Early systems used a mechanical link between the brake pedal and brakes. In cable-operated systems, when force is applied on the brake pedal, the cable pulls the brake shoes against the inner surface of the drum. This system, although simple, had its disadvantages. There was the danger of the cable snapping, resulting in total brake failure. Moreover, pressure applied on the pedal couldn’t be multiplied as on a hydraulic system, which has many advantages over mechanical linkages — it is silent, flexible and self-lubricating, and ensures that braking forces to both sides of the car are automatically equalised.
Hydraulic systems use three basic principles: leverage, force multiplication and friction. These systems use an incompressible fluid that transmits pedal pressure to the brakes. The great thing about a hydraulic system is that it makes force multiplication quite easy.
The brake pedal is designed in a manner that it multiplies the force applied on it even before it is transmitted to the brake fluid. Imagine a master cylinder (A), connected to a slave cylinders (B) by a tube filled with incompressible fluid (see Diagram 1). The slave cylinder has a diameter three times that of the master cylinder. If force is applied via a piston on the smaller cylinder, this force will be multiplied three times at the other end, the only downside being that the smaller piston will have to be moved three inches for the larger piston to move one inch. These forces combined will result in the driver having to apply only a small force on the brake pedal to bring the car to a halt. The other benefit about hydraulic systems is that the pipe connecting the two cylinders can be of any shape and length and can snake through anything separating the two systems.
A vacuum servo mechanism fitted to the braking system also reduces the effort that the driver has to use on the brake pedal. The servo unit has a cylinder that contains a piston or diaphragm. It creates a vacuum inside the cylinder using the suction from the intake manifold of the engine. Air is exhausted from both sides of the piston and atmospheric pressure is admitted on one side while a partial vacuum remains on the other. This vacuum makes the diaphragm move and it applies force on the slave piston. This reduces driver effort while braking, and the amount of assistance increases with the pedal pressure.
If the servo system fails, the car’s brakes will still work, although the pressure to be applied on the brake pedal will increase greatly. That’s why it is not advisable to coast downhill with the engine off in a car with servo-assisted brakes.
The drum brake was the next step after the transmission-mounted brake. Most cars used drum brakes on all four wheels until the disc brake was introduced.
The drum brake consists of two brake shoes, two pistons, and many springs. When the pedal is depressed, the brake shoes are pushed out against the inner surface of the drum. The brake shoes wear down with use, and the distance between the drum and the shoes increases.
For the brakes to work properly, the shoes have to remain as close to the drum as possible without touching it. If they move too far away, more brake fluid will be needed for effective operation. The pedal will sink closer to the floor every time the brakes are applied. For this reason, most drum brakes have an automatic adjuster. The adjuster has threads on it, like a bolt, so that it unscrews a little bit when it turns, and lengthens to fill in the gap. When the brake shoes wear a little more, the adjuster can advance again, so it always keeps the shoes close to the drum.
To improve braking, the shoes are also wrapped into the drum when the brakes are applied. The brake shoes have to be pulled away from the drum when the brake is released because of this wrapping action. Springs in the brake drum pull the shoes back. Other springs hold the brake shoes in place.
There are two types of drum brakes: the twin leading shoe type and the single leading shoe type. In the twin leading shoe type, the leading edges of both the shoes are pushed against the drum. Drum brakes also have an emergency brake built into them. It is activated by a simple cable linkage. When the emergency brake is actuated, the cable pulls the lever which forces the brake shoes apart and brings them into contact with the drum.
Upto the 1960s, the materials used for brake drums didn’t conduct heat as well as they do now. After repeated use, brakes heated up and the metal expanded away from the shoes. The brake pedal would sink to the floor before there was sufficient bite. To overcome this problem, high performance cars were fitted with alloy drum brakes because of their ability to conduct heat better. These were also fitted with fins for better cooling.
Drum brakes require very little servicing. Deep scores sometimes get worn into brake drums. If a worn-out brake shoe is used for too long, the rivets that hold the friction material to the backing can wear grooves into the drum.
A badly scored drum can sometimes be repaired by refinishing. Drum brakes have a maximum allowable diameter. Since the contact surface is the inside of the drum, as you remove material from the drum brake, the diameter gets bigger. Drum brakes are still used in many cars made in India, albeit only at the rear wheels.
Disc brakes are far more advanced than drum brakes and can dissipate heat much better than drum brakes, owing to their design. The disc brakes in most cars are vented for even better cooling. Such brakes have a set of vanes, between the two sides of the disc, which pump air through the disc to provide cooling. This is the main advantage of disc brakes, along with improved braking power and less weight. Disc brakes also dispel water away from the surface much faster than drums.
A disc brake consists of brake pads, brake caliper (containing the piston), and the rotor (mounted on the hub). When the brakes are applied, the piston in the brake caliper squeezes the brake pads against the spinning rotor. The friction between the pads and the spinning disc brings the car to a halt.
There are no springs to pull the pads away from the rotor, so the pads always stay in light contact with the rotor. If the brake pistons were to retract into the cylinder, it may take several applications of the brake pedal to pump enough brake fluid to engage the brakes.
If worn-out brake pads are used for too long, deep scores can form on the surface of the disc. Disc brakes can also warp because of the tremendous heat that is generated during braking (in extreme conditions). Refinishing the surface of the rotors is the best way to solve both these problems. This is done by removing material from the surface of the disc to give it back its smooth surface. Most modern luxury saloons in India are now sold with disc brakes on all four wheels.