Birla Institute of Technology, Mesra,Ranchi
Team Firebolt 2010 SAE BAJA ASIA COMPETITION CAR #82 Technical design report
Birla Institute of Technology, Mesra 2010 SAE Baja Asia
Firebolt - CAR#82
Design Report
Birla Institute of Technology, Mesra , Ranchi SAE Mini-Baja Design Report J.Pramoth, Hari Sudarshan, Japveer Singh Arora, Abhishek Ranjan, Amrit Raj, Shravan Kumar Ponnada, Abhishek Sharma, Arka Pratim Das, Rahul Bajaj, Kumar Prakhar and Anuj Sharma Mechanical, Production, Electronics and Electrical Engineering
Dr. Arbind Kumar and Mr. Arun Dayal Udai Faculty Advisors Copyright © 2009 SAE International
ABSTRACT The SAE India sponsors an annual engineering design competition that challenges students from various colleges and universities to design, construct, and race an All Terrain Vehicle with enough speed and agility to be capable of enduring various terrains. This year, the Birla Institute of Technology Mesra, Ranchi is planning to participate in the 2010 SAE Baja Asia competition. This competition requires each contending team to build a Mini-Baja® vehicle that fulfills both static and dynamic requirements as outlined by SAE. Additionally, each team must submit an analysis of their unique design in the form of a technical report.
INTRODUCTION
being subjected to extreme conditions and rugged terrain, emphasis was given to durability without compromising on performance.
Vehicle Specifications
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Overall Length:
2400.0 cm
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Wheel Base:
1960.0 cm
• The design is a result of many great design attributes including a durable and well performing front suspension, a simple and well-performing steering system, and a lightweight and robust chassis. The vehicle designed was aimed to improve on these elements, while creating and integrating a rear suspension, and increasing performance and durability throughout the vehicle. Weight, cost, and manufacturability were also major considerations. All subassemblies and components were researched and designed to meet rules set by the organizers. The car was modeled entirely with Catia software allowing easy and simple integration and mass properties to be determined. The car was designed with an ultimate goal of providing safety to the driver. Also with the vehicle
• •
Roll cage material Overall Weight
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Engine:
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Maximum Torque: Maximum Speed:
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•
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Transmission Ratio Mahindra&Mahi ndra o Maxim
• SAE 4130 chromoly alloy steel 1.25”*0.065” 250 kgs w/o driver LOMBARDINI LGA 340 10 HORSEPOWER 2.01 kg-m
60 km/hr
7.35:1 31.48:1 •
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Front suspension
A-arm
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Rear suspension
A-arm
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Front Tires:
ATV 21X 7X 10
1.00”X0.083” tube. Preference for wall thickness was chosen over OD for different strength members because less tooling would be necessary .Since the minimum wall thickness required is 0.065” the required material is selected.
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Rear Tires:
ATV 22x 8 x 10
Comparison Between 1018 and 4130 tubing
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Steering:
• • ••
Suspension: o Front o Rear Brakes:
6.4:1 Rack and
Material
1018 Mild
4130 Chromoly
O.D(in.)
Steel 1
Steel 1.25
Wall thickness(in.)
0.12
0.065
Weight(kg/m)
1.682
1.220
• 7 inches travel 10 inches travel • Hydraulic Discs (4)
CHASSIS ABSTRACT The purpose of the frame is to create a three dimensional space for the driver to fit within the steel roll cage while driving the car and still be in a safe environment. SAE provides all teams with a list of requirements that must be met in order to pass the technical inspection that the car will be subjected to upon initiation of competition activities. The requirements to be met range from material constraints to space limitations, making the material and solid model challenging to design, all of these conditions must be met while keeping in mind off road race cars concepts such as horsepower to weight ratio, car’s center of gravity, ground clearance, vehicle dynamics, height, width and length amongst many other concepts. MATERIAL SELECTION The material chosen for the manufacturing of the chassis was 4130 Chromoly Steel. Although thousands of steel alloys exist, only two types, 1018 and 4130, are readily available and reasonably priced. The most common carbon steel, 1018, is a basic structural steel that welds and forms very well; however it has a lower overall strength than 4130. 4130 Chromoly is known throughout the racing industry for its high strength and outstanding welding characteristics. As a result of these characteristics Chromoly was deemed the best material for the chassis. The tube is of 1.25” O.D, 0.058” wall thickness and circular cross section. The desired tubes shows a weight reduction over the 1018 MS 1.25”X0.065” by 10% and 19% over the 1018 MS
Ultimate 415.064 1110.055 Strength(MPa) Bending 36.17 49.66 Stresses(Kg-m) Thus from the table we see that on using 4130 Chromoly steel over 1018 mild steel we have weight reducing by 0.461Kg/m.
