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Friday, December 31, 2021

Valve Extensions come in two 'Flavors"

 Valve stem extensions come in two "flavors". One being a longer metal stem the other is some sort of "extender" that screws onto the end of a standard stem.

If you chose the long metal stem route, you probably should have them installed by a tire shop that services Heavy trucks as they have the experience and should know how to install properly. The stem for the inner tire can be straight or almost straight but needs to be long enough to come through the outer wheel "hand hole".  The long stem may look like this one.

The outer stem can be a regular brass stem like this one.

it will help if you have a "dual foot" inflator tool. Like this.

  If you have short metal stems as shown above and a "Dual-Foot" inflation adapter you should be able to get to both the inner and outer short stems. One end you Push on the inner tire. The other end you Pull onto the valve for the outer tire.

The above doesn't answer the question for those that use TPMS or want easy access to allow measurement of tire pressure. You can use the "other flavor of extender", a flexible "Hose" extenders BUT you REALLY need to pay attention to the following:

1. Be sure to tighten the hose properly. That means no leaks (test with some soapy water) and not over-tighten.  I screw the hose on till the air stops leaking then tighten about 3/4 to 1 additional turn. I then check and confirm no air leak.

2. The outer end needs to be SOLIDLY attached.  I use pop-rivets and the small 'L" bracket that comes with the Wheel Master stainless steel hose kit (#8001 or #8005 depending on wheel diameter). Like this. (shown with TPMS sensor)

3. When adding air you should hold the hose so you are not loosening the attachment point as pushing an air chuck or pressure gauge on an extender can generate a lot of force which can bend or loosen the attachment.

4. Do Not over-tighten the hose extenders. There are small rubber "O" ring gasket seals that can be torn resulting in a slow leak.

5 As with all rubber parts, I had one of my hose seals failed after 9 years.  The rubber is any seal can eventually fail due to "old-age". This applies to hoses, rubber valve stems and any other rubber part in your car, truck or RV. One advantage of my using a TPMS I was able to see the slow ( 2 - 3 psi per hour) and after a close inspection of the hose extender confirmed.

 Some people have used a stiff extender instead of the flexible hose. The downside of these extenders is that it is hard to "attach" the outer end to stabilize the extender. This might allow the extender  to vibrate  or unscrew which can develop into a leak. I think that these "extenders" potential to leak is what has led to the negative opinions of extenders..



Friday, December 17, 2021

“The Rest of the Story” - Attention to details.

 Many of us have heard that phrase used as a preface to additional facts that sometimes do not make it into a story as widely published, but once the additional information is explained we learn that we do not always get all the important information  the first time around.

What brings this up was an item in an RV magazine I stumbled upon when cleaning out some old boxes of stuff. This is from Nov 2008 but the facts and information are still important today. The author is one of that small group of RV “experts” that make a living providing information on the Internet on just about all things RV. Sometimes they are answering questions about refrigerators, then they might be answering a question on holding tanks or plumbing. From my point of view it seems that they are quite knowledgeable about most topics but sometimes there are important details related to tires that don't always make it into their posts on tires.

The owner of a 5th wheel trailer had purchased a set of tires and offered that the tire store inflated the tires to 85 psi which was the “maximum” as molded onto the tire sidewall. The owner said he had heard that using the maximum inflation from the tire as the standard pressure was advisable and wanted to know the “rule of thumb” for the proper inflation.

He also wanted to know if he needed to reduce the tire pressure when he traveled to 8,000 ft elevations.

The “expert” correctly replied that the inflation on the tire was the minimum required to carry the load on the tire, he continued with the recommendation that tire loads be measured on a truck scale and then the inflation set based on Load & Inflation tables. There was no mention of ensuring that there was at least a 10% margin between the tire Load Capacity and the actual load on the tire. This recommendation to a trailer owner would address some of the unique side loading
(Interply Shear) seen on multi-axle trailers which is significantly different than that seen on motorized RVs.

