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Friday, May 27, 2011

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.

Monday, May 23, 2011

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.

Tuesday, May 10, 2011

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.

Wednesday, May 4, 2011

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.