Tech Talk

How the rubber meets the road affects every aspect

The way that rubber meets and reacts with the road affects every component and every aspect of your truck’s performance and operation.

The footprint or contact patch affects braking, handling, ride quality, truck wear and tear, driver health, road health and our environment.

The correct footprint is essential for safety, profits, driver health, truck reliability, down time, breakdowns and your business.

Unfortunately, most people don’t understand the ramifications of incorrect tyre pressures, or even what the correct tyre pressures are. Most tyre suppliers are in the same boat, as are most truck people.

What is the correct footprint and what controls that footprint?

The correct footprint size for an 11R22.5 tyre is 240mm long, 184 mm wide. The correct area is 427 square centimetres, or 66 square inches. The tyre should deform and settle onto the road 37mm (1.46”), spreading the load over the full footprint as equally as possible.

If the load on the tyre changes, the footprint will change, unless the tyre pressure is changed to match. Load to inflation tables are available from tyre manufacturers, tyre associations and online. Load to inflation tables provide the correct pressures for each tyre load to ensure a full and correct tyre footprint, optimising performance. Unfortunately, this does not happen.

This real and scientific information is not taught and is not promoted. This poor distribution of vital knowledge even effects many major tyre reps. We were hauled over the coals by one of the major tyre companies because of our recommendations even though our information came from them.

Our government transport regulators don’t understand the damage done by incorrect tyre inflation. If they did, our world would be a lot safer, greener and more profitable.

Everyone knows that under-inflation is bad. An under-inflated tyre running on the highway can overheat from too much sidewall flexing. The heat build-up weakens the sidewall, and something will break, blowing the tyre and potentially causing an accident.

Over inflation reduces the tyre footprint. As an example, an empty tri axle semi tyre at 100 psi will have a tiny footprint around the size of a business card. That tiny amount of rubber in contact with the road will not have much grip. The typical Aussie tandem drive tyre is 35 per cent over-inflated at maximum legal load. The footprint will be 35 per cent smaller, with less traction, less braking and less directional control. Most steer tyres at 100 psi are 20 per cent under-inflated at a 6 tonne axle load.

Both over-inflation and under-inflation increase tyre wear. A 20 per cent over-inflated drive tyre on Aussie roads will wear out 22 per cent faster. Optimising the tyre pressure to the load improves tyre life by at least 30 per cent. At the same time, both under and over-inflation reduces the casing life, increasing costs again. An over-inflated tyre is the main cause of uneven tyre wear and gets 40 per cent more punctures and cuts.

Over-inflated tyres bounce off bumps and road roughness instead of absorbing the impact within the 37mm of correct deflection. Over-inflated tyres halve the life of your springs, suspension and bushes, and significantly increase fatigue loads leading to cracked axle housings, broken springs, blown shocks and breakdowns.

Wheel bearing life is severely shortened by over-inflation and by uneven tyre pressures in dual tyres.  When one tyre carries more load, the wheel bearing load is offset, causing one bearing to be overloaded, severely shortening bearing life. Of course, uneven tyre pressures also wreak havoc with tyre wear.   

Over-inflated tyres act like a rigid wheel, significantly increasing drive train loads. A correctly inflated tyre acts like a crankshaft damper, absorbing loads and vibrations, doubling diff and transmission life. Each explosion in the engine sends a shockwave throughout the drive train. Over-inflated tyres bounce that shockwave back to the engine, affecting every component. Again, a correctly inflated tyre absorbs that shock wave, eliminating wear and tear and breakages.

Over-inflated tyres double vibration levels in the entire truck affecting every component, from the load to the radiator and intercooler. Correct drive tyre pressures reduce suspension seat wear and tear by 65 per cent. Vibration kills your truck and your driver.

Whole body vibration is a proven cause of a lot of health issues with truck drivers. The continuous high vibration levels damage the ocular system (eyes), the skeletal system (back), the stomach and digestion system and the circulatory system. Drivers have the shortest life expectancy of any trade or profession.

Outside your truck, road damage is directly proportional to the tyre pressure. The higher the pressure, the more road damage. We all know that a new pot hole will develop a few metres from an existing pot hole. Our tyres simply bounce off the first and hammer the road. Even corrugations are caused by over-inflated tyres. Correctly inflated tyres iron out corrugations.

Our environment is overloaded with dead tyres. Recycling tyres is costly and difficult, as we all know when it costs so much just to get rid of a tyre. A lot of energy, minerals and material go into manufacturing tyres. Tyres are transported to us from other countries adding to the environmental damage. The CO2 waste is huge. Running the correct tyre pressures for the load is good business, and good for our world.

In fact, running the correct tyre pressure will extend the reliable working life of any truck by at least 40 per cent. That saves a lot of money, pollution and waste.

If we as a country and an industry are serious about our financial future and our countries future, correct tyre pressure is simply the smart way to work.   

The equipment to optimise tyre pressures on your truck pays for itself, usually in the first year.  The safety improvements via Chain of Responsibility really mean this equipment is essential. The improved ride, reduced driver stress and easier driving mean your staff will drive for you for more years, happier and healthier.

About the author:

Chet Cline is the owner of Air CTI, an industry-leading tyre inflation system that helps maximise vehicle and tyre life.

For more information, contact

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