Baja SAE

Diseño de vehículo Baja SAE

UNIVERSIDAD AUTÓNOMA DE CIUDAD JUÁREZ Ricardo Duarte Jáquez Rector David Ramírez Perea Secretario General Manuel Loera de la Rosa Secretario Académico Francisco López Hernández Director del Instituto de Ingeniería y Tecnología Luis Enrique Gutiérrez Casas Coordinador General de Investigación y Posgrado Ramón Chavira Chavira Director General de Difusión Cultural y Divulgación Científica

Universidad Autónoma de Ciudad Juárez

Diseño de vehículo Baja SAE

Juan Miguel Díaz Mendoza Luis Vidal Portilla Carlos Felipe Ramírez Espinoza César González Pizón

Ingeniería y Tecnología

Coordinación General de Investigación y Posgrado Lisbeily Domínguez Ruvalcaba Coordinadora de la colección

Díaz Mendoza, Juan Miguel; Vidal Portilla, Luis; Ramírez Espinoza, Carlos Felipe; González Pizón, César. Diseño de vehículo Baja SAE / Juan Miguel Díaz Mendoza, Luis Vidal Portilla, Carlos Felipe Ramírez Espinoza, César González Pizón. Ciudad Juárez, Chih. : Universidad Autónoma de Ciudad Juárez, 2013. (Colección Textos Universitarios, Serie Investigación) 32 p.; 30 cm. Incluye bibliografía Colección Reportes Técnicos de Investigación Isbn: 978-607-7953-80-7 Serie IIT, Vol. 6. isBn: 978-607-520-017-0 Contenido: 1.– Introduction. 2.– Proposal. 3.– Methodology. 4.– Results. 5.– Conclusions.

D. R. © Díaz Mendoza, Juan Miguel; Vidal Portilla, Luis; Ramírez Espinoza, Carlos Felipe; González Pizón, César. La edición, diseño y producción editorial de este documento estuvo a cargo de la Dirección General de Difusión Cultural y Divulgación Científica, a través de la Subdirección de Publicaciones.

Resumen Abstract Palabras clave Usuarios potenciales Reconocimientos

Índice

7 9 10 10 10

I. Introduction II. Proposal Background

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Theory framework Parameters Table1. Vehicle parameters Chassis roll cage requirements Engine requirements Suspension Steering system Brake system Electrical system Manufacturing

14 14 14 15 15 15 16 16 16 17

III. Methodology Chassis frame Material Table 2 Chassis design Analysis Transmission design Gear box design Figure 1 Drive system: cvt (continuous variable transmission) Figure 2 Steering design Rack and pinion Brake system Figure 3 Physical considerations Figure 4 Suspension design Manufacturing

IV. Results V. Conclusions References

19 19 20 20 20 21 21 21 22 22 23 23 23 23 24 24 24 25

Resumen

E

ste proyecto tiene el propósito de desarrollar un prototipo de vehículo todo terreno. El diseño del vehículo estará enfocado a mejorar varios aspectos del desempeño global. Por lo tanto, se establecieron varios objetivos:

ɶɶ Mejorar el desempeño con el diseño de un nuevo reductor de engranes. ɶɶ Reducir peso del vehículo con un nuevo chasis. ɶɶ Diseñar un nuevo sistema de suspensión. ɶɶ Diseñar un nuevo sistema de dirección para mejorar la maniobrabilidad. Se fabricará utilizando procesos manuales y automáticos tipo CNC. El proceso de diseño fue basado en la reglas que publica la Sociedad de Ingenieros Automotrices de Estados Unidos (SAE). Esta reglas establecen lo requerimientos mínimos del diseño con el objetivo de cumplir con seguridad del piloto en la competencia. El proyecto adicionalmente desarrolla habilidades de diseño y fabricación en los estudiantes que participan en el proyecto. Esto les permitirá adquirir habilidades para su desarrollo profesional cuando estén en la industria. El diseño fue enfoca a obtener un vehículo que fuera mas ligero como se menciono anteriormente y se obtuvo de acuerdo a los resultados. En resumen el proyecto fue terminado en tiempo y objetivos planteados al inicio del proyecto. El vehículo realizó pruebas de desempeño y resistencia básicas que cumplían con los objetivos propuestos.

