Design Report
- 2. BIT Mesra, India Car #177- SRIJAN teamsrijan@bitmesra.ac.in 2 | P a g e INTRODUCTION As a first year team, the aim of team SRIJAN to quote Mr. Claude Rouelle, is “to build a C car with an A team”. Hence, the areas of attention in design were restricted to reliability and manufacturability. The use of Engine simulation and development software, RICARDO, was highly promoted so as to lay a learning foundation for the future team. CHASSIS GOALS To improve driver visibility and comfort. Compliance with Part 3 Article T “Driver’s Cell” rules of the 2013 FSAE rule book. To improve packaging of different components. Aim at improving manufacturability of the frame. DESIGN INTRODUCTION The design of the 2013 frame began with a thorough study of the rule book to identify the rules that have to be specifically kept in mind throughout the design phase of the frame. A mock chassis was made from PVC pipes and with the driver seated, required dimensions of the cockpit and foot well was decided. MATERIAL AND TUBING SELECTION A spaceframe chassis was the best option for the team as it provided best mix of strength and stiffness, low cost and minimum weight. The material selected for the tubing was AISI 1018 steel due to its stiffness/cost ratio, ease of weldability and availability. INITIAL MODEL AND FINAL DESIGN The modeling and FEA of the frame was done using Solidworks 2012. Standard Impact Attenuator Type 13 is used, as it was felt that the cost incurred in manufacturing and testing an IA, could be better utilized in other areas. BRAKES Keeping in mind the design criteria given in the rulebook, the braking system was designed by determining the parameters necessary to produce a deceleration of 1.2g and comparing the deceleration that a known braking system would produce. The parameters calculated/assumed were: Weight distribution (40%-60%), Weight of the car (3139.2 N), Wheelbase (1.6m), C.O.G height (0.3m), Tyre-road friction coefficient (1.2), Rotor-pad friction coefficient (0.35), etc. Outboard disc brakes are used on all the four wheels actuated by dual master cylinder setup. TILTON-75 series master cylinder of bore diameter 0.7 inches and a full stroke of 1.1 inches is
- 3. BIT Mesra, India Car #177- SRIJAN teamsrijan@bitmesra.ac.in 3 | P a g e used. Pedal ratio of 5.6:1 is achieved by using TILTON 72-603 pedal assembly which has an integrated balance bar and suits the brake balance requirement of 66% FRONT and 34% REAR. The brake calipers used are fixed type WILWOOD PS-1 calipers which are compact, light (422 gm. only) and have a unique combination of strong cast Aluminum construction with a sleek low profile design. The brake rotors used are drilled discs (240 mm diameter) TVS FLAME rotors. Braided brake lines are used for steady fluid flow. ENGINE SELECTION Owing to the availability, cost and ease of access to standard components in the local area, a choice had to be made between Honda CBR 250R and Royal Enfield 500cc engine. The decision was taken based on the torque output of both engines as it is one of the critical factors for acceleration. Royal Enfield engine has a characteristic flat torque curve with a maximum of 41.3 Nm@4000rpm compared to a maximum of 22.9Nm produced by 250cc engine of CBR @7000 rpm, which meant better acceleration could be achieved at lower engine speed using an Enfield engine. INTAKE AND EXHAUST The intake and exhaust system was designed using RICARDO WAVE simulation software to optimize engine performance by minimizing power loss after imposing a working condition of a 20mm restrictor. As the engine speed is limited to a maximum of 5250 rpm, the intake and exhaust system was designed for maximum power output and optimized engine performance at lower rpm range. Calculated runner length of 478.29 mm for max power at 5000 rpm was simulated in WAVE which showed maximum power output in the 4500 to 5000 rpm range with a torque curve which peaks at 3000 rpm. A 28mm throttle body with a 20mm restrictor was chosen from ‘AT Power throttles’ which uses a shaft less blade technology improving air flow. This shaft less system can improve flow by up to 10% on common throttle body sizes. To compensate for pressure loss of intake air through the restrictor, a plenum was designed with different volumes simulated in WAVE in the range of 2 L- 4 L. Power and torque curve with minimum number of peaks and broader power curve was observed for plenum volume of 4 L. SUSPENSION SUSPENSION GEOMETRY The team decided to maintain the use of 13” wheels, to provide room for the upright and A-arm configuration, despite the added weight of this larger wheel. Pushrod suspension was chosen because of the wide range of adjustability and packaging options that it provides. The suspension points were changed owing to the change in track for both front and rear. The new geometry for the front was obtained by reducing the length of the wishbones to maintain the width of the frame while that of the rear was redesigned completely. This was backed up by running iterative simulations in Optimum K suspension software. It was determined that the lower front A-arms could be increased in length by reducing the lower footwell area. This increase in A-arm length provides improved camber control through the wheel motion. Another
