What size tire is it?

That seems to be an early response whenever you ask a question about tires. Whether you ask about price, load capacity, or inflation, the first response may be “What size is it?”  The answer you give should not be “It’s a 22.5”  or  “it’s a 225-R-15”. These are just partial answers and indicate to many that you don’t really understand much about your tires.

 The reason you need to provide the complete tire size information is simply because there are so many possible replies and starting to narrow down the possibilities by properly identifying the size is just the first step in learning the details so the person that is offering the help can provide the correct answer you seek and not just a wild guess. Knowing your tire size can be confusing and sometimes it seems as though we tire engineers and government agencies have conspired to make things difficult. What is needed is to remember there are three basic features that must be established first. Tire type, tire physical dimensions, and tire strength.

This article covers a number of different type tires. Please do not skip over any part as the knowledge provided here builds on previously covered information.

Let’s start off with TYPE. This is usually a function of the application. For most tires, there is a letter code as the first part of a “Complete” identification of a tire. For most tires sold in the US, the code for tire type is either a “P” for Passenger, “LT” for Light Truck, “ST” for Special Trailer, or no letter for commercial or heavy duty. Tire engineers sometimes call these commercial sizes “TBR”, short for Truck Bus Radial. If you are reading this article, most likely you have a Recreational Vehicle or RV of some sort so the use and application of tires on the various type of RV will be our focus. There are of course many other type tires. OTR for Off the Road or AG for agricultural or AT for All-Terrain or M for Motorcycle and others, but we will not cover those and will focus on the type, size and strength tires used in various RV application.

First, we will cover “P” type tires. Most of us own or have owned some form of a passenger car that came with P-type tires. Older and smaller trailers may also come with "P" type tires mounted by the manufacturer. When our car required replacement tires, we seldom needed to think much about the proper size nomenclature, as it was the responsibility of the tire dealer to confirm the appropriate type and size tire that was needed.  Our car tires would probably be identified as a P195/75R15 94S or similar combination of letters and numbers. The P, as you now know indicates Passenger car application. 195 is the width in mm.  Not the tread width but the maximum width. 75 is a ratio of the tire width to the tire height from the wheel to the tread. “R” stands for radial. Since there are very few non-radial “D” or “Diagonal” construction tires, we don’t need to go down the road of old tire construction. The "15" is the wheel size. Finally, there is the “Service Description” which is a combination of “Load Index” number, 94 in our example and finally, the “S” is the Speed Symbol. In the US, the speed symbol is really just an indication of the level of handling capability or steering response with increased handling potential as we move from Q to R, then S followed by T, U, H, V, W, Y and finally Z. Unlike Europe where by law you are required to replace tires with the same speed symbol, we have the option of changing the rating but should expect the steering response to get slower if we go to a lower symbol. The final bit of information concerning the use of P-type tires in RV application, the load capacity of a P-type tire must be reduced by dividing by 1.10 per tire industry design standards.

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 ’60s 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. You should be aware that in 2002 both P and LT type tires had the test and durability requirements significantly updated and improved but ST tires only need to pass the same tests as in 1970.  In ST type tires, the abbreviation of LR for Load Range is introduced. You can think of the Load Range as a replacement for Ply Rating. 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.

 For RV application you will see that Load Range starts at C (old 6 ply rating) and moves through D, E, F, and G. 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. Unlike Speed Symbol you should never consider moving to a lower Load Range than selected by the RV manufacturer. The Load Range along with the original size, inflation level, and type tire is shown on the RV Certification Label that the RV company applied to your RV.

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, they will normally 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. The Load Range (ply rating) has the same meaning as it does for ST type tires. As with the ST type tires, 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.

Finally, we move to TBR tires as seen on most Class-A RVs. Generally, these are considered Commercial type tires and not consumer level tires. If you have and need this type of tires 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 Books from the major companies that make most of the TBR tires you will see that they specify 75 mph as the maximum operating speed level. 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.

There is a lot more information on tires available in tire Data Books and Industry Standards organizations. You do need to be careful about the source of the information you are relying on. Some sources, as in tire industry publications, 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 all the time.

If you have questions about tires for your RV, it may be best to review some of the other posts on this blog.

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

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

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

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

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

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

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

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

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

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

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

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

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

Here is some info I posted Sept 14 2011

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

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

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

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

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

Summary for Newbies

This is a summary of a discussion I had on RV Forum on tire pressure that started with a question and some comments.

"I'll give my take on maybe why, we have ----- Travel Trailer owners ----- Motor Home owners that never had a TT ---- TT owners that moved up to a MH ----- MH owners that still act like they still own a TT -- maybe there is any other I can not think of right now.

THE POINT IS TIRE PRESSURES ARE FIGURED DIFFERENTLY between the RV's owner's involved.