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Abstract

T

his project aims to develop a prototype of an ATV. The vehicle design will be focused on improving various aspects of overall performance. It is therefore established several objectives which are:

ɶɶ Improve performance with a new design of gear reducer. ɶɶ Reducing vehicle weight with a new chassis. ɶɶ Design a new suspension system. ɶɶ Design a new steering system for improved handling. Which was made using manual processes and automatic type CNC. The design process was based on the rules published by the Society of Automotive Engineers of America (SAE). These rules establish the minimum design requirements in order to comply with driver safety in the competition. The project further develops skills in design and manufacturing students participating in the project. This will enable them to acquire skills for their professional development when they are in the industry. The design was focused to obtain a vehicle that was much lighter as mentioned above and was obtained according to the results. In summary, the project was completed on time and objectives at the beginning of the project. The vehicle tested for performance and basic resistance that met the objectives.

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Palabras clave

Mechanical design, manufacturing, improvement.

Usuarios potenciales

Potential users of this project are students that are interested in automotive design, projects that requires information in automated process application.

Reconocimientos Agradecemos a la universidad Autónoma de Ciudad Juárez y a las compañías BRP, El Milésimo, Y&D, Ergomed, Jidosha, Rino Metales, Ávila manufacturing, Leyco, Dr. García, que apoyaron a financiar este proyecto. También queremos agradecer a los estudiantes que apoyaron en el desarrollo de este proyecto: Mayra Nevárez, Eduardo Morales, Valentín Gallega, Ismael Pérez, Juan José Velázquez, Néstor Santos, Enrique Duarte, Felipe Jiménez, Roberto Sierra, Roberto Alamillo, Jerónimo Huerta y Jaime Mendoza. Ya que sin el apoyo de todos ellos no su hubiera logrado la terminación de este proyecto.

Diseño de vehículo Baja sae

I. Introduction

A

utomotive design is one of the areas where engineering knowledge is applied in several systems such chassis structural analysis, mechanism design (brakes, transmission, steering, etc.), stress analysis, materials selection and manufacturing. The project BAJA SAE requires applying several analyses in the design process, which also needs physical testing on order to achieve a good design. This project was developed with the purpose to design and manufacture an all terrain vehicle prototype. The University of Juarez has participated in a competition called BAJA SAE in the past three years in which the competition results have been improved each year. The project also required to use the SAE rules that each year are published for the competition and used to assure that vehicle design meets them. There are several aspects that the design must meet in order to comply with the rules requirements and participate. Based on this rules and order aspects the project was initiated and completed with the validation of the prototype. There are several aspects that the vehicle must meet with are the following: ɶɶ Must meet safety design. This means rollover protection, brakes and lighting. ɶɶ Meet performance criteria’s. ɶɶ Meet building standards to comply with safety. ɶɶ Meet materials standards to comply with safety The vehicle for this project is important to design and improve over the past prototype in several areas. The main goals in this vehicle design are the following: ɶɶ Improve performance by design a new gearbox. ɶɶ Reduce vehicle weight. ɶɶ Design a new suspension system. ɶɶ Design a new steering system to improve maneuverability.

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The goals will be achieved through several steps and work among the research team with the support of the students. Once revise the project requirements is needed to develop a work including concept design, detailed design, mechanical analysis (stress, dynamic and others), and fabrication (manual and automated processes (CNC , EDM).

