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Tyre Tips - Light Commercial Vans, Trucks and Buses

Tyre Tip 4 - Inflation Pressures

Have you ever thought of what you are asking of those two new tyres you just put on the front of your truck?


So you'd like to see them deliver, say, around 120,000 kilometers on the steer, then maybe run them out on the trailer with a couple of caps for another modest 80,000 kilometers each. No worries mate!!


But just think what these tyre have to endure to achieve these expectations before you throw them away, because make no bones about it, truck tyres will eventually die through fatigue. Would you believe that the average 11R22.5 steer tyre will complete around 38 million rotations during those above first life 120,000 kilometers? Then two caps on the trailer position will contribute another 160,000 kilometers or 50 million revolutions to the total. Thus a massive 88 million times each part of the tyre has to hit the road, deflect under load through the footprint, and resume its normal diameter out the other side!


Now consider the performance of your deep tread drive tyres. 175,000 kilometers sound reasonable? Work on approx. 50 million revolutions in its first life alone. Why not give the tyre a break when recapping, and request a shallow cap on it as it continues to spin it's way to a huge number of flex reversals. You could do yourself a favour and place the tyre in alternative applications, i.e. urban work, as it approaches it's hard to predict end of working life. That way you won't get caught somewhere between Sydney and Melbourne wondering why this goddam tyre has failed --- and after only three caps, too!

This end of working life fatigue condition is hard to predict since it depends on a number of factors, not only on rotations. The state of the road, heat, inflation, all contribute. Ever wonder why a jumbo airliner tyre can accommodate up to ten retreads? Because the tyre casing does very little work. These airliners do comparatively few kilometers on the ground, but obviously scrub the tread rather prodigiously each time they land. So the tyre casing completes relatively few revolutions and can handle a multiple number of retreads. Surprisingly, the taxiing of the aircraft with it's attendent heat build-up has more affect on the tyre casing than the actual landing.


Tyre designers try to minimise the flex, or deflection, of a truck tyre to achieve a high fatigue capability. Obviously there is a compromise here, since ride and comfort factors, impact damage, etc., need to be taken into account. They calculate around this deflection figure to achieve the ideal contact patch and obtain optimum treadwear, braking and handling performance. This is where inflation pressures become important to maintain the tyre deflection within these design parameters.


Just remind yourself that it is not the tyre that carries the load, but the air inside it. Remember, a tyre is basically a flexible air container, and without air it cannot hold it's shape let alone carry any load. So it is imperative that the correct inflation pressure is in the tyre to match the load it is carrying. Too little pressure will lead to over deflection of the tyre, which makes the tyre run hotter and may cause premature tyre failure. The service life of the tyre is shortened roughly in proportion to the reduction in inflation pressure, (Rule of thumb --- a tyre reduced by 20 percent below the recommended inflation figure equals 20 percent reduction in service life). Higher rolling resistance is generated with under-inflated tyres, which leads to increased fuel consumption. Lastly, irregular tread wear can be produced by reduced pressures.


Over inflation of tyres can also lead to difficulties such as increased impact damage potential, as well as irregular treadwear.


Of course, other factors may come into play in choosing your ideal inflation pressures. Factors such as stability, wear characteristics, operational conditions, etc. We recommend that once these ideal inflation pressures are established, high mileage fleets should be checked regularly each fortnight or so with a reliable tyre gauge (not with a boot!). Missing valve caps and leaky valves should be replaced immediately. And if you can't reach the inside dual valve to check it, don't ignore it, organise some valve extensions!