Bulges, cuts and cracks in a tyre sidewall
6 min readBy Wheel Fit
If you have found a bulge, stop reading and stop driving A bulge or egg-shaped lump in a sidewall means the internal structure has already failed and the tyre is being held together by rubber. It cannot be repaired, and it can fail without further warning. Fit the spare or call someone out.
The sidewall is the flexible part of the tyre between the tread and the wheel rim. It carries the load and absorbs impacts, and it is thin — there is no thick tread rubber protecting it. That is why damage there is treated so differently from damage in the tread.
Why sidewall damage is never repairable
A tread puncture goes through a thick, reinforced, relatively flat part of the tyre that is supported by steel belts. A plug-and-patch repair can restore it properly.
The sidewall flexes on every single rotation. There is no belt package there, the rubber is thin, and any repair would sit in the part of the tyre under the most constant movement. No repair is permitted, by anyone, under the British Standard that governs tyre repairs. Can a puncture be repaired? covers the repairable area in full.
Telling serious damage from cosmetic marking
| What you see | Severity | Action |
|---|---|---|
| Bulge or lump | Critical | Replace now. Do not drive on it. |
| Cut deep enough to show cords | Critical | Replace. Any exposed ply or cord is an immediate failure. |
| Cut or tear, depth unclear | Needs inspection | Have it assessed before further driving |
| Kerb scuffing on the rim protector | Usually cosmetic | Inspect — the raised rib is designed to take this |
| Fine surface crazing | Age-related | Assess; often a sign the tyre is timing out |
| Deep cracks in the grooves of the sidewall | Serious | Replace — the rubber has perished |
What causes a bulge
Almost always a single impact: a pothole, a kerb hit at an angle, or a sharp-edged speed hump taken too fast. The impact pinches the sidewall between the obstacle and the rim and snaps some of the internal cords. Air then pushes out into the weakened area, and you get the characteristic egg shape.
The bulge often appears days or weeks after the impact, which is why people do not connect the two. If you have hit something hard, it is worth looking at both tyres on that side afterwards — and at the wheel, since a bent rim frequently comes with it.
Weathering and cracking
Fine cracking across the sidewall surface is age and UV exposure. It is common on tyres that do little mileage — caravans, trailers, classic cars, second cars — and it is the reason a tyre with full tread can still be finished.
Deep cracks, particularly at the base of the sidewall or in the grooves, mean the rubber has perished and the tyre should be replaced regardless of tread. Check the manufacture date while you are there — how long tyres last explains the DOT code and what to look for on an older tyre.
The MOT position
What a tester looks for A tyre with a cut, lump, bulge or tear caused by separation or partial failure of its structure, or with any ply or cord exposed, is a failure. This is the same standard that makes it an offence to use the vehicle on the road at all — it is not merely an MOT technicality. See tyre faults that fail an MOT.
Common questions
- Can I drive slowly to a garage on a bulging tyre?
- It is not advisable. The failure mode is a blowout, and it gives no further warning. Fit the spare, or have someone come to the car — see what is mobile tyre fitting.
- The bulge is small. Does that matter?
- No. Size does not indicate how close the tyre is to failing; it indicates how much air has already found its way into the damaged area.
- Is kerb rash a problem?
- Usually not on its own. Many tyres have a raised rim protector rib designed to take exactly that. It becomes a problem if the rubber is gouged deeply or cords are visible.
- Can I claim on insurance?
- Some policies cover accidental tyre damage, and some local authorities accept pothole damage claims. Photograph the tyre and the location before it is replaced if you intend to try.
- Should I replace both tyres on that axle?
- It depends on how worn the other one is. See replacing all four or just one.
If you have found damage
Call 07989 606060 and describe what you can see. If it is a bulge or exposed cords the answer is replacement, and we will come to the car rather than asking you to drive it. We cover areas across England and Wales, 24 hours a day.
