Discussion

Many road agencies now assess brake performance by measuring the slow-speed brake torque levels of each axle group on roller-brake testers.

The National Transport Commission has published a National Heavy Vehicle Inspection Manual which proposes the following standard:

Roller brake test results to achieve:

• Less than 30% difference in the brake force on each wheel of an axle.
• (It is not clear whether the percentage is based on the higher or lower value).
• A minimum force level of 3.0 kN/tonne (total brake force / GVM).
• Brake drag / axle is less than 1kN for a driven axle and 0.5kN for a non-driven axle.
• Parking force to be ‘adequate’ or vehicle does not lift out of the rollers when parked.

Note that there is no explicit requirement that the brakes are working at all wheels, although this may be inferred from the maximum 30% difference requirement.

The national proposal is based on the NSW levels. Different levels are applied in Queensland.

The minimum 3kN/tonne when fully laden implies a deceleration capability (assuming that wheels do not lock up) of ~0.3g. The ADRs require that the new vehicle can achieve 0.38g from 100km/h and that it can hold its GVM on an 18o slope. The later requirement implies a parking force of about 1.8kN/tonne.

Roller brake testing is useful for assessing the road worthiness condition of the brake system. There is the difficulty that low-speed brake torque performance may be substantially different from the torque values inferred from high-speed stopping distance performance. Consequently it is unclear whether roller brake testing can be reliably used to verify certification-level performance. Further work is needed to determine whether roller brake testing standards can be reliably related to inertia-dynamometer test torque performance of foundation brakes. If so, this would allow roller brake test performance standards to be tied to certification-level inertia dynamometer test levels.

Experience has shown that side-side differences can occur on vehicles will brakes in apparently good condition. Specifically new vehicles sometimes fail the roller brake test.

Roller brake testing accuracy depends upon the tyres having rolling contact with the driven roller. This is often difficult when the vehicle is lightly laden as the tyres tend to slip under heavy braking. Reliable results only occur when the vehicle is mid or fully laden. A vehicle that fails the guidelines when it is unladen should be retested in the semi-laden condition.

There are roller testers that apply a loading force to the rear of the truck during test. These are not widely used in Australia. The existence of this technology justifies the claim that wheel-slip during testing can produce errors.

Proposal

The following proposal to send to the vehicle standards managers of each of the state road agencies:

That a national roller brake standard and test procedure be adopted by state road agencies.

The recommended actions are:

• Adoption of the roller-brake pass/fail criteria that the NTC has developed.
• Amended procedures so that a vehicle that is defected in the lightly laden condition (that is with less than 60% load compared to GVM) can be represented for test within, say 7 days (in the laden condition). The defect notice should be withdrawn if the vehicle passes in the laden condition.