This article considers some importance safety aspects of heavy vehicle tyre performance: Standards and Certification.
There is no Australian Design Rule that specifies the tyre and rim ratings that must be used on new commercial vehicles. This is left to vehicle manufacturers.
The in-service requirements for tyres and rims are in the Heavy Vehicle National Law (vehicle standards regulations).
Surprisingly, these regulations do not refer to the Australian Tyre & Rim Association Manual, which is the ‘national manual’ of the tyre-supplier industry. This manual is aligned with overseas technical specifications and manuals.
The in-service regulations do require that the tyre and the rim are suitably rated for the axle gross max rating. The maximum air pressure that can be used is 825 kPa and the speed rating should exceed 100 km/h.
The original manufacturer’s specification is not mentioned in the regulation (HVNL regulations 23 – 26). This creates doubt about whether a tyre or rim change is a modification or not.
The vehicle manufacturer is no longer required by an ADR to provide a tyre placard on a new vehicle, but it may in the truck manual.
The requirement was dropped about a decade ago.
An advantage of a tyre placard was to identify the manufacturer’s tyre size options.
This defines the manufacturer’s specification, which was relevant to defining whether changing a tyre was a modification or not fitting a different make, model or size of tyres is usually done without manufacturer’s approval or AVE oversight because it is not considered to be a modification, but it is.
The maximum Australian inflation pressure of 825 kPa was set based upon tyre-inflation pressure equipment limits in work-safety regulations and does not indicate the maximum safe tyre pressure.
Size: The international (ISO) size designation is best illustrated by example: 295/80R22.5 with load rating 152/148 at 750 kPa, and speed rating = N (140 km/h).
The assignment of the load index and speed ratings can be found on the internet. The 295 designation means tyre width measured just above the rim in mm.
The 80 designation is the aspect ratio, which means the nominal sidewall/tread width at the top. So, the sidewall height is nominally 295mm x 0.8 = 236 mm at correct inflation pressure with rated load.
A ‘low profile’ truck or trailer tyre will have an aspect ratio of 60 – 70. The 11R22.5 ‘imperial’ tyre size in common use has an aspect ratio = 88 per cent.
The rated load with index 152=3,550kg, or if used in a dual tyre set 148=3,150kg. While the individual tyre is rated at the higher level, use in a dual set introduces a risk of rubbing that could heat tyres, so a ‘factor of safety‘ of 1.125 is applied.
Minimum load ratings are set in Tyre & Rim manuals, but tyre manufacturers can determine higher load ratings. If the tyre is regroovable, it will say so on the sidewall.
Rolling Radius: Tyre rolling radius directly affects brake performance. Each wheel-end brake produces torque according to the brake level determined by the driver.
The retardation force applied to the axle is inversely proportional to the tyre radius during braking. So, if a 385/65R22.5 tyre was fitted to a steer axle instead of a 295R80 tyre, the rolling radius would be reduced slightly by about 2.5 per cent and the brake force potential would increase by that amount. Whilst these changes may be minor, the problem is that the manufacturer’s brake certification is altered, and it is a modification. Load Ratings and Inflation
Pressure: A 295/80R22.5, which is commonly used as a steer axle tyre on Australian heavy trucks, is likely to carry about 3,000/kg per tyre (statically).
At that load, the inflation pressure is likely to be about 750 kPa (109psi). If the load on the tyre is reduced to 2,230kg (77 per cent), the appropriate inflation pressure is 500kpa (72.5psi).
The pressure ratio 500/750=67 per cent. This illustrates the significant challenge to set the correct pressure depending upon load. For a trailer that ratio of laden to unladen mass on the axle is likely to be ~ 4 which would require a near quarter of the optimum laden pressure of 656kPa (95psi).
Setting tyre pressure to optimum levels reduces stopping distance of a lightly laden vehicle by about 15 per cent and also reduces side-sliding tendency.
Sidewall Condition: Tyre inflation pressure does not hold up the axle. Air pressure keeps the tyre in a suitably rigid form. That form should be independent of the mass on the tyre, so the air pressure needs to change to keep the form correct.
As the air pressure applies equally in all directions at the cylindrical centre of the wheel rim, no net force results on the rim. It is the side wall that supports the mass on the axle. The bottom sections of the sidewall push up on the rim and the top sidewall pull upwards on the wheel rim.
The axle is lifted via the sidewalls and the lip of the wheel. Consequently, the strength and condition of the sidewalls and wheel rim lips are important. Rigid sidewalls give better road handling. Correct inflation pressure helps.
Rolling Resistance: The force needed to distort the tyre with each turn generates heat in the tyre. The main contribution is from the tread because of road friction, but the sidewalls also absorb energy, particularly when under-inflated.
Low loss tryes are available that have additives, such as silicon, added to the rubber (polymer) tyre material. This can reduce internal heating (which is called hysteresis) during distortion with each rotation. Additives do not alter tread heating.
Over-inflation reduces the size of the contact patch and the amount of sidewall distortion, which reduces losses. However, there is a road safety penalty with over-inflation as discussed above.
Another Australian consideration is direct heating of the tyre via the contact patch when the road is hot.
The hotter the tyre rubber, the more compliant and the more internal heating will occur with each turn. Over-inflation reduces this heating.
Total tyre power loss tends to be proportional to vehicle mass, so, because long-distance Australian trucks tend to be heavy, the tyre loses are substantial and probably exceed the aerodynamic power loss.
Road Friction: The peak road friction between a truck tyre and a road is avaialble when the wheel slip is 15-20 per cent. This is illustrated in the figure.
During wheel slip the tyre is being distorted at and around the contact patch. The condition of the road surface, including water on the surface significantly changes the available road friction.
An Antilock (ABS) brake, is likely to intervene when the wheel slip is about 10 per cent or more.
The purpose is to prevent the wheel locking up so that the maximum braking and road handling forces are achieved, albeit with ABS cycling delay.
Truck tyres running on sealed bituman roads probably have a peak friction level of about 0.8, but this depends on the road and whether the tyre is correctly inflated and the tread is good.
Rainwater probably reduces the peak friction level to about 0.4, half the dry value. A loose surface and corrugations reduce the friction even further.
The peak emergency braking deceleration that can be achieved is limited by the peak tyre-road friction. As a guide achievable deceleration (in g terms) is a bit greater than half of peak friction because braking effort is never perfectly balanced between all tyres and some friction is wasted.
That is, the maximum practical emergency braking level of a (combination) heavy vehicle on a good road is probably about 0.5g. From 100 km/h the best case stopping distance is 77m.
In the wet it is about 152m. Drivers should double these estimates to provide a safety margin. The ‘simple’ tyre is a crucial safety and efficiency component!
Peter Hart
ARTSA-I Chair
