Team VEGA JSS Academy of Technical Education Preliminary Design Report SUPRA SAEINDIA 2014 ABSTRACT This project required immense research and study of various systems incorporated into a formula race car and also the design methodologies used in today‟s industrial scenario. It also included the study of the previous models built by other teams. It challenged our knowledge of the engineering principles and practices. The process started with the identification of the major goals of the project and conversion of those objectives into measurable quantities. After this various constraints were identified. The team was then divided into various sections and work was allotted to each section according to their field of expertise. The design for each component was iterated until the desired objectives were fulfilled.
of motion and a compromise has to be made between the kinematically and practically optimum geometry. Team VEGA decided to go with the fourwheel independent Double A-Arm suspension design. A wheel size of 13 inches (330.2mm) was chosen in order to meet the demand for acceleration as well as handling. The team‟s plan was to pack the entire upright and kingpin assembly within the wheel therefore a wider wheel size with large offset was chosen. As our wheel was 330mm in diameter hence the upper and lower ball joints could not be more than 300mm apart. As for the packaging of brake assembly and the uprights, suitable clearance was maintained to locate the lower ball joints inside the wheel.
SUSPENSION DESIGN
After putting the variables in place, kinematic design was started. This included the length of the desired front view swing arm (FVSA). Once this was located, a desired location of the roll centre was marked on the centre line of the vehicle. The basic goal was to find the optimum location of the RC and to minimize RC migration. Now as the vehicle is cornering, lateral forces at the Centre of gravity (CG) of the frame tend to create a moment about the roll Centre, the RC hence should be as close to the CG as possible, however a high RC would create undesirable “jacking” forces. After a number of iterations a RC height of 45 mm above the ground was chosen. Once the RC is determined, a line between the Centre of the tire patch and the RC is drawn to find the location of the IC, this also finds out the upper arm angle. Finally the data is fed into a suspension modeling software which calculates the roll Centre location for various upper arm lengths and body roll angles. After numerous iterations a suitable arm length is chosen. A suitable camber angle was decided to ensure maximum contact of tires with ground.
The suspension system of a car serves a dual purpose – contributing to the car's handling and braking for good active safety and driving performance, and keeping vehicle occupants comfortable and reasonably well isolated from road noise, bumps, and vibrations. However keeping in mind the flat event track the suspension was designed to improve the vehicle level. Firstly the rulebook was studied to ensure compliance to SUPRA rule. As per the rulebook the car must have a wheelbase of at least 1525mm (60 inches) and must be equipped with a fully operational suspension system with shock absorbers, front and rear, with usable wheel travel of at least 50.8mm (2 inches), 25.4mm (1 inch) jounce and 25.4mm (1 inch) rebound, with driver seated. Also the wheel size is restricted to a minimum of 203.2mm (8 inch). Once the rulebook had been studied the goals and constraints were identified to initiate the design of suspension geometry. Suspension geometry is governed by kinematic laws
CHASSIS AND FRAME DESIGN The Chassis design is done keeping in mind the requirements of high torsional stiffness, driver‟s protection, manufacturability, serviceability, compactness and packaging of all other components. The overall structure is compiled to reduce the complexity and fulfillment of systems packaging and so the suspension pick-up points are calculated taking consideration of the diveness and Squatness for chassis members. IS 3074 (ERW) steel spaceframe is chosen over 373MPa Yield Strength and Roll Hoops including Main Roll hoop and Front Roll hoop with ASTM 106 (Seamless) due to bending strength and basically the availability in the desired wall thickness in nearby region was primary reason. With optimized amount of triangulation and keeping the weight as low as possible, the chassis is targeted for 30 kg including brackets and mountings. Pick-up points of suspension and engine produces significant amounts of forces, thus they were attached to the frame at triangulated points. The frame is designed using CATIA V5 CAD and analysis work is done on CATIA V5 FEA and ANSYS 14.0. All harness and safety attachment points are given space to the requirements according to the Rule Book. The design is implemented to provide load paths and stress regulation throughout the extremities of suspension mountings including front impact members and side impact members to ensure complete safety to the driver. The farther material is from the axis of twist the stiffer the frame will be in bending and torsion. This concept is implemented by adding Structural Side Pods to the basic frame. ERGONOMICS: Properly incorporating the driver into frame design can be very difficult because of wide variations in driver sizes. Each driver interface has to be designed so that it is comfortable for accommodating drivers who range in height from 1.55m to 1.90m.Comparison of different material properties is shown below: Material Candidates IS 3074 AISI 4130 ASTM 106 AISI 1018
Design Requirements UTS (N/m2) TS (N/m2) 432 373 670 435 480 300 634 386
Density (Kg/m2) 7860 7850 6100 7800
TORSIONAL STIFFNESS: Torsional stiffness is defined as the resistance of the structure to torsional loads. The frame is subjected to frequent torsional strains while cornering, this also changes the wheel loads as calculated while designing of the suspension. Torsional rigidity of the frame is targeted for above 2000 Nm/degree. Otherwise undesirable changes in camber and toe can occur if the frame lacks stiffness. Each of the systems of a car must be packaged within the frame. The placement of the components limits the available paths for tubes, which is usually detrimental to the chassis stiffness. FRAME FINITE ELEMENT ANALYSIS: The assumption made in using beam elements is that the welded tubes have stiffness in bending and torsion and inclusion of transverse shearing effect. The analysis was done by taking the Linear Tetrahedral Geometry for meshing. Then results are analyzed and iterated for Von Misses yield Stresses and optimized accordingly for considerable factor of safety. Sr. no. 1.
