In short. A bulge on a tyre's sidewall means the cords inside the tyre have broken, usually after an impact with a kerb or a pothole, and only the outer rubber is holding the air in. It is not safe to drive on: tyre makers say to fit the spare, and a bulge cannot be repaired. In Great Britain the MOT classes it as a dangerous defect, and the law forbids using such a tyre on a road. A narrow, even dent in the sidewall is a different thing: tyre makers describe it as a normal feature of radial tyres.
The US spelling is tire, the British one tyre; the advice below is the same on both sides of the Atlantic, and where the rules differ by country we say so.
What a sidewall bulge actually is
A tyre keeps its shape because of its carcass: layers (plies) of textile cords embedded in rubber, running from one bead to the other. The cords hold the pressure. The rubber of the sidewall is mostly a cover.
Michelin's Canadian page puts it plainly: "A bulge or bubble on the sidewall of a tire generally indicates damaged cords caused by a severe impact." Once cords are damaged, air gets between the plies and pushes the rubber outwards, which is the bulge you see. Michelin adds that damaged cords are often accompanied by a visible break in the inner liner, the inside face of the tyre, which can only be seen once the tyre is off the wheel.
Pirelli's UK page describes a bulge as a swelling or distortion that may start small and grow over time, and says it is not to be ignored.
What causes it
An impact, most of the time
Michelin ties a sidewall bulge to "a severe impact". Pirelli lists potholes and speed bumps taken at speed, kerbs hit repeatedly while parking, and accidents, and notes that front tyres are affected more often because they meet kerbs more often.
Our reading of the mechanism is a pinch: on impact the sidewall is crushed between the obstacle and the rim flange, and the cords can break while the outside rubber stays unmarked. Continental warns that driving against steep or sharp-edged kerbstones "can lead to non-visible tyre damage which can cause problems later", and that the tyre "may fail when running at high speeds". So a tyre can be damaged without showing it at first; none of the sources we read says how long a bulge takes to appear.
Pressure
Continental says that if pressure is too low, heat builds up in the tyre and can damage it internally, and adds that internal damage is not put right by raising the pressure afterwards. Our reading: an underinflated tyre also deflects further, so its sidewall reaches the rim sooner on a hit. Pirelli, for its part, lists over-inflation first among the common causes. The correct pressure is the one on your car's placard or in its handbook: our tyre pressure guide explains where to find it.
A manufacturing defect
Pirelli says manufacturing defects can "occasionally" be responsible for a bulge. None of the makers we read gives a share. It matters for the warranty (see below).
Low profile tyres and large wheels
For roughly the same overall wheel diameter, a bigger rim means a shorter sidewall, so less rubber and air between the obstacle and the rim. Our calculation on five textbook sizes:
| Tyre size | Sidewall height | Overall diameter | Sidewall vs the 16 inch |
|---|---|---|---|
| 205/55 R16 | 112.75 mm (4.44 in) | 631.9 mm (24.88 in) | 100 % |
| 225/45 R17 | 101.25 mm (3.99 in) | 634.3 mm (24.97 in) | 90 % |
| 225/40 R18 | 90.0 mm (3.54 in) | 637.2 mm (25.09 in) | 80 % |
| 225/35 R19 | 78.75 mm (3.10 in) | 640.1 mm (25.20 in) | 70 % |
| 225/30 R20 | 67.5 mm (2.66 in) | 643.0 mm (25.31 in) | 60 % |
Sidewall = section width × aspect ratio; overall diameter = rim diameter × 25.4 mm + 2 sidewalls. These are worked examples, not sizes for any car: Onyx Forged does not recommend a size or an offset, and the sizes allowed on your car are the ones its maker approves. To compare your own sizes, use our tyre size calculator.
Between 16 and 20 inches the sidewall loses 45.25 mm, or 40 % of its height. That is less travel before the sidewall is pinched against the rim. What we do not know: none of the sources we read measures how much more often low profile tyres bulge, so we cannot give you a figure. What changes with a 19 inch wheel in comfort and tyre choice is covered in our guide to tyre sizes for 19 inch wheels.
