Bubble or lump in the tread: belt separation

Park the car. A raised bubble or lump in the tread means the steel belts inside the tire have started to come apart, and a separating tire ends in a blowout rather than a slow leak.

It is not repairable. The tire has to be replaced, and the car should not be driven on it in the meantime.

What has gone wrong inside

Under the tread there are two or more steel belts bonded into the rubber. They hold the tread flat against the road and stop the tire growing under pressure and centrifugal force. They are the tire's backbone.

Separation is the bond between those layers letting go. Once a patch is loose, the air inside pushes the tread up in that spot, and each rotation peels it a little further. It does not heal. It gets worse with every mile, and then the tread lifts off the casing at speed.

Causes, in rough order of frequency: an impact, usually a pothole, that bruised the casing months ago; running the tire badly underinflated, which overheats the belt edges, and heat is what destroys the bond; driving on a puncture, or a bad outside plug repair that let moisture reach the steel; overloading; and, rarely, a manufacturing defect.

NHTSA identifies tread separation as one of the tire failures that correct pressure, load limits and regular inspection exist to prevent.

How to be sure that is what you have

Feel before you look. Put your palm flat on the tread and run it slowly all the way around the tire, turning the wheel or rolling the car a few inches at a time. A separation reads as a dome that rises and falls under your hand, typically a few inches across. Your hand finds it long before your eye does.

Then check three things:

1. Is it in one place, or all the way round? One place means separation. A pattern that repeats evenly around the tire is a wear pattern, not a structural failure.
2. Does it stand proud, or is it a dip? A bubble stands proud. A scalloped dip is cupping, a suspension and wear problem, not an immediate danger.
3. Is the tire also making a noise? Separation usually comes with a rhythmic thump that keeps time with wheel rotation. If it does, read thumping noise from a tire, which separates the three common causes.

A bulge on the smooth outer face rather than in the tread is the same failure in a different place, and the same answer. That is covered on bulge in a tire sidewall.

How far can you move the car?

As little as possible, and not on a road if you can avoid it.

We are not going to give you a mileage figure, because nobody can. A separation is not a component with a known remaining life. It is a tear that is already propagating. The honest answer is that some hold for a few hundred miles and some let go in five, and there is no way to tell from outside which one you have.

Moving the car within a parking lot or off a driveway so it can be worked on is a reasonable risk. A highway is not. If you have a spare, fit it, and stay inside the limits printed on the spare itself.

Why no repair is possible

The US Tire Manufacturers Association allows repairs only in the tread area, only up to a quarter of an inch, only with the tire off the wheel so the inside can be inspected, and only with a plug and a patch together.

Those rules describe fixing a hole. A separation is not a hole. It is the tire's structure coming apart across an area, and there is nothing to plug and nothing sound to patch onto. Anyone who offers to fix it is wrong.

What we do

We bring the tire to the car, because the vehicle should not be driven to a shop. We supply it matched to the door placard, fit it, balance it and torque the wheel to spec. We do not fit tires bought elsewhere, and we do not do alignment, rotation alone or balancing alone.

Worth doing at the same visit: check the pressure on the other three against your tire pressure placard, since the underinflation that killed one tire has usually been at work on the rest. If the car needs more than one, how many tires do I need covers the drivetrain rules, and mobile tire replacement cost sets price expectations.

Questions people ask

• Is a bubble in the tread repairable? No. Belt separation is a structural failure across an area, not a puncture, so no plug or patch applies. The tire must be replaced.
• How far can I drive on a tire with a tread bubble? Treat it as not driveable. A separation grows with every rotation and nobody can predict when it fails. Move the car only to make it workable on.
• What is the difference between a tread bubble and cupping? A bubble stands proud of the tread in one place. Cupping is a repeating pattern of scalloped dips around the whole tire and is a suspension or balance problem rather than an immediate failure.
• What causes belt separation? Usually an earlier impact, long-term underinflation, heat, or moisture reaching the steel belts through a bad outside plug repair. Manufacturing defects happen but are uncommon.
• Will my warranty cover it? If it is a manufacturing defect, the tire maker's warranty applies and we help you file it. If it followed an impact or underinflation, it is road hazard or wear and normally is not covered.
• The tire still holds pressure. Does that mean it is fine? No. Separation is a failure of the structure, not of the seal. Holding air tells you nothing about whether the tread will stay attached.