How is a tire like Potato Salad?

If you have attended one of my seminars you already know the answer but for those who haven’t, I thought it was time I explained this idea to others.

I tell the story of a friend Bob, whose wife Mary makes the best tasting potato salad I have ever had.

Others agree and her salad has even won awards. One day last summer in the middle of a major heat wave with temperatures well above 95°F, Bob and Mary had a picnic.
Of course Mary made a big batch of her potato salad. As the afternoon wore on it got even hotter.
“How hot was it?” you ask. Well it was so hot we were thinking of testing the idea that you could fry an egg on the sidewalk. Really, it was so hot that we ended up moving inside into the AC cooled home.

As everyone moved indoors Mary reminded Bob to be sure to put the remaining potato salad into the fridge. Well wouldn’t you know it but Bob got side-tracked with keeping the beer on ice so he forgot the salad. That was until the next morning when he was cleaning up the deck and discovered the bowl of potato salad, so not wanting to get on Mary’s bad side he just put the bowl in the fridge and figured it would get cold again after being out in the sun all afternoon and outside with the temperature still over 85°F that night.

In my seminar, after relating this story, I ask who would like to sample some of Mary’s potato salad that sat outside in the heat for many hours. Strange thing is, I have yet to find anyone that thinks it would be a good idea to eat the remains of the salad, even though Bob had placed it back in the fridge the next day. For some reason no one thinks that the salad would be OK to eat even though Bob had “fixed” it by putting it back into the fridge.

I then point out that a tire is like Mary’s salad. If you run a tire overloaded or underinflated or faster than it’s design limit you do not “fix” the damage you have done to the tire by re-inflating it or taking some of the load off or slowing down. You have done permanent and irreparable damage to the structure and like potato salad which was “damaged” by being left out in the heat, the tire will not fix itself after being damaged.

Now some of you will ask how much overload can a tire tolerate or how much under inflation or how many miles it can run at too high a speed. Well the reality is that just as there is no exact number of minutes or hours the salad can sit out in the sun before it goes bad enough to make you sick. The thing to remember is that just because the tire looks OK it does not mean that there has not been some microscopic damage done.
A tire, like Mary’s award winning salad, has a finite life no matter how well it was designed or manufactured. Leaving the salad out in the heat consumes the normal life of the salad just as running under inflated consumes the normal life built into a tire.

Bottom Line:
If you remember nothing else from any post on this blog or from what I teach in my seminar I ask that you simply remember "Potato Salad".

If you always keep your tires properly maintained and within their designed usage limits for load, inflation and speed you should be able to get the maximum life from your tires with no problems. If however you occasionally run your tire 5% or 30% low on air or a few hundred or thousand pounds overloaded or a few or a dozen mph over their speed limit, you will be “consuming” a portion of the tires normal lifespan every mile you run so you should not be surprised if you do not get the suggested normal life out of your tires.

Do you need to balance your motor home tires?

Do you need to balance your motor home tires?

There is some controversy on this topic. Some insist it is not necessary while others say you should always have your tires balanced. Here are my thoughts.
If you are not experiencing any vibration of your seat, through the floor boards or in the steering wheel due to the imbalance of your front end components, I see no reason to spend the money to have your tires balanced. Now if you have some vibration, one of the first things you need to do is to try and identify the cause. Is it due to tire wheel assembly imbalance and not some other front end component or source? To learn the answer you need to do a little investigation and give a little thought to what you discover.

Vibration can be due to a number of different sources and depending on when and where you feel the vibration and if you can make the level or frequency change you will know what the possible source is.
Do you have a large generator up front as normally found in “pusher” Class-A? Does the vibration change or go away when you run or turn off the generator? If so then one of the areas I would first look into is the generator mountings. This can include the rubber bushings that attach the generator to the frame around it. Are the bushings old?
Remember the general advice to change your tires when they get to be 10 years old?
Well motor mounts are also rubber and that rubber can age or get oil soaked and sometimes even tear. Spend a few minutes with good lighting and check each mount to be sure the bolts are tight and the rubber appears to be in good condition and not overly soft or hard. If your generator can slide out for service, be sure the mounts of the slide frame are firm when the generator is locked in place for travel.
Next ask yourself; does the co-pilot feel the vibration or just the driver? Is the vibration felt mostly through the steering wheel? Does the wheel just “Buzz” or does it oscillate back and forth? Steering wheel motion can many times be traced to worn steering components so you need to do a thorough inspection of the steering shaft mounts, steering box and all the links that go to the front suspension.

Moving on, when there is vibration felt through the floor and possibly the seat when both the driver & co-pilot can feel it, does the vibration feel most pronounced at low speeds or above say 40 mph? Low speed vibration can many times be caused by tire “Flat Spots”. A Flat Spot is a localized condition where a tire has taken a set from long term parking and can be more pronounced when the tire was parked when it is hot. Driving on a tire that has a large flat spot sometimes is noticeable at less than 10 mph. Also sometimes a flat spot can go away after 30 minutes to an hour of highway speed driving after the tire warms up. You can minimize flat spotting by ensuring you are carrying more than the minimum inflation needed to carry the load. If you are going to park for a few weeks or more I would recommend you inflate your tires to the max pressure shown on the tire sidewall. If you have the ability to remove some of the load on the front tires that will also decrease the tendency to flat spot. Balancing a flat spotted tire will not solve the ride problem. Remember it is possible to balance a cinder block but I doubt you would ever get a smooth ride driving on one. Old worn shocks can allow normal short term flatspot vibration to get to the passengers so you might need new or better shocks.

Another possible source of floor vibration with front engine motor homes when not felt through the steering wheel could be problems with the drive-shaft, U-Joints or support bearing. These can be checked by any truck service company and with smaller Class-C units possibly a large Ford or Chevy dealer depending on the manufacturer of the original cutaway van chassis.