Test Static Shear
2.
Static Overall Vertical Bending
3.
Static torsional loading
4.
Acceleration Analysis
Boundary conditions Clamp rear suspension mounts Clamp- front and rear suspension mounts Clamp- rear suspension mounts Clamp- front and rear suspension mounts
Force moments
Magnitude
Downwa rd force at front bulkhead Uniformly distributed loading
1440 N
Clockwise Moment at bulkhead side
310 Nm
Force applied towards rear
840 N
1500 N
Table: Boundary conditions of various tests.
STEERING SYSTEM The next way-point in the design process is the steering system. Steering is another important aspect of the vehicle‟s handling. If the system is not properly designed the wheel may toe in or out during suspension travel, this is known as „bump steer‟.
In order to minimize this event the IC of the tie rod must coincide with that of the control arms. This ensures that the tie rod travels in the same curvature as that of the control arms and there is no unintentional relative movement between the kingpin and the tie rod. GEOMETRY: Ackermann geometry is generally used in production cars in order to minimize tire slip. While this geometry is very effective in slow speed turns, it is not suitable for high speed cornering. During a high speed turn the inner wheel provides for very little cornering force therefore its slip is irrelevant and can be neglected. Most of the frictional force is generated by the outer wheel and therefore it accounts for much of the steering. Parallel or even reverse Ackerman geometry is used in these cases. A compromise between parallel and Ackermann geometry was chosen by the team. STEERING RATIO: Overall steering ratio is an important parameter because it determines how "quick" the steering is. Like many parameters, steering ratio is partly a driver preference, but some guidelines do exist. A very fast ratio will be more sensitive to the driver‟s steering inputs, but extreme sensitivity may increase driver fatigue or make it difficult for the driver to keep the car pointed in the desired direction when road inputs jerk the driver‟s hands. At the other extreme, too slow a ratio will require large hand movements, which is not desirable for the tight SUPRA SAE 2014 courses. Driver does not have to shuffle his or her hands around which would again reduce the car‟s maneuverability. In order to negotiate the tightest corners (m outer diameter) for the SUPRA SAE circuits with less than one rotation of the steering wheel, a steering ratio of about 8:1 was decided by Team VEGA. CASTER ANGLE: Positive caster angle induces a self-cornering force that provides straight line stability, but increases steering effort. Toe is the angle made by the wheel with respect to the longitudinal axis of the vehicle. Static toe of front and rear wheel is taken as zero to maintain directional stability.
