17/10/2019
In the UK, the annual MOT (Ministry of Transport) test is a critical benchmark for ensuring your vehicle meets essential road safety and environmental standards. While modern cars are complex machines, it's often the seemingly simple components that lead to a failed MOT. Among the most frequent culprits are issues with tyres, particularly uneven wear patterns. This week, we delve into the dangers and implications of outer edge tyre wear, a common yet often overlooked problem that can lead to an MOT failure, compromising your safety and potentially costing you a significant amount in fines or replacements.
Your vehicle's tyres are its only point of contact with the road, making their condition paramount for maintaining grip, stability, and braking performance. Even a slight deterioration can have a profound impact on handling, especially in adverse weather conditions. Uneven tyre wear, which manifests in various forms, is a clear indicator that something isn't quite right with your vehicle. One of the most prevalent types of uneven wear, and a significant cause of MOT failure, is wear concentrated on the outer edge of the tyre's tread.
Understanding Outer Edge Tyre Wear
Outer edge tyre wear occurs when the rubber on the exterior shoulder of the tyre wears down more rapidly than the rest of the tread. This contrasts with inner edge wear, which affects the opposite side, or centre wear, typically caused by overinflation. While some degree of outer edge wear can be normal, especially on front tyres due to cornering forces, excessive or rapid wear indicates an underlying issue that needs immediate attention. Ignoring it not only jeopardises your safety but also guarantees an MOT failure and could lead to substantial fines.
This type of wear is particularly concerning because it reduces the tyre's effective contact patch with the road, diminishing grip and increasing braking distances. Furthermore, once the wear becomes severe enough to expose the secondary rubber (the structural, non-tread rubber) or the internal belting material, the tyre's structural integrity is severely compromised. Driving on such tyres is incredibly dangerous and illegal. The penalties can be steep, with fines of up to £2,500 per tyre if you're stopped by the police, not to mention penalty points on your licence.
Primary Causes of Outer Edge Tyre Wear
Identifying the root cause of outer edge wear is crucial for preventing its recurrence and ensuring your tyres wear evenly, maximising their lifespan and maintaining your vehicle's safety. Several factors can contribute to this issue:
- Incorrect Wheel Alignment: This is arguably the most common cause. If your wheels are not perfectly aligned with each other and the road (i.e., incorrect toe-in/toe-out, camber, or caster settings), it can cause the tyres to drag or scrub across the road surface, leading to uneven wear. For outer edge wear, an excessive 'toe-in' (where the front of the wheels points inwards too much) or negative camber (the top of the wheel tilts inwards) can be culprits, though positive camber (top of wheel tilts outwards) is more commonly associated with outer wear.
- Underinflation: While often associated with wear on both shoulders, consistent underinflation can sometimes contribute to outer edge wear, particularly if combined with other issues like aggressive cornering. An underinflated tyre will have a softer contact patch, causing the shoulders to bear more of the load.
- Worn Suspension Components: Components such as worn bushes, ball joints, or control arm bushings can allow excessive movement in the suspension system. This instability can lead to the wheels not maintaining their correct position relative to the road, resulting in uneven tyre wear.
- Aggressive Driving Habits: Regularly taking corners at high speeds puts immense stress on the outer edges of your tyres, especially the front ones. Over time, this can accelerate wear on these areas.
- Worn Steering Components: Play in steering components can also affect wheel alignment and lead to uneven wear.
The MOT and Your Tyres: What Inspectors Look For
During an MOT, the inspector will meticulously examine your tyres for several key criteria. Any failure in these areas will result in a 'major' or 'dangerous' defect, leading to an MOT failure. The primary checks include:
- Tread Depth: The legal minimum tread depth for cars in the UK is 1.6mm across the central three-quarters of the tread around the entire circumference of the tyre. Outer edge wear often means that while the central tread might be legal, the outer section falls below this crucial threshold.
- Condition of the Tyre Wall and Tread: Inspectors look for cuts, bulges, cracks, or any signs of internal damage. As mentioned, exposure of the secondary rubber or belting material due to severe uneven wear is an immediate failure.
- Tyre Pressure: While not a direct MOT failure point, incorrect tyre pressure can lead to wear patterns that will fail the MOT.
- Suitability: Ensuring tyres are appropriate for the vehicle and are of the correct size and speed rating.
It's important to remember that if your tyres are deemed unsafe due to excessive outer edge wear, you cannot legally drive your vehicle on public roads until the tyres are replaced and the underlying issue rectified. This underscores the importance of proactive tyre checks.
Preventing and Rectifying Uneven Tyre Wear
The good news is that many instances of uneven tyre wear, including outer edge wear, can be prevented or rectified if caught early. Regular checks and timely maintenance are your best defence:
- Routine Visual Inspections: Make it a habit to visually inspect your tyres at least once a month, and certainly before any long journeys. Look for any signs of uneven wear, cuts, bulges, or embedded objects. Run your hand across the tread to feel for irregularities.
- Check Tyre Pressures: Maintain the correct tyre pressure as specified by your vehicle's manufacturer (usually found in your owner's manual, on a sticker inside the driver's door jamb, or on the fuel filler flap). Correct inflation ensures the tyre's contact patch is optimised for even wear.