I have friends that own MH's and suffer blow outs all the time and their tires look good at the pressure they are using and not a Run Low Flex Failure that will most likely cause a blowout.

Maybe Tireman9 will answer some of the differences between TP's of TT's and MH's to clear up some of the misconceptions people have."

Here is my answer to the broad question

Yes, the proper inflation for MH and "tow-able" RV are different. There is actually strong science behind why there is a difference. THIS blog post is a short explanation. You can read the technical info HERE.

The other issue is that some folks just do not want to hassle with vehicle maintenance required with RV ownership. They are used to cars that have thousands of man-hours of engineering design, testing and development to make those vehicles extremely reliable.

Some of us are old enough to remember that when we learned to drive we were taught how to change a tire, as 10,000 miles was good tire life. Many probably learned how to change oil or adjust a carb. Some even knew how to set the engine timing and change a set of spark plugs. Nowadays it's hard to even see the plugs but with their life now at 50,000+ and the ignition computer controlled and many tires good for 40 to 60k miles who needs to know how to maintain a car?

RVs have a number of systems that do need maintenance. The tires on RVs are an outlier. While top tier tire companies use essentially the same rubber compounds and tire building equipment. This blog post explains the concept of "Tiers" in the tire industry. If you have a MH you will probably recognize the brands and names of the tires on your RV. 

However if you have a "towable" and by this, I mean both 5th wheel and more basic "Travel Trailers" many of you probably have tire brands that don't even fall into the 5th tier level. Some of these off-brand tires may not have a lot of up to date engineering built into them but IMO the main problem is the selection of tire size and load/speed capacity made by the RV company.

Again IMO many RVs are sold based on low cost and the unit has lots of "bling". I don't think I have ever heard of a salesman touting the benefits of the tires that come on an RV. I doubt that many even know the size or maybe even the brand provided on the various units they sell.

So the bottom line is that tires on RVs need more maintenance than the tires that came on their cars. Partially because of their usage but also because there is such a small margin of extra capacity provided based on the tires being undersized to keep costs down. Maybe if RV companies spent more, or even some time evaluating better options, i.e. had better quality for the tire sizes and type they offered, the incidences of tire failure on RVs would be less frequent just as they are less frequent on today's cars. 

##RVT818

Change Tire size

When considering a change in tire size there are many different areas you should consider.

Tire load capacity. You need to be sure you go with tires that have equal or greater load capacity than original.
It is possible that the original tires delivered a very small load capacity such that the coach is easily overloaded. Recalls may have been ordered or initiated but sometimes a manufacturer went bankrupt so no recall action took place or no new tire certification label was provided.
So it is up to you, the current owner to do your homework and get the facts.

- You need to learn the real load capability of the axle by contacting the manufacturer if there is no plate on the axle with GAWR info.
- You also need to learn the real loads you are placing on your tires with your coach fully loaded. Here is a worksheet with instructions on the steps and calculations you need to do.

Now load capacity is not the only information you need. You also have to be sure the tire will never rub and contact any portion of the coach or its suspension. part of this is obvious and can be discovered by rolling on your back with a flashlight to confirm sufficient clearance around all your tires. Don't forget to check both front tires with wheels turned completely both directions. I have read that 3" on all sides is a reasonable clearance.

Rim width - There is a list of approved widths for each tire size. You must stay with the dimensions listed by the tire company

One final clearance check that is sometimes overlooked is the clearance between dual tires. This requires more than just looking at the rear tires. In tire company specification charts there will be a dimension called "Minimum Dual Spacing" for the tires you may be considering. This dimension is controlled by the wheels. Sometimes this information may be marked on the wheel but most likely you will need to contact the wheel manufacturer to learn the specification. This is CRITICAL, as too small a clearance can result in tire damage or even failure.


I can't address if a specific size will fit but simply going up one size would be reasonably close to the 1/2" width increase.

For example if your current size was an ST215/75R15 going to run ST225/75R15 would give you a tire that was approx 10mm ( about 3/8" wider over all)
Now it is important to remember that tire dimensions are approximate and if you change brands the actual with may be a little different.

Also don't forget that the OD would also increase which means the clearance between the tread and wheel well or the companion tire would change also.

You should be able to find published OD and width dimensions from your tire companies web page.

One other item is that a change in wheel width (as published in the literature) will affect the tire width by about 40%. So a tire with a published width of 10" on a 6" wide rim would be about 10.2" on a 6-1/2" wide rim (40% of the 1/2" rim change = about 0.2")


 But in some cases on older coaches tires may have been changed in size or even the size designation such as an old 7.50-15.  If you find yourself in that situation you might consider dropping me an email so I can help walk you through the things you need to consider.



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Are you smarter than a tire dealer? Part 1

With apologies to the TV show "Are you smarter than a 5th grader"...

