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Tuesday, July 12, 2016

Change Tire size

When considering a change in tire size there are many different areas you should consider.

Tire load capacity. You need to be sure you go with tires that have equal or greater load capacity than original.
It is possible that the original tires delivered a very small load capacity such that the coach is easily overloaded. Recalls may have been ordered or initiated but sometimes a manufacturer went bankrupt so no recall action took place or no new tire certification label was provided.
So it is up to you, the current owner to do your homework and get the facts.

- You need to learn the real load capability of the axle by contacting the manufacturer if there is no plate on the axle with GAWR info.
- You also need to learn the real loads you are placing on your tires with your coach fully loaded. Here is a worksheet with instructions on the steps and calculations you need to do.

Now load capacity is not the only information you need. You also have to be sure the tire will never rub and contact any portion of the coach or its suspension. part of this is obvious and can be discovered by rolling on your back with a flashlight to confirm sufficient clearance around all your tires. Don't forget to check both front tires with wheels turned completely both directions. I have read that 3" on all sides is a reasonable clearance.

Rim width - There is a list of approved widths for each tire size. You must stay with the dimensions listed by the tire company

One final clearance check that is sometimes overlooked is the clearance between dual tires. This requires more than just looking at the rear tires. In tire company specification charts there will be a dimension called "Minimum Dual Spacing" for the tires you may be considering. This dimension is controlled by the wheels. Sometimes this information may be marked on the wheel but most likely you will need to contact the wheel manufacturer to learn the specification. This is CRITICAL, as too small a clearance can result in tire damage or even failure.

I can't address if a specific size will fit but simply going up one size would be reasonably close to the 1/2" width increase.

For example if your current size was an ST215/75R15 going to run ST225/75R15 would give you a tire that was approx 10mm ( about 3/8" wider over all)
Now it is important to remember that tire dimensions are approximate and if you change brands the actual with may be a little different.

Also don't forget that the OD would also increase which means the clearance between the tread and wheel well or the companion tire would change also.

You should be able to find published OD and width dimensions from your tire companies web page.

One other item is that a change in wheel width (as published in the literature) will affect the tire width by about 40%. So a tire with a published width of 10" on a 6" wide rim would be about 10.2" on a 6-1/2" wide rim (40% of the 1/2" rim change = about 0.2")

 But in some cases on older coaches tires may have been changed in size or even the size designation such as an old 7.50-15.  If you find yourself in that situation you might consider dropping me an email so I can help walk you through the things you need to consider.

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  1. You did not address the pressure that the tire rim is rated for. Some of the higher capacity tires are rated for 110 psi for maximum load carrying capacity. The tire rim may not be rated for 1110 psi.

  2. You are correct. Rim ratings are one of the number of items that need to be considered when looking at changing size or Load Range of your tires. I would hope that anyone considering such a change would review a number of my blog posts to gain the knowledge needed to make an informed decision.
    Sorry but I can't cover every aspect of every topic in every post. If you have a question on wheels I would direct you to review the posts on "Wheels" found in the list to the left.


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