There is a lot to be gained in a trucking operation, if we spend a little time thinking about tyre pressure and getting it right. The results can be improved fuel use, lower tyre cost and fewer unschedules delays.
Checking the Pressure
Just checking all of the tyre pressures on something like a B-double set can be quite a task. Not only are there all of those tyres, but many of them can also be hard to get at, especially the inner tyres on dual tyre axles. It is often a chore which a driver knows they have to do, but quite often feel they don’t have enough time to get the job done properly and don’t do it themselves. If the tyre pressures aren’t checked regularly, the operator can be certain there will be disparity in pressures across the vehicle.
How do you know the what the right pressure is?
Some tyre users can find the correct tyre pressure for their vehicle using the 4psi rule. This entails setting the tyre pressure at a point you think it should be set at and then running the vehicle for 100 km. The pressure needs to be checked again and if the tyre pressure reading is more than 4 psi above the original pressure the starting tyre pressure was too low. If the tyre pressure reading is less than 4 psi above the original tyre pressure, the original tyre pressure was too high. If the tyre increases in pressure by 4 psi, the pressure was just right. Running this experiment a few times should enable you to get the right pressure for your vehicle. If the truck is running on dirt roads, then the rule becomes the 6 psi rule.
The Big Wheels website comes up with a choice of two methods.
Calculate the load that the tyre is required to carry and then refer to the Tyre and Rim Association Manual, which will give you the correct pressure for that particular tyre.
For example, 295 80r 22.5 Steer Tyre. Maximum weights over Steer Axle = 6000 kg’s. For instance: 3000 kg’s per tyre. The pressure required is 750 KPA or 109 PSI.
Static Laden Radius. Each tyre manufacturer will specify this dimension. In relation to a steer tyre the manufacturer will specify, e.g.497mm.
The measurement is taken from the road surface to the centre of the Stub Axle when the tyre is inflated, to give us as per the example 497mm. We now have the correct pressure in the tyre.
The cost of over inflation
A report by the Logging Industry Research Association of New Zealand looked into the correct pressures to run tyres on their trucks. The study came across tyres being run at pressure between 95 and 110 psi. The hypothesis made in the report was that tyres could be run at much lower pressures to good effect.
Running through a case study of an operator running a semi over a 100 km route and the effect of changing tyre pressure from 100 psi to 80 psi. Tyre life was found by LIRA to be 60,000 km at 80 psi, but 55000 km at 100 psi. When compared to the amount of work done by the tyres, the equation came down to show the truck running at 80 psi would cost $0.55 per tonne km but the actual of cost running at 100 PSI would be $0.60 per tonne km.
Over Inflation issues
In a report by Safe Work Australia into work related fatalities involving trucks in Australia from 2003 to 2012, the report found nine per cent of the people killed while servicing a vehicle were killed in tyre incidents, in most cases when the sidewall of the tyre exploded. The actual number of people killed was five.
Tyre Wear from Mismatched Pressures
It is important not overlook the effect that mismatched pressures in dual tyres have on tread wear. An inflation mismatch of greater than 5 psi will result in two tyres on a dual wheel axle being significantly different in circumference. A 5 psi difference creates an eight mm difference in tyre circumference. Over 160,000 km this eight mm difference causes the smaller tyre to be scuffed for 393 km.
Because they are bolted together, duals have to cover the same amount of road in a single revolution. As a result of the larger diameter tyre dragging the smaller one with less inflation pressure, both tyres can develop fast or irregular wear; usually the smaller one develops the worst wear.