Beatingheat

A DRIVER does not perceive the relationship between brake lining temperature and stopping distance from the comfort of a modern airconditioned cab. That is until brakes are required in an emergency, and having been used excessively instead of external retardation systems, the friction-coefficient has literally plunged ‘off the cliff’. 

The chart on the right graphically demonstrates how braking efficiency halves from a cool brake to an overheated situation at 400°C. The other outcome is how brake lining wear also accelerates beyond 250°C. 

The next consequence of super-heated brakes is ‘heat-soak’ where the heat transfers from the brake linings into the wheel rims and tyres. Tyre bead components cannot stand exposure to consistent abnormal temperature of 120°C-plus, resulting from excessive braking or binding brakes. Heat transferred from a brake drum to a rim and thence onto tyre beads can lead to bead failure – the bead will be damaged through continuous exposure due to these high temperatures. When a tyre bead fails a tyre is scrap. 

Trailer brake hand control – rearmost heat build-up 

Where trucks are equipped with an independent trailer brake hand control, drivers tend to use this to stretch a vehicle combination on down-grades – that’s why this brake control is also known as a ‘stretch-brake’. The result is that lightly dragging trailer brakes builds up heat in trailer brake linings with loss of trailer brake efficiency. Under emergency, a prime-mover stops efficiently with an inefficient trailer jack-knifing. ABS cannot make up for loss of brake lining efficiency. 

The message is clear – drivers must understand the consequences of their actions and not just how to use the controls at their disposal.