How to check tyre tread depth & what to look out for.

We all know that our car will fail an MOT if there is not enough tread on our tyres.

With this check in place some drivers pay little or no attention to their tyres until the MOT or, sadly, when their tyres fail on the road. Some of you may think there is little you can do if a tyre 'blows out', many will say it is just bad luck however, the real reason in almost all cases is that the owner/driver has paid little attention to the one thing (all four) that gives them the contact with the road, maintains traction and dispels water from under the tyre to prevent aquaplaning. Aquaplaning is a phenomenon where the tyre fails to displace the water and builds up under the tyre effectively raising the tyre off the road surface so you will not be able to steer, brake or accelerate effectively.

Tread is often misunderstood as the thing that gives you grip on the road, if we look at Formula 1 tyres they have no tread pattern at all and achieve maximum grip, not forgetting the aerodynamic enhancements too, they have all the available rubber on the road, that is until it rains – pit stop! The wet tyres (with tread pattern) are fitted and off they go racing again.

Obviously we cannot pit stop on the public road, so our road tyres are optimised to work in the dry, and the wet, and the very reason we have tread. A normal road tyre will displace, on average, around 10 litres of water per second (varies on tyre size, width) Formula 1 wet tyres displace around 60 litres per second. For normal road use that 10 litres is more than enough in most circumstances and some drivers will have experienced a small aquaplane going through a puddle or standing water with even new tyres. Tread is vital to displacing water and as the tyre wears that displacement becomes less efficient, add to that incorrect tyre pressure and the tyre will be operating outside of the design parameters and will displace less water as the 'footprint' on the road is not changes. As your tyres wear so your braking distance increases exponentially. Typically, at 50mph you will stop an extra 33 metres further on with tyres with 1.6mm of tread compared with tyres that have 3mm of tread.

Tyres normally have around 8mm of tread when new but vary slightly between manufacturers.The legal minimum tread depth for cars is 1.6mm across the central three-quarters of the width of the tyre, and across the entire circumference. Once your tread gets as low as this, you need to replace the tyre although many motoring organisations would recommend the tyre be changed at 2mm because of the reduction in efficiency as it wears and gets closer to the legal limit. Police vehicles in Lincolnshire have a limit of 3mm.

So what is the easiest way to check the tread depth? There are tyre tread depth gauges available but by far the easiest method I have found is using a 20p coin.



Place the 20p coin in the main groove of the tyre and if the part of the coin with 'TWENTY PENCE' written on it disappears, then your tyres have enough depth. You should carry out the check in 3 or 4 different places around the tyre just in case it has worn unevenly, not forgetting the inner part of the tread that maybe out of reach. Move the car forward, safely, to check the bottom of the tyre.

That said, tyres have Tyre Wear Indicators (TWI), these are small raised blocks (2mm) in the bottom of the tread that enable you to see, when the remaining is tread is level with the outside diameter, it is time to change the tyre. However, when they are level your tyres could be at the legal limit or lower and not the recommended minimum tread depth. . If you are unsure contact a specialist

Checking the tread depth is one issue, while you are there check for damage, cracks or lumps in the side wall and items stuck in the tread, be careful though, if your tyre is badly worn it may have exposed the steel wires that are in the casing, be careful, these are thin enough (like needles) to cut your bare hands. If you see, or feel, wires protruding from the tyre it must be changed. If you find anything embedded in the tyre change it for the spare, if you have one, then ask a specialist if it can be repaired. Some punctures can be repaired, but only by a specialist.

When in doubt seek expert advice.

Bush Tyres Branch Finder 

Winter tyres, if you have them fitted, need more tread depth for them to operate correctly and should be replaced with 4mm of tread left. This is becuase winter tyres are designed to hold snow in the tread so when the tyre comes into contact with the snow on the ground a superior grip is achieved. It sounds like a hard concept to grasp, but the next time it snows, grab two snowballs & try twisting them together & you will find they just lock to each other. Typically, with winter tyres, you can stop around 11 metres sooner even at just 20mph than on a normal tyre so it is essential they have a minimum of 4mm tread depth to operate correctly.

So, what if you get it wrong and your tyres are worn past the legal limit?

Besides the legal penalties where you could receive 3 penalty points on your licence for each tyre and a possible £2500 fine, in the worst case you also put yourself in danger of having your car unfit for the road which then puts other road users at risk. If you have a collision and someone is seriously injured, or even killed because you failed to maintain your tyres, the Collision Investigation Team, who investigate all serious and fatal incidents on our roads, will present a prosecution file to Crown Prosecution Service and, if you are found guilty, you could be sent to prison.

3 points for each tyre and upto a £2500 fine 

At the start of this article I mentioned the contact a tyre has with the road, your tyres play such an important role in your safety but we (some drivers) happily bounce up and down kerbs, drive through potholes or just do not look at any debris, or what is on the road surface, that may damage tyres. Look after your tyres and they should look after you. If you need help, or are unsure, seek specialist help – Make sure your tread depth is at a safe depth.


 

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John Siddle

Communications Manager

From a mechanical engineering background John Siddle has worked for Lincolnshire Police for over 10 years. He is the spokesperson for all road safety issues in Lincolnshire and has spoken at a national level of how road safety and media work together. A qualified Road Safety Practitioner he holds driving licences for HGV, motorcycle as well as car. Having lived and worked in the Far East, Middle East and Central America he has had to drive through jungles and deserts, sometimes navigating by the stars, in his 45 years of driving.

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