- Wheel Alignment Checks: If you notice any signs of uneven wear, or if your steering feels off-centre, or the car pulls to one side, have your wheel alignment checked by a professional. This is a relatively inexpensive procedure that can save you significant money on premature tyre replacement. It directly addresses the most common cause of outer edge wear.
- Tyre Rotation: Regularly rotating your tyres (e.g., every 5,000-8,000 miles) can help promote more even wear across all four tyres, as different positions experience different stresses. If outer edge wear has just begun and is mild, rotating the tyre to a different wheel end position might help prolong its life, provided the underlying cause is addressed.
- Address Worn Components: If a mechanical issue like worn suspension bushes or steering components is causing the uneven wear, these must be replaced. Ignoring them will only lead to repeated tyre problems.
Any tyre whose structure has been undermined by abnormal wear, particularly if secondary rubber or belting material is exposed, should be replaced without delay. There is no 'waiting until the next service' when it comes to safety-critical components like tyres.
Beyond Tyres: Other Common MOT Failure Points
While tyre issues are a leading cause of MOT failures, it's beneficial to be aware of other common areas that often trip up vehicles during the test. A proactive approach to these can save you time, money, and stress:
| System/Component | Common Failure Points | Prevention/Check |
|---|---|---|
| Lighting & Signalling | Blown bulbs (headlights, brake lights, indicators), incorrect headlight aim, cracked lenses. | Regularly check all lights, including brake lights (ask someone to help), and ensure headlights aren't pointing too high or low. |
| Brakes | Excessive wear on pads/discs, fluid leaks, imbalanced braking effort, faulty ABS warning light, ineffective handbrake. | Listen for grinding noises, check fluid levels, ensure handbrake holds firmly. Get professional checks if unsure. |
| Suspension | Worn shock absorbers, broken springs, worn bushes in suspension arms, excessive play in joints. | Listen for clunks/rattles, check for excessive bounce after going over bumps, look for visible damage or leaks. |
| Steering | Excessive play in steering wheel, worn steering rack, fluid leaks from power steering system. | Check for excessive movement in the steering wheel before the wheels respond, listen for unusual noises during turning. |
| Exhaust System | Leaks, corrosion, loose mountings, excessive emissions. | Listen for loud exhaust noises, check for visible rust or holes, ensure no excessive smoke from the tailpipe. |
| Windscreen & Wipers | Cracks or chips in the windscreen (especially in the driver's line of sight), ineffective or damaged wiper blades, empty washer fluid bottle. | Regularly clean windscreen, repair chips promptly. Replace worn wiper blades annually. Keep washer fluid topped up. |
| Seatbelts | Cuts, fraying, inability to latch/unlatch, not retracting properly. | Inspect belts for damage, ensure they click securely into place and retract fully. |
| Number Plates | Incorrect font, spacing, colour, or damage making them unreadable. | Ensure plates are clean, legible, and conform to legal requirements. |
| Bodywork & Structure | Excessive corrosion, sharp edges, damaged chassis. | Regularly wash your car, inspect for rust, especially around sills and wheel arches. |
Frequently Asked Questions About MOTs and Tyres
- Can I legally drive with minor outer edge tyre wear?
- While mild wear might not immediately be illegal, if it causes the tread depth to fall below the legal 1.6mm limit in the central three-quarters of the tyre, or exposes structural components, it is illegal and will fail an MOT. It's always best to address any uneven wear promptly for safety.
- How often should I check my tyre pressures and condition?
- It's recommended to check your tyre pressures at least once a month and before any long journeys. A visual inspection for wear and damage should be done at the same time.
- What is wheel alignment, and why is it important?
- Wheel alignment involves adjusting the angles of your vehicle's wheels to the manufacturer's specifications. It's crucial for ensuring even tyre wear, optimal handling, and fuel efficiency. Misalignment is a primary cause of uneven tyre wear, including outer edge wear.
- What is the legal minimum tyre tread depth in the UK?
- The legal minimum tread depth for cars is 1.6 millimetres across the central three-quarters of the tread width and around the entire circumference of the tyre.
- Will a tyre with small cracks on the sidewall fail an MOT?
- Yes, if the cracks are deemed significant enough to compromise the tyre's integrity, it will fail. This includes cracks that expose cord or are deep enough to be a concern for immediate safety.
- What happens if my car fails its MOT?
- If your car fails its MOT, it is illegal to drive it on public roads until the defects are rectified and it passes a retest. The only exception is driving it to a pre-booked appointment for repairs or to an MOT retest centre.
In conclusion, understanding and proactively addressing issues like outer edge tyre wear are fundamental to ensuring your vehicle remains roadworthy and passes its annual MOT. Beyond avoiding fines and legal repercussions, it's about safeguarding yourself and other road users. Regular checks, correct tyre pressures, and timely professional inspections for wheel alignment and worn components are investments in your safety and the longevity of your tyres. Don't wait until your MOT is due to discover a problem; make tyre care a regular part of your vehicle maintenance routine for happy, safe motoring.
If you want to read more articles similar to Common MOT Failures: Are Your Tyres Safe?, you can visit the Maintenance category.