do not understand tire markings.

It would be nice if we could all simply depend on the tire salesperson to know exactly what we need for our RV. However, with numerous tire companies making hundreds of tire designs in countless combinations of type, load range, size, and tread design, that’s an almost impossible task. With a little effort on our part, we can help the salesperson offer a smaller, more reasonable selection and end up getting a set of tires that better fits our individual needs.

What brought this to mind was a question from a reader. They said, “I got new tires yesterday, but as I was looking through my records they have a different number vs. my old tires. The new ones are P235 /75 R15 108S XL WW and the old ones are P235 /75 R15 105S OWL. So why is it different and will it be a problem?”

The salesperson may make a mistake or just be trying to sell the tires they have in stock.

While the above question is about passenger car tires, I think we can use it as a learning example.

Translating tire information

First, let me help everyone translate that tire information and nomenclature.

The “P” stands for Passenger. They’re usually used on our regular cars but sometimes used on smaller lightweight trailers. Most trailers and 5th wheel RVs have “ST” for the type. Truck tires would normally start with “LT”. This type is found on pickups, Class B and Class C motorhomes. If there are no letters before the three-digit tire width number, that along with the wheel diameter and Load Range would indicate Truck-Bus or Commercial-type tires as found on Class A RVs.

“235” is the overall width in mm. This works out to about 9-1/4″. The 3-digit metric width number could run from 205 up to 445, depending on the type and expected usage, but the meaning of width is the same.

“75” is the “Aspect Ratio” or the relationship between tire width and the distance from the wheel to the tread as a percentage. So, in this example, the tire would be about 75% as tall above the wheel as the tire is wide or .75 x 9.25″ or about 6.9″. The 2-digit Aspect Ratio number could run from 35 to 85.

R is for Radial

“R”—I do hope that everyone knows this stands for Radial, which is the type of construction used on modern tires. It might also be a “D” for Diagonal, which is the construction used in the older-type “Bias” tires.

“15” is the wheel diameter in inches, with most wheels running from 13″ to 20″ for Passenger-type tires. Light Truck-type tires come in 14″ to 24″ diameter, with most being 16″ on current Pickup trucks used to pull RV trailers.

The “108” or “105” is the Load Index, which is defined as “numerical code associated with the maximum load a tire can carry at the speed indicated by its Speed Symbol under specified service conditions” by the Tire & Rim Association. That is the standardizing body for the tire, rim, valve, and allied parts industry for the United States.

Passenger tires are not used in “dual” or side-by-side position, so they will have only one Load Index. But LT and Bus-type can be used in Dual position, so they will have both Single and Dual Load capacity. If they have a Load Index, there will be two numbers.

Maximum load capacity on all tires

The “S” after the Load Index is the Speed Symbol. Not all tires have the “Load Index” number, but all tires do have the maximum load capacity stated in both Pounds and Kilograms. Also, not all tires have a Speed Symbol. But industry literature found in technical data books suggests a max operating speed in RV application of 75 mph (Speed Symbol L), no matter what the Speed Symbol says.

The “XL” stands for “Extra Load.” No Load Range implies the tie is “Standard Load”—for Passenger tires. For ST, LT, and the Bus-type, the “Load Range” letters start with “C”, “D” and go on up to “H” or “J”. These letters replaced the old “Ply Rating” when tire construction changed from Bias to Radial.

Finally, the “WW or “OWL” stands for Wide White and Outline White Letter. Those are just the fancy white decoration seen on some Passenger tires.

Wheels can be more complicated

Wheels get a bit more complicated as some can be 14″, 15″ or 16″. Then there is a different group that ends in .5, as 17.5, 19.5, and 22.5. These .5″ wheels would be considered HD Truck/Bus type and would all be found on larger and heavier trailers and motorhomes.

So, when shopping for tires you should start off by knowing the “Complete” Size Designation which includes all the numbers and letters from your Original Equipment tires such as “LT235/75R16 114 / 117 L LR-E “. Most, if not all, of this information can be found on your RV Certification Placard or “Sticker”.

If you are going to change any of the above numbers or letters on your tires when buying replacements, be sure the new tires are rated equal or higher for all the Load, Inflation, and Speed symbols. You should never go to a lower load capacity with new tires. In the original example, the new tires are XL and 108, which means higher inflation and higher load capacity.



Friday, September 9, 2022

RV weighing worksheet and selecting PSI for YOUR RV.

You have probably heard about the advisability of learning the actual load on each tire position on your RV. This is many times called "4-Corner-Weight" in reference to the basic position of tires on vehicles, Right & Left, Front and Back.   If you have a towable you may have one, two or even three axles so while the number 4 may not apply the concept is the same. You should learn the load on tires on each end of each axle.

If you get to a large RV convention such as FMCA events (check Conventions link) or Escapees SmartWeigh there are some companies that offer the special service and use special portable scales that can give the load on individual tire positions. Weigh To Go

Also I understand that National Indoor RV Center (Dallas & Atlanta Locations) offers scale weighing.

You might check with your local Moving & Storage companies, Builder Supply and even Sand & Gravel pits.