High speed vibration that changes as you increase speed does point to a rotating component. This is not always just the tire as the imbalance could also be the wheel or the brake drum/rotor or hub or even a wheel not properly centered on the hub. The rim hub fit should look as seen in this picture.









Also your tire might not be properly centered on the wheel. One thing you can check yourself is tire centering on the wheel. Most tires have what is sometimes called a Rim Centering Rib.




This is a feature molded on the sidewall which should be visible just above the rim and uniformly spaced around the tire.











If you see variation in the RCR to rim spacing you need to have the tire inspected and possibly de-seated and re-inflated to ensure the tire is concentric with the wheel. The tire shop that mounted the tire is responsible for ensuring the tire is properly seated. You should not attempt to fix this yourself as it requires the use of a tire safety cage.

Another thing to check is the run-out of the tire. This should be less than 0.035”. If there is more than this then you need to fix the centering or mounting issue that is probably causing the run-out. If all the components are properly centered then you might need to have the tire trued if the vibration is a real problem. Remember that cinder block.

If all else checks out then I suggest you have an on vehicle balance done as this will identify any imbalance in the rim, hub, brakes or tire. Off vehicle balance does not confirm the brake drum/rotor and wheel mounting or hub is in balance.
If you need to have the rotating assembly balanced external weights are usually used and may be needed on both the inside and outside of the rim to achieve full dynamic balance. If you choose to have the tire balanced by adding one of the products to the inside of the tire be sure to get a full warranty, in writing, that the balance material used will not damage the wheel, tire, valve or TPMS. If you use one of these products and have a TPMS valve you will also need to purchase a special valve core to prevent the material from plugging the small sensor hole.

I hope this clears up any confusion on how to achieve a smooth ride.

Some links with more information. NOTE I am not recommending any of these companies or products. Just showing you that a quick GOOGLE on truck tire truing will find the options available to you.
GardenSpot
Cooktire
TruckmodCentral
KeenTransport

Don't damage your tires when you clean them

Recently there was a Tech Article in an RV publication about cleaning the tires on an RV. The reply reminded me of some investigations I did a number of years ago on some strange looking sidewall damage. After doing a confirming test, where I was able to duplicate the damage, I concluded that the damage was done by a High Pressure Power Washer.

Here are some examples of the damage. You can click on the image to see an enlarged view.








Another example is seen here:











In sample three you can see the cut that was made to confirm the depth of the damage.



I have never seen a advisory posted on one of these High Pressure Power Washers warning of the damage that can be done if you get too close. There are many different pressure ratings so I cannot give an exact limit on how close you can get without doing damage but I would certainly stay more than a foot away if I felt I needed to use one.

This damage has been documented in Tire Industry publications so if you do cause this type of damage, there is a good chance you may not be compensated under the tire warranty, so be careful. The best way to clean your tires is to use the same soap you use to clean your car or RV and a sponge or cloth just as you would on the paint on your vehicle. I would not use a stiff brush or anything like steel wool on the tire as you will scrub off the protestant waxes built into the tire which will accelerate the tire sidewall aging and could promote sidewall cracks. I would limit the pressure I use to what I get out of a garden hose.

Why should I weigh my Trailer tires individually?

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


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

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

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

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

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

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

Quick post on rubber valves

I continue to see posts from RV owners saying they have "Rubber" valves. This even though most rubber valves are only rated for 65 psi cold. There are some rubber valves rated for 80 and even some rated for 100 psi but these are not interchangeable.

How can you tell if you have the correct rubber valve for your application without dismounting the tires?

A quick examination should give you an idea of what you have.


What to look for

First unscrew the cap. See if the the only metal you see is the threaded part right under the cap. This is just over 3/8" long.


This valve is usually about 1-1/4 long outside the wheel. Is designed for a hole in the wheel of 0.453 and is rated at 65 PSI Max Cold inflation.


If you see metal even with the cap on like this

It is either a 600HP series LIKE THIS which is rated for 80 psi Max Cold inflation or a 801HP series rated for 100psi Maximum Cold inflation.


NOTE these are NOT interchangeable as the 600 is designed for a rim hole of 0.453" while the 801 needs a hole diameter of 0.625".

Now even if your cold tire pressure does not exceed the rated capacity for the valve you have I strongly suggest you consider switching to a bolt in metal valve with the appropriate rim hole size as if you run valve extenders or hoses or external TPMS you are probably applying more side load to the valve that it was initially designed to accept.

Also you should be running metal valve caps not the cheap plastic ones that probably were OE for if the valve core leaks due to a bit of dirt the metal cap can hold much more pressure than the plastic one.


NOTE if still in doubt you should have the valves inspected by a tire dealer. Call and ask if they have a Technician ASE Certified on tire inspection. If not, select another dealer.

Careful when you do research or before you buy

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

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



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


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

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

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

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

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.

“I had a Blowout”

That’s an all too often statement heard from RV owners.

According to some data a significant portion of RV owners have experienced a tire failure in the past two years. All too often the tire is blamed but my experience from examining thousands of “tire failures” is that the vast majority of these failures can be traced to the service conditions and not some so-called defective tire. Let’s look at a few examples.

Here we see an LT type tire that is now in two pieces







This picture shows the classical “Three piece Flex Failure”. I bet you see a tread ring like this almost every day in your travels.




This tire was one I did an “autopsy” on. Looking at the “broken edge of the sidewall we can see hard body ply cord.





Taking a microscopic look at just one end we easily see the fused cord.