TURNING RADIUS: According to the rulebook, the turning radius of hairpin turn is 4.5m. So we designed our steering geometry to take a turn of minimum 2.5m radius of circle. Table below shows the different steering and suspension geometry. KPI
3°
CASTER
4° positive
CAMBER
2° negative
TOE
0°
STEERING RATIO
8:1
BRAKES Brakes of a vehicle matter a lot in the performance of a vehicle. To get maximum heat dissipation team decided to use disc brakes. Parameters which define the performance of braking system were defined and calculated below. WEIGHT TRANSFER: To get a desire tractive force on drive wheels (i.e. rear wheel in our car) we fixed the static weight ratio of front and rear as 40:60 and it becomes 54:46 at the time of retardation in brake test as given in rulebook. BRAKE EFFICIENCY: Braking efficiency is defined as the ratio of maximum wheels-unlocked vehicle deceleration to tire-road friction coefficient. The braking efficiency expresses the extent to which a given tire-road friction coefficient available to a vehicle is transformed into maximum wheelsunlocked deceleration. Brake system is designed as to achieve maximum retardation of “g” .The distance travel by the car from 40kmph to 0kmph is 6.9meters.For µ(coefficient of friction)=0.7,brake efficiency is 88% which is fairly good and it becomes 100% at µ=1. BRAKING FORCE: Brake system is designed to achieve the ratio of 1.27:1 braking force on disc by calipers in front and rear wheels (which obtained according to the longitudinal weight transfer). Selected pedal ratio was 4:1 due to which pedal force require for this is 207.5N which can be easily apply by the driver. Rotors having diameter of 240mm and 220mm were selected to use in front and rear disc respectively.
ENGINE AND DRIVETRAIN ENGINE: For the purpose of SUPRA SAEINDIA 2014 competition our team has chosen an engine from a HONDA CBR 250r motorbike. Our team´s intention is to build a small and a very light Formula car and this compact power train unit perfectly fits to our car conception. It is a four stroke water cooled single cylinder SI engine with a displacement of 250cc and it weighs just 35.4 kilograms. This engine will be overcharged by the intake manifold using the ram wave effect.
Engine specification: 250 cc 76 mm 55 mm 10.7 25bhp@8500rpm 22.9@7000rpm
DRIVETRAIN: The drivetrain system consists of every component from the engine output shaft to the wheels. Our goal is to achieve a speed of 80kmph because in the endurance race, average speed lies in the range of 47-57kmph and usually we don‟t see speed greater than 80kmph although the maximum speed is 105km/hr. Looking at our speed calculation, we see that our car is capable of reaching this speed with the final drive ratio of 4:1(56/14).Table shows the different gear ratios of drivetrain with maximum traction force, acceleration and velocity for peak engine torque. gear
1 2 3 4 5 6
Gear ratio 3.33 2.117 1.571 1.304 1.115 0.962
Tractive force(N) 3443.113 2188.909 1624.363 1348.94 1152.87 994.627
Acc. (m/s2) 10.43 6.63 4.92 4.08 3.49 3.01
INTAKE RESTRICTOR DESIGN: The restrictor is a very significant part of the intake system being modeled. The restrictor is the ultimate restriction on the amount of air which can flow into the intake system, and thus, the amount of power produced by the engine. Consequently this segment of the intake manifold is the logical place to commence simulations. A thorough understanding of the flow through this section will allow the team VEGA to improve the design as much as possible, giving the best possible air flow into the plenum. In order to get the maximum mass flow rate and minimum pressure drop across the restrictor, converging-diverging geometry is selected with throat section of 15mm. From the data gathered through the numerous simulation in CFD fluent, it can be observed that the optimized value for converging and diverging angle of restrictor were found to be 10º and 6º respectively. Optimization of converging-diverging angles for minimum pressure drop is shown in tabular form:
Velocity(m/s)
4.72 7.43 10.02 12.07 14.12 16.37
Fig.2.1 shows the plot between tractive force and road speed. The "Constant Engine Power" line is equal to the maximum power of the engine, which is
Converging angle
Displacement Bore Stroke Compression ratio Engine power Engine torque
the upper limit of tractive effort available, less any losses in the driveline. For maximum acceleration performance the optimum shift point between gears is the point where the lines cross. The area between the lines for the different gears and the constant power curve is indicative of the deficiencies of the transmission in providing maximum acceleration performance.
Row value
Diverging angle
column value
4º
6º
8º
10º
12º
8º
0.21
0.192
0.198
0.21
0.22
10º
0.2
0.193
0.195
0.214
0.22
12º 14º 15º
0.199 0.22 .192
0.197 0.196 0.184
0.193 0.20 0.21
0.196 0.205 0.196
0.198 0.213 0.20
CAD MODEL
377.7 mm 1300 mm
FRONT VIEW
508 mm
TOP VIEW
330 mm
43.29°
SIDE VIEW
