SUMMER INTERNSHIP REPORT
Roll Cage Design for an All-Terrain Vehicle For Baja SAEINDIA 2012
Ali Qasim Khoyee 0109 – 616 Department of Mechanical Engineering University College of Engineering (A), Osmania University
ABSTRACT Roll Cage Design for an All-Terrain Vehicle – For Baja SAEINDIA 2012 Baja SAE India is an intercollegiate engineering design competition where each team’s goal is to design and build a prototype of a rugged, single seat, off-road recreational vehicle, which should be able to negotiate rough terrain without damage. One of the most fundamental components of the vehicle is the roll cage, which is essentially a minimal threedimensional space surrounding the driver. Its primary function is to provide safety to the driver, especially in the case of a rollover, and it also serves as the chassis for the vehicle, providing mounting points for all other major systems and aggregates of the vehicle. It therefore goes through intense loading scenarios, and so making it high-strength is absolutely necessary. At the same time, all teams are provided with the same low power 10 HP engine, and therefore performance is directly dependent on weight. These two aims namely, high strength and low weight are difficult to achieve simultaneously, and a compromise between the two is inevitable. This project involved making of the roll cage right from conceptualization, through the design process, to the fabrication and validation, with a special emphasis on durability. Weight reduction, though important, was not the primary aim of this project, and it is something that can be achieved through constant optimization over the next few years. For a first-year Baja team, it was essential to ensure that the basic requirements of the roll cage were met. To this end, we focused mainly on the rulebook, packaging constraints, ease of fabrication, driver safety and ergonomics. After an extensive study and a preliminary analysis of these design considerations, 2D drawings were made, which were then translated into an initial 3D model. To make sure that this design was consistent with what we had in mind, a physical full scale prototype made from PVC pipes was made. Choices for the roll cage material to be used were discussed and one was zeroed on based on our priorities. The model was then virtually put to the test using finite element analysis software, and various loading scenarios were simulated. This was an iterative process - modifications were made in each cycle of tests as per the results. When the design was finalized, we proceeded towards fabrication. Here, we followed a computer-aided approach, wherein we used the 3D model to determine the notching profiles for the tubes to be welded. Tubes were cut and notched using these templates to produce accurate fitment for the weld. Gas Metal Arc Welding (GMAW) was used to weld the roll cage structure together. A comprehensive design verification procedure was conducted to verify whether or not the specifications set at the design stage were fully achieved. A plan for design validation was also outlined, which will be taken up when the vehicle is fully completed. Finally, some possible improvements and alternative ways of going about this task in future years were discussed.
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ACKNOWLEDGEMENTS
I owe many thanks to: Prof. V. Nageswara Rao (Associate Professor at Department of Mechanical Engineering), for his ideas and invaluable insights into structural design, loading scenarios for the roll cage, and finite element analysis, which formed the initial major core of the project. Mr. C. Pandu Ranga Rao (Technical Director at VAMA Industries Ltd, former Head of Vehicle Engineering at Mahindra & Mahindra), for his help on industry standards, and also for his encouragement and appreciation of the work. Mr. V. Uma Maheshwar (Associate Professor at Department of Mechanical Engineering, Project guide for Baja SAEINDIA 2012), for being a constant source of motivation. My teammates, for their important contributions. And finally to my parents, for putting up with my bizarre working hours during the entire period.