This is much like what we have all seen when we cut Nylon or Poly rope and then melt the end with a match. Click on the picture for a better view.
It takes excessive flexing at a high rate to generate sufficient heat to melt the cord like this.
This Body Ply cord melts at approximately 495°F. I usually say the tire was operated at less than 20% of the proper inflation and at normal highway speeds to generate this level of heat.

Before you say “Ya but I have steel body ply so it’ can’t melt”, just think about how easy it is to break a paper-clip by flexing it back and forth.




Here you are looking through the hole in the sidewall of a 22.5 size tire that ran low at highway speeds but lost it’s air because of a bad valve leak.


So was this “blowout” the fault of the tire? Or was there some reason the tire lost most of its air pressure which let the tire flex excessively? I say that based on a majority of tires I have examined, "Blowouts" like these are really Sidewall Flex failures from being operated at highway speed while the tire has lost 3/4ths of it's inflation



Next time you see a tread ring by the side of the road or hear someone say “I had a blowout” you now know one of the probable causes.

Finally, You might ask if there is a way to prevent a sidewall flex failure. I believe that if you have a properly functioning TPMS it should warn you before the tire has lost 20% of it's air. If still inflated to this level it isn't considered officially "flat" and I have never seen a tire that can generate the heat necessary to fail a sidewall if it hasn't been run "flat". If you can set your TPMS to warn you if you either loose 10% of your air pressure or it sees a temperature over 170°F in most cases you will have sufficient warning to avoid this type of catistrophic failure.

Tires - Dull or Dynamic? Part two

Following is the second part of an article I was asked to write for a tire industry trade publication in the UK. “Tyres International.” I thought you might find it a bit interesting.

“To the untrained eye it might seem like the tire industry has not done much to improve John Dunlop's original. But looks can be deceiving.

Part two
A cost/performance comparison on the improvements in wet and snow traction, ride comfort, low noise, blowout resistance and handling response is significantly more difficult, as the tire of 1920 was so poor in many of these areas as to be considered useless when compared to today's tire.

When one considers high-speed capability it is a little easier. In 1920 most cars were not capable of a sustained 85mph, yet today that is considered an absolute minimum high speed capability for a tire to be sold for use on the public highway.

Material World
Some would have us believe that today's tire engineer is not willing or even encouraged to look at new ideas or materials. Even the use of rubber, both natural and synthetic is used as an example of the perceived lack of advancement in tire technology.

I know from personal experience that alternate materials for both wheels and tires have been investigated but either lack of performance or poor reception in the market prevented those ideas from making it into production. I have seen non-metallic wheels that were too flexible to hold air at high temperatures and too stiff so they shattered at low temperatures. New tire sizes with improvements in safety and mobility were not accepted because non- standard wheels were required. Even well engineered items such as the mini-spare, still meet with resistance and ridicule despite the fact that they can deliver acceptable performance when needed. They even have an impact on vehicle fuel economy when not needed

Rubber band
The basic material in Mr. Dunlop's tire, natural rubber, comprises less than 10 per cent of today's modem passenger radial. Today's tire, with 25-30 different materials, made up of hundreds of different chemicals, is one of the more complex components in a modem automobile. This is especially surprising when you consider that some materials are considered contaminants and are incompatible with other materials in a tire yet we have managed to make these incompatible materials work together to deliver improved air retention and blow- out resistance.

It is likely the concept that tire materials have not changed is a concept only held by those with little training in the an of tire design. There are few materials that are capable of 300 per cent strain for tens of millions of cycles over an operating range of temperatures from -20°F to +200°F while at the same time having a coefficient of friction of 0.8 or higher. Today's steel belted radial could be improved upon and even have its weight lowered, with increased use of rayon, fiberglass or other materials as belt material. There are however, restrictions on pollution or customer resistance to materials other than steel which have so far proven insurmountable obstacles to broad appeal for the average consumer.

Some of our biggest challenges will come in the next decade as we are asked to change from making a product that will last indefinitely under extreme conditions as the tire industry has been asked to do for more than a hundred years. We are now being asked to design a product that will be almost indestructible until the user wants to change it, then the tire should, as if by magic, become easy and inexpensive to deconstruct into its chemical components. Some OEMs are even starting to suggest that old tires should be able to be recycled into new tires with no loss in any performance characteristic.

I have every confidence that the tire industry will rise to this new challenge and methods will be developed to address the disposal and reuse of materials in a tire. It is unlikely the recycled materials will be used 100 per cent in another tire just as the OEM will not be able to recycle the leather car seat into a good as new' leather car seat, but we will incorporate an ever increasing percentage of recycled materials in tires and we will find acceptable methods of recycling them into some usable material or product at the end of their useful life as a normal tire.

To the uninitiated it is easy to look at John Dunlop's tire of the late 1800s and say that since today's tire is still made of "rubber' it is not really any different. Thus some would consider this sufficient proof to postulate the tire industry is not capable of looking at history, learning from it and moving on.

It is my belief that this thinking ignores the advancements in both the materials and performance delivered at a very low cost to the often uncaring consumer.

Tires - Dull or Dynamic? part one

Following is an article I was asked to write for a tire industry trade publication in the UK. “Tyres International.” I thought you might find it a bit interesting.

“To the untrained eye it might seem like the tire industry has not done much to improve John Dunlop's original. But looks can be deceiving.

Many people think the tires on their car are just a necessary evil. Most seldom, if ever check their inflation while alignment is something few ever consider. Then after 40,000 miles, they complain about the fact they have to buy new tires. Even within the automotive industry, there are those who don't appreciate the dynamic nature of tire design, and the significant improvements and changes made since John Dunlop invented the forerunner of today's modem tire.

