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Wednesday, December 21, 2011

A wheel is just a wheel, isn’t it?

That wasn’t the exact phrase used in a recent post but it seemed to be the thought behind some of the comments I saw. Today’s topic will be of interest to anyone who has changed tire size or Load Range (ply rating to some). It also covers some important safety concerns that you must consider if contemplating such a change. This is a TECHNICAL matter which can be boring to some but there are Explosions in some of the videos to keep everyone interested and awake.

We and others have covered the information molded into the sidewall of all highway tires concerning the Maximum Load and Maximum Inflation limits for your tires. What you may not realize is that wheels have similar limits. If you are lucky these limits are stamped or cast into your wheels. If there, this information is not hidden by the tire but may be on the side of the wheel mounted toward the inside of the vehicle or in some cases on the surface that is bolted against the hub or brake drum.

Sorry to say that many of you will only find part numbers and manufacturing date code stamped into the wheel but nothing that looks like load or inflation. If you are in that boat you will need to contact either the wheel manufacturer or the chassis manufacturer. I would not depend on any verbal information from the average RV salesperson, but only accept some published information that answers the question based on the wheel part number marked on the wheel.
Here are the links to a couple wheel manufacturers. Accuride

Now you may be asking why go to all the trouble. This was the basis of the question I was asked by an RV owner that wanted to know if there would be any problems using a Load Range E tire that had a rating of 80 psi on a wheel that originally had a passenger tire mounted on it.
So we get to the point of this post. WHEEL FAILURE.

OK first off this answer has not been approved by any lawyer.

Luckily wheel failure is an infrequent occurrence but if it happens it can not only ruin your day but as those ads on TV for new medications warn, the side effects could include serious injury or even death.

NOT KIDDING HERE. Every year more than one person manages to kill themselves by improperly inflating a tire when something goes wrong. The forces of compressed air are much like a bomb. The failure can happen while inflating or minutes to weeks later. One thing that can happen is the wheel flange bends or breaks and the tire exits sideways taking out anything or anyone in the way. If you want to have a better understanding of the forces involved I have collected a number of videos.
Safety info HERE
SAFETY video. This is multi piece rim but a flange rupture will do similar damage.

Example of what can happen. Another and many videos HERE

If you are lucky you will only blow your fender off when it lets go.
Now before you say you aren’t using tires as shown in the videos, I want to assure you that when a wheel fails from fatigue it could in all probability react similarly to what is seen in the videos. A fatigue failure usually occurs after many thousands of cycles so if you exceed the max inflation rating for a wheel, you can decrease the number of cycles it takes before the wheel might fail.
If interested, you can learn more about metal fatigue HERE.

Bottom Line
You should never set the cold inflation at a level that is higher than the Max for either the tire OR THE WHEEL.
Both the tire and wheel manufacturers take pressure increase due to operation heat into consideration so don’t bleed off hot air out.