Some of the above have extensive experience with RV and can offer suggested levels of inflation needed but others can simply tell you the scale reading. You do not need the accuracy of "Certified" scale as offered by CAT at truck stops as almost any heavy duty "truck scale is good enough for our needs.

But you ask how do i use the scale readings to learn the minimum inflation I need for my RV. That's where this RVWorkSheet comes in. Simply download the PDF file and print it off. (You should also save a copy for future use or to share with other RV friends.

The worksheet allows you to enter the various scale readings for your type of RV and with simple calculations you learn the heavy end of each axle.

Using the Load & Inflation tables for your tires as found HERE you check for the MINIMUM inflation required to support the heaviest loaded tire for each axle. If you can't find your exact brand or size tire in my list you can use the information provided by Goodyear, Bridgestone, Michelin as you will see that all the tables give very similar (+/- 5 psi) numbers for identical size, type and load Range tires.

You can read a number of my posts on how to do the final calculations in THIS group of posts. Some even show examples of how to do the calculations and adjustments to get the final PSI including a suggested "Reserve Load".



Friday, August 26, 2022

Can you do a tire "Autopsy" at home?

 When you have a tire failure many people are only interested in replacing the tire as soon as possible. But there are others that want to understand the "why?" of the failure so they can take the steps necessary to avoid a re-occurrence of the problem for if you don't know the real basic reason for the failure any corrective action you might take may not actually "fix" the problem.

Think of a car not starting. If you jump to the conclusion that the reason for the non-start, is a bad battery when it really is the first signs of a failure of the starter motor or a failure of the ignition switch, the new battery will not "fix" the no start problem will it?

Now I am not saying that everyone needs to have the tools and skill to do the "autopsy" shown below but when I was working as a Forensic Tire Engineer I was expected to dig deep and to learn the real reason for a tire to fail. 

It is in my basic nature to be inquisitive, so when I was contacted by an RV owner who had what he thought was a tire about to fail I decided that I could help him learn and understand the "why" when he started asking questions as many engineers do.

The following is what we did and what we learned.

As you can see the tread is very round.


This is not normal for a steel belted radial tire.

John, the owner measured the OC of all 4 tires and found a big difference with one tire being an inch larger than the others.

Clearly something was different this tire.

After removing the tire from the rv and looking closely at the tire he noticed that one portion of the tread seemed to "bulge" outward.


His next step was to confirm the tire was no longer round.


 After de-mounting the tire and inspecting the interior he said he could not find any indication of failure. At that point John contacted me and after some back and forth I told him how to dissect the tire so he could ship the tread portion to me for a more detailed inspection.

You can see the work he had to do to get the tire tread area cut from the rest of the tire.


This was John's effort to

find the belt separation,

but he was not sucessful.

When I received the tread my experienced eye quickly found the location of the belt separation.

This level of effort on John's part was extraordinary but we got the evidence to prove a belt separation as the reason for the bulge in the tread area.

Now you don't need to go to all this effort as a simple "Free Spin" inspection as seen in this YouTube video will provide the evidence you would need to decide if it is time to replace your tires.

I have recommended that owners of RV trailers do a "Free Spin" inspection once a year or every 2,000 miles. This schedule might line up with when you inspect brakes or re-pack the wheel bearings so the Free Spin is not any additional effort and if you discover a wobble or bulge in your tire you know that it's probably time to replace it rather than wait for it to come apart on the side of the Interstate. 

Wednesday, August 17, 2022

Weigh the RV each trip?

Was following a thread where some people were saying they adjust tire inflation each trip based on how much "stuff" they are carrying. I didn't think this was a good policy so offered the following: 


Unless you are making significant (1,000# or more) changes in what you pack in your RV, I see no reason to be messing with tire inflation once you have finished adjusting axle alignment and been on a scale to learn the actual load on your tires. The general advice for weighing is to have the RV loaded to the heaviest weight you expect to travel with. This means full of cloths, books, tools, water, fuel, food, spare parts and toys you might travel with. There is really only one inflation number you need to keep in mind and that is the MINIMUM inflation required to support your load. We are trying to protect the tire from failure and with data from RVSEF showing that over half of RVs historically have been running their tires in overload. Damage to the tire structure is caused by the rubber bending and stretching past an elastic limit at the molecular level. This stretching actually breaks some of the chemical bonds and once broken the resultant cracks never repair or reform themselves. They can start small as seen here.

 The cracks only grow. Once formed the cracks will just get larger and larger till eventually they get large enough to result in a failure of the structure which many times is in the form of a belt detachment from the body of the tire as seen here.