There are many challenges facing the tire engineer today, but one of the most challenging is to make continual improvements in rolling resistance. In North America, the Original Equipment Manufacturers, otherwise known as "Detroit", demand the improvement or lowering of the rolling resistance value of the tires they approve. The rolling resistance of tires has a direct impact on the fuel economy of the vehicle. Many would think the primary reason for an interest in rolling resistance would be to allow the manufacturer to advertise good gas mileage for their vehicles.

Stopping the gas guzzle
While good fuel economy is something that can be advertised, a more concrete example of why this is a major concern of the OEM is the avoidance of what is known as the "Fuel Guzzler Tax". This tax can amount to many tens of millions of dollars as the OEM must pay for each 0.10 mpg their vehicles are over a government mandated rating.

There is some data that shows that a one per cent reduction in rolling resistance could be worth US$60 million on certain car lines. Despite this major interest by Detroit, I have never been asked about the rolling resistance of a brand or line of tires by any individual considering the purchase of a set of tires.

Significant strides have been made in improving the rolling resistance of tires going to Detroit. Figure 1 shows the trend as well as the ultimate value possible with a steel wheel on steel rail. It is obvious to see the majority of improvements have already been made and while we may expect some level of continual improvement, we will not see the dramatic improvements of the past 20 years continue in the future.
RRC or Rolling Resistance Coefficient is one way of comparing a variety of sizes and adjusting for vehicle load as well.


Figure 1


Pricing opens the purse strings
At the same time as these improvements have been made and despite great strides in ride quality, crisper handling, improved snow and wet traction and other various measures of performance, Detroit also expects us to lower our price a few per cent each and every year. The fact that many people are willing to spend more for shoes, simply because some athlete wears the same brand, than they are willing to spend on a tire points out the level of disdain and disinterest most feel toward tires. Seldom does a driver consider that it is the tires that must deliver strong performance in emergencies to help protect them and their family from harm.

History in the making
A quick look at the advancements in tires during just the last 80 years can be very instructive. In 1920. your normal auto-mobile tires cost between US$25 and US$60 each. This tire was advertised as being capable of delivering 6,000 miles. This translates to about US$0.007 per mile. With normal inflation considered, these sale prices translate to US$200 to US$500 per tire in today's dollars. When we consider that today's normal tire is capable of delivering 40,000 miles, yet can be purchased for about US$70, we can see that the cost per mile is now only US$0.0018 per mile.”

We will conclude this article in the next post.

Special considerations for duals

Highways March 2011 had a post titled "Tire Snafu" on pg 40

A person mentioned that both rear duals tires got hot due to air loss and subsequent overload. One tire was identified with a leaking valve and was replaced but there was no suggestion that its mate might have also suffered internal structural damage from running hundreds to thousand miles in an overload condition.

Few tire dealer salespeople or even tire technicians have the training to see the tell tale signs of overload.

Here is a picture I call "Shades of Black".


We know the tire in the picture was run overloaded as it had an improper repair (plug) and had continued to slowly leak air for many hundreds of miles. By "reading" a tire when doing a true forensic level examination the experienced tire engineer will notice the darker inner sidewall portion of the tire. This darker shading comes from increased level of flexing that can occur when run in overload or under inflated condition. The darker black is physical proof that the subject tire had been operated for many miles while under significant overload.

This operation resulted in degradation and weakening of the tire's structure which meant that in all probability, it could fail catastrophically with little or no warning hundreds or thousands of miles later.

This type of structural weakening is not visible from the outside and is difficult to see even with a proper inspection of the inside of the tire which takes more than the few seconds most tire techs are given to do an inspection.

I don't expect the magazine writer to know this but when responding to complaints about tires I strongly suggested that they touch base with tire technical resources and to provide a little education to readers.

When one tire in a set of duals is run "flat" i.e. more than 20% under inflated, it means the mate was also operated in an overload condition. Unless there are some special circumstances, the operator seldom knows how many miles, at what speed, at what % overloaded the tire was operated. Only the most experienced tire engineer and not tire store personnel, have the training necessary to pass judgment of the probable condition of the tire. Many times the mate to the "failed" tire should also be replaced or retained as a spare for temporary use.

Failing to warn readers of this potential safety issue is not a good policy.

Gage Accuracy, Rotation, Alignment, Fuel Economy

Went back and tried to review comments and questions. I know some have been answered in depth but some others maybe not enough to make the point clear so here are some short replies and thoughts.

May 15 Quampers asked about tire gauge accuracy.
Quampers, I currently travel with two digital gages that are accurate to 0.5 psi when compared to ISO certified pressure gauge in a tire testing lab. I can compare readings between these two gages and as long as they are within 1.0 psi or less of each other I can be pretty confident they are still giving me the correct pressure. I will have the gages checked on a two year cycle even if I find no change in their readings. These are the gages I will use to compare to others at my Tire Basics seminars. FMCA at Madison, WI Aug 11 and at Gypsy Journal Rally at Celina, Ohio Sept 27.

June 25 Tom asked about tire rotation and where to put the newer tires if you don't buy a complete set.
I have heard it said by others that if you don’t have a problem with irregular wear on your RV you don’t need to do an alignment or rotate tires. I guess that may be true as long as 1. The manufacturer of the coach bothered to do an alignment at the end of the assembly line and 2. You keep a close eye on the wear of your tires.
Now I can tell you that in my experience it appears that some manufacturers may be cutting corners when it comes to spending the time to set the alignment. After all your warranty is only good for a year and in that time most RV drivers will not get enough miles on their tires to see alignment problems. The second point is that even if you do complain the mfg will probably tell you it is your responsibility to ensure the coach or trailer is in align, even though there is no mention that as a new owner one of the first things you would need to do is to drive your new rig to a heavy duty alignment shop and do the work the manufacturer should have done in the first place.
When it comes to the question of where to place the new tires there are a couple of things you must consider. If your RV has dual tire position you need to be sure the Outside Circumference difference on a pair of tires in dual position is ¾” or less otherwise there will be more load transfer from one tire to the other than is advisable. Not Outside Diameter buy OC which is a more accurate measurement to get if you don’t have special tools. So for Tom it will depend on how many tires he needs to replace as to where they go.
When I recently rotated my tires because of miss alignment on fronts.
They looked like this in inner shoulder and this on outside shoulder













I measured all seven tires (including spare) then put the most worn tire as the new spare. It worked out that the still new original spare ended up on the front and I was able to match my duals to within ¼” for each pair.

