Automotive Brake Systems: A Comprehensive Guide
Functions and Classes of Brakes
Foot Brake
The foot brake, controlled by the driver, is designed to slow down or stop the vehicle as needed.
Parking Brake
The parking brake prevents a stationary vehicle from rolling.
Service Brake Mode
This mode maintains a vehicle’s speed on long downhill stretches.
Hydraulic Brakes
Hydraulic brake systems use fluid to transmit force from the brake pedal to the wheels, allowing for efficient braking with minimal effort.
Front Brake Pressure Regulator
This component maintains a pressure of 0.5 to 2 bar on the front brakes, preventing premature engagement and ensuring proper release.
Compensation Port
The compensation port balances brake fluid volume changes due to temperature fluctuations. This prevents unintended braking caused by fluid expansion.
Types of Master Cylinders
The master cylinder generates pressure in the brake circuits and is activated by the brake pedal.
Tandem Cylinder
This type consists of two separate master cylinders connected in series.
Twintax Cylinder
This type features two pistons that are not directly connected.
Wheel Cylinder
The wheel cylinder converts hydraulic pressure from the master cylinder into mechanical force, pushing the brake pads against the drum or disc.
Brakes on Tires
Friction-based brakes, such as drum or disc brakes, are commonly used in automobiles. Drum brakes are often found on the rear axle of passenger vehicles.
Drum Brakes
Drum brakes, typically of the internal shoe type, consist of several key components: brake backing plate, brake shoes, and actuating mechanisms.
Advantages of Drum Brakes
- Internal force propagation creates a self-energizing effect, enhancing braking action.
Disadvantages of Drum Brakes
- Brake lining replacement is more time-consuming compared to disc brakes.
- Adjustment mechanisms can be complex and costly.
- Heat dissipation and removal of brake dust are less efficient.
Types of Drum Brakes
Simplex Brake
This basic type has a single inlet and outlet, with shared components like a double-acting cylinder, brake cams, and expanding levers. Brake shoe lining wear is uneven.
Duplex Brake
This type features two independent actuation mechanisms for each brake shoe, requiring separate input forces. Only the rear shoe provides braking action in reverse.
Duo Servo Brake
This type offers equal braking performance in both forward and reverse directions and requires minimal actuation force.
Brake Drum
The brake drum is made from materials with high resistance to abrasive wear, even at elevated temperatures. It also exhibits a high friction coefficient, minimal tendency for brake fade, and excellent rigidity and deformation resistance.
Brake Linings
Brake linings have a friction coefficient between 0.3 and 0.5 and can withstand temperatures up to 450°C. They are typically made from composite materials containing asbestos and zinc alloys.
Brake Shoes
Brake shoes are shaped to fit the drum and are usually constructed from cast aluminum or sheet steel.
Brake Backing Plate
The brake backing plate supports the brake cam, hydraulic wheel cylinder, and brake shoes while enclosing the drum.
Brake Cams
Brake cams are used to expand the brake shoes. S-cams with a notched end for adjustment are commonly employed in modern vehicles.
Recoil Spring
The recoil spring, with a traction force of 150N to 300N, retracts the brake shoes after braking.
Adjusting Devices
As brake shoes wear, automatic or manual adjustment devices maintain the proper clearance between the shoes and the drum. Automatic adjusters can operate in stages or continuously, while manual adjustment typically involves an eccentric pin on the brake backing plate.
Disc Brakes
Disc brakes are often used on wheels but can also be mounted separately. Partial disc brakes are the most common type.
Advantages of Disc Brakes
- High temperature resistance and minimal brake fade.
- Simple maintenance and brake pad replacement.
- Uniform wear and automatic adjustment.
- Effective self-cleaning due to centrifugal force.
Disadvantages of Disc Brakes
- Smaller braking surface area compared to drum brakes, requiring higher clamping force.
- Brake pad wear can be significant.
- Localized heating of the pads can cause vapor lock in the cylinders.
Fixed Caliper Disc Brake
In this type, the caliper and brake cylinder are fixed, while the disc rotates with the wheel.
Floating Caliper Disc Brake
This type uses a single-piston brake cylinder that presses against the brake pads, which in turn clamp onto the disc during braking. It does not have bolted connections.
Brake Disc
Brake discs are typically made from cast iron, spheroidal graphite cast iron, or cast steel and often have a casserole shape.
Brake Fluid
Brake fluid possesses several key properties: low compressibility, high boiling point (above 230°C), resistance to aging, low water absorption, low internal friction, and good lubrication. It is typically alcohol-based, hygroscopic, and can damage paint.
Tubing and Sleeves
Brake tubing has a smooth, rust-free inner and outer surface. Common outer diameters include 4.75mm, 6mm, 8mm, and 10mm. The tubes are galvanized both internally and externally with a 12 to 15 μm layer.
Sleeves protect the sections between the tubing and fixed parts of the vehicle’s axle and suspension, allowing for relative movement. Important considerations for sleeve installation include:
- Avoid overtightening to prevent warping or twisting.
- Do not route sleeves near the exhaust system.