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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.


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