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CONTENTS Abstract Acknowledgements Chapter 1
Introduction 1.1 Background 1.2 Objective 1.3 Methodology
Chapter 2
Design Considerations 2.1 Rulebook 2.2 Ergonomics 2.3 Suspension & Powertrain
Chapter 3
Preliminary Design 3.1 2D Drawings 3.2 3D Model 3.3 Prototype
Chapter 4
Material 4.1 Material Requirements 4.2 Availability 4.3 Choice Comparison & Material Selected
Chapter 5
Finite Element Analysis 5.1 Pre-processing 5.2 Post-processing 5.3 Scenarios Tested
Chapter 6
Fabrication 6.1 3D Model Refinement 6.2 Templates 6.3 Tube Cutting & Notching 6.4 MIG Welding
Chapter 7
Design Verification & Validation 7.1 Assumptions 7.2 Verification Procedures 7.3 Plans for Validation
Chapter 8
Conclusion 8.1 Summary 8.2 Recommended Enhancements for the Future
Appendices A B
Baja SAEINDIA 2012 Rules Example FEA force calculation
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1
INTRODUCTION
1.1
Background
Baja SAE India is a national intercollegiate engineering design competition. Its website states: “The objective of the competition is to provide SAE student members with a challenging project that involves design, engineering, planning, manufacturing and marketing the tasks found when introducing a new product to the consumer industrial market. Teams compete against one another to have their design accepted for manufacture by a fictitious firm. Students must function as a team not only to design, build, test, promote, and run a vehicle within the limits of the rules, but also to generate financial support for their project and manage their educational priorities. Each team’s goal is to design and build a prototype of a rugged single seat, off-road recreational four-wheel vehicle intended for sale to the nonprofessional, weekend off-road enthusiast. The competition goals include that the vehicle must be safe, easily transported, easily maintained and fun to drive. It should be able to negotiate rough terrain in all types of weather without damage.”1
1.2 Objective The objective of this project is to design and manufacture the roll cage of the all-terrain vehicle. It is one of the most fundamental components of the vehicle and is essentially a minimal three-dimensional space surrounding the driver. One of its primary functions is to provide safety to the driver, especially in the case of a rollover, which is where it gets its name from. The other important function is that it serves as the chassis for the vehicle, providing mounting points for all other major systems and aggregates of the vehicle. It is therefore essential to get the design absolutely spot on, in order to eliminate any risk to the driver, and to maintain optimal relationships with other subsystems of the all-terrain vehicle. It can be said that roll cage design directly affects almost all other subsystems of the vehicle, and so any error in design will get compounded and transferred to other subsystems. The vehicle goes through intense loading scenarios on rough terrain, with the worst-case being rollovers and crashes with obstacles or other vehicles. This necessitates that the roll cage be designed to impart it with as much strength as possible. At the same time, since all teams are provided with the same low power 10 HP engine, any difference in weight will directly impact vehicle performance. This is significant as the roll cage is one of the biggest contributors to weight in the vehicle. These two aims namely, high strength and low weight are difficult to achieve simultaneously, and generally a compromise between the two is inevitable. Our priority in this project was more on durability than on weight reduction. This strategy makes sense for a first-year team, as it is important to first focus on the absolute fundamentals and to ensure that the basic requirements of the roll cage are met. As the vehicle begins to evolve over the coming years, weight reduction will naturally take place. To this end, our objective was to come up with a design for the roll cage while focusing mainly on the rulebook, packaging constraints, ease of fabrication, driver safety and ergonomics – the most fundamental points that need to be satisfied for a vehicle to be successful. 1
“Baja: Objective”, Baja SAE India, www.bajasaeindia.org/about-us/objective 5
1.3 Methodology
Fig 1.1 Roll cage design methodology Fig 1.1 outlines our methodology for the roll cage design. The rulebook was central to our design as it laid out various rules and guidelines that constrained how far we could go in terms of pushing the limit for maximum functionality and optimum packaging. We divided the roll cage into three main parts, namely front, rear, and cockpit. This distinction is not without good reason, as they involve different challenges coming from different subsystems of the vehicle. The cockpit was designed taking into consideration proper comfort for the driver in the seated position and also for ergonomic reach to various controls like the dashboard, pedals, levers, etc. The front of the cockpit was largely dependent on the input from the suspension team. After the suspension was optimization, the hard points obtained were used to determine the width at the front of the roll cage. The rear design was particularly challenging as it involved not only suspension, but also constraints from the powertrain, and it took many rounds of discussion among the three teams of roll cage, suspension, and powertrain to come to a satisfactory conclusion for the design. When we had these three designs sorted out, we proceeded to build 2D sketches, which we then translated into a 3D model on CAD software. We then built a full scale prototype out of PVC pipes in order to actually experience in real what we had built in the software. This was followed by testing of the model in finite element analysis software to simulate various loading conditions on the roll cage, and optimising as seemed fit.
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2
DESIGN CONSIDERATIONS
As mentioned previously, the challenge in the design process was to simultaneously take into consideration the constraints imposed by different subsystems of the vehicle. The main factors that we decided to pay the most attention to are explained here.