I sometimes wonder if RV owners are or at least need to be smarter than the average tire salesperson.

I bring this up based on a few recent observations and experiences. Let me give you some examples and then some suggestions to help you get the tires and service you want and need for your RV.

I have read a number of examples where RV owners had their tires inflated by a tire dealer only to later discover a variety of issues such as too much air, too little air, loose or missing valve caps or even loose valve cores. While I am sure that many dealerships want to provide good service, I think it important to remember that many times it is the "new kid" that gets the job of doing a pressure check and the on the job training may be done at your expense.
I would suggest that if you are going to have a dealer do tire inflation, it is in your best interest to provide the tire tech with clear instructions on the pressures you want. If you have more than a single inflation (fronts different than rears for example) maybe just an index card with a simple graphic along with the inflations you want will avoid confusion. You might even consider specifying a few psi higher than your real Cold Inflation Pressure or CIP so the next morning when you do your check with your Master Gauge you can let the pressure down which is always easier than needing to add a couple psi.
Also I suggest a quick walk around after the inflation check to be sure you have metal caps on all your valves. A light tightening twist may not be out of line to ensure you don't loose a cap on the way home.




Now the biggie Tire Buying.

 Next to going to the dentist, buying tires may be one of our least favorite activities in life. I do not expect to change your mind on this and turn the act of visiting a tire store something you look forward to, but I would like to offer some advice to make the task less stressful and possibly extend the time between visits so you have to go tire shopping less often.


Here is where some fact based knowledge can pay off in making the activity less stressful and might even end up saving you some money in the long term for I believe that a better purchasing decision will lead to lower probability of problems.

Lets start off by deciding if we are simply "replacing" out tires with more of the same or if we are "shopping" for tires that might deliver better performance, fewer problems or longer life.

Tire Replacement
If the tires you currently run have delivered satisfactory service, why would you think you need to make a change to a different size, type or brand? To me this would fall under the "It ain't broke, but I'm gonna try and fix it" approach. If your plan is to simply replace the tires you have with a newer set then you have it easy, simply shop for tires identical to what you already have.
When shopping I suggest you first go out and read the sidewall of your current tires. Note the Brand i.e. Goodyear, Michelin, Maxxis, Duro, Cooper etc. Next the type or design. This might be a name like "Marathon" or a number like "S637". Finally and maybe most important the COMPLETE tire size this includes the letters and numbers in front of and after the dimensional numbers and would also include the Service Description part, if any, that comes after the rim diameter. Service Description has two parts: Numbers for Load Index and a letter for the Speed Symbol or Speed Rating

For size something like "235-15" isn't correct and if that is what you tell the tire dealer, what you are really doing is sending the message that you really don't understand tires so an unscrupulous dealer may try and take advantage of you.
Technically an "LT235/75R15 107/110Q Load Range D"
is a different tire than an "LT235/75R15 Load Range D".


If you are not completely clear on this you might want to review THIS post that covers the differences for Passenger and Light Truck type tires.


Trailer or "ST" tires will have markings similar to LT type tires


Class-A Truck Bus Radials or TBR type and European commercial tires will also have size information similar to LT but with no letters in front and of course bigger numbers like "275/80R22.5 Load Range G". For TBR type tires the Service Description part is optional. If your current tires have a Service Description I would include that information when identifying the tires you want to the dealer.

You may find that shopping on the Interweb an exercise in confusion as I find many tires listed with incomplete or even incorrect tire information. Poorly designed web site may not be an indication of the competence of the dealership but it is not a good sign. This might be the first sign that you actually are smarter than the tire dealer.




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Why don't tire companies make more tires of the size I need?

I have noticed that every now and then people post variations of the question above. This happens because they have decided it is time to replace the tires on their RV and with only a few weeks before they expect to travel they discover that the size they have is "Out Of Stock" and "On Back Order". Sometimes they discover that there are a few tire dealers with the size they want but due to the shortage the rules of "Supply and Demand" have taken over and the price of the few tires that are available has gone up by 25% more than a no-name brand or maybe 50% higher than the normal price for a tire of a specific brand. So I decided to do a little investigation.


Lets put the RV tire sales volume in perspective.
In 2014 there were about 310,000 trailer sales. A majority of these came with ST type tires and some with LT type tires. While I could not find specific figures I believe that almost all ST type tires were imports. I would estimate this represents almost 1 Mil. OE tire sales

In the motorhome market I think it's reasonable to assume that Class B and Class C basically use LT type tires. There are some exceptions such as the new Super-C class but these are not broken out in the data I could find.  These two type RV would represent about 130,000 LT tire sales.

Class-A with about 22,000 units produced probably represent OE sales of about 140,000 total spread over maybe 15 sizes and Load Ranges. I am certain some sizes are relatively common and popular with a few that may represent 20,000 units in total annual sales and potentially a couple sizes with as few as 1,000 tires a year.

