A bulge is a raised, bubble or egg-shaped area on a tyre, and it is one of the most serious faults a tyre can have. It signals that the structure inside the tyre has broken, and a tyre with a bulge can fail without warning. There is no such thing as a safe bulge.
What a bulge actually is
A tyre holds its shape through layers of fabric and steel cords, the carcass, sealed inside the rubber. A bulge appears when those internal plies break and the air pressure inside pushes out against the weakened spot, ballooning the rubber outward. It is not a surface blemish; it is the visible sign of a tyre coming apart from the inside.
Because the damage is structural, the amount of tread left makes no difference. A nearly new tyre can carry a dangerous bulge.
What causes them
Bulges come from impact. The usual culprits are:
- Potholes hit at speed
- Kerbs, struck head-on or at an angle while parking or turning
- Road debris or sharp objects
A single hard knock can be enough to fracture the cords inside without leaving an obvious mark on the surface. Tyre makers note that ordinary speed bumps, designed to be driven over gently, do not cause bulges, though they can affect suspension, whereas kerbs and potholes very much do. After hitting one hard, it is worth having the tyre checked even if nothing looks wrong.
Why they are so dangerous
A bulge is a weak point where the tyre can rupture. At motorway speed, or when the tyre is loaded over a bump, that weak spot can give way as a blowout, a sudden, total loss of pressure that can cause a loss of control. The risk is not gradual; the tyre can look the same for days and then fail in an instant.
One detail catches people out: a bulge can form on the inner sidewall, facing the car, where it is invisible when walking round the vehicle. Often the first sign is an abnormal vibration through the car that was not there before. That vibration is a reason to have the tyres inspected promptly rather than waited out.
What to do
The course of action is simple and not negotiable: replace the tyre. A bulge cannot be repaired, and no reputable fitter will try.
If a bulge is found while the car is parked, it is safer to drive slowly and directly to a tyre fitter, or to have a replacement brought to the car by a mobile service like Tyres.co.uk, than to drive any distance at speed. In the eyes of the law a bulge is also a serious defect, a lump, bulge or tear caused by structural separation is a dangerous MOT failure, so it is not something to leave until the next service.
From the workshop: the ones that worry me are the inner-edge bulges people cannot see. A customer comes in about a vibration, we put it on the ramp, and there is an egg the size of a thumb on the inside wall. That tyre was one motorway journey from letting go.
Sources and accuracy. The cause, the danger and the legal status described here reflect tyre-maker guidance and UK MOT rules at the time of writing, which can change. Anything safety-critical should be confirmed against current official guidance. If anything here looks wrong, get in touch and we will check it and put it right.
Common questions
Is it safe to drive on a tyre with a bulge?+
No. A bulge means the internal structure of the tyre has failed and air pressure is pushing the rubber outward. It can blow out without warning, especially at speed, so the tyre must be replaced and not driven on at speed.
What causes a bulge in a tyre?+
Impact damage. Hitting a pothole, kerb or debris hard enough breaks the internal carcass plies inside the tyre, and the air pressure then pushes the weakened rubber out into a bubble. A single sharp impact is often all it takes.
Can a tyre bulge be repaired?+
No. A bulge is structural failure, not a puncture, and it cannot be safely repaired. No reputable tyre shop will attempt it. The only safe course is to replace the tyre.
Why is there a bulge if my tread is fine?+
A bulge has nothing to do with tread. It comes from impact damage to the casing, so a tyre with plenty of tread can still develop a dangerous bulge after hitting a pothole or kerb.