Tire bulge: Is it a defect or damage? Sidewall bulges can be difficult to diagnose. Sometimes it is even possible to misinterpret a depression as a bulge. So let’s start off today’s lesson by clarifying the words.
According to Dictionary.com, “bulge” means a “rounded projection, bend, or protruding part; protuberance; hump.” I think we can all agree that this is a sidewall bulge:
.JPG)
A “depression” would be the opposite, or “sunken place or part; an area lower than the surrounding surface.” Sometimes I may use the terms “bulge in” and “bulge out” just to be sure people have a clear understanding.
Look closely at this shot and I think you can see that this is a depression in the sidewall:
Here is another shot showing sidewall depression.
OK. So now you are probably asking why are these two conditions in tires and are they defects or what? Let’s step back for a moment and consider how tires are made.
The basics apply to all tires, be they small 10″ or 12″ as seen on micro cars, or 22.5 or even large mining tires like this one:
What can cause a depression
If you are not familiar with how tires are built, you might want to check this video showing the basic process of wrapping layers of fabric (or sometimes steel cord) that is in a sheet of rubber, around a drum. The place where the builder starts and stops has a “splice,” as seen at about 2:01 in the video. The goal is to have a strong enough joint to keep the uncured rubber together until the tire is cured. In some constructions, this means a small overlap of one to maybe four cords. If the overlap is larger than desired, there is a doubling-up of the cord, and this is what creates the depression.
I know this is counterintuitive, but you need to remember that when a tire is inflated the rubber stretches and the textile cords stretch a small amount. However, if the splice is “heavy” or larger than desired with more cords “doubled up,” the forces from inflation are resisted by twice the normal amount of cord and rubber, so the stretch is less than in the rest of the tire. There is nothing wrong here other than a visual depression.
What can cause a bulge
A bulge is just the opposite. If the splice is “open” or there are cords missing, then that area will stretch out more as there is only sidewall rubber resisting the air pressure, so the sidewall stretches out just like a balloon. A bulge from an open splice is noticeable as soon as the tire is inflated.
If you see this on a new tire, point it out to the tire dealer right away and confirm the bulge is below the level of concern for that make of tire. This will probably be less than 1/10 inch above the surface of the rest of the tire and less than 1/2 inch wide. If larger, I would request a different tire, unless the dealer is willing to put in writing that the tire is safe. Get a nice close picture of the tire for your records and be sure the bulge does not get any larger.
Another cause of a bulge
The other thing that can cause a bulge is broken body cords from some sort of impact such as a curb, pothole, or from hitting something on the road. Here are a couple of shots of a 22.5 that suffered an impact.
One thing to point out is how I know this was not a factory defect. I have yellow arrows pointing to the small amount of irregular tread wear (below). You will note that this level of wear is fairly uniform around the tire. If the defect had been in the tire from when new, I would expect the sidewall bulge to affect the tread wear. Since it didn’t, that indicates to me the break of the body cord is relatively recent.
In case you are wondering what a broken cord might look like, here are a couple of shots of broken polyester cord from a smaller tire:
.jpg)
I hope everyone now understands the difference between cosmetic depression and a bulge due to a tire impact.
If you have a bulge that looks anything like the examples above, I would not drive on the tire. If we are talking about a high-pressure tire (75 psi or higher), I would not even stand near the tire while waiting for service. A tire explosion can be damaging or even injure someone.
Roger Marble
Check out my Blog www.RVTireSafety.Net
Have a tire question? Ask Roger on his RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.
Read more from Roger Marble on RVtravel.com.
 ##RVT1107
Roger, I always look forward to and much appreciate your informative articles. Pictures REALLY help! And thank you for the tire mfg video link. Very interesting.
I have always wondered about those depressions as I have seen them many times on LT tires. Now I know they aren’t anything to be concerned about.
Once again an excellent and informative article. Thanks, Roger.
The sidewall depressions description was excellent. I bought 4 tires to replace my 2017 dually tires then 2 more for the front 2018 tires 6 months later – all the same tires, just different manufacturing dates. None of the rears have the depressions but both of the fronts do. Good to know it’s not a problem. Thanks.
Roger. Thanks for the detailed explanation of what causes depressions in tires.
I looked over my new Michelins and found a depression on 2 different tires (out of the six).
Nice to known that this isn’t a defect in the product.