While I advocate that trailer owners get their rig weighed so they can confirm they are not overloading any individual tire, as it is not unusual for one tire or axle to be 500 to 1,000Ibs away from a theoretical 50/50 weight balance. I do strongly suggest that all multi-axle trailers run the inflation shown on the tire, which in most cases is the inflation shown on the RV placard as provided by the RV manufacturer. This will help reduce the side force  overload seen by trailers but not by motorhomes.

The expert did correctly advise that while the tire pressure will change with elevation, but said that unless the owner was checking his inflation daily, "as recommended", it was not necessary to adjust inflation when traveling to high elevations.

When I originally read this reply I was concerned and did send a letter to the expert pointing out the missed opportunity to educate RV owners about the importance of setting proper inflation, and that a stronger statement on more frequent or constant inflation checks by using a tire pressure monitoring system ( TPMS) was needed.

I do not consider daily inflation checks sufficient, especially for towables as too often the driver of the tow vehicle has no idea a tire is loosing air till it is too late to save the tire and there has been a "blowout" from a puncture or leaking valve which could cause thousands of dollars worth of damage to the RV. I also would not imply that when traveling to high elevations it isn't necessary to adjust your inflation whenever the inflation is checked.

As you can see the general RV expert provided mostly correct information, but he left out some considerations that I, as a tire engineer, do believe to be very important.



Friday, December 10, 2021

DOT Tire Safety Regulations

 Occasionally I will get questions about some specific feature in a tire or asked why tire engineers didn't design tires to perform in a different manner. Sometimes some even want to question why I didn't design a tire to perform some task such as supporting more load so the RV owner wasn't forced to buy a more expensive tire. The primary reason is that there are numerous federal Safety regulations that all tires sold in the US for highway use must pass or face fines that could be as high as $1,000 per tire.  For those that think they can do a better job of designing tires I can save them some time when they do the research themselves so they can ignore people such as myself who do have experience on the subject.

For those interested in the various safety regulations as they pertain to vehicles you can start HERE:

Then start digging down to the appropriate specific standard.

 For my 40 year career, I had to live every day under Chapter V National Highway Traffic Safety Administration, Department of Transportation and most of the time focus on section 571 ( but occasionally confirm others sections such as 569, 573, 574 and a few others.

I found myself most concerned with Sub Part B and 571.109, 571.119 and of course 571.139. Our focus would of course change depending on the type and size tire we might be tasked with developing or evaluation.

I doubt that you want to have to learn all the appropriate sections but if you don't want to accept my experience and expertise you are welcome to read and research all of 571 and then contact the appropriate department in DOT to get a clarification on some of the apparent contradictions.

Good luck with your research and education.

Friday, December 3, 2021

"65 psi is too much"

 "65 psi is too much" was the opinion posted by Tom when he said 

"Ok, I can’t find any info here so one last time. The tire pressure says 65psi cold on the side wall of the tires on my travel trailer. That sounds like a lot to me. What tire pressure do you run your camper at?" on an RVTips FB page. 

I'm not sure where he did his search. On the FB page, he did get some answers such as "60"  and "110" and "80".

 I responded:

 As an actual Tire Design Engineer, with 40 years of experience including decades of trailer ownership, I can explain why you need to follow the Science of proper tire loading and inflation. You have a Certification sticker or label on your TT that was applied by the RV company based on safety regulations. The sticker tells you the correct information for tire size, type, and Load Range. It also tells you the GAWR. You should not exceed the GAWR because you can break wheels, bolts, axles, hubs, springs, and related parts. Known strength limits of these parts were used to establish the maximum load you should put on your axles. The tire industry has tables that cover the dimensions and load limits for tires for different levels of inflation. These tables have been around since the 70's and all tires sold for highway use, in the US, are required to be capable of passing a number of different strength and durability tests. The test conditions specify both load and inflation levels for the different tests. The RV company has the responsibility to provide the information on the certification sticker such that the specified tires are rated to support at least 110% of GAWR when they are inflated to the stated inflation when the tires are at Ambient Temperature, i.e. "cold". You didn't offer what tire engineering experience you bring to this discussion so I don't know how you arrived at your conclusion that 65 psi cold (ambient) was "too much" but I sincerely doubt that the RV company would select LR-D tires if that level of load capacity was not needed. You can learn more about tires by reading some of the 500+ posts on my blog       www.RVTireSafety.Net



Friday, November 26, 2021

Adjusting tire pressure because it's cold outside

 I ran across a series of posts on an RV forum on the need to adjust tire pressure because of changes in ambient temperature. 

