Your Ad here
Be sure to sign up for the weekly RV Travel Newsletter, published continuously every Saturday since 2001. NOTE By subscribing to RVTravel you will get info on the newest post on RV Tire Safety too
. Click here.
Huge RV parts & accessories store!
You have never seen so many RV parts and accessories in one place! And, Wow! Check out those low prices! Click to shop or browse!

Saturday, March 29, 2014

"I don't want to weigh my RV"

That is the statement I occasionally get from a few who say they have no intention of getting their RV weighed each time they go on a trip so they see no reason to ever get the RV weighed as they are certain they know they are not overweight.

With over 40,000 data points RVSEF can show that over 55% of the units they have weighed have one or more tires and/or axles overloaded, I am pretty comfortable in suggesting that RV owners need to learn the individual tire position weights at least one time.
It is not unusual for an RV to be out of balance by 1,000# or more.
Trailer owners need to do this to confirm they are not overloaded due to axle to axle imbalance or side to side balance. They still need to run tire inflation at the tire max.

Motorhome owners need to learn the "corner loads" before they calculate the minimum cold inflation for each axle.

Once you have learned the actual loads and done the calculations and table look-up you are good to go unless or until you make some significant change to your RV such as adding a residential refrigerator or possibly granite counter tops.

Not understanding the difference between "Measurable and Meaningful" some incorrectly think I am suggesting that they need to get their RV re-weighed every time they go on a trip as they may pack different items. Not wanting to do this they just throw up their hands and don't get the RV weighed.

Improper inflation can lead to overloading and excess heat generation. This is bad for fuel economy and tire durability and in extreme cases could lead to blowout failure or tread separation.

I have had my small Class-C 2008 coach (seen above) weighed three times.

My first time was just after I bought the RV and not having access to corner weighing I still wanted to know what my weights were with the "stuff" I had loaded for my first trip.

I knew I would pass a CAT scale about 15 miles from home so pulled in and got the following results
Front 3,940#  Rear 6,580#   Total 10,520# Based on these results plus the margin of inflation pressure I wanted I then knew what to set as my cold inflation.

Three months later I attended a large RV Rally and had RVSEF do corner weighing
LF 1,750  RF 1,850   Total Front  3,600#   -340#
LR 3,400  RR 3,250  Total Rear   6,650#   +40#             Total 10,250  +270#

It has been a few years since my RVSEF weighing and I have added a few items as "permanent" take along stuff and a few minor modifications to the RV that I felt had increased the load when preparing for a longer trip. I again decided to do a quick check so again visited a convenient CAT scale.
Front 3,820#   Rear7,020#   Total 10,840#   This indicates an additional 220# on the front axle and an extra 370# on the rear from the RVSEF weights

As you can see the RV has gained a some weight over the years, A bit like its owner I am sorry to say.

The good news is that the new weights do not require a change in cold inflation as the extra 320# is not large enough to put any individual tire into a higher inflation bracket. Plus my margin easily accommodates this change

As you can see it is easy to confirm that no significant change in weight has occured. There have been and will continue to be changes in the tire loading due to weight changes but those changes, while measureable are not meaningful so are not of concern. I also think you can see that excuse for never getting your RV weighed because you don't intend to or want to weigh the RV each trip is not a reasonable excuse.

Monday, March 24, 2014

What you need to do to get a tire failure replaced under Warranty

Along with this Blog I try and monitor a number of various RV forums. Some of you may have seen my posts. There have been a number of times when a topic comes up and I make a reply that I think would be a good topic for my blog but it is on a short side. I have decided that just because an answer to a question is not 600 words long, that should not stop me from sharing the answer so over the next few weeks I plan on posting a number of items where I provided an answer or comment on questions posed by other RV owners. As they say the Names have been changed to protect the innocent.

