By Roger Marble
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. In some cases it’s on the surface that is bolted against the hub or brake drum.
You may not find all of the information on the wheel
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. Only accept some published information that answers the question based on the wheel part number marked on the wheel.
If you can’t find the wheel inflation or load rating marked on the wheel, the best I can suggest is to confirm the ratings from the OE tire as found on your RV Certification Label. Consider those numbers the maximum for load or inflation until you learn otherwise from the wheel or RV company.
Why go to all the trouble?
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. They 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 video. This is “zipper” failure from running a steel body tire when flat. But it shows the forces involved. Example. Be warned. This is a disturbing video. The Example shows what can happen when a large tire lets go. The failure of a wheel has similar forces.
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.
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 normal pressure increase due to operation heat into consideration, so don’t bleed off hot air out.