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Brake System Design for BMW M3

December 1 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, 4 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.


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 and Phase 2

Phase 1 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.

2.0 Design Constraints

Constraints will set limits on the potential designs of the braking system. The designs can then be gauged using the constraint limit as a guideline or benchmark and the best design can be selected by Crambo engineers.

2.1 Brake Performance

The braking system designed by Crambo must perform better than the current production M3’s system.  A typical test done to measure brake performance is stopping distance from 60 mph.  The current system’s stopping distance has been tested to be 112 ft from 60 mph, Crambo’s design will be better [5]. 

2.2 Spatial Considerations

The dimensions of the braking system (calliper, discs, cylinders, etc) can only be large enough such that the stock wheels will fit over all the brake components with enough clearance. The stock wheel sizes can be found in table 2.

2.3 Government Regulations

The Federal Motor Vehicle Safety Standards (FMVSS) regarding braking systems and parking brake systems must be met.

2.3.1 Parking Brake

According to The Federal Motor Vehicle Safety (FMVSS), states then when engaged, the vehicle's parking brake and parking mechanism, should be capable of holding the vehicle stationary (to the limit of traction of the braked wheels) for 5 minutes, in both forward and reverse directions, on a 30 percent grade.

2.3.2 Brake System with Booster

From FMVSS, a maximum pedal force of approximately 50N to 70N (11 to 17 lb) should provide a deceleration of 0.9 to 1 g [1]. The associated pedal travel should not exceed 75 to 90 mm. The booster characteristic should increase linearly with pedal force and travel. In order to ensure proper brake force modulation, a pedal force not greater than 5N (1lb) should be required to start the brake boost. And finally the boost ration must be within 6 to ensure safe vehicle deceleration in the event of a boost failure [1].

2.3.3 Failure Performance

The braking system is the most important system in the car with regards to safety. A brake failure can leads to disastrous results. The brake system designed by Crambo must meet or exceed the safety performance of the stock M3.

The Federal Motor Vehicle Safety Standard (FMVSS) provides certain limits on pedal force and stopping distant in the event of partial brake failure [3]. A maximum pedal force of 100N (22lb) should achieve a deceleration of 0.3 for a loaded vehicle, in the event of a boost failure, or circuit failure [1].

3.0 Design Criteria

Design criteria are important to define as they give direction to the Crambo engineers and an idea of where the design should be in terms of performance, quality and cost.

The brake system in development will fit directly on to the M3 with little modification. It will work with the onboard traction control and ABS brake actuation systems.

3.1 Maximum braking torque vs. actuating force

For optimum braking, the braking torque must be large enough to lock all four wheels at any speed. Ideally the wheels should be at the threshold of locking/sliding since ustatic > usliding sliding, but this is difficult to achieve in practice.

3.2 Heat Dissipation

The thermal energy created during braking must be absorbed by the braking system and the surrounding components. Most of the heat should be dissipated to the free air stream to minimize heat conduction by surrounding parts. Cooler rotors minimize brake fade, and prevent critically heating other components, or causing brake fluid vaporisation.

3.3 Cost

The cost must be low to be competitive between our competitors, other braking after market specialists, such as Brembo and Stop Tech.

3.4 Weight

The mass of any vehicle requires energy to accelerate or decelerate. Reducing the vehicle mass improves acceleration, and requires less energy to be dissipated during deceleration.
Therefore decreasing the mass of all the components of the brake system, decrease the overall weight of the car, and hence improves performance [2].
Furthermore, rotating mass has inertia, and requires additional energy in order to increase or decrease its speed of rotation. Therefore, decreasing the mass of the disc reduces the rotational inertia, and has an even greater benefit [2].

3.4.1 Unsprung Weight

This is one of the most critical factors affecting a vehicle's road holding ability. Unsprung weight is that portion of a vehicle that is not supported by the suspension and therefore most susceptible to road shock and cornering forces. It includes hubs, wheel, tires, uprights, brakes, and half of the weight of the suspension links [3]. It is desirable to have a low unsprung weight for improved performance and handling.

3.5 Energy Absorption

The rotors must also absorb the energy from braking without detrimental effects such as warping, and must be under the permissible operation temperature dictated by the brake pads.

3.6 Maintenance

The brake system should require as little maintenance as possible and should operate at or near optimum conditions at all time. This includes regular maintenance, such as cleaning or rotors, checking fluid levels, and any repairs that may become necessary.

3.7 Simplicity of Design

A simple design is often desirable as it is easy to manufacture. Ease of manufacture is critical, as it will reduce costs if full time production is to take place. Complicated geometries can be difficult to fabricate and may require the use of specialty tools and machines, ultimately increasing production costs. Further simpler designs will allow for faster assembly and maintenance times, as well as rapid preparation of spare parts.

4.0 Vehicle Specifications

The vehicle specifications are shown below in Table 1.

Table 1 Basic Statistics of the e46 BMW M3 [4]






Vehicle Price

US dollars


Vehicle Curb Weight



Weight distribution, front/rear






Track, front/rear



Length x width x height


176.8 x 70.1 x 54.0

Aerodynamic drag coefficient






24 valve, 3.2 litre, 6 cylinder


hp SAE net

333 @ 7900 



262 @ 4900



Acceleration, 0-60 mph, manufacturer’s data



Top speed


155 (electronically limited)

* An additional weight of 180 lbs for driver of car is to be added.

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