Diseño de vehículo Baja sae

II. Proposal

T

Background

his technical report consist on the description of the four-wheel vehicle that was being designed and constructed with the purpose of participate on the competition Baja SAE 2011. The design objective is a light but strong vehicle based on the regulations for the competition and dedicated to reach the expected results on performance and cost. The society of Automotive Engineers (SAE) is the organization that sponsors and organizes the Baja SAE competition. This is an international competition that promotes design, manufacturing and project management skills on the students that participate on the project with support of a academic advisor. The competition was established in 1976 in the university of South Carolina under supervision of the Dr. J. F Stevens. Since then, this event has become a premier design event for engineering students. The main objectives of this design project are to enhance vehicle performance and cost, meeting the rules requirements. Some of these requirements require using engineering knowledge and tools such manufacturing technology applications, CAD/CAM/CAE, cost analysis, etc. Also the design requires performing validation and test to assure vehicle will stand the completion tests. The vehicle design consist on a frame chassis that main objective is to design the cockpit to withstand rollover situation which prevents that pilot suffers any injuries. A key part of the performance is the engine, which is the same for all participants. Finally there is the score process for each of the events and a final score is giving to each participant and ranked among all registered in the event. Usually 100 or more universities participate on each of the events that SAE sponsors on the BAJA SAE category. Design in the automotive industry is one of the most challenge for a new product because requires a good design process and fabrication which will assure a good performance and reliability. Since the car has several systems, each requires a particular approach in order to meet car design requirements.

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These systems, which we list as follows: ɶɶ Chassis: the main structural element in the car. ɶɶ Suspension: System, which controls the ride, comfort as well the maneuverability of the car. ɶɶ Power train: This system provides the energy to move the car at specific speed based on weight and dimension. ɶɶ Transmission: Provides the ratio between power and tires assuring a good traction and speed. ɶɶ Brakes: This system is very critical for the safety of the vehicle when used by the final user of the vehicle. ɶɶ Lights: System the provides lighting capability and safety since is used an visual control to others. ɶɶ Steering: The steering system is a key part of the front suspension since enhance the maneuverability of the vehicle. Considering all these requirements for the car the design process was developed for the car, which included body panels.

Theory framework Parameters The parameters that are defined for the car are established in the BAJA SAE rules, which are shown in the following chart. Table 1. Table1. Vehicle parameters Vehicle Dimensional Requirement MIN

MAX

Wheelbase

AD

108 IN

Overall height

52 IN

AD

Front track

AD

64 IN

Rear track

AD

64 IN

Engine power

AD

10 HP

Ground Clearance

AD

12 IN

Material

1 IN DIAM

AD

Others requirements are hitch points, electrical system, safety guards, brake system, fuel system, fasteners, and suspension and steering system.

Diseño de vehículo Baja sae

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Chassis roll cage requirements Parameters for the chassis requirements are several which will be describe in order to properly design this structure and analysis from the mechanical and stress performance. Cockpit. The cockpit is the key section of the vehicle since has the most important job in the car design: protect the pilot’s physical integrity during a rollover situation. So should be very stiff and light at the same time. Some these requirements are as follows: ɶɶ Safety of the pilot in case of rollover ɶɶ Safety in case of frontal or rear crash ɶɶ Dimensions to fir 95% of the percentile of the users. ɶɶ Material that meets specifications of steel SAE1018 with 1in diameter and 3.00 mm thickness. Additionally structural analysis will be required. Engine Requirements The vehicle engine requirements are established in the SAE rules(1) which mention that only Briggs & Stratton (B&S) engines with the following parts number should be used: Baja Acceptable Engines 20S332 0036-F1 205432 0536-E9 205332 0536-E9 205332 0536-B1 The engine is rated as a 10 hp power and according to the rules no modifications are allowed the modify engine performance. An other key point is rpm must regulated to top 3800 rpm since B&S will verify it prior to test. Suspension The suspension system requires a new design that allows better maneuverability and better steering. The basic requirements are to withstand track terrain and design. Since there are no additionally requirements in the rules, are at the team option the improvements in design and manufacturing requirements.

II. Proposal

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Steering system The steering systems is also requiring a basic approach, which is safety of the vehicle, and withstand track terrain and design. No further requirements are requested. Brake system The requirements of the brake system are two and they consist in the following: ɶɶ Able to block all four wheels at the same time. ɶɶ The hydraulic system must have a master cylinder with independent circuits for front and rear wheels. Must be shown the proper selection was made in order to assure safety of the person and vehicle. Electrical system The electrical system requires the following items: ɶɶ Should use to killing switches for emergency shutdown and the switches must be placed on in front and one in the back of the vehicle. ɶɶ For lights an additional battery should be used, and the lights used must meet SAE “U” or “S” rated. Finally there are other areas where is require to meet engineering standards which will assure safety of the vehicle. Some of these are: ɶɶ Fasteners. Should meet a minimum of SAE grade 5 according to the rules. ɶɶ Safety equipment: Pilot restrains systems must meet either SFI Specification 16.5/16.1, or FIA specification 8853/98. ɶɶ Guards: the body panels and other guards should be design to protect pilot from coming injured during a crash situation and protect moving parts from injure a person in case of failure.

