Bubble in Tire Sidewall: What It Means and What to Do

What you're looking at

You're loading groceries and notice it: a rounded bump on the side of one tire, like a small egg pressing out from inside the rubber. It wasn't there last week.

That's a sidewall bubble, also called a bulge. It looks minor. It isn't. Here's what it means, how to tell it apart from a harmless dip, and what to do next.

What a sidewall bubble means

A tire's sidewall gets its strength from layers of cords, called plies, running inside the rubber. Michelin explains that a bulge or bubble on the sidewall generally indicates damaged cords caused by a severe impact. Once cords are broken or the plies separate, the air pressure inside pushes out against the rubber, and the rubber alone forms the bump you can see.

In other words, the bubble is the visible part of damage you can't see. The structure that holds the air in is compromised at that spot. Michelin warns that this exposes you to a rapid loss of pressure if the bubble bursts.

Bubble or indentation? How to tell

Not every irregular spot on a sidewall is a bubble. Michelin describes sidewall indentations, shallow dips where the cords overlap inside the tire, as normal on radial tires. They don't affect performance.

Running your fingers lightly over the spot helps: a bubble stands proud of the surface. If you can't tell which you have, treat it as a bubble until a technician says otherwise.

What causes them

Michelin names the usual cause: a severe impact, such as driving into a pothole or hitting a curb. The cords can be damaged without any cut you can see on the outside.

That's why a bubble can seem to come from nowhere. The impact might have been a pothole on a flooded street where you couldn't see the edge, or a curb in a tight parking garage. After a hard hit, it's worth walking around the car and looking at both sidewalls.

Can a tire bubble be repaired?

No. There's no patch for broken cords.

• USTMA's repair guidance says punctures or cuts in the sidewall or shoulder of a tire can't be repaired, because that part of the tire flexes the most and lacks the reinforcement of the tread area.
• Michelin says a tire with a bulge or bubble can't be repaired and must be replaced.

Be wary of anyone who offers to fix one. The only fix is a replacement tire.

What to do next

1. Put the spare on. Michelin's advice is to replace the tire with the spare immediately and have it inspected. Know what kind of spare you have first: a compact spare has speed and distance limits printed on its label and in the owner's manual. Our guide to spare tire types explains each one.
2. No spare, or can't change it where you are? Call for help rather than driving on the bubble. In South Florida, Extreme Tires' mobile vans install tires at your home, office or other parking spot across the local install area. See mobile service or call (954) 773-1896.
3. Don't wait for a convenient day. The damage behind a bubble doesn't heal, and a bubble can lose air suddenly.
4. Check the wheel too. An impact hard enough to break cords can also bend a wheel or knock the alignment out. A new pull or vibration after you replace the tire is a reason to have both checked. Extreme Tires does alignments at the Sunrise shop.

Replacing one tire: match what's on the car

You'll often be replacing a single tire, and it needs to fit with the other three. USTMA's guidance is that replacement tires should be the same size, load index and speed rating recommended by the vehicle maker, as shown on the door placard, with a load index equal to or above what the placard requires. USTMA recommends replacing all four at once for best performance, and if you replace only one or two, keeping to those same specifications.

All-wheel-drive vehicles can be stricter: some makers limit how much the tread depth can differ between tires. Your owner's manual has the rule for your car.

To find a match, shop by size or bring the old tire's size, and have the other three measured when the new one goes on.

Quick reference

NHTSA's advice is to look for bulges, cuts and cracks whenever you check your tire pressure. A bubble caught on your driveway is a lot easier to handle than one found on I-595. And if you have a nail in the tread rather than a bubble on the side, that's a different story: see plug vs patch.