First lets be sure we all are using the same definition for "Ambient temperature" For our purposes we should consider it "the outdoor air temperature in the shade".

An RV owner noticed that his tire pressure had dropped based on his TPMS readings, almost 7 psi when the temperature dropped to the 40's. He did not provide his "normal" pressure or temperature. But he did claim " But within 10 driving miles they are back to normal pressure." He asked if he needed to add air, when it is cold, to bring pressure back to normal. There followed over 100 posts which is not uncommon when tire pressure is the topic. As expected, some offered correct information on the need to adjust pressure to accommodate the normal drop when it gets cold or increase in pressure when the ambient temperature rises. This immediately lead to various suggestions on the amount of adjustment needed.

I was heartened to read the comments from many, that knew it was important to adjust tire pressure to compensate for changes in ambient temperature. There were a couple of people that thought there was some special temperature for adjusting pressure. Replies to these people were quick and correctly pointed out that there was no special temperature such as 68F or 72F when tire inflation could be set but that a tire pressure can be set when the tire was not "warmed" by being in sunlight or having been driven on more than a mile.

Adjusting pressure when traveling

A few offered information on how to calculate the amount of air pressure that needed to be added but this concept quickly into discussions of Pressure vs Temperature formulas.

Bottom Line:

Tire inflation pressure will change about 2% for each change in Ambient Temperature of 10°F (6°C).

I advise that you use your TPMS to check inflation at the start of each travel day. This might be in the Morning but whenever you check you need to ensure that no tire has been warmed from being in direct sunlight for the previous couple of hours. Then set your tire inflation when the tires are at Ambient Temperature. It's just that easy.

There are charts on the Internet that cover how to adjust tire inflation under extreme conditions such as the RV being a garage heated to 65°F but the outdoor Ambient is way below freezing. I posted a chart last March but I bet that there are not many RV owners that need this information.


Friday, November 19, 2021

Another question on "4 corner weights"

 On Tue, Nov 9, 2021 Andy  wrote:

Good afternoon Roger,

 I wanted to double check myself regarding tire position weights.  In reading your blogs you said for double-axle trailers the tire pressure should be the max cold pressure amount as stamped on the tire because of trailer sway and turning forces on those tires is different than on a motorhome.  Is it important to get individual tire position weights for trailers?

 As I just got a new 5th wheel trailer and I am getting ready to install a Tire Tracker tire pressure monitoring system I have a few questions:

1.      Do I need to get 4 corner weights (or in this case 8 tire position weights)? 

2.      Is load on each tire position important for trailers vs. motorhomes, or is axle weight sufficient?  (I can get overall and axle weights easy enough.  And, I just got an appointment for tire position weights with a SmartWeigh club in FL, if you think it is important.)    

3.      When I go to get the weights, should ALL holding tanks be full or only Fresh Water? (The SmartWeigh group literature states that they only want the fresh water tank full.)

 The 4 tires on my 5th wheel trailer are: (And the spare, too.)

Carslie    CSL16   ST235/85R16

Load index 132/127

Speed Rating: M (81 MPH)

Max Load Single 4400 lbs. at 110 psi. cold

Max Load Dual 3860 lbs. at 110 psi. cold



The 6 tires on my Ford F450 DRW are:



Max. Load Single 3970 lbs. 110 psi. cold

Max Load dual 3750 lbs. 110 psi. cold

Then they have LRG in an oval followed by 128/126N in another oval and then DOT B6 YB NFL X 0621 in various ovals


I don’t have weights on the trailer yet but the GAWR is 7,000 lbs.   GVWR is 17,000 lbs.

