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Tuesday, December 6, 2016

Important consideration if changing tire size: "Minimum Dual Spacing"

Knowing about the extra details of dual tire application is not anything car drivers were ever called upon to know. When you joined the RV world your first RVs were probably trailers and you don't have to know about special considerations with duals. If you now own a motorhome that has two tires on each end of the rear axle, then you have "duals" and need to know and consider some new information. I have written a number of posts on this topic and I suggest you take a look at each post by clicking on the word "dual" on the list to the left.

If you are considering changing tire size on your motorhome, there are a number of items you need to consider. I have previously covered the importance of matching tire OD if only changing one tire of a pair of duals. While you may remember reading that POST, you may be telling yourself that you don't need to worry about the matching dimension if you are changing all your tires. While you may understand the need to pay attention to load capacity and the physical dimensions of the tires themselves, there is also a CRITICAL dimension that you can't measure with a tape measure and that is called "Minimum Dual Spacing". You may or may not have noticed this dimension when looking at a table with tire dimensions. Not all tables have this dimension but it is very critical.



I have highlighted the Min Dual Spacing information and the rim width dimension which is also critical. It is important to remember that not all tires are approved for a specific rim width when looking at the upper range of load capacity.

 
Tires in a side-by-side, i.e., dual position MUST have a certain amount of clearance between them or the tire-to-tire contact of the sidewall may lead to failure of BOTH tires. Going from a 255 to 275 wide tire will probably mean you will not have the specified clearance unless you also change wheels. Only way to know is to check the wheel specifications from the manufacturer.
Here is what that dimension means.
It is the distance from the center of one tire to the center of its mate, when mounted on the actual wheels you will be using. Now there is no single place you can make this measurement on a wheel as it requires calculation, as the mounting surface of the inner dual wheel is not the same surface as the mounting surface of the outer wheel.

Tire charts and dealers can tell you the minimum dimension needed as seen above but you need information from the wheel manufacturer to learn the dimension from the mounting surfaces to the wheel center-line.

So, along with load capacity, OD and tire width you also need to confirm your wheel width meets the tire specification as well as calculate the dual spacing from the dimensions provided for your wheels.

Special warning. I know that some motorhomes have steel inner wheels and aluminum outers for the dual position. You will quickly learn that the offset spacing is different for steel vs aluminum.


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