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.

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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.