Brake for Success: Understanding the Components and Functions of Automobile Brakes

Brake System

• Motorcycle brakes can be classified as drum brakes (internal expansion) and disc brakes.
• Both are operated mechanically by means of steel cables and rods, and hydraulically by means of fluid pressure.
• Generally, the drum brake is operated mechanically and the disc brake hydraulically.

A. Drum brake

• The drum brake has brake shoes (with a lining) which are forced against the rotating drum (wheel hub), using friction to stop the wheel. This is called an "internal expansion lining brake".
•  This type of brake is most widely used on general motorcycles, and can be classified into the following two types by brake shoe operation.

Operation of Mechanical Braking System (Drum Brake)

1. Before operation

Before operation lever is in release position.

At release position drum rotate freely. Because there is gap between brake drum and brake shoe lining.

At that time > No friction is made.

Half brake

During half braking operation lever is in slightly pull.

Brake shoe are pushed slightly to touch the brake drum by cam.

At that time > Small friction - Small brake force

3. Full brake

During Full braking operation lever is in slightly pull.

Brake shoe are pushed fully to reach the brake drum by cam.

At that time > More friction - More brake force

4. Brake released

During Brake release lever is completely released.

Brake shoe are pulled back to original position by return springs

At that time > No friction - No brake force

2. Type of drum brake 

a. Leading-trailing type

• Only one cam is used to force two brake shoes. 
• The shoe having the stronger braking effect is called the "leading shoe", and the other is the "trailing shoe".
• Principle of leading shoe operation when the leading brake shoe (which is pushed in the rotational direction of the drum) is forced by the cam against the drum, the friction force created between the shoe and the drum produces an increased braking force. At the same time, the trailing brake shoe is pushed against the rotational direction of the drum, so its braking effect is reduced. However, when the drum is forced to rotate in, reverse, for instance, on a slope, the trailing shoe acts as a leading shoe. Hence.
• The leading-trailing shoe type brake is always used as the rear brake.

b. Two-leading shoe type

• Two cams are used to force both brake shoes so that, both shoes can act as leading shoes. Hence this type of brake produces a braking power of about 1.5 times that of the leading trailing shoe brake. 
• It is mainly used as the front brake on sport bikes but recently it is being replaced by the disc brake.

3. Brake drums

• The brake drum is made of cast iron and integrated with the hub. 
• When the brake is applied, friction heat is generated and hence the friction force on the brake lining is reduced. 
• To counteract this, the outer surface of the hub is fitted with cooling fins made of an aluminum alloy which has excellent heat dissipation.
• The drum is grooved for water and dust proofing. If water enters, it flows through the groove and out of the drain hole, so that the inside of the drum can be kept free of water.

Fading

• This phenomenon occurs when the surface temperature of the brake shoe or disc pad rises and the friction coefficient (degree of slipping difficulty) of the brake shoe or disc pad is reduced.
• Although the feeling of the brake action does not change, the brakes suddenly do not have any effect.

B. Disc brakes

• The disc brake has a stainless steel disc plate that rotates with the wheel. 
• When the brake is applied, the disc plate is gripped by pads driven by hydraulically operated pistons.

1. Principle of the disc brake

• The hydraulic force of the pistons in the caliperscan be calculated using Pascal's law. Pascal's law A confined fluid transmits externally applied pressure uniformly in all directions, without change in magnitude.
• When force F1 is applied to the master cylinder which has a cross-sectional area S1, F2 exerting on the cross-section S2, of the wheel cylinder (caliper) is given by S1: F1 = S2 F2 F2 F1XS2/S1 Therefore,
• when the ratio S2/S1 is large, an increased F2 force can be obtained by applying a slight F1 force.

2. Operation of disc brakes 

• When the brake lever or pedal is actuated, the master cylinder converts the force applied into fluid pressure.
• It consists of a reservoir filled with brake fluid and a cylinder in which fluid pressure is produced. The reservoir is normally made of plastic, cast iron or aluminum alloy and is integrated with the cylinder. 
• The master cylinder piston end is fitted with a rubber cup to provide a better seal with the cylinder. The other end is also fitted with a rubber cup to prevent fluid leakage.

Operation

• When the brake lever is squeezed, the piston overcomes the return spring and moves further. The piston cup on the end of the piston closes the return port and the piston moves further. 
• The fluid pressure in the master cylinder rises and the fluid forces the calipers through the brake hose.
• When the brake lever is released, the piston is pushed back by the return spring. and the fluid returns to the reservoir through the return port.

1. Before operation

• Brake fluid pressure : Zero 
• Pads do not touch disc.

2. Beginning of operation 

• Brake fluid pressure: increases
• Pads touch disc slightly. 
• Friction: small
• Brake force: small

3. During operation

• Brake fluid pressure : high 
• Pads squeeze disc hard.
• Friction large
• Brake force: large

4. Brake released

• Brake fluid pressure : Zero
• Each pad returns to its original position. 
• Brake force : Zero

3. Disc pad auto-adjustment 

1. Before operation

2. Operation

• Piston seal is deformed.
• Piston does not slip on the piston seal.

3. Returning

• Piston seal pulls back piston to its original position.

4. Auto-adjustment

• When pad is worn and becomes thinner, piston slide on the piston seal and only the worn portion moves forward.

5. Advantage of the disc brake

• Since the disc is exposed, it dissipates heat faster and thus has less tendency to fade. Hence, a stable braking effect can be ensured.
• It will not self-energize like a leading shoe brake. When two disc brakes are used, there is no difference in braking power between right and left brakes. Thus the bike does not pull to one side.
• Even if the disc expands due to heat, the clearance between the disc and the pads will not change much and the brake lever and pedal can be operated normally. 
• When the disc gets wet, water is quickly dissipated by centrifugal force and the recovering of brake effect is fast. Because of these advantages, the disc brake is used extensively as a front brake. The reason is that when the brake is applied, most of the load is imposed on the front wheel and so emphasis is placed on the front brake. To increase braking power, the double-brake system is used. Recently, many bikes are using disc brakes for rear wheels.

C. Brake fluid

The brake fluid must meet the following requirements: 

• It does not damage rubber or freeze.
•  It must also have a high boiling point and not be corrosive.
• The brake fluid tends to absorb moisture in air and as a result, the boiling point causing vapor lock. Therefore, the brake fluid must be replaced periodically.

a. Characteristic (nature)

• Damage paint and plastic.
• High hygroscopicy. 
• Don't mix different types of fluids.

b. Standard for brake fluid

• DOT: Department of Transportation (of U.S.A.)
• JIS: Japan Industry Standard

c. Vapor lock

In this phenomenon, heat from the disc pad is transferred to the brake fluid, which boils at high temperatures and generates bubbles. Even though the brake lever is squeezed, the bubbles are compressed, so the brake lever operation seems light and brake effectiveness is poor.