Clearly the low volume needs of some Class-A sizes is where the shortages in supply develop. I have never heard of anyone having problems finding a 16" LT tire which is probably 95+% of the Class B & C market spread over fewer than 5 to 8 size & Load Range variations.


Now to tire production:

First off a little background is in order. Tires are basically made in batches of a couple hundred to a few thousand tires, then the manufacturing plant switches to a different size or design. The only exception to this would be in OE passenger or Light Truck items that are scheduled for delivery to a major car or truck manufacturer. Lets look at the numbers so you will understand why this is.

In the US there were about 7.6 Mil cars and 8.7 Mil light trucks and 400 thousand heavy trucks produced in 2014.  There were about 253 Mil. passenger tires plus 33.6 Mil. LT tires and 23.1 MIl medium and heavy truck tires sold at replacement and OE. These figures include import tires. So you can see that Class-B and C combined represent about 1.5% of the LT market an Class-A size tires represent less than 1% of truck tire production.

When tires are made they are cured in molds that cost $30,000 to $100,000+ with truck sizes being at the high end of costs. Companies also need cure presses as seen in this video on tire production

You can click here to see the 7 min video on truck tire production and the press in operation.

The press holds the molds and the press system can cost upward of $1 Mil each. It also may take an hour to cure each tire so a balance needs to be reached between cost of molds, press utilization and production volume. I think you can understand that tire companies don't want a large number of very expensive molds or curing presses just sitting around and not being used so scheduling of production in the 7/24 curing room operations is very complex.

So why do people have problems finding a specific size from a specific manufacturer. It's simply a matter of scheduling, equipment utilization and priority. A tire company may have two molds in a specific size available and can produce about 40 tires a day in one press with two molds. They may schedule just one weeks production to build and cure 200 or so tires in a single batch than move to a different size/design tire. Don't forget your annual sales is only 1000 units a year and your customers do not want tires that are more than a few months old so you probably limit production to a single week every other month.

I would suggest that short supply of a specific size for an RV application is the result of the RV company's choice to go with a low volume tire and not a desire on the part of the tire company to make customers unhappy. From the comments I see it is always the tire company that gets blamed for a shortage in supply of a specific tire as no one ever seems to hold the RV company responsible for their selection of a hard to find size.

If you were a tire company where would you focus your production efforts on RV tire sales that represent less than 1% of your total sales and then on a specific size that is less than 5% of the RV market?

Maybe a better thing to do is to think about tire size availability when you make a RV buying decision. This can also have an impact on your ability to get a replacement if you get a puncture while on the road and your need to carry a spare which I mentioned in this video.

As I suggest in the video, have you bothered to check on the ease of finding replacement tires in the size and Load Range you want BEFORE you are in need?


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Much confusion on some RV forums

 Much confusion on some RV forums on de-rating tire load, Passenger tires and Michelin LTX tires

As some of you know I make an effort to follow posts on a number of different RV forums. Some forums are RV brand specific, some cater to the members of a specific "club" others cover the gamut of Class-A motorhomes to pop-up trailers and even slide-in truck campers. One thing they all have in common are posts from RV owners with questions about tires. Sometimes the questions receive correct and informative answers. BUT I am sorry to say that all to often incorrect information spreads like a virus with posts from some well meaning but uninformed RV owners .

I seem to be posting similar corrections over an over on a couple of topics so felt it better use of my time to cover a couple of points of confusion here in the hopes that if you find this helpful you can provide links to these facts when you come across confusion and incorrect information on other forums.

First off lets cover the use of PASSENGER tires on SUVs, Light Trucks, Multi-purpose vehicles and trailers.  "Passenger type" tires have a size designation that starts with the letter P such as P235/75R15 105T. There are also "Metric" size tires designed primarily for passenger car application made in other countries and sometimes imported into the USA. This tire might have a size 205/60R15 but to help you confirm that this is a standard tire intended for passenger car application you should be able to find the tire has an inflation level of 240Kpa or 35 or 36 psi.
If you apply a passenger type tire to something other than a passenger car (this means an SUV, Light Truck, Multi-purpose vehicle or trailer) you MUST de-rate the load capacity by dividing by 1.10.

Example a P235/75R15 105T would show Max of 2,028# at 35 psi on the tire sidewall. If used on a trailer etc the load capacity is really 2028/1.1 or  1,844# MAX when inflated to 35 psi cold.

The De-rating only applies to Passenger type tires even though many people are incorrectly posting that this De-rating also applies to LT tires.  I believe I have found the reason for this confusion and will cover that topic in a moment.

LIGHT TRUCK type tires have a size designation such as LT235/75R15  LR-E
Note the "LT before the numbers and the "LR" or words "Load Range" followed by letters such as "C", "D" or "E". The Load Range letters replaced "Ply Rating" numbers 6, 8 and 10 decades ago.

