Tech Talk

Tyre traction, wear, handling and pressures

Transport is an amazing business. We invest millions of dollars, work super long hours, put up with other drivers that should never have gotten a license, suffer from over-zealous regulators, and then are ripped off with high rego bills, stamp tax, tolls and fuel tax. Sometimes I wonder why we do it?

In an increasingly complicated world, we are forced into GPS tracking, electronic work diaries, speed cameras and seriously complicated trucks. The regulations continue to become more complicated. And then COR and other workplace safety issues mean more paperwork, more risk and more fines.

Yet where is the education system for anyone to improve our knowledge, performance and skills? No one teaches a full course. Getting a truck license is a joke. Where can young ones learn the skills? Where can us older ones learn the latest requirements and knowledge that COR demands?

For instance, look at the humble heavy truck tyre. What an amazing amount of engineering, but where do we learn about tyre care?

For decades we have inflated our truck tyres to 100 psi.  Some use 110 psi.  Is this correct for every tyre and every load? How do we know what is right? Where is the information to guide us toward best practice?

Do we trust our tyre fitter? What education has he had? Did he do a tyre pressure course at some university or with one of the tyre manufacturers? No, of course not. What is his motivation? Let’s face it, he wants to sell tyres, fit them and send you on your way, with minimal risk to him. If he puts more air pressure in the tyre than it needs, this is an insurance policy for him. If the tyre is over-inflated by 20 psi, it can lose a couple of psi per week, with no recourse. Same with tyre manufacturers. Why bother preaching best practice tyre management practices when no one listens. And, if the tyre wears out a little faster than it should, they sell more tyres anyway.

When I started AIR CTI some 22 years ago, Michelin engineers were very helpful, both in discussions and providing some fabulous information. My old ‘Michelin Truck and Bus Technical Data Book, Australia and New Zealand’ has a lot of information about matching tyre pressures to the load on the tyre. A similar age Michelin book for North American truckies had similar information. They state categorically that the tyre pressure must be adjusted to suit the load on the tyre for best tyre life and performance.   

Unfortunately, this information is left out of more modern technical books.

Load to Inflation tables are available from most tyre manufactures on Google. Simply Google ‘heavy truck load to inflation tables’, then find your tyre size. (There will be two choices: single or dual. Single is a single tyre like on a lazy axle, or a steer tyre. Duals are duals, like your drive tyres.) Then work across the correct line to find your axle load. Look up the chart to find the correct tyre pressure for that load and your tyre size. Run that tyre pressure until you change the load.

The benefits are:   

• More tyre grip which improves braking, steering, handling, tracking and safety.

• Longer tyre life which reduces costs, reduces downtime, saves money and lets your truck earn more money.

• Almost all uneven tyre wear is eliminated.

• Better ride because your tyre is now absorbing bumps and road roughness, instead of amplifying the bump.   

• Better ride, lower vibration and better traction reduces driver stress, both mentally and physically, keeping them healthier, happier and safer.

• By absorbing bumps, the suspension, axles and truck lasts longer, with reduced maintenance.

• Safer means less accidents.

Yes, I know, there will be a lot of tyre fitters and truck owners that will say this is bull dust. The science is totally solid. Michelin invented radial truck tyres. They know their game.  Every tyre manufacturer provides load to inflation tables. Every tyre and rim association provides load to inflation tables.

And common sense will prove it to you.

It is the rubber that grips the road that works. That contact patch or footprint is where all of the engine horsepower is transmitted to the road. It is where all the braking happens. It is where all the steering and cornering happens. That footprint size and shape is extremely important. For an 11R22.5 tyre, that footprint should be 430 square centimetres, or 67 square inches in my old language. That tyre will deflect around 33mm. The length of that ideal footprint is around 230mm, and 185 wide.

The tyre engineers go to a lot of work to tune that footprint to get an even distribution of load across the entire footprint. If that footprint changes, the load distribution changes, with high pressure in some areas, and lower pressures in other areas. This incorrect footprint will squirm and wear.

The only way to get this ideal footprint is to adjust the tyre pressure to match the load. If the tyre pressure is too high for the load, the outer part of the tyre will lift off the road, concentrating the load in the middle of the footprint. This will increase tyre wear on the outer portion of the tyre, often scalloping or rib punching. This tyre will ride rough, take longer to stop, and hammer your driver, your truck and the road into an early grave.

If the pressure is too low for the load, this will increase the load on the outer areas of the tyre, with reduced pressure toward the centre. The tyre sidewalls will flex more than they should, potentially overheating, which can lead to blowouts. The tyre will brake well, but handling suffers. Tyre wear increases and casing life is often ruined.

To add proof of the correct tyre pressures, the TERNZ PBS review completed last year for the NHVR recommended 120 psi for steer tyres, 75 psi for tandem drives, and 55 psi for tri axle trailers loaded to the maximum legal loads running 11R22.5 tyres.

Aussie transport is wasting a lot of lives, money, and time while increasing costs, downtime, and risk.

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