Paper Number 03 Design of Formula SAE Brake Systems Eric Hamilton 2008 FSAE Team Member Eric Klang Faculty Advisor Wolfpack Motorsports College of Engineering North Carolina State University Raleigh, NC 27695-7910 Copyright © 2009 Wolfpack MotorsportsIntroduction The Wolfpack Motorsports team designs, builds, and races a car in the Formula SAE Collegiate Design Series. Formula SAE [FSAE] is a competition organized by the Society of Automotive Engineers that draws over 100 international schools to the main competition in Detroit, Michigan. Wolfpack Motorsports has a history of strong finishes in the competition and continues to work towards winning. The objective of this project was to expand the student’s knowledge of mechanical engineering by developing software tools to help design a FSAE brakes system. These tools were be used to help design the brakes system on the 2008 Wolfpack Motorsports FSAE car. The software was used to determine the pedal ratio, master cylinders, calipers, and the number of rotors and their geometry. The guiding factors of the design process were maximum braking power, minimal weight, and manageable temperatures while maintaining reliability. These factors were analyzed in the student developed software program. The brake rotors for the 2008 car were designed in Solidworks and analyzed with Cosmo Express based on the results of this analysis. This report aims to explain the details of the software program as well as each step of the design process for the brakes system. It also serves as a reference to future members of Wolfpack Motorsports for brake system design. Design Objectives The primary design consideration of a FSAE brakes system is the number of rotors and subsequently their placement inboard or outboard. The Wolfpack Motorsports car has primarily been a three rotor car with one foray into a 4 rotor (Front outboard, Rear inboard) design in 2006. There has been much debate with judges and amongst the team as to which setup is ideal and as fine tuning the handling and team members understandings of vehicle dynamics have improved the 4 rotor design has come back in 2008 with outboard rotors front and rear. This allows for more tunable and controllable dynamics - especially braking in corners. It also helps with packaging issues to run the two smaller rear rotors outboard. Also associated with the rotors is their mounting. As of 2007 Wolfpack Motorsports has started using a floating rotor with a fixed, opposed-piston caliper setup. This allows the rotor to center itself in between the pads to ensure efficient braking as well as a quick return to non-braking once the brake pedal is released. This also narrows down the caliper selection, which is generally determined before sizing any other components. Another important factor in a brakes system is the balance of the car under braking. A car that is effectively balanced will be able achieve the maximum braking the system components are capable of as maintain stable handling under braking. A bias bar allows for moderate adjustment of the balance which is used primarily for fine tuning the handling of the car. But to be utilized correctly the car’s brake system must be designed around the geometry and mass of the vehicle. Thus the sizes and number of rotors, diameters of master cylinders, and caliper pistons must be calculated with regard to the CG height of the car, wheelbase, and mass of the car and driver. Design Process Component sizing and selection: The Matlab program was written to streamline the component sizing process. It allows a basicconfiguration to be entered and will iterate to help determine what size components to use. As more variables are determined, the braking of the car can be fine tuned further. This section will discuss the functionality of the program in relation to how it is used to size components. The program consists of three windows: Car Properties, Kinematics, and Heat Transfer. They are accessed one at a time with buttons at the bottom of the program window. Also at the bottom of the program window are buttons to save and load configurations. These buttons will either save or load all variables contained in the program. The Car Properties window contains all the variables related to the car that do not change due to changes in the brakes setup but that said setup depends on. The Kinematics window contains the bulk of the program and is used to size components and determine balance and braking forces. The Heat Transfer window is used for checking the rotor designs to make sure they meet fade and thermal heat failure requirements. The background color of a box is white if it requires a user input and grey if the program calculates the value. The following flowchart explains how to use the program. Details of each section follow the flowchart. Figure 1: Flowchart for Brakes Program Car Properties: When the program starts it opens to the Car Properties window. All of these values are necessary to perform the brakes calculations and should be determined in advance. The first box will show what, if any, file is loaded. This program allows all the variables to be saved and loaded to streamline comparison between different vehicles. Weight distribution will automatically be calculated as values are entered. The coefficients of friction for the tires are estimated values. 0.65 has been used for the kinetic value and 1.35 for static. The mass moment of inertia can be determined with Solidworks, but 3.85 lb ft s^2 should be a fairly accurate assumption. Figure 2: Car Properties Figure Window Kinematics: The kinematics window contains all the calculations relating to the dynamics of the vehicle under braking. This is where most of the work will take place. It calculates the deceleration of the car in G’s based on the force of the pedal, the configuration of the bias bar, the master cylinder sizes, caliper sizes and configuration, pad type, rotor geometry, and the interaction of the tires wit the road. For the program to calculate the braking values and help the user determine the optimal setup values must be filled out in every white box. Start: Either start with a blank setup or load a setup. Loading an old setup should be easier for setting up a new car as many of the variables will not change from car to car Car Properties: Input all properties of the car that do not depend on the brakes setup Figure 3: Kinematics Program Window The first box