Diseño de vehículo Baja sae

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Manufacturing Manufacturing engineering is technology that applies techniques or equipment to improve processes. During the project three main processes will be used to manufacture the main components. The three processes are: ɶɶ MIG welding.- This welding process is defined as metal inert gas process which uses automatic wire feed in the welding process. This process has given good results in previous projects. ɶɶ Turning.- This is one of the process that many of the parts will be fabricated. The current application will be on manual process. ɶɶ CNC process. This automated process has different field such, turning, milling, drilling, EDM (electro discharge machine) and others. The project is considering milling and wire EDM mainly on suspension and gear box assemblies.

II. Proposal

III. Methodology

T

he design development and fabrication was made in a team scope where the professors and students design and manufacture the key components of the car. Each of the systems was developed having in main the objectives of the car.

Chassis frame The chassis of a vehicle is the armor on which are mounted and holding all the mechanisms, supporting the weight of some of them and some other components are hanging of it. The most important objective of our chassis design is to ensure driver safety, but it is also a priority for the structure to be as light as possible. For those reasons, the two most important things were considered on the makeup of the structure are the weight and the stiffness.

Material Due to the SAE rules, the frame must be made of circular steel tubing with an outside diameter of 25mm (1 in) and a wall thickness of 3mm (0.120 in) and a carbon content of at least 0.18% (1018 steel), but we can also use a steel shape that exceeds the bending stiffness and bending strength of 1018 steel and having at least a wall thickness of 1.57mm (0.062 in). The frame of our car is made of chromoly 4130N tubing whit an outside diameter of 31.25mm (1.25 in) for the main roll cage members and 25mm (1 in) for the rear some roll cage members, both having a wall thickness of 1.625mm (0.065 in), which meets the requirements. Besides, it was calculate the bending stiffness and bending strength of chromoly tubing and demonstrated that exceed the requirements (See table 2).

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Table 2 1018 Steel

4130N Chromoly

Outside diameter (m).

0.025

0.03125

Wall thickness (m)

0.003

0.001625

c (m)

0.0125

0.015625

Yield Strength (MPa)

365

778

Modulus of elasticity, E (GPa)

205

205

1.277x10-8

1.6641x10-8

Bending Stiffness (Nm2)

2619.42

3411.51

Bending Strength

373.107

828.615

I=

π (D -d ) 4

64

4

(m4)

Chassis design One of the goals was to create a good design. It was needed to create a small structure that allowed to have more maneuverability, but also to create an ergonomic design. When designing the chassis, was ensure to act in accordance with all specifications that SAE rules demand. In addition, was designed thinking about the manufacturing; because of this, it was optimized the model having less bent members. Moreover, the design counts with front and rear bumpers, to avoid any collision damage in some element of the chassis and/or the suspension. The dimensions of the chassis are the followings: from rear to front bumper the distance is 2.11m (83 in), the width is 0.91m (36 in), and the height is 1.21m (47.5 in). These are dimension only of chassis, without considering the suspensions and tires.

Analysis To ensure that chassis of the vehicle is tough; it was made some FEA (Finite Element Analysis) using Solid Works Simulation software. It was analyzed the structure for three different collisions: front and rear impact, and rollover. For simulation, was defined a material with chromoly specifications. Besides it was made real destructive collision test using a prototype of the chassis. Front impact simulation – In this test the chassis was subjected to a force of 12000 N over the front bumper with a direction normal to the right plane. The results show that the minimum factor of safety (FOS) is 2.7 on this section. Rear impact simulation – For this test, also was use a force of 12000 N this time over the rear members that are at the end of the vehicle. The result of this test showed a FOS of 1.68. Rollover simulation – This time, to simulate a rollover of the vehicle in the completion, it was applied a force of 15000 N over the RHO members, with and angle of 60 degrees to the top plane. The FOS obtained in this test was 1.58.

