Chapter 5 — Section 4: Tyres and road wheels

Tyres are the only part of the car actually touching the road, which makes Section 4 one of the most safety-critical parts of the test. The car is raised on a lift so the tester can rotate each wheel and check the inner sidewall as well as the outer. Wheels aren't removed, but the tester assesses condition, suitability and tread depth on every tyre fitted, including a spare if it's being used as a road wheel. Most failures here reflect a lack of routine checking, since tyre defects are usually easy for an owner to spot beforehand.

01Tread depth and measurement

The legal minimum for most passenger cars is 1.6mm across the central three-quarters of the tread width, in a continuous band round the whole circumference. Testers use a calibrated gauge; if any point in that band falls below 1.6mm, the tyre fails.

• A Major defect is recorded where tread is below 1.6mm but the tyre hasn't reached structural failure.
• A Dangerous defect applies where wear has exposed the cords or carcass.
• Testers often flag an advisory between 1.6mm and 2.0mm, since the tyre is likely to fall below the limit before the next test.

Measurement is taken across the tread pattern and doesn't include wear right at the shoulder unless it reaches into the central 75% band. If shoulder wear is severe enough to show the textile cords, though, the tyre fails regardless of what the centre reads.

02Sidewall damage and structural integrity

Sidewalls take a lot of heat and pressure, so checking them properly takes a careful eye. Not every mark is a failure. A cut that doesn't reach the internal cords is usually an advisory unless it looks deep enough to compromise the tyre; a cut that exposes the ply or cord is a Major defect.

• A bulge or bubble points to internal delamination, usually from hitting a kerb or pothole, and is an automatic Dangerous defect.
• Perishing or crazing from age and UV exposure, common on low-mileage cars, is only a failure once it's deep enough to expose the casing cords.
• If a tyre rubs against any fixed part of the car, such as an arch liner or brake hose, at any point in its travel, that's a fail too.

03Tyre mixing and axle consistency

The Construction and Use Regulations govern how different tyre types can be mixed. Here 'type' means the internal structure, radial, cross-ply or bias-belted, and mixing a radial with a cross-ply on the same axle is a Major failure. Mixing brands or tread patterns on the same axle is legal, if not ideal for handling. Mixing summer and winter tyres of the same structure is also legal, though a tester who judges the combination makes the car unstable can still fail it under general condition. Tyres fitted against their directional or asymmetric arrow markings are a Major defect too.

04Load index and speed ratings

Load index and speed rating have to suit the vehicle's maximum weight and top speed, which matters particularly for vans and performance cars. The tester checks each tyre's load capacity is at least half the maximum axle weight shown on the VIN plate; falling short is a Major defect.

• On vehicles first used on or after 1 January 1991, the speed rating must suit the car's top speed.
• Light goods vehicles generally need commercial-rated (C) tyres; fitting a standard car tyre to a heavy van fails on load capacity.
• Tyres over ten years old are a failure item on the steered front axles of heavy goods vehicles and buses, though this specific age rule doesn't currently apply to standard Class 4 cars, even though it's often sensible advice regardless.

05Road wheels and fixings

Wheels themselves are checked for cracks, distortion and secure fixings. Steel wheels are prone to corrosion and dents, alloys to cracking, often on the inner rim where it's hard for an owner to see.

• A cracked wheel is a Major defect; if the crack looks likely to cause the wheel to fail structurally, it's Dangerous.
• A missing, loose or wrong-type wheel nut (for example a tapered nut on a radiused seat) fails the car.
• Damaged or loose trims that could come off, or that have sharp edges, also fail.
• Heavily kerbed alloys usually only fail if the damage has broken the tyre seal or left a sharp edge that could injure a pedestrian.

06Where the Dangerous category applies

The distinction between Major and Dangerous matters here more than most sections. Major allows the owner to drive away and get the car repaired, provided the previous MOT is still valid; Dangerous means the car shouldn't be driven at all. Exposed cords, large bulges, or a wheel loose enough to be at risk of coming off all fall into Dangerous, on the basis that a tyre or wheel that's already failed structurally isn't fit for even a short trip to the garage.

Most Section 4 failures are preventable with a five-minute monthly check of tread and sidewalls. As the only components governing grip and stopping distance, the standards here are deliberately unforgiving.