 Thank you very much,  Andy

I wrote:

The "4 corner weight" is usually talking about Class-A motorhomes and the data shows that some number of those units can be significantly (1,000# or more) unbalanced side to side on an axle,

Smaller RVs can probably get away with just learning the weight on each axle with the RV and TV loaded to its heaviest.
Then calculating 51% or 52% for each axle and then using that weight number to confirm you are not exceeding the load capacity of the tires.
For towables, including 5vers, your calculated heavy weight should be no more than 90% of the tire capacity when inflated per the Load & Inflation tables. I covered the reason for this 90% limit in  my blog post on Interply Shear and the RVIA (see that Gold sticker near the door on the trailer) has a requirement that tire capacity should be = 110% of GAWR.
The max load capacity for a tire is load in pounds shown on the tire.

Your Dry weight is not important when we are talking about tire loading. The GAWR is just a number for all the tires on any one axle. The tire on the Passenger side has no idea what the load on the driver side is so you can't average the axle weight to learn the max weight on the heavier loaded tire.

"Dual" loading is when 2 tires are mounted side by side on one end of an axle as we see on the rear of most Class-C motorhomes and you have on "Dually" Pick-up trucks like your F450. Dual does not apply to RV Trailers. LT tires do have different load capacities if mounted on the front (single) or mounted on the rear (dual).
Yes, you do not need both freshwater and holding tanks full. Think of your loading when you start a trip. Fresh water is normally full, so is propane, gas or diesel and food pantry but holding tanks are empty, don't forget your tool box. That would be what I would call "Heaviest expected weight".

This is when you get on a truck scale and get readings with just one axle on a scale pad. You may need a couple of readings depending on pad spacing and your axle locations. Get both trailer and truck weights as you should also confirm you are not overloading your TV tires either.

Example:  Suppose you have tires that say 3,900# @ 80 psi. Your GAWR on the certification label is 7,000#   RVIA requires the tires have a capacity of 110% of the  7,000 or 7,700 total or 3,850# each so you might think you are OK but if your RV axle is "out of balance" side to side by 1% or 3,885# on the heavy end you would have a tire in overload. Not a great deal but in overload assuming the scale is accurate +/- 1% and you have a 100% accurate pressure gauge.  What if your axle is unbalanced by 200# or 300#?  TV are normally more balanced so just take the axle load and divide by the number of tires on that axle.

BUT the RVIA 110% requirement went into effect in Nov 2018 so there are many RVs out there that only require that tires be capable of supporting 100% of GAWR so that 10% "cushion" is gone.

I strongly support the RVIA's 110% load capacity. In fact my Interply Shear data suggests that something closer to 125% would be desirable but the RV companies simply have not designed their vehicles with large enough wheel wells to allow that large of a tire even if the cost penalty was less than $200 an RV.

Back to your original question: You do not need to learn the individual loads on each tire position. BUT I do suggest you assume at least a 1% out of balance and apply that to the weight reading you can get at truck scales (just need to pay attention to where each axle is on which scale pad)


Friday, November 12, 2021

Are IR guns good for detecting low inflation in tires?

 Comment of Facebook about RV tires

"I carry a heat gun with me. When I stop I check all tires. If one is a lot hotter than the rest I have a problem."

and another

"A cheap harbor freight gun is a good investment. We carry two when I tow my boat to the keys. Each stop my buddy and I have a gun in each door. We each shoot all four wheels then compare noted about what we got as far as readings."

 My response:

As a tire engineer I can advise that IR guns are OK for checking the temperature of metal items such as wheels, hubs or brake drums that conduct heat but rubber is an insulator and very poor conductor of heat so no IR hand gun I know of can detect a meaningful temperature at a rest stop as it isn't the average temperature of a tire that can result in a failure but a spot, maybe smaller than a quarter that you have to worry about. What testing have you done to learn that exact spot over the entire surface of your tires?

