Bridgestone South Africa

Passenger Tyre Inflation

The importance of correct tyre inflation pressurescannot be over-emphasised. The inflation pressure determines the amount of flex in the sidewalls and the carrying capacity of the tyre. Incorrect inflation will encourage poor wear, performance and maneuverability.

Passenger Tyre Inflation - inflation.gif
Pressure errorIncrease in tyre loss
20% Over
20% Under
30% Under
40% Under

You will notice that over inflation, though not as severe in its effect as under-inflation, still causes some loss in tyre life.

Bleeding tyres

When the tread rubber comes into contact with the road surface, the load causes the rubber to become compressed like a spring, and, like a spring, mechanical energy is converted into potential energy. When the rubber has passed through the contact area, the potential energy is released as the rubber returns to its original thickness.

When rubber is deformed in this way, not all of the potential energy is regained on release. Some of the potential energy is converted into heat which is known as the hysteresis loss. Friction and flexing in the sidewalls also cause heat build up in the tyre.

Coupled to the fact that heat is generated from within the tyre, is the fact that the various rubber components that make up the tyre are all poor conductors of heat. It takes a passenger car tyre between 30 minutes and one hour of continuous driving for the generation and dissipation of heat to equalise. It is at this point that the tyre reaches its designed operating temperature.

The thickness of the tread and sidewall rubber dictates the running temperature of the tyre, and in passenger car tyres, this is around 80 degrees C.

As air warms up, it will try to expand, but because it is confined in the carcass of the tyre, it cannot. The heated, expanded air raises the internal pressure by about 15% - 20%, and is catered for in the design.

The danger of not knowing this fact is that "hot" inflation pressure might be reduced. This will mean that the tyre is working at too great a deflection for a hot tyre. The increased deflection will generate even more heat and the tyre may again reach the higher pressure, but it will be operating at a higher temperature. Tyre breakdown is incremental to a temperature increase and a complete breakdown occurs at temperatures in the region of 170 degrees C.

Checking inflation pressure

There are two dangers that need to be considered when checking inflation pressures. The first is the relatively common problem of variance between pressure gauges from one service centre to the next; the best way around this for the motorist is to have their own, reliable, hand-held pressure-gauge.

The second problem to look out for – and which is more likely to occur where no valve caps are used – is when grit is blown into the valve mechanism, causing a slow leak. Always recommend the fitment of airtight metal valve caps!

Bridgestone South Africa

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