Brakes are arguably the single most critical safety item on a heavy vehicle, but often too little thought is given to braking performance, and brakes can be overlooked during routine maintenance. This is becoming a concern now more than ever to heavy vehicle safety as productivity increases and higher payloads place increased demands on braking systems.
The Australian Vehicle Standards Rules (1999) set out safety standards for in-service vehicles and combinations. Part 5 of Rule 128 sets out the average deceleration requirement for a vehicle or combination with a gross mass of 2.5 tonnes or more. The rule requires that the braking system of the vehicle must achieve an average deceleration of at least 2.8 m/s/s when the service brake (i.e. foot pedal) is applied.
A heavy vehicle travelling at 85 km/h needs to be able to come to a complete stop within 100 metres to satisfy this requirement. The performance of a properly-maintained heavy vehicle would satisfy this requirement; however vehicles with brake system defects, or with poorly-maintained braking systems, may not.
Joint efforts between VicRoads, WorkSafe and Victoria Police in recent years (e.g. Operation Hazard, Operation Trishula) have highlighted the alarming number of heavy vehicles in operation with defects in critical safety systems, including the brakes.
“Joint efforts…have highlighted the alarming number of heavy vehicles in operation with defects in critical safety systems, including the brakes.”
Common brake system defects include worn linings and drums, and over-stroked brake booster rods. As brake linings wear out, the linkages that apply the brakes are required to move further each time. The further the linkages have to move, the further away from the ideal geometry they depart. This affects not only the maximum force that can be applied by the brake boosters (due to the increased stroke), but also the resultant force by the linings on the drums (due to the non-orthogonal angle between the brake booster and the slack adjuster).
Ratchet-type ‘automatic’ slack adjusters continually operate to correct the brake system linkage geometry and ensure that maximum braking force can always be achieved. However, not all heavy vehicles are fitted with automatic slack adjusters, which means that regular inspection and maintenance are of critical importance. I’ve personally been involved in several crash investigations which have highlighted this issue.
Putting braking system defects into real world terms, a heavy vehicle that meets the in-service standard and travels at 85 km/h will require 100 metres to come to a complete stop. Add the commonly-accepted driver perception-reaction time of 2 seconds, and this increases the stopping distance to 147 metres (a total stopping time of over 10 seconds). A defective braking system could increase stopping distance and stopping time significantly above these levels.
“…this increases the stopping distance to 147 metres (a total stopping time of over 10 seconds).”
Proper selection of vehicle braking systems with the highest-level of technology available, backed up by a sensible service schedule which is diligently followed, a system for conducting in-service tests and fault reporting, and proper care and attention given by drivers and operators should be considered as the minimum requirements for any modern transport company.