Is it safe to drive on a tyre with a bulge?
No. The makers we read agree, each in its own words:
- Michelin (US): "Replacement is not optional for a tire with a confirmed bulge — driving on it risks a sudden blowout."
- Michelin (Canada): replace the tyre with the spare immediately, then have it inspected by a retailer.
- Pirelli: a damaged tyre can burst and cause a loss of control; high speed, heavy loads, potholes and hard braking all raise the risk.
- Continental: "Damaged tyres can burst."
None of these sources gives a speed, a distance or a bulge size under which driving would be safe. A small bulge is not a minor bulge: it is the same broken cords, at an earlier stage.
There is one difference in tone worth knowing. Michelin's US page allows that, if there is no spare and you must drive, you should "keep the journey very short and at low speed to get to the nearest service center". Its Canadian pages do not mention that option, and one of them says there is "no safe temporary fix for a sidewall bulge". In Great Britain, using a tyre with a bulge on a road is against the law (see below), so the safe choice there and everywhere is the spare or a recovery truck. If the bulge sits next to a fresh kerb mark on the rim, have the wheel checked too.
Bulge or harmless dent: how to tell
Not every uneven sidewall is damaged. Radial tyres often show narrow, regular dents (indentations or undulations) where the ends of a ply overlap during manufacturing. Michelin explains that where the cords overlap "there is often slight indentation", and that indentations "are quite natural on radial tires (most tires) and will not affect performance". The MOT manual asks testers to distinguish "normal undulations in the carcass, resulting from manufacturing" from lumps or bulges "caused by structural deterioration".
The rule of thumb that follows from these texts: a bulge goes out, a ply joint goes in.
| What you see on the sidewall | What tyre makers say | Great Britain (MOT 5.2.3) |
|---|---|---|
| A bubble or bump standing out from the surface | broken cords, cannot be repaired, fit the spare (Michelin) | "lump, bulge or tear caused by separation or partial failure of its structure": Dangerous |
| A narrow, regular dent running from rim to tread | ply overlap, normal on radial tyres (Michelin) | normal undulation, not a defect (our reading of the manual) |
| A cut deep enough to reach the cords | have it inspected; damaged tyres can burst (Continental) | cut beyond the limits and reaching the ply or cords: Major; cords exposed or damaged: Dangerous |
A look from outside does not replace an inspection. Only a fitter who takes the tyre off the wheel can see the inner liner, where Michelin says the break often shows.
How to spot one, including on the inner side
A bulge can form on the inner sidewall, the side facing the suspension, which you will not see walking round the car. Continental says vibration through the steering wheel can point to tyre damage, and that all tyres should then be checked.
- After a hard hit, look at both sidewalls of that wheel the same day and again over the next days. To see the inner side of a front tyre, turn the steering to full lock with the car parked.
- Never go under a car held only by a jack. If you need to look underneath, the car must be raised and resting on axle stands.
- If in doubt, have the tyre removed and inspected. Continental recommends having your tyre dealer check the tyres if you are unsure of their condition, and inspecting tyres, spare included, at least once a month.
What to do when you find a bulge
Before you set off: do not drive on it. Fit the spare if you have one, or call recovery. A space-saver spare has its own speed and distance limits, printed on the wheel or in the handbook; our guide to spare wheels and repair kits covers them, and a sealant kit does nothing for broken cords. Pirelli says that if there is no spare, call a tow truck.
If you notice it while driving: slow down gently, without hard braking, and stop somewhere safe. Pirelli lists hard braking among the things that raise the risk of a blowout.
The tyre is replaced, not repaired. Pirelli says a bulge cannot be safely repaired because the tyre's structure is no longer stable; Michelin says a tyre with a bulge or bubble "cannot be repaired". Industry repair rules point the same way. The USTMA puncture repair procedures limit repairs to the tread area, to injuries no larger than 6 mm (1/4 inch), and say: "DO NOT make repairs where the injury damage extends into the shoulder/belt edge area"; any doubt and the tyre "must be taken out of service". Michelin explains why the sidewall is off limits: any repair material on that flexing surface would be under the same repeated stress. For what can be repaired in the tread, see our guide to slow punctures and our plug vs patch guide.