The hottest location (most likely to fail or cause problems is also about 1/4" deep inside the structure). Heat is the result of running overloaded or under-inflated. Scale measurement is the only reliable way to know if you are overloaded. Pressure readings are the only reliable way for a consumer to know if they are running under-inflated. You can pick up a nail or cut or have a leaking valve and be underinflated within a few minutes and have a failure a few miles later. The tire I covered in my detailed post where I did a visual "Autopsy"reportedly had it's air pressure checked just 50 miles before the catastrophic "Blowout" run low flex failure. A leaking valve core from improper seating is one possible reason for the air loss the led to the failure. A TPMS is the only reliable way to know the actual running pressure unless you stop every 5 to 10 minutes and check with a gauge and confirm your valve core has properly sealed after you check the pressure. A TPMS can even sometimes warn of brake or bearing problems because the heat is transferred through the metal parts to the metal base of the TPM sensor. 

I do think these low cost temperature sensors have other uses such as checking the temperature of your furnace output or confirming the electrical connections in your 'fuse box" are not getting  overheated. Even the heating element in you absorption refrigerator can be checked. I did use my IR gun to check the surface temperature of my tires when I did my test on tire covers but running tires have a wide range of temperatures as seen here with the output from a $10,000 IR camera. Note the wide range of operating temperatures (80F to 130F) in close proximity ( 0.10") when the passenger tire was run with 27 psi.

It only takes a few seconds for the surface temperature to drop significantly which would be less time than it takes you to come to a stop in a rest area so your readings would not reflect the important temperatures near the tire hot spot.


Friday, November 5, 2021

Why so much confusion on tire inflation vs Max Inflation vs my recommendation of +10%

A question posted on an RV forum

Tom said "So, I see some who are saying to set pressure to max cold pressure recommended, and others talk about “minimum +10%”…I’m confused."

Starting at the end.

Assuming you know the actual load on each tire from your measurement on a scale (ya I know about assuming but every RV owner has been told at least once to learn their actual loading).
You take the load on the heavy end of an axle as there are almost zero percent RVs with the load exactly at 50/50% side to side.

The load number is then found in the Load/Inflation charts for your size tire and you go up (to the right) till you find a block with at least or more load than what you measured on the scale. NEVER go lower than your scale reading. DO NOT average the reading from each end of the axle weight measurement. DO NOT try and calculate a pressure between the 5 psi increments.    Then look up in the chart to find the PSI.  That is the MINIMUM inflation you should ever run in the tires on that axle.

I suggest you add 10% to that inflation number to offer some "protection" in case the temperature drops. If you have added my recommended 10%, you will probably see that you do not have to add air every day the temperature drops 10 degrees.

RVs have Certification labels AKA Tire Placards that have tire size, type, Load range and inflation numbers. They also have GAWR which is the MAXIMUM load you should ever have on that axle.  The RV company is required by DOT, to post on the sticker, an inflation number that is sufficient to support 100% of the GAWR.  RVIA (a standards organization's sticker on the side of your RV now requires inflation level good enough to support 110% which is better than the DOT requirement)   Because of these load capabilities most RV companies select the smallest (lowest cost for them) tire that can just barely meed these requirements. The result of this purchasing decision is that you will need to inflate your tires to the level needed to support the tire's MAXIMUM load capacity which is the number on the sidewall of the tire.

Side issue. The wording on the tire sidewall is confusing. The reality of what it means is that any given tire has a MAXIMUM load capacity and an inflation (minimum) required to support that load. What is not printed on the tire sidewall is the fact that there is no increase in inflation that will result in that tire ever being capable of supporting more load. Therefore  the "max inflation" wording that was decided upon by some committee 50 years ago.


Friday, October 29, 2021

It's the air pressure that supports the load, not the tire structure

As part of a discussion on tire inflation and cold weather I spotted one special post. With the permission of the author Cushing Hamlen, who has a PHD in engineering who said his education translates into many, many classes in thermodynamics, including statistical thermo.  Which is a real mind bender (conceptually and mathematically).

Here is what he said about tire inflation supporting the load:

"A plane wing flies because of two things: the curved top of the wing which produces lower pressure on the top of the wing than on the bottom (the Bernoulli effect - which is is a pressure thing and has nothing to do with density), and the angle of attack of the wing (where, when the front of the wing is tilted upward and the wing pushed forward, air hitting the bottom of the wing is deflected downward, which exerts an upward force on the wing (newtons third law - when an object exerts a force on another object, the second object exerts an equal and opposite force on the first - strictly a density/mass thing, and has nothing to do with pressure).