FRAME With a limited amount of power the focus is primarily on the power to weight ratio of the vehicle. With the engine limitation the only means to improve this critical parameter is to reduce the overall vehicle weight. Safety for the driver and ergonomics throughout the design of the frame has been given utmost importance. Use of CATIA V5 also allowed the team to plot full scale prints of each individual part which provided a quality control check of each part that had to be out-sourced. The components of the frame are the RRH, LDB, RHO, FBM, LC, LFS, SIM, FAB, and FLC (See Figure A-1 and the Acronym list for member clarification). Per SAE Competition Rules, the RRH, LDB, RHO, FBM, and LC material properties were required to have a bending stiffness and a bending strength equal to or greater than that of 1018 steel with an O.D. of 1 in. and a thickness of 0.12 in. Members LFS, SIM, FAB, and FLC were required to have a minimum wall thickness of .035 in and a minimum O.D. of 1 in. All frame members with a bend radius greater than 6 in. may be no longer than 28 in. unsupported. Clearance guidelines dictate a minimum of 6 in. vertical distance from the driver’s head to the
bottom of the RHO and 3 in. clearance between the rest of the body and the vehicle envelope. The frame was designed according to the specified rules. Throughout the design of the chassis a 3D solid modeling program called CATIA was used. CATIA allowed our team to visualize the design as well as integrate all of the systems with the chassis before manufacturing begins. ANSYS allowed our team to perform finite element analysis on the chassis to determine the strength and stresses of the design. These tools decrease waste, and increase productivity by determining all issues with potential designs before any material is machined. Fig2. Illustrates the von Misses stress for a side impact The team was familiar with static linear FEA analysis using ANSYS. The analysis concentrated on estimating the linear von Misses equivalent stresses in the planes of the pipe elements from which the model was constructed.
Fig 1. The above fig. illustrates the von Mises stress for a side impact. As predicted theoretically the maximum stress occurs at the joints and the vehicle does not undergo very high levels of stresses thereby protecting the driver in case of inadvertent conditions.
and the corresponding stress are shown .The above condition can occur when the car is being hit by another baja vehicle at top speed.
Fig 3. Illustrates the von Misses stress for a rear impact and the corresponding stress are shown .The above condition can occur when the car is being hit by another baja vehicle at top speed.
DRIVETRAIN The goals for the powertrain and driveline were: • •
Reduce cost of the components Increase the performance of the system, by eliminating losses
Harnessing the power of the 340cc, 10 Hp Lombardini LGA 340 engine and efficiently delivering this power to the tires is essential for peak performance. The primary reason for our choice of Mahindra & Mahindra champion alpha transmission as it was widely used in the previous Baja events and it has withstood the harsh conditions that the event track posed. Finally we implement a chain type drive in the design of the powertrain.
BRAKING The goals for the braking system were: • • •
Reduce weight in the overall system, Increase reliability Improve performance.
