Tyre Failures on the MOT

The annual MOT test is a crucial part of vehicle ownership in the UK, ensuring that your car meets the minimum safety and environmental standards. While many drivers associate MOT failures with more complex mechanical issues like braking systems or exhaust emissions, it's surprisingly common for tyres to be the culprit. In fact, worn or damaged tyres are one of the most frequent reasons for a vehicle to fail its MOT. Understanding the specific tyre-related reasons for failure can help you proactively maintain your vehicle and avoid unexpected costs and inconvenience.

Tyre Tread Depth: The Legal Minimum

One of the most straightforward reasons for an MOT failure is inadequate tyre tread depth. The legal minimum tread depth for car tyres in the UK is 1.6 millimetres across the central three-quarters of the tyre's breadth. This depth is not arbitrary; it's essential for providing sufficient grip, particularly in wet conditions. When the tread wears down, the grooves that channel water away from the contact patch between the tyre and the road become less effective. This significantly increases the risk of aquaplaning, where the tyre loses contact with the road surface due to a layer of water, leading to a loss of steering and braking control.

During the MOT inspection, the tester will examine the tread depth of each tyre, including the spare if it's fitted and available. They will use a calibrated tread depth gauge to measure this accurately. If any tyre falls below the 1.6mm legal limit, it will result in an MOT failure. It's important to remember that even if your tyres are above the legal minimum, but showing significant signs of wear or uneven wear, they might still be flagged as advisory or even a failure if deemed unsafe.

Sidewall Damage: A Critical Safety Concern

Beyond tread depth, the condition of the tyre's sidewall is also a critical factor in the MOT test. The sidewall is the part of the tyre between the tread and the wheel rim. It's responsible for supporting the vehicle's weight and absorbing road shocks. Any damage to the sidewall can compromise the structural integrity of the tyre, making it prone to sudden failure, such as a blowout.

Common types of sidewall damage that can lead to an MOT failure include:

• Cuts and Cracks: Deep cuts or extensive cracking in the sidewall indicate that the rubber has degraded or been damaged, potentially weakening the tyre's structure.
• Bulges and Blisters: These are often caused by internal damage to the tyre's structure, such as a separation of the plies. A bulge or blister signifies that the tyre is under internal pressure and is at high risk of bursting.
• Perished Rubber: Over time, rubber can degrade, especially if exposed to sunlight, ozone, or if the tyres are old. Perished rubber can appear dry, brittle, and cracked, even on the sidewalls.
• Exposed Casing or Reinforcement: If a cut or abrasion is deep enough to expose the internal cords or reinforcement material of the tyre, it's a serious safety concern and will result in an immediate failure.

The MOT tester will carefully inspect the sidewalls of all tyres for any such damage. If any of these issues are present and deemed to be a safety risk, the vehicle will fail its MOT.

Tyre Age: Not Directly Tested, But Crucial

While the age of a tyre is not a direct failure point in the MOT test itself, it's an important factor for tyre health and safety. Tyres degrade over time, regardless of tread depth. The rubber can become harder, more brittle, and more susceptible to cracking. Many manufacturers recommend replacing tyres after six years, even if they appear to have plenty of tread left. An MOT tester may flag older tyres as an advisory if they show signs of ageing, such as small cracks or hardening, which could lead to a failure in the future.

Tyre Size and Specification: Matching Your Vehicle

Your vehicle manufacturer specifies the correct tyre size and type for your car. These specifications are usually found in your vehicle's handbook, on a sticker inside the driver's door frame, or on the fuel filler flap. Using tyres that do not meet these specifications can lead to an MOT failure.

Reasons for failure related to tyre size and specification include:

• Incorrect Tyre Size: Fitting tyres that are a different size to what the manufacturer recommends can affect the vehicle's speedometer readings, handling, and stability.
• Incorrect Speed Rating: Tyres have a speed rating that indicates the maximum speed at which they can safely carry a load. Fitting tyres with a speed rating lower than that recommended by the manufacturer is a failure.
• Incorrect Load Rating: Similarly, tyres have a load index, which indicates the maximum weight a tyre can support. Using tyres with a load index below the manufacturer's recommendation is also a failure.
• Mismatched Tyres on the Same Axle: In the UK, it is a legal requirement that tyres fitted to the same axle must be of the same type, size, construction, and load/speed rating. Mismatched tyres on the same axle will result in an MOT failure.