- 4. BIT Mesra, India Car #177- SRIJAN teamsrijan@bitmesra.ac.in 4 | P a g e improvement this year is the packaging of rear pushrod and the design of rear bell cranks. The new bell crank geometry achieves static wheel rate. A-arm spherical housing was designed in order to increase the accuracy of the A-arm dimensions during fabrication. The housings on the lower wishbones also incorporate the mounting points of the pushrods. This brings the line of action closer to the spherical bearing thus reducing the bending moment on the A-arm. UPRIGHTS The front and the rear uprights were designed independently due to the interferences present due to the bolts required to attach the steel inserts in the rear hub. The design is such that the mounting plate of the upright is parallel with the A-arms while at static ride height enabling a greater range of relative motion before the parts interfere. The steering clevis was integrated in the upright to reduce the number of components. For the rear upright the toe base was increased to maximum possible value to increase the toe stiffness. Finite Element Analysis (FEA) was conducted on the uprights to ensure these components could withstand the necessary applied loads with the material selection of 6082-T6 aluminium. HUBS The front hubs were designed to reduce the bearing diameter, in process increasing its width. This allowed the use of single bearing instead of two. The hat was integrated with the front hubs to reduce the number of components and to ease the assembly. Threads were made on the hub spindle to seat the wheel retention nut. Opposite hand threads were made on the left and the right hub to ensure that the retention nuts do not loosen on driving the car. The rear hubs were made to accommodate larger wheel bearing. The tripod profile machined on the hubs itself eliminating the use of stub axle at the rear. Similar to the front hubs the hat was integrated with the hub to reduce the number of components. Wheel retention provision was similar to the front hubs. Finite Element Analysis (FEA) was conducted on the hubs to ensure these components could withstand the necessary applied loads with the material selection of 6082-T6 aluminium. STEERING & ERGONOMICS STEERING The steering system uses a rack and pinion arrangement. The rack is “rear-floor” mounted (behind the kingpin axis) to lower the CG of the vehicle, to eliminate bump steer and to prevent driver’s leg from obstruction. The steering ratio of rack is 6.4:1 and a c-factor of 130 mm providing quick steering response to driver. 100 % Ackermann geometry, 77 mm steering arm length and 11.25 inch rack length is used to keep the bump steer to a minimum of 0.14 degrees. Rack is center mounted and the mounts are supported using 3 steel tubes on frame. Two universal joints are used as maximum bend angle can be only 32 degrees exceeding which can lead to locking of the u-joints. A 7 inch steering wheel is used to provide enough space for driver’s leg movement in cockpit area.
- 5. BIT Mesra, India Car #177- SRIJAN teamsrijan@bitmesra.ac.in 5 | P a g e ERGONOMICS The driver seating position is in accordance with the 95th male percentile rule. The position of driver is 24 degrees reclined keeping the proper ergonomics, driver comfort and providing 200 degrees field of view. The seat is made of fiberglass and epoxy resin keeping the cost and easy manufacturability in mind. The pedal box contains 3 pedals and is made adjustable to accommodate drivers of different heights. The throttle and clutches are actuated mechanically by push pull cable. ELECTRONICS Original wiring Diagram of the Royal Enfield Classic 500cc EFI engine is used in the car. After a thorough investigation in the field of reliability and compatibility, Race Dynamics ‘PowerTRONIC RR’ model Engine Control Unit (ECU) was selected. A digital speedometer is designed using ARDUINO UNO R3 development board (microcontroller used ATMEGA328) which will also indicate the wheel’s RPM on a 16X2 LCD screen. DRIVETRAIN Out of the three choices available for the differential i.e. spool, open and limited slip, Quaife Limited Slip Differential of Honda Civic make, was chosen. Apart from the conventional benefits of LSD, Quaife’s Automatic Torque Biasing unit eliminates Torque Steer as a result of unequal length half-shafts. Stock Royal Enfield engine has a countershaft sprocket having 18 teeth and uses a 530 O-Ring chain. The same was used for the present drivetrain which eliminated the need to modify the countershaft sprocket. Compatible sprockets from Taylor race were available in three sizes which had 45, 46 and 50 teeth. Out of these, the 46 teeth sprocket was chosen, as it provided the best compromise between vehicle speed and available torque at the wheels. Calculations were done using OPTIMUM LAP Software. The differential was mounted using CNC machined aluminium upright of 6061 – T6 grade. Turn buckle was used for chain tensioning.
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- 9. BIT Mesra, India Car #177- SRIJAN teamsrijan@bitmesra.ac.in 8 | P a g e Optimized Engine 1-D Model in Ricardo WaveStock Engine 1-D Model in Ricardo Wave
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