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BRAKE SYSTEM DESIGN – PHASE 1

October 27, 2003

 

1.0 Introduction

The approach taken during the design of the braking system for the BMW M3 is to act as an aftermarket company that specializes in performance brake designs and is improving the current design of BMW M3’s braking system. Considerations will be given to braking performance, safety, the parking brake sub-system, and the actuating mechanism.

Company Background
Crambo specializes in aftermarket braking solutions for performance cars. Through technology, innovation, design, and engineering excellence, Crambo continues to push the envelope in brake systems. Crambo brake systems have one of the highest reliability and safety records in the industry.

Vehicle Background
The 2003 e46 BMW M3 is a 2 door, 5 passenger luxury sports car available in two trims – convertible or hard top. The “e46” refers to the model year of the vehicle. For example, this BMW (e46) is of production model year 2000 to present. The design of these brakes will focus on the hard top vehicle as this has the highest production numbers of the two trims.

Brake System
The brake system of a car is one of the most critical components and as such it is important to take into account factors such as safety, reliability, quality, performance, and cost, to efficiently design a brake system. At Crambo, engineers ensure that these factors are mated together with perfection.

Problem Statement
The problem Crambo will discuss in this report is to propose a design that is superior to the current brake system in the current production e46 BMW M3. The key objectives will be to determine the current brake specifications of the M3, discuss brake torque, temperature rise, energy dissipation and parking brake force. Also, an analysis of the various methods of improving the current M3 components will be required. A final design will then be proposed.

Justification
BMW enthusiasts often enjoy taking their cars to races and autocross events. These enthusiasts demand the highest performance from their cars in all aspects. In order satisfy and profit from this niche, Crambo has undertaken a design project to replace the stock BMW M3 brake system with a performance braking system that is both race worthy and street worthy. An improved braking system will reduce lap times, since the car can approach a corner faster before applying the brakes. The improved braking system will also improve stopping distance and braking response.. This enhances safety and performance both on the track and on the street.

Phase 1
Phase 1, this report, will discuss and analyze the many options available to improve the e46 M3 brakes such as disc material, disc diameter and thickness, disc heat dissipations characteristics, brake lines, etc. Final selections will take place completing phase 1. Full calculations of braking force, stopping distance, and actuating force will be performed in phase 2 of the report to confirm the final selection of phase 1. If a different design is justified through phase 2 the proposed design of phase 1 will be replaced.

2.0 Background and Terminology of Braking Systems

2.1 Functions of a Brake System

A vehicle is connected to the roadway by the traction forces produced by the tires. Any braking force must be generated by the small tire tread area contacting the road surface. Only forces equal to or less than the product of the normal force and the tire-road friction coefficient can be transmitted by the tire treads and wheels. Even an ideal braking system cannot utilize more traction than what the tire and the road provides [1]. Brakes convert the vehicle’s kinetic and potential energy in to thermal energy.

The basic functions of the brake system are to slow a vehicles speed, to maintain speed during downhill operation, and to hold a vehicle stationary. A good braking system determines how much control of the car a driver has while driving.

There are two types of brake systems; service brakes, and parking brakes. Service brakes are used for standard braking while driving, whereas parking brakes are used to hold a vehicle stationary, but may be used for slowing down in an emergency.

2.2 Components of a Brake system

There are two types of friction brakes; drum brakes and disc brakes. Drum brakes use brake shoes that are pushed out in a radial direction against the drum (figure 1). Disc brakes use pads that are pressed axially against a disc (figure 2). The primary advantage of a disc brake is that it is lightweight, easy to manufacture, low cost, has better thermal dissipation, and performs better. Thus disc brakes are essential used in all modern performance and sports cars. For these reasons Crambo only works with disc brakes.
 

Figure  1 Typical Drum Brake Components [3]

Figure  2 Typical Disc Brake Components [3]

A typical automotive brake system consists of service brakes and the parking brake. The service brake consists of several key components. The brake pedal connects to the master cylinder, and converts driver’s pedal effort into hydraulic force. Brake boost assist systems are used to add pressure to the master cylinder, which affectively multiplies the driver pedal force. Thus the driver has to apply less force on the pedal to achieve a larger braking force. The hydraulic fluid in the system flows through flexible brake hoses from one component to the next. After the boost, the hydraulic fluid flows into each of the brake callipers. The callipers work like a C-clamp to squeeze the brake pads on to the rotor. The pad rubs against the rotating disc to slow it down, effectively converting the rotor’s kinetic energy in to thermal energy. See figure 3 for an overview of a general braking system.

The parking brake is a cable-activated system used to hold the brakes continuously in the applied position. The parking brake activates the brakes on the rear wheels. Instead of hydraulic pressure, a cable (mechanical) linkage is used to engage the brake discs. When the parking-lever is pulled, a steel cable draws the brake pads firmly against the rotors. The release lever or button slackens the cables and disengages the brake pads.

2.3 Layout of a Brake System

A typical brake system layout is shown in figure 3 below.

Figure  3 Typical Brake System Layout [3].

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