Tyres are the most important safety aspect of the car because the tyres are the only thing keeping your car in contact with the road; because of this the law can be strict in terms of penalties.  In theory the fine for the driver can be up to £2,500 per tyre and 3 penalty points per tyre, although commonly a fixed penalty of £60 per tyre is given although it depends on the circumstances, this includes the spare which should also be legal.  Remember it is the drivers’ responsibility even if the car is not theirs e.g. a company vehicle.

front-wheel-stafford

The three functions the tyres provide are linked to the friction; braking, accelerating and steering, all of which can be jeopardised without frequent checks. 

The Tread 

These are the grooves cut into the tyre, technically they reduce the contact the tyre has with the road in dry conditions; a slick tyre has the most friction between the road and tyre.  However in wet conditions these grooves are critical in order to displace the water away from the tyre so the tyre is touching the road, not floating on the surface of the water – this is called aquaplaning, very similar to icy conditions.

The depth of this is critical in how effective the tyres cut through water.  The legal minimum limit for a car or light van is 1.6mm in the central three quarters of the tyre and around the whole tyre; this is really easy to check. 

tyre-tread-depth

Look at the picture, the arrow is pointing at a built in tread depth indicator, it is set at 1.6mm if your tyre wears down to this point it would be at 1.6mm.

The tread depth significantly effects tyre performance particularly on braking the lower it is, generally it is recommended to change the tyre when the get to 3mm or before.  There are issues with environmental impact and some say that can be inefficient, and to use the tyre until about 2mm or even the 1.6mm limit.  Of course this is up to the individual however this can put you are higher risk of skidding on the road, which can have far worse affects.

It is also worth noting the edge of the tyre should have visible depth in order that the tyre effectively displaces water too.

 Tyre Pressures

Tyre pressure refers to the pressure of air inside the tyre there are some significant issues to over and under inflation.  The specified pressures are written in the owners handbook, on the A pillar or in the fuel filler cap. 

 Over Inflation

Where the tyre is inflated more than the specified pressure will reduce the surface area of the tyre touching the road because it turns it into a balloon shape.  Only the centre part of the tyre is in contact with the road; this has serious implications to grip increasing the possibility of skidding.  It also makes the ride very heavy as the ride is not softened by the air as much as it should.  It makes the tyre wear quicker in the centre than the outside, meaning it is likely to need replacing sooner. 

The benefit of over inflation could be increased fuel economy, but this is not worth the loss in grip and uneven wear on the tyre.  Tyre pressures should be checked weekly with a reliable pressure gauge when the tyres are cold, hot tyres would give a false reading, as pressures change with temperature.

Under Inflation

Where the tyre is inflated less than the specified pressure can increase the surface area of the tyre touching the road because it turns it into a wider spread out shape.  The edges of the tyre are in contact with the road more than the centre; this has serious implications to handling and increasing the possibility of the tyre coming off the rim.  It also makes the ride very soft as the ride is not softened by the air as much as it should.  It makes the tyre wear on the outside quicker than the inside, meaning it is likely to need replacing sooner. 

The benefit of under inflation could be increased grip in very icy or snowy conditions, but this is not worth the potential loss of control, and it can use a lot more extra fuel as drag in increased.

Condition of the tyre

Rubber determinates significantly over time; tyres should be replaced every few years.  Rubber which has deteriorated can cause loss of pressure, and can reduce grip with the road surface due to the rubber loosing its properties.   Cuts causes by hitting the kerb, normal knocks, wear and tear significantly changes the tyres shape and effectiveness. 

Valve caps should be replaced to prevent dust from getting in, and air from getting out.

Remember to check the spare regularly. 

Changing a tyre

Changing a tyre by the road side is a risky business, for example your life expectancy on the hard shoulder of the motorway is about two and a half minutes!  Breakdown services should be called for tyre changes by the roadside; it is very risky to try this yourself.  Wheels are tyres are also heavy and can pull your back, if you fitted your tyre incorrectly you could also cause further danger.

To ensure your tyres are kept legal and safe

  • Visually check the tread depth and for punctures daily or every journey
  • Check tyre pressure weekly/every fill up with a reliable pressure gauge (most petrol stations have them) remember to check your spare
  • Check your spare at least once a month
  • Change your tyres at 3mm or sooner
  • Ensure you have breakdown cover for punctures by the road side.