2.1
Rulebook
The rulebook was the single most important factor to consider in our design process. Since any deviation from it would jeopardise our participation in the event, it was crucial for us to stick to all of its rules and guidelines, especially when most of these rules come from the standpoint of safety. Some of the rules specified regarding roll cage are as follows: •
•
• •
2.2
Sufficient driver clearances – Driver’s helmet to be 6 inches away from a straight edge applied to any two points on the roll cage – Driver’s torso and limbs must have a minimum of 3 inches clearance Guidelines on dimensions – Rear Roll Hoop: width and angle – RRH Diagonal Bracing: angles and distances – Roll Hoop Overhead members: lengths – Side Impact members: height – Front Bracing members: angle – Roll Hoop Bracing: lengths and angles – Overall vehicle dimension limits: 64 inches maximum width Safety guidelines – Driver exit time of maximum 5 seconds Minimum material specifications – Outer diameter, thickness, carbon content – Equivalency to 1018 steel
Ergonomics
As far as cockpit design is concerned, ergonomics was the driving force behind all design decisions. Variables like cockpit lengths and widths, placement of gear rod, hand brake, pedals, etc were decided based on average human dimensions. Seat height and recline angle were also subject to the same statistics. For example, dimensions like elbow to elbow width in a relaxed position were used to obtain positions for hand levers, hip breadth was used to determine seat width and consequently roll cage width in that area, and many others were used to determine appropriate clearances for the driver from the insides of the roll cage and other aggregates. In utilising ergonomic data, it is important to use average data of Indians, rather than the readily available international data which would actually lead to oversizing of the vehicle. Table 2.1 lists out data for the 95th percentile Indian male, which we used for our cockpit design.
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Table 2.1 Indian Anthropometric Dimensions: 95th Percentile Indian Male2 Description Dimension (mm) From top of seat to top of head (relaxed posture) 893 From top of seat to top of head (erect posture) 905 From buttocks to front of knee 615 From top of knee to heel 567 From bottom of knee to heel 471 Shoulder to shoulder width 482 Elbow to elbow width (closed) 489 Elbow to elbow width(relaxed) 644 Knee to knee width (closed) 235 Knee to knee width (relaxed) 535 Hip breadth 405 Weight 76 kg
2.3
Suspension & Powertrain
Suspension plays a critical part in the performance of an all-terrain vehicle. So rather than transferring the constraints of the roll cage to the suspension team, the coordinates from the optimised suspension geometry were taken as input for roll cage design. This means that while there could be some compromise in terms of packaging of aggregates and roll cage strength, there should be none for the suspension. This is a significant departure from conventional design procedure, but one that we think will help us achieve greater suspension characteristics to be able to negotiate the unforgiving terrain. With a double wishbone setup, packaging becomes a greater concern. The wishbone arms often require large lengths for favourable performance. While in the front, this is not a major problem and the roll cage can be brought to almost any width as required by the suspension system, in the rear it presents a mutually opposing constraint with the powertrain. This is because the overall width of the vehicle is limited to only 64 inches by the rulebook, which leaves suspension and powertrain jostling for space. However, this is where we had no choice but to compromise on suspension, as the powertrain compartment took up a large area of space. Specifically, in order to minimise the space required for powertrain, we opted to shim the whole powertrain assembly up by about 1 inch, which created more than 2 inches of horizontal space for the wishbones. This meant that the half-shafts had to run at a higher angle then previously designed for, but our calculations showed that this would be within the limits provided by the CV joints even at full droop of the tyres.
“Indan Anthropometric Dimensions: For Ergonomic Design Practice”, Debkumar Chakrabarti, National Institute of Design 2
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PRELIMINARY DESIGN
3.1
2D Drawings
Once the basic design issues were resolved, 2D wireframe sketches of parts of the roll cage were drawn.
Fig 3.1 Rear Roll Hoop Figure 3.1 shows the 2D sketch for the Rear Roll Hoop (RRH) and some important dimensions. The top of the seat surface has been positioned at 6 inches above the belly pan. 27 inches above the seat surface, the width of the RRH has been put at 30 inches, which is just greater than what is required by the rulebook. The vertical members of the RRH are nobreak members, i.e. they are continuous tubes bent rather than weld points. Two diagonal bracings are used and they lie within the angle limits as in the rulebook.
Fig 3.2 Roll Cage Side View Figure 3.2 shows the roll cage from the side view. There is a 9 inch clearance provided from the seat top surface to the Side Impact Member (SIM), and more than a 6 inch clearance from 9
the driver’s helmet to the Roll Hoop Overhead (RHO) for a 95th percentile Indian male. A front Fore-Aft Bracing (FAB) was used to connect the Lower Frame Side (LFS), the SIM, and the upper Front Bracing Member (FBMUP) within the 45 degree angle requirement. The end of the RHO is greater than 12 inches from the start of the seat. Furthermore, the RRH is inclined at an angle of 5 degrees in the side view, which is under the limit of 20 degrees either way.
Fig 3.3 Lower Frame Side Members The members constituting the LFS members are shown in Figure 3.3. A width of 18 inches in the front as decided by the suspension team was allowed. This is enough for mounting of accessories inside the roll cage, like steering rack, battery, master cylinder, etc. The main cockpit area was given an overall length of 43 inches, with the mid of the area being angled outwards by 2 inches for larger space for aggregates like gear rod, brake levers, etc.