In Europe and other countries, they have "Commercial" tires but do not have "LT" as the first part of their size designation. Some may have LT at the end. The inflation may range upwards to the 85 psi level or 600 Kpa.

======
One recent series of posts involve converting from ST type tires to LT type of even P type tires. All to often only partially correct information is passed on so confusion gets compounded.

Here is an example
Michelin lists 42 part numbers for the line called "LTX® M/S2". This line of tires is specifically aimed for "SUV/Crossover, Light Truck application" and that's where the problems start. With "LT" as part of the design name some think these tires are just like Light Truck tires. Others have managed to read the fine print that says "* Passenger sizes used in Light Truck/SUV applications have reduced load capacity. This will differ from the maximum load branded on the tire sidewall." and incorrectly assumed that all the tires in the LXT line are LT tires so these folks start passing around information that LT tires should be de-rated.


The majority of the 42 items in the Michelin list are P type tires with a few LT type and even a few Euro-Metric passenger type tires mixed in. The listing even includes two "Xl or Xtra Load passenger type tires.

The LT tires have a speed symbol of "R" and all appear to be Load Range "E" or 10 ply rating for those that prefer to use the out of  date nomenclature.


"LTX MS2" is a design name much as the Bridgestone Dueler H/T 684 or Goodyear Wranger SA and I would never accuse Michelin of intentionally misleading people into thinking of this line of tires to be LT tires but the reality is that even when I engage some individuals about the incorrect and confusing information they are posting I get responses such as "I think my tires are Michelin LTX M/S2 P235/75R15 Load Range D (2150lbs)"  Here we clearly have a passenger tire but the writer has applied Light Truck "Load Range D" and the load capacity of 2,150 is for a Light Truck tire in Dual application at 65 psi. I have to wonder what inflation he is actually running.


The confusion is not limited to Michelin LXT. Earlier today I had a poster tell me that he checked the load capacity of a Mastercraft HSX which is a European passenger tire with American "XL" designation (don't ask, I have no idea how that is legal. Probably just a confused Customer Service rep I talked with)
and a Michelin Primacy of "the same size" and he claimed the load capacity was different. I pointed out that the Primacy is a Standard Load (35 psi rated) passenger tire so it would of course have a lower rating that an Xtra Load tire at 50 psi.

======================

Much of this confusion can be avoided if people would simply take the minute or so to read the tire size information on their tires and write it down.




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Soften the blow to your wallet when buying tires

Recently read a blog post about the "sticker shock" of the cost of "truck type" tires for the large RV. I would like to offer a suggestion on how to soften the financial hit. This strategy can work for all RVs be they trailers, Class-B, Class-C or Class-A size units.

It a simple plan and will only take a few minutes to lay-out. With the planning behind us we will be prepared to take advantage of any special deals or short term sales we might come across.

Now to follow this plan there are a few bits of  information you need to have:

You need to know the exact size, Load-Range and DOT serial date for your current tires.
Sorry but just thinking you have 235-16 size tires that you bought in 2009 doesn't cut the mustard.

If you are not sure how to get this information I suggest a quick refresher is in order so you might read the post on "Reading a Tire". So lets assume you have collected the details on your tires so you know exactly what you will be looking for over the next few months.

Here is how my suggestion works.
If all your tires are the same size I would suggest you plan on replacing the two oldest tires first. The new tires would go on the front positions and the others in dual position in the back.

Next year I would buy two more new tires, place them on the front and move the tires that were on the front back to one pair of duals.

Then the third year buy two more tires, place them on the front position and move the older fronts back to the rear.

The above schedule serves a few purposes:

1. It spreads out the purchase over a three year period.
2. It has you running on the newest tires on your front position.
3. With this plan you can spend months shopping for the best deal and avoids the need to accept tires that may be years old before you even buy them. Since you know you have a plan and you know that over a six to 10 month time period to do your shopping.
4. When you are done you know your oldest tires are three years old and if you plan on changing tires when they get to seven or eight years you now have a four year period to set aside some cash for when you start the cycle over again. You can continue this schedule for as long as you own your RV and never be confronted with having to buy more than two tires in any year.


Now if your fronts are different size than your rears as seen on some large Class-A units you can't do the front tire rotation but you can still limit the purchase of your rears to just two a year

With a little planning no one should have to buy more than two tires in any one year. It just takes some thought and planning.



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Real quick post on considerations when changing tire type or Load Range

When changing the "Type" tire be it P-metric, LT, Euro-metric or Euro Commercial or ST or even Low Platform trailer you MUST consult the appropriate Load \ Inflation tables  AND when passenger type tires are involved make the required load adjustment based on the type vehicle you are looking at.

