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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



Friday, October 1, 2021

What "Size" Tires do you have Part 2?

Last week in Part 1 I covered Passenger car type / size tires. This week we move on to LT and ST type.

The next type tire is “LT” or Light Truck. These will be found on both Class-B and Class-C and possibly a few small Class-A RVs. Since these RVs are larger and heavier vehicles, they will normally have tires that come in larger physical size and stronger Load Range. For example a popular tire for Class-C motorhome might be an LT225/75R16 115/112 LR-E. The double number 115/112 is the different Load Index for single (front) application and dual (rear) application where two tires are mounted side by side. Note that some RV trailers may have two axles, one in front of the other, this is not called “dual” but would be a “Tandem axle” and the tire loading is considered “single” when looking up the capacity in the tables. The Load Index number can be applied to a group of tires that have similar but not identical load capacities. When replacing tires, you should not go to a lower Load Index number.

In LT type tires, the abbreviation of LR for Load Range is used. You can think of the Load Range as a replacement for Ply Rating. For RV application you will see that Load Range starts at C (old 6 ply rating) and moves through D, E, F, and G. This is really an indication of the strength of a tire to hold the inflation pressure, not the ability of a tire to support additional load. It is important to remember it is the inflation pressure and not the tire construction that supports the load. This is why we have Load and Inflation tables not Load and tire construction tables. You should not consider moving to a lower Load Range than selected by the RV manufacturer. The LR along with the original size and type tire and minimum inflation is shown on the RV Certification Label. Basically, the Load Range identifies the highest level of cold inflation to be used starting at 50 psi and moving up to 100 psi or even higher as identified on the sidewall of your tires. Like Speed Symbol you should never consider moving to a lower Load Range than selected by the RV manufacturer.

Do you know where your Certification Label is for your RV? If not, you can contact your RV dealer or post the question on almost any RV Forum on Facebook. The tires on this RV are Load Range C. 



Once you find the label / sticker, I suggest you take a picture as there is important safety information the label that you should keep along with other important papers like your Registration and Insurance card.

Next, we will cover "ST" or Special Trailer tires. This is a special type tire, unique to the US market. It was developed and introduced in the late 60’s for exclusive use on trailers. In fact, it is against safety regulations to use ST type tires on vehicles designed to carry passengers. In this category we might find an ST205/75R15 101K LR-C. For these tires the ST205/75R15 101K is similar in meaning to what we saw in the Passenger type tires. The primary difference is that the Speed symbol K, L, or R is lower for these applications than for passenger type vehicle. The K stands for up to 68 mph. Because the ST type tires are expected to carry higher loads at higher inflation levels the trade-off is the restriction to be operated at lower speeds. The load formula used by tires engineers when designing ST type tires is based on a stated upper operating speed of 65 mph. While today highway speeds can be significantly higher than 65 we should remember that when ST tires were introduced we had a nationwide Speed Limit of 55 mph so a tire design limit of 65 mph was not unreasonable.

While on the topic of Speed Rating people need to understand that the test is designed for Passenger car tires and only requires a new tire be capable of running up to the stated speed for 10 minutes while running a reduced load of 88% of the tire stated max.

You should be aware that in 2002 both P and LT type tires had the DOT test and durability requirements significantly updated and improved by requiring more durability for those tires, but ST tires only need pass the same tests as in 1970. Many consider this increase in performance and durability desirable as it can be considered a measure of “Quality”.

Next time we will cover the heavy duty tires used on Class-A Motorhomes. These include 19.5 and 22.5 rim diameter and a few others.