As part of their daily car-care routine, motorists should inspect their tyres for irregular wear or damage. Look out for sidewall damage in particular; if there's a protruding bulge on the sidewall, this is a sign of imminent tyre failure and must be replaced immediately.
Why the need for urgent action? Because this type of bulge is known as an impact break, where the cords inside the carcass (or casing of the tyre) have severe structural damage. There is a high probability that the tyre will soon fail, either as delamination of the tread or plies or disintegration of the tyre sidewall.
If this occurs while driving your car, there's a high risk to your safety and other users of the road.
An impact break is usually caused by driving a car over objects like kerbs, railroad crossings or speed bumps at excessive speed or the wrong angle.
When the tyre comes into contact with a sharp edge, the force from the weight and speed of the vehicle applies pressure onto a small area on the tyre. In so doing, the carcass becomes over-stressed and can cause individual cords to break.
Most often, the impact that caused the damage was not severe enough to be noticed by the driver. The rim of the wheel remains intact, for example, and there is no puncture requiring immediate attention. But it will have been strong enough to damage the tyre.
The result is a bulge on the sidewall, which means air is leaking from the inside of the tyre into the carcass of the tyre. The size of the bulge depends on the speed and angle of impact and the size of the obstacle. The only thing preventing a complete tyre failure is the thin sidewall plies, which is highly unsafe.
Motorists can usually avoid causing this type of damage with careful and steady driving when behind the wheel. In normal circumstances, something like a kerb or speed bump should only be driven over at a wide angle and appropriately slow speed.
An impact break is inevitable only in very exceptional cases, for example with a pothole in the road when driving in poor visibility, or if debris suddenly appears under the wheels of the vehicle.
Important to note is that no category of tyre is immune to an impact break, including winter tyres. And low-profile tyres with a shorter sidewall are easier to damage.
The short answer is no; you cannot repair a tyre after an impact break. That's because the tyre will flex while driving, and a patch will not stay in place.
The bulge is also evidence of structural damage to the tyre, which means it's well beyond repair. A tyre in this condition could fail without warning and mustn't be driven on. Motorists should instead use their spare tyre until such time they can fit a new tyre to the wheel.
Please note: a sidewall bulge should not be confused with a sidewall indentation. If in doubt, check with your local mechanic or MOT specialist.