While the vast majority of tires use the same Load/Inflation tables there are a few exceptions. Usually Michelin but also some ST tire brands have Load & Inflation numbers that are a bit different that the rest of the industry. Some ST tires even has special ratings that are not published but are covered by a special dispensation letter from NHTSA so reviewing the original Loads & Inflation for ST tires and comparing to the Tire & Rim tables or tables from other companies such as Goodyear, Bridgestone\Firestone or MAXIS are in order. In my experience these companies seem to line up 100% of the time with Industry guidelines. If your numbers are anything different that means you MUST do more investigation or face the posibility of being in an overload situation and not knowing it.

It's not hard to do but you do have to make an effort to cover ALL the requirements. While most of the info you need is covered in the Load/Infl tables some considerations such as speed rating limitations and "De-Rating" of the load capacity are things that even many tire dealers are not trained to do so you do need to consult a knowledgeable source.

Please remember that probably 90% or more of tire applications do not involve a change in type or application so the above is definitely NOT the norm even for the folks on the 1-800 Customer Support lines.



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Do you want to change your RV's wheels?

by Roger Marble

Here are some things to consider regarding changing the wheels on an RV.

For most RV owners, the wheels that came with the RV when new should work just fine. If you are running the original size tire and have confirmed you do not need to increase the Load Range of your tires to carry more load with a higher inflation, there is no reason to change your wheels, unless you have damaged one.

Now, if you want to change the look of your unit and switch to special chrome or aluminum wheels, then there are a number of things you need to consider:

What is the maximum load capacity of the new wheels?

What is the rated inflation of the new wheels?

Are they the same width and flange contour? This means the official size is identical, such as 16x7J. Note the letter is the shape of the area that contacts the tire. You should not change letters such as changing from a J to a K. One is not better than the other, but tires are designed for a specific flange shape.

Finally, if you run duals, then the "offset" dimension is very important. If you go smaller, your tires may rub, which could cause a problem.

All of the dimensions and ratings need to be stamped into the wheel or in writing from the manufacturer. I strongly urge you not to just take the word of the person selling the wheels.

If you think you need to change the wheels because you are changing tire size or rating to carry more load, you need to work closely with the supplier to be sure you are not overloading the axle, springs or other suspension components, the dimensions of the new wheels will properly fit the hub and bolts, and the offset will not allow the tires to rub.

Tires intended for dual application have specified clearance called "Dual Spacing," so be sure to confirm that dimension from the tire manufacturer before you go wheel shopping.


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TPMS batteries, Changing tires, Tire Size info, ST type tire speed rating and RV control with tire Blowout

Few quick topics in this post.
Saw a few posts on RV forums on TPMS sensor replacement.

It seems some people have had their TPM systems long enough that they need new sensors

because their batteries are low. So some folks are confronted with spending $200 to $300 on a set of 6 sensors.  I suggested that they look into the TireTraker system as it has low cost watch batteries so the cost is only a few bucks for a new battery rather than $35 to $50 each sensor. Full disclosure. TireTraker is a sponsor of this blog but I did buy my TPMS from them at a rally a couple years ago based on their features.
I suggest you do a "Life Cysle" cost comparison based on 5 years. Include the initial system cost, the cost of replacing the batteries if your system allows that or the cost of replacing all the sensors if you can't replace just the batteries. You might also want to consider the length of the initial warranty.

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Another person said he had 8 years on his Class-A tires and was asking "What brand should I buy"? 

This of  course immediately started a flurry of "I have had great success with Brand-X" or I had a failure with

Brand-X so will never buy another one of their tires, etc.
I asked the poster why he was considering changing brands if he had had 8 year  good service from the tires he had? When it comes time to consider new tires I suggest you make a list of the Pros and Cons of your current tires.

 Be sure not to include things like the puncture with the roofing nail as that can happen to any brand tire.

 I often tell people you are buying a tire company and their dealer network not just a set of tires. If you get a great price from Billy-Jo-Bobs Cheap Tire Emporium and Bate Shop but there are only a handful of dealers in the country where you can get a replacement and you have to pay shipping back to Billy-Jo if you want to make a warranty claim I doubt that the total price of owning that set of tires is as good of a deal as you first thought.

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When asking a question on an RV forum, it helps if you include the complete tire size designation. Some folks say nothing about the size but want specific answers on load capacity. Others provide only part of the size such as 235/75R16 and leave off if they are talking about a "P" type or "LT" type or "ST" type. The answer to these questions will probably depend on which type of tire we are talking about. It also helps if you include the Load Range as in LR-D or LR-F or whatever is molded on the tire sidewall.
Sorry but indicating that you have a Mountain Top Rambler RV doesn't help as there are just too many makes and models of RV out there for anyone to know all the tire options that migh be used.