Diseño de vehículo Baja sae

Destructive collision test – Additionally of using FEA software to analyze the structure, to assure that the chassis is really tough, a prototype was built to make a destructive test. The prototype was made of industrial tube with an outside diameter of 25mm (1 in) size 18. The structure was fixed to the ground, and another Baja vehicle crashed on it at 30 km per hour speed. The figures 1 show the results of the test. The destructive test was good; showed that will stand a frontal and rear crash. With all this information it was concluded that chassis structure design meets the objectives and requirements.

Transmission design To make the vehicle move, the power of the engine must be transmitted and augmented in some way; for this reason the car has a gearbox (transmission). The gears system augments the final torque that the engine transmits to the wheels so they can start moving.

Gear box design According to the needs the gearbox was designed with a double stage gear reduction box. The calculation of design was to transmit a final torque of 198Nm (1750 lb-in). Figure 1

III. Methodology

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Gear and shaft specifications - These are the characteristics of the four spur gears of the transmission uses: for the first reduction it has a 19 teeth gear and a 57 teeth gear with a diametrical pitch of 12, both of them with 12.5mm (0.5 in) of face width, and for the second reduction it counts with a 18 teeth gear and a 45 teeth gear with diametrical pitch of 8, both with 18.75mm (0.75mm) of face width. These gears give a first stage reduction of 3:1, and a second reduction of 2.5:1, multiplying this values we have a final reduction of 7.5:1. In addition, the shafts were designed to support all the stresses to which they are subjected, the entrance and the intermediate shafts have an outer diameter of 23.44mm (15/16 in), and the final shaft has 29.69 mm (1 3/16 in). For both gears and shaft is used a 4140 steel which is appropriate for this application. Covers design - The covers of the gearbox were designed to have an easy and quick assembly, to be strong enough to support all the loads on them, and to be as light as possible. The material used on the covers is aluminum 6061; this is kind of tough but at the same time is light, so is option for the design and manufacturing.

Drive system: cvt (continuous variable transmission) The revolutions of the engine must be transferred to the wheels somehow and part of that function is done by the CVT. The CVT is a transmission that can change effectively trough an infinite number of gear ratios between maximum and minimum values (called shift ratios). For this vehicle it was use a Polaris P90 that transfers engine torque and maximize the rpm that are delivered to the gearbox. Part of the innovation for this year was to modify the edge through a small cut to reduce weight while increasing the speed at which RPM increases these with the intention get better results on the back shifting of the CVT under certain conditions. Flyweight’s average weight is 82.5gr Figure 2 shows the flyweights and spring. Figure 2

Diseño de vehículo Baja sae

Steering design One of the key systems is the steering system of an automobile as a set of mechanisms that are responsible for guiding the front wheels for the vehicle to take the desired path; these mechanisms over time have been improved to achieve three main things for a management system, such as security, softness and precision. Taking into account these needs the design was done in conjunction with the front suspension system due to the great relationship they have with each other and thus be more efficient when they are manufactured, another point of the system is the principle that was based, called the Principle of Ackerman or Ackerman geometry. Which is used in the design of steering systems on all vehicles in series or competition.

Rack and pinion The Rack and Pinion that will be used has the following description, for a more comfortable driving wheel is used a diameter 10 in. This will add one item to the ergonomics of the vehicle, high efficiency, accuracy and greater control in both straight and curves whether open or closed. Figure 3 Rack and Pinion steering. Specifications – 5/8-36 Spline Stub, 15 in Center to Center (Adjustable), 1/2 Bolt holes.

Brake system For the new design it’s very important to have an optimum braking system. To make that, the team used formulas to calculate the distance, the time and the force necessary to stop the prototype. Figure 3

According with the rules of this year, and the help of the team’s last year experience, it was decided to use a CNC dual cylinder brake pedal with reverse swing mount of 3/4in pistons, that features an adjustable balance bar for front and rear. The decision it’s the best ergonomic option for the pilot’s comfort. Figure 4.

