Tyres

Tyre pressures

The correct tyre pressure is important for two reasons:

Safety - Can be reduced due to the higher risk of over/understeer, increased braking distances and higher risk of aquaplaning

Cost reduction - tyres with incorrect pressure will have a reduced life span and increase fuel consumption

Checking tyre pressure

Bluebellanycar recommends checking tyre pressures (including the spare tyre) monthly and before any long trip. Tyres should be checked regularly as they are susceptible to drops in pressure which can be caused by:

• natural leakage of air through the walls of the tyre
• drops in ambient temperature
• slow puncture

Tyre pressures should be checked cold (tyres not having run for at least 2 hours, or having run for less than 2 miles at low speed). If tyre pressures are checked hot, add 4 to 5 psi (0.3 bar) to the recommended pressures. Correct the tyre pressures if they no longer correspond to the pressures recommended by vehicle manufacturer.

Information on the recommended tyre pressures can be found in the vehicle documentation, and often on a sticker fixed to the vehicle, for example on the door jamb or on the fuel filler cap.

In case of unusual pressure loss, have the internal and external condition of the tyre checked, as well as the condition of the wheel and valve.

Low pressure affects the balance of the vehicle

A reduction in pressure on the front axle will tend to increase the chances of understeer.

A reduction in pressure on the rear axle with tend to increase the chances of oversteer.

Low pressure and the effect on aquaplaning

Michelin conducted tests to see how much of the tyre is in contact with the ground when it was progressively deflated by driving a car through a fluorescent dye on a toughened glass window and photographed the results. As you can see below, the lower the pressure, the less the tyre is in contact with the road. This will dramatically increase the chances of aquaplaning in wet weather.

If the tyre with correct pressures has a nominal surface contact are of 100% then the progressive reduction in contact can be seen.

Low pressure and the effect on braking distances

Braking distance in the wet increases by up to 11m; that's more than the width of 2 pedestrian crossings and could easily be the difference between hitting something and not.

Low pressure and the risk of rapid deflation

Prolonged running at reduced pressure causes a build up of excess heat in the tyre, and in exceptional cases, can cause the tyre to fail and cause a rapid deflation.

Low pressure and the effect on tyre life

A tyre that is under inflated by 20% will have a reduction in life of just over 20% (depending on use). This could mean changing the tyres up to 5,000 miles sooner than normal.

Conversely, over-inflation will also increase the incidences of abnormal wear and these will be particularly accentuated in the middle of the tyre.

Low pressure and the effect on your fuel consumption

A set of under-inflated tyres will have the effect of causing the engine to work harder and this will increase your fuel consumption or decrease your fuel efficiency. A set of tyres that are 10psi under-inflated will have the same effect as increasing the cost of fuel by 4p per litre (based on £1.37 per litre). This could lead to needing an extra tank of fuel per year.
bluebellanycar recommend that you check tyre pressures weekly, or at the very least, monthly.


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