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ST type tires have a normal Max Speed of 65 mph. You should consider this like the redline on your engine. While it is possible to exceed the red-line it isn't good for long term durability. When covering this topic some point to the Goodyear Tech Bulliten that indicates you may increase this max speed rating up to 75 mph if you also increase the tire inflation 10psi above the inflation associated with the max load on the tire sidewall. While Goodyear may be willing to stand behind the warranty of their Marathon ST type tires at this higher speed you should not do the same for other brands of tires unless you get something in writing from that tire MFG. Tire company Tech Bullitens only apply to the brand and line of tires mentioned in the bulliten.

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Finally I had a question on what to do if you have a  blowout on your RV.  Michelin has a good video on this topic covering Motorhomes. Similar driver reaction if you are driving a tow vehicle would probably apply, so there isn't much I can add. The question however asked about blowout on an RV trailer. Now I have not tested this myself but I have seen more than one example where the impact on vehicle control was so small the driver never knew he had a trailer tire failure till someone flagged the driver down.
As explained in the Michelin video the forces would be drag on the trailer which would keep it generally straight behind the tow vehicle with only a little side offset.
If your trailer did start to sway I would use the manual brake control to slow the trailer down. This should quickly stop the side to side sway and allow you to bring the tow vehicle and trailer to a safe stop.

How to get your tire questions answered correctly

I have offered to answer specific tire questions on this Blog. I respond to questions on four different RV web sites that have forums where tires seem to be a hot topic. One things is a constant with many who ask questions is that they seldom provide enough information with their question such that I am not forced to ask for more information or guess.
I and the other tire experts on the web have a request.


When asking about specific loads, inflation, alternate size, or specific tire problems it would really help if basic information was provided.

While I and the others are more than willing to consult the standards books or provide an opinion we really can't spend hours trying to first solve the problem of knowing what tire we are being asked about. One thread I am following has over 275 posts but at least 25 - 40% are replies that show the need for more accurate information is needed from the poster.

Please provide the "Complete" size. That would include the Service Description part that comes after the rim diameter as well as the type tire which is the letters up front if any.

235-15 isn't correct but many times that is what we get.
Technically an LT235/75R15 107/110Q Load Range D
is a different tire than a LT235/75R15 Load Range D.

Also concerning the load on the tires. As a minimum you should have a real scale measurement of the total load on all the tires. While you may feel you know a lot about your "Death Valley Special" RV not all of us know if it has one, two or three axles.
Ideally you should have at least once had your RV weighed with certified scales where you can get the individual loads on each tire.
Almost ALL trailers are out of balance axle to axle in addition to side to side with a good portion (usually the ones with tire problems) discovering that they are at least 5% off and sometimes as much as 15% out of balance.
So in addition to the complete tire size how about:
"I guess the 4 tires on my trailer carry 10,000 pounds"
or
"The tires on my front axle carry 5500 and the rear axle 4500"
or best of all
"RF 3300 LF 2200 LR 2700 RR 1800"

Motorized RVs are a little better but still have some out of balance.

Finally if you really want to do a First Class job then the tire brand and design as molded on the tire, the max load at what inflation as molded on the tire and picture -in focus please- of the placard would be of great help.

I know that if given the choice of who to answer first I would answer the person that supplies the most information first.

OK I am stepping down off my soap box now.

Class-A TBR type tires

Previously we talked about the full tire size nomenclature for Passenger and Light Truck applications.

Large RVs usually run what are known as Truck or Bus type tires. With most new applications being of Radial construction we use the acronym TBR. Here are some examples:
255/70R22.5 LR-G 138/134 M
In this example, the various numbers and letters are similar to those for Light Truck except the Load Index numbers and Speed Symbol are optional so may not be marked on the tire sidewall as part of the complete size nomenclature. The Load Index 138/134 and the Speed Symbol "M" make up the Service Description. If this is on your tire please include it when asking a question.

11R22.5 LR-H
This is an old “Inch” size description with the tire being about 11” wide. This would be a tubeless type tire.

10.00R20 LR-H 146/142 L
This is an old “Inch” size description with the tire being about 10” wide. This would be a tube type tire. Again the Load Index and Speed Symbol are optional

8R19.5 LR-F 124/122 L
This is another old “Inch” size description with the tire being about 8” wide. This would be a tubeless type tire with the example showing the optional Load Index numbers and Speed Symbol.
The two Load index numbers signify that the tire is rated for a higher load in single application and a lower rating when in a dual fitment.

The Speed Symbol for TBR tires are associated with the following MAXIMUM operation speeds.
F = 50 MPH G = 55 MPH J = 62 MPG K = 68 MPH and L = 75 MPH
If no Speed Symbol is marked on your tire you need to consider the tire is rated for a MAXIMUM of 65 unless you can find printed documentation from your tire manufacturer stating some higher speed limit is acceptable.
If you are writing to ask a question about your tires please include the complete nomenclature including Load Index, and Speed Symbol if marked on the tire.