III. Methodology

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Physical considerations

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According to the summary of moments in the front tire of the vehicle normal force on the front rim is 1366 Newtons and normal force in the rear wheel is 884.67 Nektons. For the calculations, consider the mass of drivers = 69.5Kg and vehicle mass = 229.5kg. Total mass = 299 Kg. According to the calculations the strength to stop the vehicle equals (mass) (deceleration) = (299Kg)(4.905m) = 1466.5 Newtons. The BRP stainless steel brake lines of 1/8in were chosen to resist the 12MPa pressure that offers the master cylinder. The two piston BRP calipers were the best option to fit in the new design. With a force of 7KN, the piston pushes the brake pads against the rotor, making a uniform brake. Considering the magnitudes of the force, the material has to be such as resistant as the force applied in each part of the system. In short was needed a force of 1466.5N to stop the vehicle and have a force of 7000N as the vehicle stops whatever the condition of the floor. Figure 4

Suspension design The suspension is one of the most important things in a car, it helps in every way, if well design it could make the car overcome any obstacle on the road. Basically it is made up of a rigid structure with shocks and springs. DESIGN.- Once the purpose is taken, it is time to design the suspension according to the needs. The suspension should be enough short to make the car light but still strong to make a 12 inches travel. With this sketch at hand every change made could be seen in the overall movement of the tires.

Diseño de vehículo Baja sae

The objectives of this suspension are: ɶɶ The most ground clearance. ɶɶ A good position of front tires when steer. ɶɶ A 12 inches travel in front suspension The ground clearance was obtain with a front suspension of double A arm, having the lower A arm almost horizontally, making the size of the tire the height of the front part of the vehicle. Suspension geometry affects steering, that was solve having the front tires tilt depending to the turn so that the tires could dig the ground and force the car to turn. All without sacrificing the long travel of the tire. The front suspension has a 8º of caster and 5º of kingpin. The caster angle was determined by having the tire tilt to the turn side, and the kingpin angle for reducing the tire horizontal drag when moving vertically without restricting steering. The rear suspension is a swing arm, which it restricts the rear tires movement by only vertical displacement, which means 0º camber and it helps for a better traction. The limits of the rear suspension are the continuous variable joints, because their length will determine the width and frame will put the base for it, giving the length required.

Manufacturing The manufacturing process for the vehicle was based in three main processes. MIG welding process, manual milling and turning process and milling CNC and EDM for key components. Parts design was based in cad/cam/cae software to develop parts and process them in our manufacturing systems laboratory. During the process key steps were considered to assure the quality and functionally of the assemblies. One key step was to make additional prototype parts to get experience mainly on welding.

III. Methodology

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IV. Results As was mentioned in the project objectives, which are the following: ɶɶ Improve performance by design a new gearbox. ɶɶ Reduce vehicle weight. ɶɶ Design a new suspension system. ɶɶ Design a new steering system to improve maneuverability. The vehicle was improved in overall on each of these goals. During the process of testing and validating these goals were met. During test brake was better, cornering was much better than expected. In the other hand the gearbox was slim and lighter as planned. However, still some work was needed on suspension. It failed during some to rough terrain. The steering system was also giving good performance during testing. The vehicle was improved in suspension. During testing better corning was better that previous version.

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V. Conclusions The project was concentrated in one of the principal goals, to reduce weight. In every system was made a reduction of weight without sacrificing the performance of the new vehicle. There were several improvements including, a better CVT Shifting, Suspension and Steering. Also the results obtained during testing and competition allowed to evaluated furthermore the design improvements areas, in order to enhance vehicle performance.

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References 1. 2011 Baja SAE Competition Rules 2. MOTT, Robert L. Machine elements in mechanical design. Fourth edition. 2006. 3. Miliken, William F., Milliken, Douglas L. Race car vehicle dynamics I. 1995. 4. Miliken, William F., Milliken, Douglas L. Race car vehicle dynamics II. 1995. 5. Indios Baja SAE Design report. 2011.

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