Another performance enhancing feature of the vehicle design is the braking system. The design team must decide upon an effective braking system which will be reliable for the life of the vehicle. There many different types of braking systems available for our application. However, the ease of manufacturability and performance are the driving forces of this design. The two main types of braking systems that the design team has analyzed are drum and disc brakes. The system must also meet the SAE braking requirements. This requirement simply states that the vehicle’s braking system must be capable of locking all four wheels on a dry surface. The first option the team explored was the use of drum brakes. A drum brake system consists of a rotating cylinder which is stopped through two brake shoes. The brake shoes surround the outer surface of the rotating cylinder. The braking system consists of a pedal mechanism which applies a force to the brake shoes through the use of a cable. The brake shoes then apply pressure to the inner surface of the cylinder, which in return, stops the wheel from rotating. It was determined the use of a braking cable arrangement is not feasible as the lack of area on the vehicle’s chassis. Another disadvantage of the drum brakes is encountered in an off-road application. Since the vehicle will be traveling through mud, there is a high possibility of mud and debris to gather in the space between the shoe and the drum.
The other consideration is the disc braking system. Braking with this system can be obtained both mechanically and hydraulically. However, the same problems of the drum brake occur with a mechanical disc brake system. A hydraulic disc braking system uses fluid displacement to engage the brake calipers on the rotor. This is a more ideal system because there is no need for a mechanical system. Therefore, available area along the vehicle’s chassis is not required. Another advantage is mud build up is no longer a limiting factor. From analysis of the different systems, the team has decided to use four wheel hydraulic disc brakes. This will allow the vehicle to meet SAE’s braking requirements, while enhancing vehicle performance. On using aluminium pedals instead of mild steel pedals the weight reduces to a certain extent.
STEERING The steering system was to be chosen with the main focus being handling, durability and less complexity. For this two different systems were considered. Firstly, the standard direct link Pitman arm gear which requires more linkages with heavier mounting. On the other hand the design chosen is a simple rack and pinion steering gear which has refined roll cage along with space liberation and integrated mountings without compromising the safety of the driver. The principal of rack and pinion steering is very simple. Turning the wheel results in the turning of a pinion located in the steering box, which is matted with a rack. The linear movement of the rack pushes and pulls the tie rod ends, causing the wheels to rotate, and therefore turning the Baja. There are two large advantages to a rack system: it is generally easier to steer and less susceptible to failure. This is due to the fact that it uses mechanical advantage to spin the wheels and rugged. Also, the tie rod ends are generally shorter than in a direct steering system, making them less likely to fail. In choosing the rack and pinion steering system, we decided that device durability was the most important feature. A steering system
failure would significantly reduce our chances of winning the competition. Another important factor is ease of steering, where a rack and pinion system has the advantage. The system can also be bought out of the shelf, requiring less design time. This steering platform is designed to increase impact harshness isolation, handling, and durability characteristics of the vehicle suspension STEERING WHEEL The design of the steering wheel did not require any engineering calculations; rather it was an exercise of intuitive judgment. Since the wheel would be rotated over 180° to either side, a traditional circular style was chosen, as opposed to a butterfly or ¾ circle. Two diameters were readily available: 25 cm (10 in.) and 36cm (14 in.). The 25 cm diameter steering wheel was chosen to allow more room for drivers to enter and exit the vehicle. It was also judged that the car was easy enough to steer that a larger lever-arm was unnecessary. Even though using intuitive judgment as a design tool involves some risk, it can never be avoided in any design task. A hard eco-friendly rubber would be the material used to make the wheel. STEERING COLUMN The steering column would be an electronic steering column consisting of an electronic motor and a rotary valve. The rotary valve is present to detect when the force is applied by the driver on the steering wheel which is only during turning. INNOVATIONS These are some of the modifications that we plan to implement in the rack and pinion system of steering•
To improve and quicken the steering pushback after we turn, a spring system on the rack and pinion is used.
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Variable pitch( no. of teeth/cm) rack would be used with pitch towards centre on both sides of the pinion is less and more towards the end which helps in providing a lesser steering ratio.
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Reduction in size of rack and tie rods to help in achieving a smaller turning radius.