Your density altitude thing is mostly a result of the angle of attack of a wing allowing it to "push" downward on the air - the denser (colder) the air, the stronger the upward force (because the air molecules are closer together, and the wing pushes more molecules downward for a given amount of forward motion .... kind of like throwing downward two baseballs versus one ... it takes more force to throw down two of them.

Inside a tire, there is no such "pushing" of air, and so its density becomes a non-issue. The ONLY thing acting inside a tire is the pressure the air exerts on the tread, walls, and rim of the tire. This works because a given pressure pushing on the tire "stiffens" the tire, and limits how much the sidewalls of the tire will deflect for a given load. If the pressure is lower, the tire sidewalls are not held stiffly in place, and can deflect more (very much like a very underinflated balloon is easy to squeeze and deform, but a highly inflated balloon is very stiff, and difficult to deform - it can support more weight without deforming.

To understand pressure - you really need to understand statistical thermodynamics .... but the simple explanation is that pressure is the result of lots and lots of gas molecules hitting the inside of the tire .... it is nothing more than that. It is the summation over time of many, many small "balls" (molecules) each with very very small mass and momentum hitting a wall. So .... the fewer the number of molecules inside the tire (like letting air out of the tire), the fewer will be hitting the wall in a given time, and the pressure is lower (the opposite is true when you add air to the tire.

As for temperature - it turns out that the speed a gas molecule flies through space is directly dependent on the temperature (the maxwell-boltzmann distribution). So for a tire with a certain amount of air in it, if the temperature goes down, the speed that the gas molecules are moving at goes down, and they each hit the inside of the tire with less momentum - and the pressure (and thus stiffness of the tire) goes down - for a given amount of weight on the tire, the tire deforms more. The tire may technically be supporting the weight, but upon each revolution it deforms more than if it were supported by a higher pressure - and it is this ongoing increased amount of deformation that causes increased stress and damage to the tire."

So I hope this clarifies why I have been saying for years that it is not tie tire construction that supports the load but the air pressure.

 If you doubt this, then please explain where the "Construction/Load Capacity" tables are as all I can find are "Inflation/Load Capacity" tables. 


Thursday, October 28, 2021

Quick post on trailer axle alignment and tire wear.

 RVTravel, the sponsor of this blog published an item about trailer tire wear. Thought the readers of the blog who do not read RVTravel might be interested.

Friday, October 22, 2021

It's just a wheel, What can go wrong

 A wheel is just a wheel, isn’t it?

That wasn’t the exact phrase used in a recent post but it seemed to be the thought behind some of the comments I saw. Today’s topic will be of interest to anyone who has changed tire size or Load Range (ply rating to some). It also covers some important safety concerns that you must consider if contemplating such a change. This is a TECHNICAL matter which can be boring to some but there are Explosions in some of the videos to keep everyone interested and awake.

We and others have covered the information molded into the sidewall of all highway tires concerning the Maximum Load and Maximum Inflation limits for your tires. What you may not realize is that wheels have similar limits. If you are lucky these limits are stamped or cast into your wheels. If there, this information is not hidden by the tire but may be on the side of the wheel mounted toward the inside of the vehicle or in some cases on the surface that is bolted against the hub or brake drum.

Sorry to say that many of you will only find part numbers and manufacturing date code stamped into the wheel but nothing that looks like load or inflation. If you are in that boat you will need to contact either the wheel manufacturer or the chassis manufacturer. I would not depend on any verbal information from the average RV salesperson, but only accept some published information that answers the question based on the wheel part number marked on the wheel.
Here are the links to a couple wheel manufacturers. Accuride 

If you can't find the wheel inflation or load rating marked on the wheel the best I can suggest is to confirm the ratings from the OE tire as found on your RV Certification Label and consider those numbers the Max for load or inflation till you learn otherwise from the Wheel or RV company.

