AA Car Care: How to know when your tyres have given up
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Tyres are incredibly important – they play a vital part in the safety and comfort of every car journey. Here are a few tips on what to check and how to know they could be due for replacement.
How your tyres might fail a WoF
The inspector or service technician will check your tyres and advise if they show damage that is likely to compromise safety. That could include:
- A lump or bulge that is likely to be caused by separation or partial failure of the tyre structure
- A cut or crack in a sidewall or tread more than 25mm long that reaches the cords
- Exposed or cut cords
- A re-treaded tyre that shows signs of separation
- Nails or other sharp objects embedded in the tyre
- Significant perishing due to age, moisture or exposure
What can I check myself?
Vehicle maintenance does not just mean just checking items under the bonnet, but extends to looking after tyres and other key vehicle components that can often be overlooked – including the spare tyre (if your car has one).
Setting the correct tyre pressure is important for several reasons:
Tyre life – Under or over-inflation increases wear. A flat tyre will scrub the road surface and wear prematurely, while over-inflation could increase wear.
Fuel economy – Under-inflation increases fuel consumption: a flat tyre doesn’t roll as efficiently.
Safety – Under or over-inflation will affect grip and braking performance.
Ride – Over-inflation can result in a harsh, uncomfortable and skittish ride. Under-inflation can result in steering wheel wobble, increased steering effort or unstable cornering.
The 20c coin check
Almost all tyres have wear indicator bars in the groove of the tread, so check that the tyre isn’t worn down to this level. The wear bars appear when you only have 1.5mm of tread left, at which point your tyres are on their absolute minimum legal tread depth to pass a WoF.
In the wet, or when travelling at high speeds, grip and stopping distances are seriously compromised with less than 1.5mm of tread.
A simple check is to insert a 20c coin into the tread. The base of the number 20 is approximately 2mm from the edge of the coin; if you can see the whole of the number, it’s time to think about replacing your tyres.
Regular checks are best
Aim to check tyre pressures each month, ideally when the tyres are cold. Be sure to use a reliable tyre pressure gauge, and don’t forget to check the spare tyre if you have one.
You can find out the right tyre pressures for your car in the handbook or on a sticker inside the fuel filler flap or driver's door. In general, the handbook or stickers will show two figures: one for normal use, and a higher figure for full loading. If you can't find the correct pressures, it's best to contact the vehicle manufacturer. For more information, visit aa.co.nz/tyres.
When you check the pressures, also check the tread for wear and look for cuts or bulges on the sidewalls.
Tyres may be rotated with every service. During rotation, each tyre and wheel is removed from your vehicle and moved to a different position to ensure that all tyres wear evenly and last that little bit longer. If you have a vehicle manual there might also be a rotation plan set by the vehicle manufacturer.
How long should my tyres last?
This is the magic question, but it’s dependent on many factors. Some are controlled by the driver, but others are not.
Factors in the driver’s control are mainly the servicing of the tyres, pressure checks, rotation, balance, and wheel alignment. How you drive is also key - high speed cornering and harsh stopping can quickly eat up tread and reduce tyre life.
Factors outside the driver’s control include where you drive and road surface types. Some drivers may get as little as 25,000km on a set of tyres, while others could easily see double that.
As tyres age, sidewalls begin to crack. This happens when UV light oxidises the rubber, causing it to dry out. Although tyres contain anti-oxidizing chemicals to slow this process down, these only work when the tyres are moving. If your car is left unused for some time or your tyres are stored, they’ll deteriorate faster.
It’s because of this that tyre companies don’t like to predict how long tyres should last. They do, however, recommend that all tyres (including the spare) should be removed from service and replaced with new tyres if they are over 10 years old.
The AA has comprehensive advice on how to look after your tyres here.