MOT Testing Tyres: tyre tread depth water diagram

Detailed information on aspects of the MOT Test.

The road safety dangers of worn tyres…

Motor Industry Research Association (MIRA) Tests

One year after the introduction of tyre labelling, Rezulteo, the purchasing guide and tyre comparison website, wanted to find out whether European consumers were more familiar with the system and if their purchasing behaviour had changed.

In 2004, the Motor Industry Research Association carried out work for Continental Tyres into stopping distance and tyre tread depth on a wet road and discovered that when tyres had worn down to below about 3mm of tread depth, stopping distances at 50 mph significantly increased.

For a Ford Focus, at 3mm tread depth the stopping distance was 91 metres compared to 135 metres when the depth was down to just 1.6mm, the current legal minimum. That’s almost half as far again. It should also be noted that in this carefully controlled experiment, the experimenters ground down the tyre evenly across its width for the Test. The MOT ‘pass’ criteria of 1.6mm, on the other hand need only be 75% of the tread width. By that criterion it is likely that the stopping distances in the wet between 3mm tread depth, and 1.6mm tread depth would be even greater.

MOT Testing Tyres: tyre tread depth water diagram

The diagram above shows how tread depth particularly affects braking distance in wet conditions; with the shallower tread depth on worn tyres, standing water on the road easily fills up the grooves in the tyre, and reduces the tyre’s contact with the road. 

MOT Testing Tyres: stopping distance tread depth

As a result, braking distance is significantly increased, as shown for various vehicles and various tread depths above.

The MOT Test – a ‘minimum’ safety check…

What this also highlights is a serious misunderstanding of what the MOT is really all about by a high proportion of motorists. A large number of motorists believe that if their car passes the MOT, their vehicles are perfectly safe to drive into the future – in fact that isn’t necessarily so.

The MOT examination is a safety check, to a very minimal standard of a defined number of so-called ‘testable items’, which of course includes tyres. It is also only applicable ‘at the time of the test’. So if the tyres are worn down to say, 1.7mm on average – within a couple of weeks they would be illegal. Also, those tyres might be ‘safe’ in most driving conditions, they would not be ‘completely safe’, especially in conditions of heavy rainfall.


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