Now if we are talking about a passenger car the two new tires should go on the rear to give you better wet traction and decrease the potential of a rotating skid and spin-out in an emergency. This is less of an issue for heavy RVs due to the weight cutting through the water better.
On multi axle trailers I would put the new tires on the front as this location is less likely to suffer a puncture and if you are going to lose a tire due to puncture I would rather lose the older tire. Hope this didn't confuse too many people. As you can see there are a number of things that need to be considered.


June 29 Bob asked about LRR or Low Rolling Resistance
LRR spec tires are designed to trade off a number of performance characteristics to improve fuel economy. Rolling Resistance is the force it takes to roll a tire under load. The more force the more energy needed to make the vehicle go down the road. I have not heard of LRR tires in RV sizes yet as most of this work is aimed at Hybrid cars getting 40 MPG or more.

What is the minimum air pressure you need to run?

Simple question but there is much confusion on the answer.

In the March 30 post we covered the fact that it is the air that carries the load not the tire.
The April 16 post showed what can happen to tread wear if you are over-inflated or under-inflated by a significant amount.

May 10, the relationship between temperature and pressure was covered.

May 23, the tire certification label or “Placard” was covered.

July 8, I covered the effect of elevation (altitude) on tire pressure.

But after reviewing all these posts it has been pointed out that there was no clear indication on the minimum inflation you should be setting your tires at when you check before a trip or travel day. There have also been questions about the minimum level for the warning from your TPMS.

I have no way of knowing every car, pick-up trailer or motorhome tire application so I can’t give you a number but I can provide step by step instructions so you can determine the values you need.

You need to create a page with the following information:

1. The tire size and Load Range.
Be sure to include the letters that are part of the size. Tires on most cars will start with a “P” but some imports have no “P”. SUV and small Pick-Up may also have a “P”. Larger Pick-ups with probably start with “LT” and many Class-B or Class-C RV will also have tire size that starts with “LT”. Many trailers will have tire size start with “ST” and most Class-A RV will have no letter before the numbers. We call these TB (truck-Bus) type.

2. The Minimum inflation and maximum load molded into the tire sidewall.
You will find that LT , TB and ST type tires will have numbers for both Single (front) and Dual (rear) position. Write them all down.

3. The Minimum inflation as provided on your Placard.
Note you may have different minimum inflations Front vs Rear. Some Pick-up may provide a light load inflation and a heavy load inflation. Anytime you are towing you need to consider that heavy load.

4. Your actual side by side load for each axle.
Here a guess may be off by 5% to as much as 20% so you need the facts.

5. Write down the minimum inflation for all tires on each axle based on your actual load for each position found in #4 above. Remember all tires on any axle should have the same inflation, so the number for the warning level from the load tables would be based on the heavier side. You can get the minimum inflation based on sacale weight from the Load & Inflation tables available from your tire manufacturer or other sources. Well over 95% of all tires of the same size have the same values as they generally follow Industry Standards, so if your tire manufacturer doesn’t have tables available on the internet find a table from another tire company and use that number.

Okay, so now you have all the information needed.

Your minimum cold inflation is the number shown on your Placard. This inflation has been established by your RV manufacturer as the inflation needed to carry the design load of the RV and to provide the best compromise of ride, wear and vehicle stability.

Your minimum for the warning level from your Tire Pressure Monitor System should be the higher of 80% of the minimum inflation on the Placard or the minimum inflation needed to carry your actual load based on the Load & Inflation tables.

This may sound complicated but here is an example from a “Super-C” application:

1. The tire size and load range.
245/70R19.5 LR-H Note this is a TBR type tire

2. The minimum inflation and maximum load molded into the tire sidewall.
120 psi 4805 Lbs single 4540 Lbs Dual

3. The minimum inflation as provided on the placard.
95 Psi

4. The actual side by side load for each axle.
LF 3242 RF 3652 LR 6914 RR 7279

5. Minimum inflation for all tires on each axle based on actual load for each position found in #4 above.

The table gives the following loads at various inflations:
Infl. 80 85 90 95
Single 3540 3740 3890 4080
Dual 3415 3515 3655 3860

So we can see that based on the heavier RF we need a minimum of 85 psi to carry the load in the front axle and 90 psi in the rear based on the heavier RR.

I would suggest that you set your warning levels at the 85 to 90 psi levels but you should be setting 95 psi as your minimum morning inflation before each trip and each day of travel as this is the manufacturer’s recommended minimum inflation for the vehicle.

Remember if you lose 20% of your inflation it is considered to have been run flat and needs to be dismounted and have the interior inspected by a knowledgeable tire tech or engineer. 80% of 95 is 76 psi and you would like to avoid the cost and time involved in having a complete inspection so having the warning go off at 85 to 90 psi provides a nice margin.

With all this information you also have learned that if you discover a tire a little below 95 psi but still over the inflation needed for the load you can plan on driving a short distance at a reduced speed to the nearest location that can provide the 95 psi needed. If you do this don’t forget to compensate for the warm tire. (See Temperature & Pressure May 10).

A final point.
Many suggest that you over-inflate your tires by 5 psi to provide a cushion for day to day temperature variations and I support this idea as long as you do not exceed the maximum rating for the tire or wheel. Many wheels are marked with their max inflation rating and if yours are not marked you need to contact the wheel manufacturer for the specifications.

