There's nothing more important on any school bus than the braking system. It is in keeping with a concern for safety that the four-wheel disc brake system has become the brake of choice for the school bus industry. Disc brakes are long lasting and provide safe, even stopping. Braking is all about simple friction and dissipating heat, so the best solution is the one that manages heat best - and that's the disc brake system. Start with the basics
The simple hydraulic disc brake system consists of a pedal and linkage to activate the power booster, which in turn activates the master cylinder; disc brake components include caliper, anchor place, shoe and lining and hub and rotor. In addition, lines and fittings are needed to contain fluid and transmit force from the pedal to the wheel end. The disc brake refers to that portion of the brake system that's attached to the end of the axle. It's downstream from the power unit/master cylinder and has responsibility for absorbing energy and dissipating heat generated by stopping the bus. Disc brake systems use a two-piston sliding caliper at the steer axle and drive axle; older buses may utilize a single-piston caliper. Service on the single- or twin-piston caliper is essentially the same. The caliper attaches to and slides within an anchor plate or bracket. Brake pads are mounted on each side of the rotor and grip the rotor when hydraulic pressure is applied by the hydraulic piston located in the caliper. The cast disc (rotor) is bolted to the hub assembly. Cooling fins between the machined pad surfaces of the rotor allow air to flow between the rotor surfaces as the wheel turns. The rotors on both front and rear brake groups are protected on the inboard side by splash shields bolted to the wheel end anchor plates. Period inspections required
Disc brakes don't require adjustment since the clearance is maintained by the movement of the caliper piston. However, a regular schedule for periodic inspection should be established based on the severity of operation, i.e., frequent stop and go driving in urban traffic. While disc brakes don't need adjustment, wheel bearings do. Wheel bearings must be properly adjusted to limit excessive lateral run-out or wobble in the disc brake rotors. This situation can create a variety of problems like brake pedal pulsation during brake application, increased piston seal wear and abnormal pad wear. Periodic adjustment to manufacturer specifications is required. Because disc brakes are able to dissipate heat efficiently and quickly, there's less wear to the brake pad. The shoe and lining assembly consists of a steel backing with attached lining material. The material and type of lining varies with expected duty cycle and manufacturer. Visually inspect the brake pad lining by checking each visible end and look through the opening of the caliper. If lining thickness at the thinnest point appears to be 3/16 inch (4.76 mm) or less, the pad must be replaced. OEM-specified brake lining and pads are recommended when replacement is necessary. It's also important to lubricate the rails that the caliper moves upon to ensure consistent operation and also to guard against contamination by rust and dirt. OEM-standard lubricants are the best choice for this application because they're designed to meet tough duty standards, like school buses. Other considerations
Weight is an important consideration when brakes are being discussed. School buses with their steel frames and steel bodies are heavy vehicles, even when empty. Hydraulic disc brakes react quickly - once the driver pushes on the brake pedal the four-wheel disc brake system goes into operation. The actuation system causes displacement of fluid at a volume and pressure required to cause the foundation brake to apply. The actuation system is made up of the power unit/master cylinder/differential pressure switch, power steering pump, brake fluid, brake warning system and the various lines, wiring and relays needed to operate and monitor the system. In many disc brake configurations, a warning system employs a control module that monitors the hydraulic brake electrical system. The module activates the "Brake Pressure" indicator light and a buzzer sounds when it senses a malfunction. Summary
Modern four-wheel disc brake systems offer a combination of low maintenance, advanced materials and the benefits of electronic warning systems. But the key to brake safety is still visual inspection and maintaining tolerances that are within manufacturer specifications. John Fay is school bus marketing manager for Navistar International.