Now you may be asking why go to all the trouble. This was the basis of the question I was asked by an RV owner that wanted to know if there would be any problems using a Load Range E tire that had a rating of 80 psi on a wheel that originally had a passenger tire mounted on it.
So we get to the point of this post. WHEEL FAILURE.

OK first off this answer has not been approved by any lawyer.

Luckily wheel failure is an infrequent occurrence but if it happens it can not only ruin your day but as those ads on TV for new medications warn, the side effects could include serious injury or even death.

NOT KIDDING HERE. Every year more than one person manages to kill themselves by improperly inflating a tire when something goes wrong. The forces of compressed air are much like a bomb. The failure can happen while inflating or minutes to weeks later. One thing that can happen is the wheel flange bends or breaks and the tire exits sideways taking out anything or anyone in the way. If you want to have a better understanding of the forces involved I have collected a number of videos.

SAFETY video. This is "Zipper" failure from running a steel body tire when flat. But is shows the forces involved Example.

Be warned,
This is a disturbing video Example shows what can happen when a large tire lets go.  The failure of a wheel has similar forces.  similar.

If you are lucky you will only blow your fender off when it lets go.
Now before you say you aren’t using tires as shown in the videos, I want to assure you that when a wheel fails from fatigue it could in all probability react similarly to what is seen in the videos. A fatigue failure usually occurs after many thousands of cycles so if you exceed the max inflation rating for a wheel, you can decrease the number of cycles it takes before the wheel might fail.
If interested, you can learn more about metal fatigue.

Bottom Line
You should never set the cold inflation at a level that is higher than the Max for either the tire OR THE WHEEL.
Both the tire and wheel manufacturers take normal pressure increase due to operation heat into consideration so don’t bleed off hot air out. 


Friday, October 15, 2021

Tow vehicle tire wear and TT tire inflation

 A question from the owner of an Airstream. My answer would apply to other brands TV and TT.

"After driving my new trailer home from the dealer, a 316 mile drive, I was alarmed at how badly things got shaken up. I noticed that Airstream recommends one pressure for all models and all loads. So I got on the Airstream forum, contacted Airstream, and contacted Goodyear. Airstream offered no logic for there 80 psi recommendation even though I penetrated fairly deep into the organization. 80 psi is on the placard and that's our answer. Goodyear referred me to the chart we all know but also discussed my concerns for the ride and my expected travels with my trailer. We concluded 40psi.
I tow my trailer with a Mercedes GLE450. I have a 600 lb. equalizer hitch.
I follow Mercedes tire inflation recommendations. Going from 36 PSI front and back for normal load to 39 front and 50 back at full load. I ran the rig fully loaded over a CAT scale and for the life of me can't find the numbers but I was very pleased with the numbers. I was 150 lbs under max payload for the car and well under the GVWR for the trailer of 6000. The equalizer put all the weight back on the front wheels confirming my wheel well to ground measurements
I have now put 11,000 miles on the rig since march. I compared tread depth using an improvised depth gauge. I can not detect any difference across the tread, from RF side to LF side or front tires vs back. However I can not claim great resolution.
I'm happy with the wear on the tires. I'm thrilled that things remain in place even when driving unmaintained roads. So take that for what it is worth.
I do have a question for the group. The Rear tires of my Mercedes are very close to the wear bars while the front tires show very little wear. I'm disappointed that the dealer did not catch this during the "A" and 'B" services. Rotating or even inspecting the tires is not included in either service schedules. The car has 33,000 and was purchase in March.

I'm I too late to rotate them or just buy two new tires for the back and be sure rotate sooner."

The rear axle tread-wear on TV, especially with the OE spec tires is almost always going to be lower than the same vehicle if not towing  and OE tires tend to not deliver the same wear mileage as replacement tires.

