Why do truck brakes fail downhill?
Over the years there have been several runaway truck incidences on the South Eastern Freeway down-track, two with devastating consequences including fatalities.
So, what causes trucks to lose control going downhill and how can heavy-vehicle drivers manage the speed of their vehicles when descending.
Just like a car, truck brakes involve two brake pads squeezing both sides of disc rotor, or brake shoes pushing out against the inside of a drum.
This creates friction which slows the vehicle. Trucks are much heavier than cars, so their brakes can overheat with sustained use during long, steep descents, like the section of freeway below Crafers. This causes loss of friction resulting in brake failure.
Selecting a much lower gear before starting their descent helps truck drivers control the momentum of their vehicles, when done in conjunction with non-friction braking, like compression-release brakes (also known as Jake brakes) and exhaust brakes.
These systems effectively turn the engine into an air compressor, which makes the engine do much of the braking work.
There are also different types of transmission retarders, which are designed to slow the vehicle at the driveshaft.
Downhill demerits
Australian Road Rule 108
When heavy-vehicle drivers see a sign reading “trucks and buses must use low gear” they need to choose a gear that allows them to control the vehicle’s speed without using the primary foot brake.
A truck is a vehicle with a Gross Vehicle Mass (GVM) over 4.5 tonnes, except a bus, tram or tractor.
A bus is a vehicle with a seating capacity greater then 12, including the driver.
The rule isn’t intended to prohibit any use of primary friction brakes, but to ensure drivers choose a gear low enough so they’re not relying solely on primary brakes to slow their vehicles.
Rule 108 applies on several South Australian Roads including:
- Willunga Hill, Willunga
- Main South Road, O’Halloran Hill
- Old Belair Road, Mitcham
- James Road, Mitcham
- Ocean Boulevard, Seacliff Park
- Sothern Expressway, O’Halloran Hill
- The Grove Way, Salisbury East
- Western Approach Road, Port Lincoln.
Penalties for breaking Rule 108 include:
- $436 plus $102 Victims of Crime Levy
- three demerit points.
Road Traffic Act Rule 45C
Road Traffic Act Rule 45C applies to the last 7km of the South Eastern Freeway down-track and attracts severe penalties for drivers exceeding the heavy-vehicle speed limit by 10km/h, or more, or not selecting a gear low enough for the vehicle to be driven without using the primary brake.
Penalties include:
- up to $5000 and minimum six months licence disqualification for a first offence
- up to two years imprisonment and minimum 12 months licence disqualification for a second offence
- up to two years imprisonment and minimum three-year licence disqualification for subsequent offences.