Having decent quality car tyres that help you drive safely is essential. Among various factors that determine the quality and condition of your tyres, the level and pattern of tread wear are extremely important.
Checking your tyre tread wear indicators properly is the best way to know when your tyres should be replaced or repaired. It is also worth knowing to be able to recognise when your tyres are completely worn down or worn inappropriately and must be replaced. But when is that?
What is a worn tyre?
In short, a worn tyre can be defined as:
- A tyre with a tread depth that has reached the tread wear index marked in several places around the circumference, usually 1.6 mm for passenger cars.
- A tyre with damage that makes it impossible to use.
- A tyre so old that it is no longer safe or usable.
Similarly, tread wear comes in many different forms, many of which mark a tyre as unusable. These include:
- A tread shallower than 1.6 mm, the UK legal minimum.
- Split tyre elements, such as a separated tread.
- Deformed or exposed bead wire (the element that holds the tyre onto the car wheel).
- Stains or other discolourations on the butyl layer, similar to a visible marble pattern.
- Bulges or deformations of the sidewall caused by broken threads in the tyre carcass.
- Any cuts or tears in the sidewall, even if superficial, that damage the carcass.
If you notice any of these symptoms, you should consult a professional tyre shop to see if it can be repaired or if you need new car tyres. Being familiar with different forms of tyre tread wear patterns can help you identify the condition of your tyres more easily.
Tyre tread depth and tyre traction
A tyre with a minimum tread depth performs far worse than a tyre with new treads, especially when wet handling and resistance to aquaplaning are concerned. A tyre with a 4 mm tread has only 65% of the grip that an 8 mm thread would have on a wet surface. Compared to a new tyre, a tread at the legal minimum (1.6 mm) only has 40% of the wet grip available.
The TWI symbol is a car tyre tread wear indicator. It shows the tyre is worn down to 1.6 mm.
Types of tread wear
With this in mind, remember that there are different types of tread wear. It is good if you know how to recognise them. The most common types of tread wear are:
- Normal wear
- Asymmetrical wear
- Round wear (shoulders)
- Wearing along the middle (concave)
- Spot wear
Tread wear bulges in the sidewall. Photo: Michelin.
Normal wear refers to situations where the tyre is worn evenly across the whole width of the tyre as well as along the whole tread. This happens with time and is normal – all tyres become worn through use.
Asymmetrical wear is the result of one side of the tread wearing faster than the other. The inner or outer tyre tread wear can be the result of an incorrect car toe or camber, causing the tyre to be at an irregular angle to the road.
Shoulder wear, also known as rounding, means that the edges of the tread are higher than the middle.
Wearing along the middle of the tread, also known as concave wear, is caused by over inflating the tyres. This incorrect tyre pressure causes the contact patch to be limited to the middle of the tyre, which is bulging, causing this section to wear much faster.
Spot wear is often caused by sudden braking with locked wheels, which can occur in cars without ABS. This causes intense wear in one part of the tyre, as the locked wheels mean the tyre cannot rotate. Similarly, it can also be caused by the use of a low quality rubber compound in the tyre construction.
Tread wear: two typical tread wear patterns caused by improper inflation.
Middle wear – probably caused by prolonged driving with overinflated tyres or an aggressive driving style.
Side wear – probably caused by prolonged driving with underinflated tyres and aggressive cornering.
Tread wear: two tread wear patterns caused by the car’s suspension or incorrect balancing.
One-sided wear – probably caused by incorrect toe or problems with the car suspension system.
Spot wear – probably caused by sudden braking, incorrect balancing or alignment.
Feathering is also a frequent tread wear pattern. Found on many tyres, it indicates that the tread blocks have been worn down unevenly. This results in a saw-tooth pattern that is both visible to the naked eye and can be easily felt with your hand.
Ultimately, this effect is inevitable, but it can vary in its intensity, depending on a number of factors, including:
- Mismatched tyres and car suspension
- Incorrect tyre pressure and loads
- Mismatched speed index and load index for a given car
- Conditions with the suspension
Heavily feathered tyres may be bothersome due to their vibration levels and, consequently, high tyre noise. In order to keep this problem to a minimal, acceptable level, you should rotate your car wheels every 6,200 miles, observe the correct pressure, speed index and load index, as well as take proper care of your suspension.
A partly used tyre tread with signs of feathering.
How to check your tyre tread wear
Knowing what kind of tread you’re working with, you can more readily test its depth and determine whether it satisfies the legal tread depth requirements. There are several ways to check the depth of your tread.
You can do it using a tyre depth gauge, a small and cheap gadget available in every car shop. A coin is also a popular tool often used to check your tyre depth. If, upon inserting a 20-penny into a tread groove, you cannot see the rim surrounding the coin, your tread is deep enough and the tyre is safe to use. To take a visual reading, you can also look at the tread wear indicators or wear bars which are evenly placed across the main tyre tread grooves. If the tread is on the same level, your tyre is worn.
Whichever method of checking the tread wear you go for, remember to perform that check in many different locations around each tyre. This is extremely important as it tells you how much tread is left at the wear bar at certain parts of the tyre helping you to notice any potential signs of uneven tyre wear.
When should I return my tyres?
When it comes to various tyre faults, you cannot rule out the possibility that it was the result of a factory fault. If you are sure you have been using your tyres correctly, observing all recommended pressure and load values and keeping your car in a good condition, and yet a tyre still wears down in an unusual way, this may suggest problems with its original quality. In these instances, you may want to take your tyres to an expert to have them inspected.
Tread wear and road safety
When it comes to safety on the road, tread wear causes two specific problems:
- First and foremost, it influences how the tyre behaves and handles, thanks to altering its parameters. A shallow tread, for instance, creates a bigger risk of losing control of your vehicle. This is especially true on wet roads, as a thinner tread has a greater susceptibility to aquaplaning.
- Secondly, there is a higher probability of causing tread wear-related incidents. This includes cracks, tread separation and even the tyre falling off of the rim.
An example of a broken tyre tread
How to control your tyre tread
Tyre tread wear is largely dependent on what we, as drivers, do. Numerous factors can determine how long you can use your tyres for.
- Ensuring the correct pressure for the load of the car
- Observing and keeping under the maximum load tolerances
- The driving environment, such as city roads, motorways or dirt roads
- The frequency or corners and hills encountered while driving
- Your driving style, which may include sudden braking or excessive acceleration
- Your car and tyre storage conditions, as well as the position of the tyres being stored
- Driving over obstacles, such as holes and curbs, at high speeds
- Controlling the technical condition of your car, such as the wheel alignment, toe and suspension
Checking regularly if your tyres should have the right pressure values recommended by the manufacturer for the given conditions is extremely important. The pressure should not be too high or too low as this can change the shape and strength of the tyre, leading to uneven tyre wear, bulging and other issues described above. For instance, underinflating a tyre by 20% can shorten the tyre lifespan by 30%. It also significantly increases your braking distances and fuel consumption.
Understanding the tyre tread wear ratings and knowing why tread depth matters is a key to keeping your tyres in the appropriate condition. That, in turn, leads to having greater control over your vehicle and, consequently, ensuring better safety on the road.