"John & Mary" wrote about the difficulty they had in getting a failed tire replaced. They were surprised that the tire company wanted to know the DOT serial, purchase date and to actually have the tire made available for inspection before a new tire would be provided. They said "When the tire company asked for the tire to be returned we were surprised. We dunno about everyone else, but we didn't keep the tires & didn't know until several months later what was required to file a report"

My response:

You are correct Mary, you can't get a tire replaced without providing a few items to the tire company. That's why I have advocated that everyone record the full DOT of each tire in the "Book of important RV Info" that we all keep, so if we ever have a tire problem and the DOT serial is destroyed we will have the information needed to file a complaint report to both NHTSA and to the tire company. We also need to keep the documents that show the date of purchase of the tire (if they were not new on the RV).

As far as keeping the tires, I understand that many RVs don't have space to store a tire but after reading this story where John & Mary were complaining that the tire company wanted the failed tire before it would issue a refund or compensation, I feel that people need to have a basic understand of their responsibilities in the event of a tire failure and wanting to make a claim so they can take the steps needed to increase their chance of getting a replacement tire when appropriate.

I have to wonder why people think that when the failed product and its serial number and some proof or purchase is required by every other company when requesting compensation or replacement, be it a toaster or refrigerator, that for some reason tire companies would be the only company to not ask for the failed product and Proof of Purchase..

Now if you don't have space to store the tire, then I would suggest that you need to contact the manufacturer at the number given in the warranty book you received when you bought the RV or tires and before you leave the tire service shop doing your changeover, you confirm what the tire company requires. You can then decide if the potential for a refund or adjustment is worth the effort and cost involved.

I do have space for a spare tire in my RV. That was a requirement when I was shopping for a new RV. I also run a brand of tires that has thousands of stores & dealers around the USA, another requirement when selecting a tire brand. So I am confident that I can always get to a store and would be able to process the paperwork needed to file a warranty claim as well as leave the tire at the store so a company engineer can inspect it or have the tire shipped back to the appropriate location if needed.

Monday, March 10, 2014

Wheels and how to change them

Sometimes an RV owner wants or needs to change their wheels. Just as with tires there are engineering terms and strength considerations that need to be understood so when you make a change you will not be compromising the safety or durability of your RV, or its tires.

As with tires there are some terms that we require a clear understanding before proceeding. A "wheel" has two parts, even if cast as a single piece of aluminum. The "Rim" is the part that contacts the tire and the air chamber in the tire. The "Disk" is the part that has the bolt holes and center hole that attaches to the vehicle. Each parts served different purpose and has different design requirements.

 Here is an engineering drawing that identifies the important dimensions of the "Rim" portion of a wheel. Each of these dimensions is specified in the published standards books such as the Tire & Rim Association Yearbook here in the US. If you look at the dimensions in both the European ETRTO and Japaneese JATMA you find the dimensions are identical. This is because the intent is to ensure proper interchangeability and fit no matter where the tire is manufactured. Most of these dimensions are specified to a tolerance of 0.01".
Two items may be of interest. Both of these are specified in inches, no matter where they are made. Even in countries that only use the Metric measurement system. While the Diameter is specified to a high degree of accuracy you will find it impossible to find a location that is actually measures 16.0" in diameter as the location is actually the intersection of the slope (5° in the above drawing) and the vertical defined by "A" or wheel width in the above drawing. The rim width "A" is specified in 1/2" increments and can be measured with a tape measure. When you look at tire specifications you will usually see a specific width mentioned such as 7.5". This does not mean you cannot mount the tire on a rim of a different width but again there are only a few widths that are "approved" for each tire size by the tire manufacturer so you must only select a rime that meets one of the specified widths.

Now if we look at the "Disk" part of a wheel you can see it in this drawing.

 Here we have an interesting situation as the Tire & Rim association does not specify any of the dimensions on the Disk portion of a wheel. These dimensions must match specifications from the axle or hub manufacturer. These include Bold Circle ( see the good explanation from Tire Rack). Center Bore,  Offset and Backspacing as shown in the picture.
It is important to understand that Offset and Backspacing are not identical dimensions and while Backspacing can easily be measures Offset is much more difficult.

