I am always surprised when speaking with people about the benefits of using tyre pressure monitoring systems (TPMS). It seems that many people don’t consider the costs of operating tyres on commercial fleets and there are probably more who don’t consider their own safety as being a thing to value.
One fleet operator was down on using TPMS. His freight was children, a school bus operator. He was transporting everyone’s most valuable asset, their children. He arrived at the depot in his car, which was fitted with TPMS yet he was more than happy to send his precious cargo out on buses without any way of knowing if a tyre was deflating. Not only was he jeopardising the safety of his charges but also he was costing the operation a bucket of money. How so many ask?
When a tyre is rolling down the road, it consumes energy. On a vehicle this energy is provided by the fuel. The degree of resistance to rolling is caused by many factors, the design of the tread pattern, the casing of structural members of the tyre, the road surface itself and the largest factor of them all is the inflation pressure of the tyre. Remember getting your pushbike home when it had a flat tyre? It was a lot of work wasn’t it? The road surface hadn’t changed only the inflation pressure of the tyre.
When a tyre supports a load on a vehicle it deforms under load. There is a technical term known as hysteresis, this explains the way a block of rubber is deformed and the energy it consumes. The higher the load the higher the deformation, it’s that simple. With an increase in vehicle speed comes an increase in the frequency of the deformation but there is a catch. The faster the vehicle travels the greater the deformation caused by the load. A small bump in the road at slow speed can be a large bump at high speed. So the energy consumption due to hysteresis is exponential. A technical way of saying the faster we go the more energy we consume, a lot more than we would travelling slowly.
Using TPMS we observe the pressure increase in a tyre in operation. A tyre moving about the suburbs travelling slowly, not carrying a lot of load barely increases in pressure. The same vehicle travelling at highway speed shows, with properly inflated tyres, we observe an increase of up to 15% over the cold setting. Put some load into the vehicle and suddenly the tyres are over working, the operating pressures are heading north of 15% indicating the tyre is over working.
A tyre supports the load imposed on it with the air contained inside. The higher the load the more air required. Remember the hysteresis? The faster the vehicle travels equals the more air the tyre requires, so when you’re driving around the city with a light load your tyres aren’t working. As soon as you add more load and start going faster your tyres require more air to support the load and the speed.
Using a TPMS unit a commercial driver can tell when the tyres are over working, just the same as someone who hooks up a caravan for a holiday onto the car they normally use around town can.
A tyre is formed using a process called vulcanisation. This is a one‐way process that combines chemicals and raw rubber to form what we know as a tyre. This is a one‐way process, it’s not like freezing water where when it defrosts it turns to water again. Making a tyre is like baking a cake, when it’s made that’s it. It can’t be unmade.
When a tyre reacts to work via hysteresis it generates heat. It generates a lot more heat than just sitting in the sun, you can’t tell how hot a tyre is by feeling it generally. The heat is inside the tyre structure. By the time you can smell it and think “gees that tyre is hot” the damage is done, the cake is burnt. By this time the inflation pressure will have increased probably well beyond 25% above the cold pressure, far too much. Tyre manufacturers suggest that for road use pressures should be in the 10% ‐ 15% range. Any higher than that the tyre is over working. In specialist tyres like motorsport and the giant haul truck tyres the pressures do go higher but those tyres are working a lot harder than normal road tyres.
Using a quality TPMS you should be able to set the over pressure alert level. I prefer to set mine at 19% above cold, yes there are times when I work the tyres hard, I throw 160 kgs of chook food in the back of the ute and I know this is worth 2 – 3 psi on the run home. If I get an alert I know the tyres are already in danger of overheating. The easiest way to cool the tyres is to slow down, remember that exponential increase? It works the same in reverse too, just 10 km/hr will make a lot of difference to the work a tyre does at highway speed.
Under pressure is where damage is done quickly. If I ask you to carry a heavy load you can walk well enough, but if I ask you to start running then you will perish quickly, a tyre is no different. So how does this all relate to the school bus operator?
By not understanding how his tyres are performing he is not only using a lot more fuel (remember pushing the bike home?) but he is wearing his tyres out a lot faster, there is a double loss which compounds. He has to change his tyres more often, can’t sell the casing or use them for retreading but worst of all he is endangering his cargo.
If a tyre is underinflated and it fails catastrophically, (yes like the road gators you see on the roadside) then there is a potential loss of control that could result in, well, lots of parents worried about the safety of their children. Not a place I ever want to go.
“But that won’t happen to me” is the standard statement, oh I see so you can tell what your tyres are doing when you’re travelling down the highway with the caravan hooked up behind can you? Can the truckie can see what the inside tyres on his trailers are doing in his mirrors when travelling at 100 km/hr?
Use a high quality TPMS, your tyres depend on you to take care of them so they can take care of you. When you consider there is a small patch of rubber about the size of your hand that is responsible for your safety why wouldn’t you want to look after it the best you can?
Adam Gosling heads up TyreSafe Australia; providing management and tyre safety guidance to private operators as well as fleet owners across Australia helping them to maximise safety and increase their return on the investment made in their tyres. Tyres are not a grudge purchase they keep you safe!