Help your tyres keep their cool… even when they are hot

Tyres get hot on long-distance and high speed journeys, and the heat increases the tyre pressure. Would it make sense to reduce the pressure slightly before a long/fast trip, so the extra heat simply restored them to the recommended pressure level? Pima Pumsi

No. Definitely and absolutely no. Lower pressure might partly and briefly “compensate” for heat expansion, but it will also generate even more heat! Even to the extent of causing a blow-out if the tyre casing has a weakness.

Sustained high-speed cruising generates a lot of heat, in five main ways.

One is the heat of the road surface which can readily reach 50 degrees C or more on a sunny day. Too hot to walk on in bare feet. 

Two, there is always some heat-generating friction between the tyre tread and the road surface; even at steady speed in a straight line the wheels are still being “driven” (not just rolling) with enough force to propel a load weighing more than a ton. When the car brakes or accelerates or corners that friction increases – sometimes considerably.

Three, the tyre wall flexes constantly with every rotation and causes friction between the particles of the casing material itself. The more the casing flexes (in degree and/or frequency) the more heat is generated.

At 100 kph, - even on a smooth, straight and flat road - every part of the casing is flexing at least 10 times…per second!  Add braking, accelerating, cornering and perhaps a rough surface and the degree of flex increases, too.

Four, the tread also squirms and twists and scrubs, however slightly, all the time. During changes of pace or direction that action can be a lot more than slight – enough to scrub the tread as though it was being sand-papered.

This effect can also be exacerbated if there is any defect in the castor, camber or alignment angles of the wheels or brake settings…and even defective shock-absorbers which allow the wheel to bounce. 

Five, the brakes generate a huge amount of heat (braking from 100 kph to zero generates enough heat to boil a litre of water) and in severe use conditions they can literally get red hot. 

By conduction and radiation, that can make the wheel rim (which is in constant contact with the air inside the tyre) too hot to touch.

So far from lowering the pressure at the start of a long and rapid journey, you should actually take special care to ensure the pressures are “at least” as high as the recommended level, and even consider increasing (!) it…by between 5 and 10 percent.

That gives the tyre a greater volume of air to absorb and dissipate the heat and reduces the degree of flex at every rotation. The tyre will still get warm, even hot, and the pressure will increase further, but to a lesser degree than it would if it was underinflated at the start.

You can prove that to yourself. Increase the pressure on three tyres by a couple of psi and reduce the pressure of the fourth by a similar amount. 

Drive 20 kms swiftly, then feel the temperature of the tyres. The softer-start tyre will be noticeably hotter than the other three. And remember that heat also softens the tread and makes it out wear out faster.

When making a brief stop on a long journey, it is a good “early warning” system to feel the temperature of all the tyres and their rims.

That they are all a bit hot is no cause for concern; if one is much hotter than the others, either the pressure in that tyre is too low or the brakes might be binding slightly on that hub.

Another indicator to look out for (over a longer time) is any uneven wear on the tyres. Depending on the shape of the wear, it could indicate a consistent pressure error or a defect in the alignment /castor/camber settings or maladjustment of the brakes.

Tyres are a great diagnostic tool; the checks are quick, the symptoms are clear, and finding the fault is a double benefit because fixing is always cheaper (and safer) than suffering the consequences of neglect.

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