Tyre Wear Patterns: Indicating Underlying Problems

The way a tyre wears can provide valuable insights into the health of your vehicle's suspension and steering systems. Uneven tyre wear, even if the tread depth is still legal, can be a sign of underlying mechanical issues that could lead to an MOT failure, or at least an advisory.

Common uneven wear patterns and their potential causes:

While these are often advisory points, if the wear is severe enough to compromise the tyre's safety or if the underlying issue is significant, it could lead to a failure. For example, severely worn suspension components could be a fail item.

Tyre Pressure: Essential for Safety and Wear

While not a direct MOT failure point unless it leads to abnormal wear or damage, maintaining the correct tyre pressure is vital for safety, fuel efficiency, and tyre longevity. Under-inflated tyres wear out faster on the edges, can overheat, and affect handling. Over-inflated tyres wear out in the centre and can provide a harsher ride. Always check your vehicle's handbook or door sticker for the recommended tyre pressures and adjust them accordingly, especially when the car is heavily loaded or when tyres are cold.

Preventing Tyre-Related MOT Failures

The good news is that most tyre-related MOT failures are preventable with regular checks and maintenance. Here are some key steps you can take:

• Regularly check tread depth: Use a tread depth gauge or even a 20p coin. Insert the coin into the grooves with the Queen's head facing you. If you can see the Queen's head clearly, your tread depth is likely below the legal minimum.
• Inspect sidewalls for damage: Look for cuts, bulges, cracks, or any signs of perished rubber.
• Check tyre age: Tyres have a DOT code on the sidewall. The last four digits indicate the week and year of manufacture (e.g., 3221 means the 32nd week of 2021). Replace tyres that are over six years old.
• Ensure correct tyre specifications: Always fit tyres that match the manufacturer's recommended size, load rating, and speed rating.
• Check for uneven wear: Regularly inspect your tyres for any signs of uneven wear patterns, which may indicate a need for wheel alignment or suspension checks.
• Maintain correct tyre pressure: Check and adjust tyre pressures regularly, ideally when the tyres are cold.
• Address advisories promptly: If your previous MOT report included advisories related to your tyres, make sure to address them before your next test.

Frequently Asked Questions (FAQs)

Q1: What is the legal minimum tyre tread depth in the UK?

A1: The legal minimum tyre tread depth in the UK is 1.6 millimetres across the central three-quarters of the tyre's breadth.

Q2: Can my car fail its MOT for having old tyres, even if the tread is good?

A2: While tyre age itself isn't a direct failure point, an MOT tester may fail tyres if they show signs of significant degradation, such as cracking or perishing, which can be associated with age and compromise safety.

Q3: Do I need to have matching tyres on my car?

A3: Yes, for the MOT test, tyres fitted to the same axle must be of the same type, size, construction, and load/speed rating. It is also best practice to have matching tyres on the front and rear axles.

Q4: What should I do if my tyres have uneven wear?

A4: Uneven wear can indicate issues with wheel alignment, tyre pressure, or suspension. It's advisable to have your vehicle inspected by a qualified mechanic to identify and rectify the underlying problem. This may prevent future MOT failures and ensure safer driving.

Q5: Does a puncture mean my car will fail its MOT?

A5: A simple puncture that has been repaired correctly according to British Standards (BS AU 159) should not cause an MOT failure. However, if the tyre is damaged in a way that makes it unrepairable or unsafe, or if the repair is not to standard, it will fail.

In conclusion, tyres are a significant and often overlooked component when it comes to MOT success. By understanding the specific criteria testers look for – tread depth, sidewall integrity, correct specifications, and signs of excessive or uneven wear – you can take proactive steps to ensure your tyres are in good condition. Regular checks and prompt attention to any advisories will not only help your vehicle pass its MOT but also contribute to a safer and more enjoyable driving experience.