  It doesn't take too many miles for a tire in this condition to come apart and you may end up with a nice "Blowout" unless the tire is replaced before the crack gets too big. The inflation in the Load & Inflation chart is the MINIMUM you should run but heat, age and the tearing a tire experiences from hitting pot holes can result in the rubber cracks forming even if you are running the inflation found in the charts. Running higher than the minimum inflation, can offer some protection as the tire will run cooler and bend less which means less tearing of the rubber chemical bonds. If you look at the tables you can see that each increase of 5 psi gives you a few hundred pounds more load capacity and conversely each drop of 5 psi decreases the load capacity of your tires. Tandem axle trailers place some additional stress called Interply Shear, on the tires which result is more cracking and more tearing of the rubber bonds. I would recommend against lowering tire inflation once you have learned the inflation required to support your heaviest weight. I also recommend that you set your TPMS Low Pressure warning level to no lower than 5 psi below the minimum inflation learned from your scale reading as I can think of no reason or benefit to ever run an inflation lower that what is needed to support the load. In fact, on my RV, I run my inflation at a +10% margin over the inflation from the tables which allows me to set the TPMS low pressure warning level to the inflation requires to support my heaviest scale reading.

Friday, August 12, 2022

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.

Here is another trailer having the axles measured

 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


Friday, August 5, 2022

"Defect Reporting"

I have asked RV owners who had tire failures if they have reported the failures to NHTSA or to the dealer that sold them the RV. Almost universally they said they didn't bother or were told by the dealer "it's not their job". 


The reality is that it is Federal law that tire failures be reported to US DOT. "defect reporting requirements, the focus of this memorandum, the TREAD Act requires automakers to notify the Secretary of Transportation within five days of discovery of a defect or the need for a safety recall. See 49 C.F.R. § 573.6." There are significant fines for failure to comply. NHTSA complaint needs the RV VIN and the complete tire DOT serial including the date portion.

 It is my opinion that if you have a failure or suspect failure and can't bother to file a complete report and include a picture you really have lost much of the right to complain about tire quality as you are effectively keeping the information from the tire company and from the Federal Agency that can order a recall after an investigation. No investigation will ever start because you posted a complaint on Facebook or an RV Forum.


Friday, July 29, 2022

Setting inflation pressure Step-By-Step

 While I have posted a number of times on how I would set tire pressure this topic seems to be a recurring question, so here is a slightly different approach to the answer where I use information on a specific example

First off.... You will need to adjust for your RV, your weights and your size tire. This should work for owners of Motorhomes but not for towables or 5th wheel RVs. I will try and cover them in a couple of weeks once I get some real weight information from a trailer/5th wheel owner.

So here is the question I got.

Sorry to bother you again but I just realize I should have asked you about CIP (cold inflation pressure)  and ambient temperatures. I was hoping to set my new pressures late this evening or early tomorrow morning when it coos down, but.
As I stated in my first email, we live in the California High Desert and the lowest temperatures we are going to see today, tonight or in the wee hours of the morning is a cool 75. If I remember correctly in one of your posts, you stated CIP of 65 - 70 degrees. I am sure folks in Arizona and Nevada have the same issue where low temperatures may not get below the 80's or 90's in any 24-hour period for weeks if not months. 
Do we just apply the 2% per 10-degree formula when initially setting our tire pressures? Meaning a 90 psi would then be 91.8 @ 80 and 93.6 @ 90 degrees?

For us tonight and tomorrow morning:
90 @ 7 PM
83 @ 10 PM
75 @ 6 AM
79 @ 7 aM
Here are my numbers:
2018 Winnebago Vista 29VE
Class A
Two axles, dual rear wheels

Scale Weights:
Total: 16,540 lbs.
Front:   5,620 lbs.
Rear:  10,940 lbs.

Goodyear G670 RV, 245/70R19.5G

Tire sidewall information:
Max. Load Single 4540 lbs@110 psi cold
Max. Load Dual 4410 lbs@110 psi cold

RV placard information:
18,000 LB
FRT  7,000 LB 82 psi  Single
RR  12,000 LB 82 psi Dual

Goodyear chart:
Goodyear Load/Inflation information for unisteel G670 RV:
Max Speed 75 mph
S3640374038904080 (F)419043354540 (G)
D3415351536553970 (F)411542654410 (G)

Again, thank you for the wealth of information and your incredible blog.
 My reply
Yes you can use axle weight numbers until you can get "4 corner weights"  (Weight of each tire position)

I suggest you assume one end of the front axle is supporting 52% of that axle.
Similar for the rear. So based on your numbers that would be . 
2,922# on the front tire
5,689# on one end of the rear axle or 2,845# on a tire.  Yes I always round up when calculating loads.
Using your chart the min inflation for the Front would be 80  and the min on the rear would also be 80.
BUT don't forget I also strongly recommend we use at least +10% on inflation to avoid the day to day temperature changes that will change the tire inflation. A +10% on inflation means you could experience a change in temperature of 50°F and not be forced to get out and adjust the tire inflation. Don't forget that some folks are where it might be raining in the AM and don't want to be on their hands and knees messing with tire inflation if they can avoid it.

Add 10% to 80 and you get 88 psi. and I see nothing wrong with rounding up to 90 psi

At 90 a front tire would be rated for 3,890#    3890/2922 = 133 so you would have a good 33% reserve load
at 90 a rear tire would be rated for 3,655#    3655/2845 = 128 so you would have a good 28% reserve load

You should be good to go.
I hope this real world example shows you that learning the proper CIP is not complex.