SPECIAL SAFETY NOTE: Maximum Inflation
In July, MOTORHOME magazine published an article on TBR tires. The author offered his personal opinion that it was acceptable to set inflation higher than the Max marked on the sidewall of a truck tire. Exceeding the max cold inflation on the sidewall has serious potential safety consequences. Large TBR tires have been known to explode and personal injuries and even deaths have occurred when proper inflation procedures are not followed. I can find no industry guidelines that allow this procedure of exceeding the maximum inflation when setting the pressure for normal highway application of any tire.

Rims also have a Maximum rating for both Load & Inflation.
Many wheels have these ratings marked on the rim. If you can’t find that information I strongly suggest you contact the manufacturer and obtain the ratings for your rims. Again Serious injury or even death can occur if a rim fails due to damage and overinflating or improper assembly.
Accuride Safety information and Product Literature you can consult.

Alcoa also has product literature online.



Inflation Safety
Whenever we are talking about setting the inflation in a tire, we are talking about “cold” inflation. This means the tire is at ambient temperature and not warmed from operation or sitting in the sun.
I would suggest the tire not be driven more than two miles in the last hour and not in direct sunlight for at least two hours. When I did my tire cover study – See my post from June 16 on Tire Covers, I saw almost 40° increase in tire temperature in about one hour for a tire in full sunlight.

I would strongly suggest you never attempt to inflate a TBR tire if the tire beads are no longer fully seated against the rim without using an approved Safety Cage. If there is a problem, the tire and wheel can separate and components can easily go through concrete block wall never mind the side of an RV.

Any tire that has lost more than 20% of its rated inflation is considered flat and if it was driven on in a “flat” condition it needs to be dismounted and inspected by trained tire company store technitians. Not a Mom & Pop garage but a store run by a tire manufacturer.

What size tire is it?

Too often that is one of the first responses’ I have to make when asked a question about tire replacement. The reason is that 98% of the time the person asking the question does not provide the details needed to allow me or other tire engineers to make an informed answer to the question.

A quick review of some on-line RV forums show the problem of incomplete tire size information. Current posts mention:
225/75R-16E 235/80R 16s 275/70/22.5 12R22.5 315/80R22.5 235/80 22.5 LT235/85R16E 225/75R/16E 235/80R16E

None of these “sizes” provide what I consider the complete size nomenclature.
Here are some facts from the Tire & Rim Association industry standards book that point out why having all the information is both helpful and important.

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

LT235/75R15 101/104Q LRC
1985#single 1820# Dual 50 psi 99mph

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

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

To help understand the nomenclature the “P” or “LT” or “ST” is the type tire i.e. passenger, Light Truck and Special Trailer. The numbers are related to the physical dimensions of the tire with 235 being the overall width in mm. The 75 is the ratio of the height of the tire from rim to tread and the 15 is the nominal rim diameter. Hopefully everyone knows that the “R” means radial construction.

LR stands for Load Range with P type tires being LRB which usually means the max inflation is 35 psi but no one uses that designation as it is considered “standard”. P type tires can be rated XL for Extra Load or “Reinforced” and tires with either of these identifications would have a max inflation of 41 psi marked on the tire. The rest of the letters C, D, E etc are related to the maximum inflation for that size. NOTE not all tires with the same Load Range letter are rated for the same max pressure.

An example would be the LT275/70R16 LRC is rated for a max inflation of 50 psi while the LT305/70R16 LRD is also rated for 50 psi max.
Now what about those numbers and letters such as 101/104Q? The “Q” is the maximum speed rating symbol and the numbers would correspond to the Load Index for Dual application and Single application. There is a table that identifies the load in pounds that correspond to the index. The simple way to think of why this is there and how it can be useful is to think of how you can decide if a different size tire can be used. If the Load Index number is equal to or higher than that would be an acceptable replacement.

All these examples relate to P, LT or ST type tires. I will provide another post focusing on larger Truck-Bus type tires some of which have a completely different dimension nomenclature.



UPDATE 9-17-2011
I guess I was in too much of a hurry to post to be sure I covered all the bases. Thanks to Al for pointing this out.

BOTTOM LINE
Whenever you post a question here or in another forum or in an email or even when asking for replacement information at a tire store Please be sure to include the complete size description

Examples:
Most passenger tires will have this format
P235/70R16 104S

Extra Load passenger tire would look like this
P255/60R15 105Q XL

Most newer LT tires will have format like these two
LT235/85R16 116/120R LR-E
LT265/70R17 109/112S LR-C

This would be on some older non-speed rated LT tires
LT235/75R15 LR-C

NOTE I have not covered TBR tires. I will try and do a better job in that post.