One tyre or two? Inspection rules on matching differ by country. Ontario rejects different nominal sizes left and right on an axle; Queensland rejects tyres of different size or construction on the same axle. Neither text requires the same brand. Your fitter will look at the tread depth of the other tyre on the axle; the car maker's handbook is the reference, especially on four-wheel drive cars. Our guide on replacing two or four tyres goes further.
The wheel. An impact hard enough to break the cords has usually hit the rim flange at the same spot too. Have the rim flange checked before a new tyre goes on it. Our guide to alloy wheel repair explains what can and cannot be fixed. If a pothole did the damage in England or Wales, our guide to pothole damage claims explains who to claim from.
What the rules say, country by country
Great Britain: the MOT and the road law
The MOT inspection manual (section 5.2.3, page updated on 14 October 2026) fails "A tyre with a lump, bulge or tear caused by separation or partial failure of its structure" as a Dangerous defect, the highest category; the same item covers a tyre "with cords exposed or damaged". A tyre with a cut "in excess of the requirements deep enough to reach the ply or cords" is a Major defect.
The MOT is only a check on one day. The law applies every day: regulation 27(1)(d) of the Road Vehicles (Construction and Use) Regulations 1986 forbids using on a road a car with a tyre that "has any lump, bulge or tear caused by separation or partial failure of its structure". Paragraph (c) adds cuts over 25 mm or 10 % of the section width (whichever is greater) deep enough to reach the ply or cord, and (e) exposed ply or cord. These regulations apply in England, Wales and Scotland; we have not read the Northern Ireland text.
Canada: Ontario
Ontario's Passenger / Light-Duty Vehicle Inspection Standard, used for Ontario's safety standards inspection, rejects a tyre when "ply separation is evident or body cords are exposed", when the "casing is broken or distorted", and for a "bulge in sidewall more than 10 mm high". It is the only rule we read that sets a size. It is a pass mark for the inspection, not a safety threshold: the makers we read give no size below which a bulge is safe. We read the October 2015 handbook as posted on ontario.ca in March 2022; check with a licensed station whether a newer edition applies. We did not read the rules of other provinces.
Australia: Queensland
The Queensland Light Vehicle Inspection Manual, in force from 7 July 2026, lists as a reason for rejection: "Tyres have cuts, bulges, tread separation, exposed or damaged cords, are perished, or other evidence of potential case failure." The other states have their own inspection rules, which we did not read.
United States: state inspections
There is no single national inspection of private cars; where a state inspects, its own code applies. As an example, Pennsylvania's code for passenger cars and light trucks (67 Pa. Code § 175.80(e)(1)) rejects a tyre when "There is a bump, bulge or separation" and when "A part of ply or cord is exposed." Rules in other states differ, and we read only this one.
The rules side by side
| Where | Text we read | What it says about a bulge |
|---|---|---|
| Great Britain | MOT inspection manual, 5.2.3 (updated 14 Oct 2026) | lump, bulge or tear from structural failure: Dangerous |
| Great Britain | Construction and Use Regulations 1986, reg. 27(1)(d) | such a tyre must not be used on a road |
| Ontario | Inspection Standard, section 9 (handbook Oct 2015) | reject: bulge in sidewall more than 10 mm high |
| Queensland | Light Vehicle Inspection Manual (from 7 July 2026) | reject: cuts, bulges, exposed or damaged cords |
| Pennsylvania | 67 Pa. Code § 175.80(e)(1)(v) | reject: bump, bulge or separation |
Does the warranty cover a bulge?
Three different things are called a warranty.
The tyre maker's limited warranty covers defects in manufacture and excludes road damage. The Bridgestone warranty manual for the US (tyres sold through one retailer, effective 1 January 2024) excludes "Road hazards, including, without limitation: Puncture, cut, impact break, stone drill, bruise, bulge, snag, etc." The word bulge is in the exclusion. We have not read the UK, Canadian or Australian terms of every maker: ask the seller for them in writing.