Most Trailers and Class-B RVs without dual tires, will have wheels that look a lot like this picture with the Offset being a small number in the 1" to 3" range. Larger Motorhomes with dual application will have large Positive (outward) offset as seen in this picture of an Accuride wheel.

 This positive offset is what controls the "dual spacing. This spacing is specified with a minimum dimension in the spec page of LT and TBR tires intended for dual application.

If you are considering changing your wheels you not only need to consider the load and inflation rating of the potential new wheel you also need to ensure you have an acceptable width, center bore, bolt circle and offset to meet the vehicle, axle and tire manufacturing specifications.

There is another post here on wheels that you should also review before considering making any change in wheels as a wheel failure can be quite catastrophic.

Monday, March 3, 2014

Why does my tire lose pressure? Simple and Technical answers

 As people get ready to start traveling again, some are discovering that their RV tires have lost inflation pressure over the past few months while it was parked. There are a number of possible contributors to a loss of pressure.

Some of these have been covered in my posts on leaking valve cores or metal valve O-Rings or possibly punctures or even rim corrosion. This post will cover just two of the possible causes. permeation and Temperature change.

The simple answer is that tire pressure will change about 2% for every 10°F change in temperature.

The complex answer and mathematical proof follows:

1. Normal pressure loss due to gas permeation. This is basically what happens when the molecules of gas "leak" or travel through the rubber of a tire. New passenger and LT tires sold to "Detroit" have to pass tests and meet a specification of a loss rate of less than 1% or 2% per month when averaged over a number of weeks or months, with different companies having different specs depending on the application. TBR (Truck-Bus-Radial) tires as seen on Class-A RVs will have similar performance goals. Low Cost import tires sold at "Billy-Jo-Bob's Cheep Tire Emporium and Bait Shop" may not have any such spec so might lose pressure faster because of the use of lower cost inner-liner rubber. This topic could be an entire post of its own but is quite technical and is not based on just the molecule size of Oxygen.

2. Tire inflation pressure is directly proportional to tire temperature. The "Ideal Gas Law" is a good approximation of what really happens. The general terms of the Ideal Gas Law are PV=NRT
 P is the Absolute pressure, not gauge pressure, measured in pascals
V is the air Volume measured in cubic meters. While tires do change a little bit in volume the difference is not meaningful for the purposes of this discussion.
N is the amount of gas in the air chamber in moles. For the purposes of this discussion, we can ignore this number as we are not changing the amount of gas in the tire when we are comparing the pressure difference just due to temperature difference.
R is the ideal, or universal, gas constant. R for dry air is 287.1 for N2 296.8 and for O2 259.8 for water vapor it is 461.5 so you can see that moisture in your inflation gas can significantly affect the Pressure/Temperature ratio a change from air (79%N2) to 95% N2 is not significant for the purposes of this discussion. I did a post on how to make your own "air dryer" so you can approximate the properties of dry air or dry Nitrogen.
T is the absolute temperature in degrees kelvin.

After discarding the inconsequential terms we really end up with is a simple ratio P2/P1 = T2/T1 and we can even ignore the conversion of psi to pascals as the ratio of pressure is the ratio of temperature in degrees Kelvin. Sorry we do need to use Kelvin but you don't need to do a conversion as you will soon see.

Lets see how this woks out
If P1 is 100 psi and if  T1 is 70°F or 294.261°K and if the temperature drops to 60°F or 288.706 °K we can solve for P2/P1 =  288.706/294.261 which leaves us with the ratio ot the two temperatures as 0.9811 so we had a 2% drop in pressure for a 10°F drop in temperature

30°F would be 272.039/294.261 = 0.92448 or 8% drop in pressure for a 40°F drop in temperature from 70°F to 30°F.

You may see various web pages such as this one from TireRack or Wikipedia saying 1psi for each 10°F change but you need to remember that this information is based on the assumption that the "normal" inflation is about 35 psi in a passenger tire.