Fig 3.4 Side Impact Members 10
The Side Impact Members are at a height of 15 inches from the LFS, and are given a width of 25 inches as per suspension requirements. This is where the Short Long Arm (SLA) double wishbone suspension gets its name from. Because of this higher width at SIM, the upper arm becomes shorter, thus providing increasing negative camber to the tyre relative to the chassis on increasing bump travel. The widths in the cockpit are proportionately increased as well to provide enough clearance for the driver.
3.2
3D Model
These drawings were then translated into a 3D wireframe model in Solidworks, a CAD tool. The Weldments feature was made use of to define a cross-section for the wireframe model in order to realistically present the tube dimensions.
Fig 3.5 Roll Cage 3D Model in Solidworks This model is seen in Figure 3.5. Here only the front portion of the roll cage is presented, as the rear is pending further revision after a last-minute change of engine manufacturer by the event organisers.
3.3
Prototype
A full scale prototype made from PVC pipes was built so that the virtual model could actually be seen and experienced by the driver. This is where our decisions regarding various aspects of design are scrutinised, and effective alternatives are come up with. Figure 3.6 shows the prototype model built up to exact dimensions and angles so as to provide the best feeling for the real roll cage to be built.
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Fig 3.6 Full Scale Roll Cage Prototype The prototype also provided the drivers the opportunity to sit inside the cockpit and get a feeling for the surroundings, the cockpit, and also the field of view. To do this, we raised the entire prototype to our target ride height of 10 inches off the ground. This even allowed us to test the 5 second egress time test for the driver. But more importantly, it allowed us to test for the clearances that we had designed for based upon the statistical anthropometric data. We found that we lacked some space for mounting aggregates beside the driver, so we increased the width in this area. Figure 3.7 shows the driver seated inside the cockpit at the designed seat and ride height.
Fig 3.7 Roll Cage Prototype with the Driver The building of the prototype was thus an important stage in our design process, where we rectified our design, something which would not have been possible through other means. It also provided our drivers the opportunity to give valuable feedback on aggregate positions, field of view, etc.
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4
MATERIAL
4.1
Material Requirements
The rulebook lays out strict rules out of which the roll cage can be made: (A) Steel tubing, having outside diameter of 1 inch, wall thickness of 0.12 inch, carbon content of at least 0.18% OR (B) Steel members with at least equal bending stiffness and bending strength to 1018 steel, having outer diameter of 1 inch, wall thickness= 0.12 inch Bending stiffness is proportional by the EI product. Bending strength is given by SyI/c E = the modulus of elasticity (205 GPa for all steels) I = the second moment of area for the cross section about the axis giving the lowest value
4.2
Availability
Even though a large variety of steels satisfy these requirements, not all are usually available in the market. Those available that we can consider are AISI/SAE 1018 and 4130 chromoly steel. These had to be ordered from a dealer in Mumbai, and so making the right decision was important.
4.3
Choice Comparison & Material Chosen
The table 4.1 lists out the properties of the two materials mentioned above.
Material AISI 1018 steel AISI 4130 steel
Table 4.1 Properties of Available Materials Modulus of Elasticity (ksi) Yield Strength (ksi) Elongation at Break 29700 53.7 15% 29700 63.1 25.5%
It can be inferred that AISI 4130 steel is far much better than 1018 steel in terms of yield strength and also has longer elongation before fracturing. However, the main disadvantage of 4130 steel is that it can only be TIG welded, and also requires preheat and post-welding heat treatment for the weld. These processes can be expensive and time consuming, and increase difficulty of fabrication. 4130 is also much more expensive than 1018. Keeping all these into consideration, we decided to opt for 1018, as it satisfied the rulebook requirements, was inexpensive, and could be easily MIG welded. The tube chosen had an outer diameter of 1 inch, and a wall thickness of 0.12 inch, both satisfying the minimum requirements given in the rulebook.
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5
FINITE ELEMNT ANALYSIS
Finite element analysis is a powerful tool that can allow us to virtually test all possible loading scenarios on the roll cage without the prohibitive costs of real world testing. The software used here was ANSYS 12.0.
