Do you know how to locate your Tread Wear Indicator bars and what their purpose is? Do you also know how to convert psi to bar to determine if your tyres are properly inflated? These are basic things you need to understand to ensure your tyres are always safe and legal and to avoid your tyres leading to a failed MOT.
But, as Chris Tarrant used to say on Who Wants to Be a Millionaire?…these questions are “only easy when you know the answer”.
Judging by the high proportion of cars that fail their MOT due to tyre-related problems, a lot of UK motorists don’t know what to look for when they check their tyres.
Follow these steps to help reduce the risk of your tyres leading to a failed MOT:
Tread Wear Indicator bars
Let’s start with the tread wear indicator bars. Their purpose is to monitor the tread depth of a tyre and help motorists recognise when it is time to replace it. The legal minimum tread depth is 1.6mm across the central three quarters of the tyre and around its entire circumference. If even one of your tyres has less than this, your vehicle will fail its MOT.
The TWIs are built into the base of the main grooves of the tyre and are evenly spaced around the entire circumference. They can actually be invisible or barely noticeable when the tyre is new and has a full 8mm of tread. However, they gradually begin to appear as the tread wears down. Tyre need replacing once the tread surface is worn to the same level as these indicators.
Tyre tread depth gauge
There are two other ways you can check your tread depths. The first is to invest in handy tyre tread depth gauge, which does exactly as its name suggests. You can pick up a decent one for around £5. Simply insert the purpose made tool into the individual tyre grooves to determine whether your tyres are above the 1.6mm limit.
20p coin test
Alternatively, if you don’t have a gauge, you could fish out a 20p coin and use that to give you a pretty good idea as to whether your tyres need replacing. This time you just simply insert the coin into the tread grooves on the tyre. If you can’t see the outer band on the coin, your tyres are above the legal limit. However, if you can see the band the chances are your tyres are already illegal.
As with all these tests, you need to carry out the checks at several different points, because the tread must meet this minimum requirement around the entire circumference of the tyre.
How to check your tyre pressure
Next you need to check your tyre pressure. This is the measurement of the volume of air inside the tyre when it is inflated. Formerly measured in pounds per square inch (psi), it is now usually expressed in bar (1 bar = 14.5038 psi).
A properly inflated tyre will provide longer life, quicker steering response, better fuel efficiency and a smoother ride than an incorrectly inflated tyre. Buy crucially, both under-inflation and over-inflation can cause possible tyre failure while you are driving.
Firstly you need to know the recommended tyre pressure for your vehicle. You will find this in your owner’s manual; on the driver’s side door jamb or on the inside of the fuel tank filler cap.
However, just to complicate matters, it is possible your numbers may be in bar, whereas the tyre pressure gauge you use on a garage forecourt is likely to be in psi – but don’t panic or give up!
How to convert psi to bar
You just need to convert them into a psi reading in order to use some gauges and air compressors. You can do this in one of two ways:
1. Go online and search ‘convert (your requirement) bar to psi’
The conversion will appear as something like this:
2.60 bar = 37.7098 psi
Don’t bother rounding the figure up or down, just take the first two numbers
2. Alternatively, you can calculate the conversion yourself. Basically, 1 bar = 14.5038 psi and the formula for conversion is:
14.5038 x bar
Eg: 2.6 bar = 37.7098 psi
For more detailed information about how to carry out these simple tests, read our tyre pressure checking blog which sets out what to do.
Other steps to smooth the MOT pass path
There are two other key issues to take into consideration before putting your vehicle in for its test and make sure your tyres will pass the MOT.
The general condition of your tyres will be under scrutiny during the test, so it is important to look for signs of damage, such as cuts, bulges, punctures or the cord or ply showing through. However, a punctured tyre can be repaired so that it will pass the test. etyres offer a mobile puncture repair service with prices starting from just £25, which can be a fraction of cost of a brand new tyre.
You also need to ensure all four tyres fitted to your vehicle are the correct size for the vehicle by comparing the tyre size printed on the sidewall (eg 195/55R16) against the front and rear tyre sizes specified in the vehicle handbook. Each tyre must match the opposite tyre on both of the axles.
Another more recent requirement applies to vehicles first used on or after 1 January 2012. The Tyre Pressure Monitoring System (TPMS) must also be functioning properly. A faulty TPMS will lead to an automatic fail.
This is a lot of information to take on board, but it is worth getting to grips with to avoid your tyres leading to a failed MOT and also to help you ensure they are not dangerous. Of course, if you are at all concerned, we recommend you call etyres for expert and impartial advice on all things tyre-related. We are happy to offer guidance and even a free inspection with no obligation to purchase a tyre.
Denna Bowman, Head Office