Your statutory rights against the seller. In the UK, the Consumer Rights Act 2015 says goods that do not conform to the contract "within the period of six months beginning with the day on which the goods were delivered" must be taken not to have conformed on that day, unless it is established that they did (section 19(14) and (15)). Our reading: a bulge with a kerb mark on the rim or the sidewall will be argued as road damage; a bulge on a recent tyre with no sign of impact is the case where this rule is worth raising. We did not read the consumer law of the other countries.
Road hazard or tyre insurance sold with the tyres. Its terms vary by contract; only the policy you signed says what it covers.
In every case, keep the removed tyre, the invoice and photos of the bulge and of the rim before the tyre goes for recycling. Without the tyre, nobody can examine it.
How to avoid one
The makers' advice adds up to a short list: check pressures cold, at the car maker's values (at least every 14 days, says Continental); take kerbs slowly and, where possible, at right angles, and do not drive up steep or sharp-edged kerbstones (Continental); do not overload the car, which Pirelli lists among the risks; and look at both sidewalls, not only the tread.
Frequently asked questions
Can I drive on a tyre with a small bulge?
No. A small bulge is the same broken cords as a big one. Makers say to fit the spare, and none gives a safe speed or distance. In Great Britain using such a tyre on a road breaks regulation 27 of the Construction and Use Regulations.
Can a tyre bulge be repaired?
No. Michelin and Pirelli say it cannot be repaired, and the USTMA repair procedures limit repairs to the tread area. The tyre has to be replaced.
How long after hitting a pothole does a bulge appear?
None of the sources we read gives a delay. Continental describes non-visible damage that causes problems later, and Pirelli says bulges can grow over time. Check the sidewalls the day of the hit and the following days.
Is a dent in the sidewall the same as a bulge?
No. A narrow, regular dent is usually where the plies overlap, which Michelin calls natural on radial tyres. A bulge stands out from the surface. When unsure, have the tyre taken off and checked from inside.
Will a tyre bulge fail the MOT?
Yes. A lump, bulge or tear caused by separation or partial failure of the tyre's structure is a Dangerous defect in the MOT manual (5.2.3). Ontario and Queensland inspections and Pennsylvania's state inspection also reject it.
Is a bulge covered by the tyre warranty?
Usually not. The Bridgestone US warranty we read names "bulge" among road hazards it does not cover. A bulge from a manufacturing defect is a different case: in the UK, the Consumer Rights Act presumes a fault that shows within six months was there from delivery.
Do low profile tyres get bulges more easily?
Our calculation shows that between 16 and 20 inches, at a similar overall diameter, the sidewall loses 40 % of its height, so it reaches the rim sooner on an impact. No source we read measures the effect on bulges, so we cannot put a number on it.
Do I need to replace both tyres on the axle?
Not always. Inspection rules ask for matching size and construction across an axle; the fitter will also compare tread depth. Follow the car maker's handbook, especially on four-wheel drive cars.
Onyx Forged is a French brand of made-to-order forged wheels designed to JWL/VIA standards: see the catalogue.
Sources
- GOV.UK, MOT inspection manual, section 5.2.3 Tyres (updated 14 October 2026).
- legislation.gov.uk, The Road Vehicles (Construction and Use) Regulations 1986, regulation 27; Consumer Rights Act 2015, section 19.
- Ontario Ministry of Transportation, Passenger / Light-Duty Vehicle Inspection Standard, section 9.
- Queensland Department of Transport and Main Roads, Queensland Light Vehicle Inspection Manual (effective 7 July 2026).
- Pennsylvania Code, 67 Pa. Code § 175.80.
- Michelin, symptom bulge or bubble (21 April 2026), symptom indentation and sidewall problems (US).
- Pirelli, Tyre bulge: is it safe and what can cause it?
- Continental, Technical Databook 2015-2016, tyre damage and guidelines on tyre safety.
- U.S. Tire Manufacturers Association, Puncture Repair Procedures for Passenger and Light Truck Tires (2025).
- Bridgestone Americas, warranty manual, United States (effective 1 January 2024).
All pages were read on 16 October 2026.
The Onyx Forged design office.