Tech Talk

Can we increase axle load if we work smarter?


If transport matched tyre pressures to the tyre load, heavy truck tyres would be much more ‘road friendly’. Road damage is directly affected by the unit pressure on the road.  NHVR PBS expert John dePont recently explained that the shearing damage that occurs when turning with multiple axles does not increase with load, but with the unit pressure, which is controlled by the tyre air pressure. 

Therefore, if we matched our tyre pressures to the load, the air pressures in multiple axles would be much lower, spreading the load over a wider area, significantly reducing road damage. Read further to understand the optimal tyre pressures as recommended by dePont, and all tyre manufacturers and tyre and rim associations.

Matching tyre pressures to the load significantly reduces tyre bounce. Over-inflated tyres, as is normal on all multiple axle tyres, bounce violently off every bump, amplifying the impact on our roads.

Road friendly suspensions are tested by ‘drop’ testing an axle to see how many times it bounces. Tyres with the optimum pressure for the load hardly bounce at all.

Therefore, if tyre pressures were optimised for the load, operators should be allowed an increase in maximum loads. I suggest that the load should be increased 250kg per tyre. This would increase a tandem axle maximum legal load to 18.5 tonne, and a tri group to 23 tonnes.  In fact, with air suspension, tri axle group loads should be 25 tonne, as road damage will still be less than our current old fashioned and wasteful practice of 100 psi in all tyres.

Central Tyre Inflation (CTI) is the only practical means to adjust tyre pressures easily and quickly. And AIR CTI is the only Australian made CTI manufacturer in Australia, with a proven reliability record of over 20 years.

Hence, trucks and trailers running AIR CTI should be allowed to increase axle loads. 

When our government realises the increased efficiencies of this proposal, the new recommended pressures would still be less than what we use now. For instance, tandem axle tyre pressures would increase from 75 psi to 79 psi, and tri axle trailer pressures would increase from 55 psi to 70 psi. 

The road damage will be far less at 70 or 79 psi than the damage we do right now at 100 psi. de Pont understands this totally. 

The following provides further understanding of the importance of matching tyre pressures to the load.

Why do we run 100 psi in all of our heavy truck tyres? Surely, a heavily loaded tyre needs more air pressure than a lightly loaded tyre. It just makes sense. But that is not what we do.

Steer tyres on modern trucks support at least 3000kg each. Yet, tri axle trailer tyres only support 1666kg. And an empty trailer tyre may only support 500kg, about the same as your car’s tyres support. It just doesn’t make sense to put 100 psi in every one of them.

What about the tyre footprint, or the contact patch where it rests on the road? Look at this spot on a steer tyre. Do you notice a long flat area where it meets the road? Often this flat area is over 250mm long. Then look at some drive and trailer tyres. Many have no flat spot.  In fact, some of the tread isn’t even touching the road.

The theoretical footprint can be calculated easily. Simply divide the weight on the tyre by the tyre pressure. For instance, 3000kg divided by 7 bar (metric equivalent of 100 psi) = 428 square centimetres. That is a footprint size of 185mm x 231mm, or about the size of the print on a standard A4 page.

A tandem drive tyre footprint at maximum load is 2062 divided by 7 = 294 square cm.

A tri axle tyre’s footprint at maximum load is 1666 divided by 7 = 238 square cm.

What about that empty flat top trailer’s footprint? 500 divided by 7 = 71 square cm. That is a footprint of 84 mm x 84 mm, or slightly bigger than a credit card. No wonder empty semi’s jack knife 2.5 times more.

This doesn’t make sense? What is the ideal footprint? Michelin says an 11R22.5 ideal footprint is 375 square cm. “Any other pressure will reduce tread life and reduce tyre performance.”

So, tyre pressures must match the load if money and performance are important.

In fact, this is why all tyre manufacturers produce Load to Inflation tables.

If you want the best value and performance out of your tyres, the pressure must match the load to ensure the correct tyre footprint.

By applying AIR CTI on all tyres of a standard Aussie semi trailer rig running interstate, it would save over $290,000 over the life of that truck.

The road and infrastructure damage would be halved, and the environmental footprint would be reduced significantly. At least one third less tyres would be used. Less cost, less waste, safer and greener.

Help me improve Australia’s international competitiveness, and our world. The maximum legal loads should be increased if we work smarter. Contact Chet Cline at to discuss any of the above.

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