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Monday, March 27, 2017

Question on Sidewall "bumps"

This topic concerns a potential SAFETY issue so is a bit more involved as it is important that everyone understands the situation.

Smitty asked:
I have 5 1/2-year-old 12R Michelin XZE tires on our coach. No cracking, and have performed well.

I found sidewall 'bumps' (Tire Center gent, said they're not 'bulges', as they're so slight). One maybe 3/32" of a protrusion, the other much less. The gent said they could be belts that are misplaced or broken. But he also said the XZE's have a thicker sidewall, and that the outer layer of the tire could have separated.

I know it's hard to give advice based upon a description, but I wanted to ask your thoughts on the opinion given to me by this tire gent.

He suggested that I monitor the size of the 'bumps' daily, and if they don't grow, he felt that they were safe to drive on. He commented that the tire was so overrated for what it was carrying weight-wise on my RV, that it was not working hard.

I've always thought that once a tire starts to break down, it was at risk for a catastrophic failure?

The tire was not unmounted for inspection, but if it was an outer layer separation, as he said it could be, how would they be able to see this?

Opinion? At 5 1/2 years of age, is it worth even considering monitoring and watching for growth? Or just replace and move on? 

Here is my reply:

First off, I do not feel that sidewall "bumps," "bulges" or "protrusions" occur because of misplaced or broken belts. Belts are under the tread not on the sidewall so I have to wonder what training, if any, the "gent" at the tire store has received on tire construction or inspection procedures. Also not sure what he is talking about as the "outer layer" of the sidewall. In a truck tire there is basically the thin innerliner (a bit like having a tube as part of the tire), then body ply, then the sidewall. So, if he thinks the sidewall has separated, then the tire has failed and should be replaced.

Now, some fact-based observations on sidewall bulges or outward protrusion vs. depressions that are inward toward the air chamber and are indications of two completely different causes.

The inward depressions can occur once or twice around almost any radial tire. If you take a close look in full sunlight of smooth sidewall tires you will see depressions that might be about 1/16" deep and 1/4" to 1/2" wide in the circumferential direction. These are caused by the lap splice of body ply material that provides more resistance to the slight growth seen in all radials due to inflation. These depressions will run a radial direction from the wheel to the tread and seem to disappear as you get near the tread or near the wheel. These are normal and I would only consider them minor cosmetic features of a tire.

An outward bulge may be 1/2" or as large as a couple of inches in the circumferential direction. The larger or more distinct, the more concerned I would be about the durability of the tire. Usually bulges are the result of some impact damage done to the body cords that resulted in a few being broken due to shock loading.

Here is a blog post I did with pictures of bulges and depressions to help people understand.

Here is a post on an Impact Failure I had on my personal car.

Now on some with small features, the eye plays tricks as to it being a bulge out or a depression in. I have even seen instances of two depressions being close together such that the normal sidewall between the depressions gave the appearance of being a depression. The simple tool to use is a ruler or straight edge and to lay it across the area of interest. This will quickly identify if you have a bulge or depression.

What to do:
Review my blog posts above.

You might also find a different dealer. A simple Google for "Truck Tire Dealer Town" where "Town" is a nearby larger community, should give you a number of options. For 19.5 and larger size tires I would only deal with a Heavy truck Center.

You might also contact Michelin at their Customer Contact on their RV tire page for a suggested location with people sufficiently trained in tire inspection to pass judgment on your tires.
You can tell them that you're not happy with the inspection you got from the tire guy as they don't seem to understand the basics of rtuck tire construction.



  1. When I first buy an newly mounted tire I carefully look it over, and note what it looks like, bumps on the sidewalls mostly. Then before I drive or tow I look at all the tires to see if there are any changes.

    Been driving for over 50 years, one of the "Lessons Learned".

  2. Good practice. Too many people don't give tires much thought but then are surprised if they have a problem.


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