Altitude (elevation) effect on Tire Pressure

We have previously discussed tire pressure change vs temperature change. The question of tire pressure change due to altitude (elevation) has been brought up on occasion so I guess it’s time to answer that question.

Quick answer: Driving from Death Valley to Denver will result in less than a 3 psi increase in tire pressure IF there are no other changes. So it isn’t a big deal.

Now for the engineers and technically inclined the formula is p = 101325*(1 - 2.25577 * 10**-5 * h)**5.25588 according to info at Engineering Toolbox.

With p (pressure) in Pascals and h (height above sea level) in meters. After unit conversion and if we assume 14.7 psig at sea level we theoretically would have 12.2 psig at 5,000 ft. This decrease in pressure outside the tire would translate to a 2.5 psi increase in the internal tire pressure.

In reality there would be other changes that would also affect your tire pressure as I doubt that you would find the ambient temperature to be the same.

Bottom Line.
If you are checking your tire inflation the morning of every travel day as you should, and adjusting when necessary to meet your minimum inflation as published on your RV tire placard, you will not have any issues as you travel across America, no matter the altitude (elevation) or weather.


Don't forget if you have questions you can submit via email (previously published)

or

Post a comment to the June 25 topic "Do you have questions?"

Four new RV tire videos

These four videos were just released by Goodyear.
The info in these videos is good to know and follow.

Tim, the guy in the video, and I go way back to the late 70's when we were both working on racing tires. I was at a different company :-)

Even though this information is posted by Goodyear I strongly suggest you follow the advice given as sound technical advice is good to follow no matter who is the messenger.

Do you have questions?

I can only guess at what your questions might be about. If you want to know something about tires post it as a comment here and I will make a list of topics to cover.
No guarantee, but if you don't ask, I may not think of the topic you want to know about.

Quick post on Max tire temperatures

I was recently asked "What do you recommend for high pressure setting with a TPMS?"

The person said after driving about 50 miles in 92°F weather he was seeing 148°F on his right front and the left front went to 136°F on his Class A RV. His tire pressure monitoring system (TPMS) has a setting of 158°F for high temp. He wants to know; At what tire temperature should he be concerned when driving?

A couple of quick points.
If the TPMS is one of those that screws onto the end of the valve, then it is reading lower than the hottest part of the tire. If he has extension hose on the dual then the temp reading is even lower. I have tried to find a TPMS manufacturer that has data on the temperature difference but so far either they do not answer my email or they haven't bothered to do a test.

My information indicates that a properly loaded and inflated Class-A tire will probably see 140°F to 170°F operating temperature and these temperatures are OK as the tire is designed for this temperature range.

If you have actual side to side weight and know you are not overloaded, you may see temperatures in the 140 - 170°F range. If you see higher then I would be a bit concerned and want to know why.

Setting your TPMS Max warning temp to 158°F sounds reasonable to me.

Tire Covers - Do they do any good?

I have seen some posts and comments about tire covers. One post caught my eye.

The owner was complaining about the cover degrading in the sun so he was of the opinion the cover wasn't worth the cost. I have to wonder how he failed to properly interpret the proof that the cover was protecting his tires. How would he answer the question: Which would you rather have degrade due to being in the sun? Your tires or the low cost cover?

As an Engineer I always prefer to have data rather than just someone's opinion. So I set up a quick test on my own RV.

As you can see I have covers for my tires. They were the second purchase after buying my rig. The first was a Tire Pressure Monitor System.





The Test
With one side of the RV in full shade I checked the temperature of the side of the unit.
You can see I recorded 85.1°F.




Then I took the temperature of the side of the RV in the full sun.

Here we see 107.9°F








Next the temperature of the white tire cover in full sun.

We get 98.6°F
I believe the cover is cooler than the side of the RV because air can circulate behind the cover.


I then removed the cover to see what the tire temperature was under it.


We see 99.5°F
Only 1 degree hotter than the cover.







I then waited 30 minutes to see how hot the black tire got while in full sun.

We see the black tire was at 136.1°F.








KEY POINT
There are three things that can "kill" your tires. Ozone, UV and High Temperature. The Ozone & UV directly attack the surface of the tire making it crack when flexed. Temperature works no only on the surface but deep down inside the tire structure.
Increased temperature causes continued and accelerated chemical reaction which "ages" a tire faster than when the tire is cool. A rule of thumb would be that the rate a tire ages doubles with every 18°F increase in temperature. We can see the result of old rubber on the surface. What we don't see is the more brittle rubber of the internal tire structure. As rubber gets more brittle with age it also looses strength.

NOTE Overload and Underinflation can overheat a portion of a tire to the point the rubber and reinforcement materials loose all their strength.

Based on my simple test it would appear that by covering my tires I am significantly reducing the artificial aging for all the daylight hours my RV is parked and the sun is out. If I didn't have covers my tires would be "aging" FOUR times faster than with the covers in place.

I don't have black tire covers so can only guess at their temperature. While they may provide protection from UV I would be surprised if they can offer much temperature protection.

So I will let you answer the question of Tire Covers being a good investment or not.

Tire Recalls

I have seen a number of posts on various forums of complaints about what is believed to be a failure of a “defective” tire. What I have not seen is any mention of anyone having bothered to provide details and information on what the defect is and a statement that they send the data to the US Dept Of Transportation. I have to wonder why people think there might be some government action when there is no information going to the government complaining about what they feel is a defective tire.

You can review the current tire recall list from the National Highway Traffic Safety Administration HERE.