SUSPENSION The focus of suspension design was to optimize this system to achieve the best vehicle performance and durability with less weight. To increase suspension performance, the geometries and properties of the front and rear suspensions and steering were modeled and simulated using CATIA. Durability, weight, and cost were factored in during the suspension design process to arrive at a final suspension that not only holds up to racing conditions, but gives better vehicle handling, impact harshness. SUSPENSION GEOMETRY Front Suspension The front suspension of the Baja vehicle serves several important purposes. It acts to absorb the impact of obstacles while providing full driver control, keeping the front tires in contact with the ground. A well-designed front suspension accomplishes this task while accommodating for other factors such as body roll, weight transfer, and steering geometries. A double Aarm design was chosen to accomplish this because of its versatility and simplicity. Two possibilities for the front suspension were a double a-arm and a single arm McPherson Strut suspension. The double a-arm suspension is the most common suspension for off-road vehicles and has been proven effective by testing from previous mini-baja teams. Double a-arm allows for good control over wheel angles and produces minimal camber gain over large amounts of wheel travel. Double a-arm is easy to design and manufacture as well as assemble and service. It also provides more protection to the steering linkages. Single arm with a McPherson Strut is an option to reduce weight and parts. The major compromise of the strut suspension is the large negative camber gain in jounce. Compromises in geometry and packaging were also negative factors. Based on the above analysis and past competitions the double a-arm suspension was chosen to accomplish this because of its versatility and simplicity.
pin relative to the ground. However, while the benefits of caster are obvious, the amount must be moderated. Too much caster will force the vehicle to change heights laterally, which gives the vehicle bad shock absorption and brake dive characteristics. A greater caster angle .......also requires more steering effort as the driver is trying to upset the balance of the vehicle
Starting with certain design parameters such as optimal vehicle width, desired wheel travel, and ideal ground clearance, the A-arm lengths and suspension hard points could then be established. Wheel travel is 13 inches while ground clearance at ride height is 12 inches, allowing the vehicle to maintain control over aggressive terrain. By prioritizing and achieving high camber control over other characteristics a large tire contact patch can be maintained improving vehicle performance and control. The larger tire patch provides greater tractive forces, which improve vehicle stability and driver control over rough terrain. Ideally, the front roll center should be placed close to the ground and to the vehicle’s CG. The higher the roll center is above the axial center of gravity of the vehicle, the larger the roll moment. A larger roll moment will produce a larger side force thus constraining the vehicle of the desired steering and stopping control. Consequently it is still very desirable to maintain a low and laterally centralized roll center while designing around other prioritized variables. Another consideration while designing the front suspension was caster angle. 6 degrees of positive caster is used for this vehicle. By implementing this caster, the contact between the tire and ground is behind the king pin axis (the axis about which the front knuckle rotates to steer the vehicle), which gives a stabilizing effect when driving. A second function of the caster angle is to decrease the overall stresses and impact loading on the chassis. Normally, the caster angle is measured at the wheel king
Rear Suspension The design of the rear suspension was begun by setting requirements that the suspension needed to meet. Parameters such as max width, ground clearance, and wheel travel provided a good starting point on which to begin the design. These parameters were set by the team from experience with vehicles in the past. Many of these design parameters intentionally duplicate those of the front suspension. By matching these two systems, the team could tune the suspension more consistently. Therefore much better roll steer, pitching moment, brake and acceleration pitching, and low frequency vibration isolation could be attained. Upon optimization of the rear suspension, a design was agreed upon that would give 10 in. of shocks travel and only 4.4 degrees of camber change. The roll center was tuned to be 5 in. from the ground. SUSPENSION DESIGN The front suspension was made to handle both suspension forces and front and side impacts.. The Aarms were designed around these parts. The lower Aarms contained the shock mount, so 1.0 X 0.065 inch 4130 tubing is being used for lower A-arms to add strength in tension, compression, shear, and bending.
The upper A-arms saw mostly side impact loads, which placed them primarily in tension and compression. Therefore, these A-arms could be constructed of 0.75 X 0.035 inch 4130 tubing.
TYRES In an all-terrain vehicle, traction is one of the most important aspects of both steering and getting the power to the ground. Tire configuration, tread depth, weight, and rotational of inertia are critical factors when choosing proper tires. The ideal tire has low weight and low internal forces. In addition, it must have strong traction on various surfaces and be capable of displacing water to provide power while in muddy conditions .The chosen tires are ATV 21X 7X 10 at the front end and ATV 21X 8X 10 at the rear end.
