Apparatus: the shops Christian P. Koop

I read an article in a recent Fire Apparatus & Emergency Equipment issue written by Chris Daly that compared braking efficiency between passenger cars and fire trucks.

Understanding driver reaction time and the big difference between the stopping distances of these vehicles is very important not only for their driver operators but also for the technicians that repair and service them. If more drivers were properly educated with this important information, perhaps it would reduce the number of accidents emergency response vehicles (ERVs) are involved in and at the same time save lives and reduce department liability.

There have been many changes to modern braking systems, which include antilock or antiskid brake systems (ABS) that most are familiar with. ABS was mandated by the government in the mid 1970s for heavy trucks; however, the early systems were a huge failure, and most were disconnected by the users very quickly because of the problems. I remember this distinctly because, as a technician back in the day, I worked on these systems. The evolution of ABS and other electronic chassis systems, such as rollover stability control, has undoubtedly allowed vast improvements in reducing passenger car, heavy truck, and ERV accidents.

For the most part, those in the industry are very aware of these technological advancements and the positive impact they have had. However, there have also been very important and significant changes in drum and disc brake friction (linings) material over the years and how they function that many may not be aware of and that require changes to service procedures.

Back to Basics

To understand how brakes slow down and stop vehicles, we need to go back to basics. Whether brakes are drum or disc, they are applied by either air pressure in most heavy-duty trucks and some medium-duty trucks or by hydraulic pressure in light-duty trucks, passenger cars, and some medium-duty trucks. Gradually applying the brakes in a moving vehicle forces the disc pads or brake drum shoes against the discs (rotor) or drums. In disc brake systems, brake pads squeeze the rotor in a clamping fashion. In drum brake systems, brake drum shoes are forced against the inside diameter of the drums. The greater pressure the driver applies at the pedal, the more pressure is exerted by the linings against the disc or drum.

As the vehicle slows down, heat is generated between the linings and the discs or drums, and the vehicle is brought to a stop. The energy the moving vehicle had is given off as heat to the atmosphere by the pads or shoes and discs or drums. Now, there is a limit to how much heat the linings and the disc or drum can tolerate. When the heat reaches or exceeds a given threshold, the brakes are no longer effective and cannot slow the vehicle down. This is referred to as brake fade.

Brake Fade

Drivers can experience this phenomenon if the brakes are applied excessively and overheat. For example, brake fade can occur easily in mountainous regions when going down grade. Vehicles that are overweight are candidates for brake fade even on flat terrain. Fire apparatus have notoriously had issues being overweight. I and many others have jokingly referred to this as 10 pounds of stuff in a five-pound bag. Al