NHTSA is the part of DOT that is responsible for collecting complaint information and doing the appropriate investigation. If there is data developed that would support some action such as a recall, NHTSA is the agency that can order the action. Before a tire might be recalled there is list of “complaints” accumulated and you can review that information HERE.

You file complaints HERE.

It is important to remember that just because someone may have had a tire failure and complained, that does not automatically mean there will be a recall. Many times there are contributing reasons for an individual tire to fail. Most of these contributing reasons are not part of a design or manufacturing trend which would affect a group of tires.

I can recount how my work as a Quality Investigation Engineer resulted in a recall of 4,900 tires when I discovered what was a manufacturing error. This sounds like a big number but in fact the data we assembled identified there were fewer than 150 tires total that had the problem. The reason the number grew is that tire recalls are done based on DOT serial, which as you know includes the week of production. So even though we were able to confirm the problem was caused by a single skid of material being used in the incorrect part of the tire, the entire weeks production had to be recalled. In this case none of the tires had made it to individual customers as the problem was discovered so quickly the tires were either in the warehouse or on vehicles still at dealer lots.

The reason I am telling you this is to show how the system works. A problem is discovered. An investigation is done. Sometimes by NHTSA based on the complaints received from the field and sometimes by the tire manufacturer with their in house quality department. If tires are to be recalled then the formal procedures and laws must be followed.


I have advised a number of people who complain about a tire failure that if they do not file a complaint there is no way there will be a recall or even an investigation. I do note there is a general tendency for many on some RV forums to brand all tire failures as being caused by “defective” tires but based on the pictures I have seen, I remember only one example of the failure that might have been caused by a design or manufacturing problem but since these failures were isolated to a single RV it is more likely the failures were service related.

Can I categorically state that there are currently no defective tires in the field? No I cannot.

However based on my experience of personally inspecting thousands of tires identified as "defective" by self proclaimed experts, the facts are that most tires fail because they are under inflated, overloaded, improperly repaired or suffered a road hazard, impact or even vandalism.

On a personal note I have to wonder why so many in the RV field are quick to blame a tire failure on the country of origin, tire manufacturer or complain when tires need replacing after many years of service and some abuse, while at the same time are willing to spend many tens or hundreds of thousands of dollars on an RV that has such poor quality that no one is willing to stand behind or provide a 100% bumper to bumper warranty of the product design or manufacturing for more than a few months.

I have offered and continue to offer to do a detailed inspection of failed tires and to issue an un-bias report of my findings but too often the most I get is a slightly out of focus and improperly lit photo of the subject tire with no DOT serial and documented loading and service speeds.

I will be offering, in future posts to my blog, examples and information on impacts and road hazards with photographs showing sidewall "bubbles" as well as proof of the tire being damaged in service.

If you have questions about a suspect tire “defect” I will be happy to talk with you if you attend my Tire Basics for the RV Owner at FMCA Family Reunion in Madison, WI the end of August or the Gypsy Journal Rally in Celina, OH the end of Sept.

Wheels we all have em

I was asked to comment on changing wheels.

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 change 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 and 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.

Certification Label otherwise known as a “Placard”

Along with others who deal with tire questions, I too often incorrectly assume that the information on all placards is complete, correct and consistent and that the owner knows where to find this sticker. This incorrect assumption comes from looking primarily at car placards from vehicle manufacturers that have regulatory compliance engineers working for them.

However, when it comes to the information on these labels for RVs, there have been significant changes in both the information that is included and the location for the placement of the sticker and it seems that not all RV manufacturers pay as much attention to the accuracy of the information on the placard as others.

Do you know where your placard is located?
Are you sure the information is both complete and correct?

Over the past few years both the location of the sticker and the information requirements have been changed.

Newer Cars and Pick-ups will have a color sticker with this information.

This sticker will be located on the driver’s door jam




When it comes to full size RVs you may find the placard in a closet










Some Class-A units will have the stickers near the driver’s left arm location

Note the lack of complete tire size on this older sticker with only the rim size identified.

Newer Class A should have a placard at the driver's location.

But may have more complete information in another location like a closet.


This newer Super-C seems to have all the information the owner needs.




Some owners will be lucky and have actual unloaded weights.




Finally here is an example of a placard applied by a manufacturer that didn’t follow the requirements. Clearly the trailer does not have two 12,000 Lb axles running dual tires and the tire cannot carry 6,000 Lbs each. There might have been a recall if the trailer manufacturer was still in business.


The bottom line is:
1. Find and know the location for the placard for all your vehicles.
2. Make a note of the minimum inflation as recommended by the vehicle manufacturer and add this information to your travel check-off notebook
3. Confirm you have the same size and Load Range tires as identified on your placard.
4. If you find a difference or have a question snap a picture of your placard and send me an email with your questions –tireman9@gmail.com- and I will try and help sort out the questions you might have.

Tire Temperature & Pressure - A Hot Topic

Today's key fact: Check your tire pressure in the morning, before you drive and before the tire gets warmed by the sun.

Now the background information for detail oriented.
The general guide for checking tire inflation is to do it when the tire is "cold". In this case "cold" means the tire is at the same temperature as the air and not in direct sunlight as the sun can raise tire temperature by 10°F to 50°F. The tire also needs to cool down after being driven and the rule of thumb is to wait at least two hours after driving.

Another rule of thumb relating to pressure change due to temperature change is that for a 10°F change in the temperature the pressure will change by 2% so for your big Class-A tires that means about 2 psi for each 10 degrees while on your passenger car you can figure about 1 psi for each 10 degrees. If you remember these numbers you don't need to do the math.

All of these numbers are based on the "Ideal Gas Law" and the assumption that the tire does not change volume. Both of these assumptions are valid unless you are trying to measure tire pressure to the nearest 0.1 psi which even I didn't do on my race car.