ELECTRONICS SPEEDOMETER The objective of the speedometer is to have a system in place that can display speed and ride time. The system must be able to handle the rough conditions that exist on the Mini Baja track such as water, mud, sand, rocks, and generally rough terrain. The speedometer must be able to record speeds of up to 50 km/h. Several alternatives were examined. First was a traditional car speedometer. These setups would be able to withstand the harsh terrain but were too large, expensive and bulky for use in such a small vehicle. Second was a bicycle speedometer, which was cheap and capable of recording both speed and runtime. The bicycle speedometer was chosen over the standard car’s speedometer. . The driver is now capable of monitoring speed as well as ride time easily. The system will be mounted in the cockpit right in front of the steering wheel. It is a location where it will be safe from damage in the event of a rollover, and where it can be clearly seen by the driver. ENGINE KILL SWITCH
The last function of the electrical system is to shut off the engine. The rules specify that two switches are required in the vehicle, one within reach of the driver and the other within reach of an observer. An emergency style push/pull switch is located in the cockpit as well as at the rear of the vehicle. One wire from each switch was connected to the kill wire on the engine while the other switch wires were grounded into the chassis. If either switch is flipped, the magneto in the engine is grounded and thus shutting down the engine. The system is simple and inexpensive. Its simplicity ensures that it is reliable while providing the necessary functions. BRAKE LIGHT A pressure-activated switch was mounted in line with the rear brake line. When the pedal is pressed and pressure is built in the line, the switch closes, activating the light. Power is supplied by a custom battery pack designed to fit inside the bottom firewall tube. This keeps weight low, the pack out of the way, and eliminates the need for a mount. The brake light utilized is an 11” LED third brake light bar that meets SAE specifications. While the cost is slightly higher than traditional bulbs, an LED light was used because of the low power consumption and extreme durability and life of LED’s.
SAFETY Drivers should be able to experience fast pace, exciting racing without risking major injury. Our car meets or exceeds all of the minimum safety requirements composed by the Society of Automotive Engineers and the event coordinators. In addition, a number of safety features have been added to further reduce the possibility of personal injury. • • • • •
An LED brake light warns other drivers of deceleration. A safety helmet and neck support protects the driver. Composite chain and belt guards protect fingers from accidental insertion. A Six-point safety harness keeps the driver adequately restrained. Roll cage padding protects driver’s head from impact.
2. Japveer Singh Arora – Team Captain Extraordinary measures have been taken to make driving the Mini-Baja car as safe as possible. Experience and responsible engineering have yielded a near perfect safety record and driver confidence. To further increase the safety of the vehicle, all operators should be educated in the operation of the vehicle and made aware of possible risks. An alert, intelligent operator of the Mini-Baja car should enjoy ride without any risk of injury.
[email protected] 3.Hari sudarshan
[email protected]
APPENDIX PERFORMANCE OF TUBING MEMBERS AS CONCLUSION The desired characteristics of each subassembly were carefully analyzed and designed to achieve the ideal characteristics. Catia and Ansys software packages were utilized to model every aspect of the vehicle. The design includes: •
A light and tough chassis.
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An innovative steering system. • • •
Double a-arm suspension in both the front and the rear. Hydraulic braking system in both the front and the rear. Light weight material 4130 Chromoly steel is used.
The result of this research, design and fabrication is the Birla Institute of Technology, Mesra’s baja vehicle.
ACKNOWLEDGMENTS The team appreciates the support and guidance of the Mechanical Engg. Dept. and the Production Engg Dept. for allowing and encouraging our team to participate in this all Asian SAE Baja 2010 event. We also thank our faculty advisors Dr. Arbind Kumar and Mr. Arun Dayal Udai for their knowledge and inputs.
CONTACT 1. J.Pramoth - Team Member
[email protected]
PER RULE 31.4