Towing results in more drag so more tractive force is required which results in increased slip which means faster wear. Increase load on TV tires will also result in faster wear and finally since fuel economy is a requirement for the car company to meet federal standards, that is one feature that tends to be lower on the tire design "want" list and can be behind Wet traction, Snow traction, steering response, noise and dry traction.
The rubber formulation is a compromise of various performance parameters and tire design engineers have to select the compromise that meets the goals as established by the car company. If tread wear is important to the car owner then you can look up the UTQG wear rating number published by the tire company and select your replacement design to have a higher wear rating while remembering that you will be giving up on one or more of the other performance goals that are not identified in the UTQG list. You can learn more about UTQG HERE.
When looking at UTQG numbers for different designs or "lines" of tires people need to remember that the ratings are not absolute and a comparison of UTQG numbers between two different tire companies is not always reliable as different companies use slightly different statistical models when developing the UTQG numbers. I have even heard of companies putting lower numbers on a line for marketing purposes. BUT this comparison is better than nothing or simply shopping on price.

Tire rotation, especially when towing with a SUV or car, can result in better over-all tire wear. My general recommendation for TV with 4 of the same tire, rotation schedule for non directional tires is to rotate using "forward-X" pattern at 1st oil change. Then rotate again at the 3rd oil change and again at 6th oil change and if still good at the 10th oil change. I suggest this sequence as tire wear rate slows down as they wear and the 1st and 2nd rotation are most important for minimizing irregular and rapid wear which are more likely in tires with deeper tread depth.

Here is some information on Tire rotation from Tire Rack.

Concerning the TT tire inflation.  The inflation specified on the certification sticker by regulation, must be sufficient to support 110% of the GAWR for the tires selected by the trailer company. When you run lower inflation than what the tables say is required to support 110% of the actual scale measured weights you are shortening TT tire life and may end up with failures earlier than what other owners are reporting.



Friday, October 8, 2021

What size tires. Part 3


Finally, we move to Truck-Bus Radials AKA “TBR” tires as seen on all the heavy trucks on the highway, and most Class-A RVs. Generally, these are considered Commercial type tires and not a consumer level product. If you have and need this type of tire it is expected that you have a deeper level of knowledge about tires. Most of these tires come on 19.5 or 22.5 size wheels. They do not have a letter preceding the size description and may be something like 255/70R22.5  139/134 LR-G

These tires seldom come with a Speed Symbol but if you review the Data Book from the tire company that make TBR tires you will see that they specify 75 mph as the maximum operating speed. The double Load Index numbers 139/134 relate to the single and dual application and the Load Range letters continue to identify the normal upper level of cold inflation pressure. I suggest you look at the actual load capacity numbers in pounds as the “index” is a range and you could end up with slight loss in load capacity.

There is a lot more information on tires available in tire Data Books and Industry Standards organizations. Since “TBR” or Commercial tires are generally not considered “Consumer” items you may need to educate yourself more about the loads and inflations and speed ratings for these tires unless you are getting the same brand, size and Load Range as came OE on your RV. You do need to be careful about the source of the information you are relying on. I have found errors on some listings on the internet. Remember the person you are ordering tires from, on the Internet, may not have much or even any actual on-hands experience with tires as some sellers are not much more than order takers. The technical data is available in tire industry publications and can normally be relied upon as accurate but even there, the information may not be aimed at the specific and sometimes unique needs of the RV community. There are many forums on the internet with hundreds of self-appointed “experts”. We need to be careful as just having used or sold tires for decades does not mean that all the information from that person can be relied upon as 100% accurate.


Recently I have seen the introduction of 17.5 size tires on a couple of large 5th wheel RVs, the tires were 235/75R17.5 LR H with a Speed Symbol of L (75 mph Max) If you are considering a move to 17.5 be sure you get the correct tires as some in that size are only rated for 62 mph Max which would be a J Speed Symbol. There might be others at K (68 mph Max).

Please remember that if you are changing tire size or Load Range you must do additional research to ensure you are getting the tires you want and need. If you are going from 16” LT type tires to a TBR 17.5 tire you also will be changing wheels which introduce another level of complexity as wheels have a number of critical measurements other than diameter and width. There is also Offset and center bore. Even the lug nuts might need to be changed.


OK this is the 3rd and final part of this overview of tires used in RV application. You can find Part 1 HERE at

and Part 2 here at