All this variation and "Rules of Thumb" are why I and others suggest you run your passenger tires (35 psi max) about +3psi, Your Light Truck tires ( 65 to 80 psi max) at + 5 psi and your Class-A size tires ( 100 - 120 psi max) at + 10 psi from recommended minimum. With these extra margins you don't have to worry about a few degrees temperature change or the one to three psi per month you will normally loose.

If you want the mathimatical "proof" send me an email and I will be happy to reply.

Are tire pressure monitoring systems “TPMS” worthwhile?

The short answer in my opinion is YES.

TPMS are relatively new development for passenger cars and pick-up applications. A system for notifying the driver that one or more tires were significantly under-inflated was mandated after it was confirmed that a number of vehicle crashes and even some fatalities had occurred because tires failed due to being run while under-inflated. Today all new vehicles rated at 10,000 Lbs or less come with a warning light. Some look like this

Since all Class-A and most Class-C and Class-B RVs exceed that load limit there is no TPMS required to be provided by the manufacturer new on these vehicles. It is up to the owner to select a system that will notify the driver if a tire looses a significant amount of inflation.

Now some will say that they check their tires frequently. The reality is that even if you do check your inflation, with an accurate gauge, every morning before leaving a campground there is no way for you to know if you picked up a nail as you pulled out of the driveway. Given the significant cost associated with a tire failure on an RV what with body damage and possibly damage to one or more of your on-board utilities such as plumbing, generator etc.

I consider a TPMS a good investment and it was one of the first items I added to my RV when I bought it.

I am not in a position to recommend one system over another as I have not been able to do an evaluation of actual performance. There are aftermarket systems starting as low as $90 at TireRack.com. Doran Manufacturing, a long-time advertiser with RVtravel.com, has a comprehensive website with information about its system.

Other four sensor systems run about $250 while many six to 10 tire systems for RVs can run in the $400 - $700 range. Simply Google Tire Pressure Monitor System and look for the application that fits your needs.

Mark Polk wrote a nice piece on his experiences. I have seen a number of people post their experiences on some of the RV forums so I suggest you do some research before spending too much money only to discover the system you selected offered great marketing but maybe not the best hardware.

One item I would be concerned with is the accuracy of any temperature reading with a sensor that screws on the end of the valve with a hose extender. I have not seen any data that identifies the temperature difference between the tire and the sensor, or if it is significant. I have looked into the subject and am only comfortable with sensors that bolt into the valve hole with the sensor internal to the tire. The bad news is this costs more to install. If you keep an eye on the inflation and continue to confirm the TPM sensor pressure matches your hand held digital gauge, I don't think you really need to worry about the temperature as the chance of loosing air at the rate that matches the increase of pressure due to increasing temperature of the remaining air is a low probability.

If the sensor matches your digital gauge +/- 2 psi or less you are probably just fine. If you do have temperature readings I would get concerned if I saw a temperature reading exceed 170F assuming the sensor is accurate to +/- 5F or less.

Bottom Line. I very strongly recommend all RV owners have some system of notifying the driver when a tire looses air. With the exception of a failure of the sidewall due to some damage, you will almost always get enough advance warning to save you the expense of damage to your RV. You might even be able to have the tire repaired if you stop soon enough and have not lost too much air and damaged the tire. Just one warning could pay for the system.

Finally having a TPMS is not a substitute for checking your inflation before each trip. I have read of some people having the TPM sensor fail for as we know nothing is perfect. Better safe than sorry when it comes to having sufficient inflation to carry the load in your RV.

What does “XL” mean?

In this series I am giving a lot of detailed technical stuff. For some of you this will make your eyes glaze over with boredom while others really want to know the “Why” of different tire selection. Please bear with me as I try and build a foundation of information that anyone can come back to later if they develop an interest or have a question.

Last time we discussed “P-Metric” sizes. That means tires whose size begins with the letter “P” and have metric width dimensions. P235/75R15 the 235 is millimeters and the size starts with a P so that is a “P-metric” size.

In the P-metric post we discussed a P235/75R16 105S which was rated at 2,028 Lbs at 35 psi max. Before we move to Light Truck tires where things get a bit more complex with multiple Load Ranges we need to finish up our discussion of Passenger tires. These are “Standard Load” tires with a max inflation of 35psi and “Extra Load” with a max inflation of 41 psi.

Since most passenger tires are standard load, that is the default so few tires actually have the words “Standard Load” on the sidewall. However if the tire is rated for Extra Load it will have the words “Extra Load” and possibly “XL” on the sidewall in addition to the 41 psi max information. Originally Extra Load passenger tires were used on Station Wagons that some of us might even remember. In the 60’s there were regular passenger cars and Station Wagons.

SUV’s, Crossover or other type vehicles really were not part of the market.

Since these larger vehicles were intended to be used with higher loads most of the time, it was soon discovered that there needed to be an adjustment when a tire was selected. Industry guidelines indicate that the vehicle normal load on the tire shall not be greater than 88% of the max load capability of the tire.

Additionally if a P-metric tire is used on a “multi-purpose” passenger vehicle, think SUV etc or truck, bus or trailer, the load rating shall be reduced by dividing by 1.10. The total load capability at the recommended inflation shall not be less than the Gross Axle Weight Rating or GAWR.

I guess you can see that things get a bit involved for the engineers at car manufacturers when they try and select an appropriate tire size and inflation.

Extra Load rated tires are one option used by the engineers when selecting tire size. In our example size P235/75R15 108S XL we can carry an additional 308 Lbs on each axle at max inflation.

If your tire placard or owner’s manual indicate your vehicle should use P-metric XL tires then that is what you should use.

I’ve got a short list of reader questions to answer, then we can move on to other type tires that many will find on their Pick-Up, Trailer or RV.