5.1
Pre-processing
5.1.1 Element Type The element type used for all roll cage FEA analysis was BEAM189. It’s a quadratic, 3-node element, with 6 DOFs at each node. It is based on the Timoshenko beam theory which also takes into account shear deformation effects, and is generally used to analyze slender to moderately stubby beams. The slenderness ratio is calculated to determine whether this element can be used in our case. Table 5.1 shows the changes in the result of a Timoshenko beam over a normal Euler-Bernoulli beam for different values of slenderness. Normally, a minimum value of 30 for the slenderness ratio is required for using the BEAM189 element. Table 5.1 Slenderness Ratio vs δT/ δEB Slenderness (GAL2/(EI))
ratio
δ Timoshenko / δ Euler-Bernoulli
25
1.120
50
1.060
100
1.030
1000
1.003
5.1.2 Sections BEAM189 element does not require defining of real constants. Rather, it requires a definition of a section. The software contains readymade common sections available, and other sections can also be defined by the user. Here, a tube section with the OD and wall thickness values was defined. 5.1.3 Material Properties The values of Young’s Modulus and Poisson’s ratio were entered. Yield strength value is not inputted to the software, and so the user has to compare the obtained Von Mises stress with the yield strength of the material he has in mind. 5.1.4 Meshing Usually in beam elements, increase in number of elements or decrease in the mesh size does not affect the result. However, BEAM189 did require proper selection of mesh size. It was observed that an element size of 0.1 inch provided good convergence results. Also, the crosssection mesh size is assumed by default by the software.
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5.2
Post-processing
The results that were checked were nodal deflections and Von Mises stress in the roll cage members.
5.3
Scenarios Tested
In total, five major loading scenarios were tested on the roll cage. The forces to be applied were decided based on the typical velocity of the vehicle at such loads. 5.3.1 Frontal Impact – Full Width
Figure 5.1 Frontal Impact Full Width Phase 1 (Impact Loads)
Figure 5.2 Frontal Impact Full Width Phase 2 (Inertial Loads)
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The frontal impact was divided into two phases as can be seen from Figure 5.1 and Figure 5.2. The first phase was the application of impact loads on the front part of the roll cage when it initially comes in contact with an obstacle or another vehicle. Constraints were applied to the rear wishbone points, as that is where the major weight of the vehicle is concentrated and so provides the most resistance to the impact force. The force obtained from the calculations was divided over 4 nodal points in the front, and applied towards the rear direction. In the second phase, inertial loads were applied. When the vehicle crashes into an obstacle, after the front has experienced the impact forces, the front part is assumed to come to a complete stop, and loads due to all aggregates on the vehicle are applied on their respective positions due to their inertia. Here, the four front nodal points are constrained, and loads and moments are applied as shown in Figure 5.2 5.3.2 Frontal Impact – Offset
Figure 5.3 Frontal Impact Offset Phase 1 (Impact Loads)
Figure 5.4 Frontal Impact Offset Phase 2 (Inertial Loads) 16
In this scenario, the obstacle or the other vehicle was assumed to hit the front of the car at an offset, so that the load is distributed to only 2 points on one side of the vehicle. Similar loads and constraints were applied in these two analyses as well. 5.3.3 Rear Impact
Figure 5.5 Rear Impact For the rear impact test, a worst-case scenario of constraining in the front, and applying forces on the rear was considered. Even though this is not too accurate a depiction of the problem, it still provides us with a worst-case. 5.3.4 Side Impact
Figure 5.6 Side Impact Case 1
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Figure 5.7 Side Impact Case 2 Side impact was analysed in two cases, one where the tyres are impacted, and the other when the roll cage directly comes into contact with the obstacle. In both cases, the other side wishbone points were constrained. In the first case, loads were applied to one side of the wishbone points, while in the other, the load was applied directly on the member connected the SIM and the LFS. In this case, both sides wishbone points were constrained. 5.3.5 Rollover
Figure 5.8 Side Rollover Impact Rollover was again divided into two cases. One is where the vehicle turns onto its side during high speed cornering and when the inner tyres go over an obstacle like a boulder. The forces are applied on the RHO as a distributed load, and the same side wishbone points are considered to be constrained.
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Figure 5.9 Front Rollover Impact In the front rollover impact, the front part of the RHO is subject to a distributed load, while the front wishbone points are constrained. This scenario is seen when the vehicle is airborne while going off a cliff and landing on the front tyres. Here, there is a possibility of the vehicle overturning to the front, and this loading is tested in this case. 5.3.6 Factors of Safety During these analyses, we had to keep modifying the design a little to get better results in each loading scenario. After all these optimizations, we ran the FEA tests once again, and obtained sufficient factors of safety as shown in Table 5.2. Table 5.2 Factors of Safety After Optimization Loading Scenario Factor of Safety Frontal Impact Full Width Phase 1 1.81 Frontal Impact Full Width Phase 2 3.55 Frontal Impact Offset Phase1 1.53 Frontal Impact Offset Phase 2 1.24 Rear Impact 1.67 Side Impact Case 1 1.51 Side Impact Case 2 1.89 Side Rollover Impact 1.74 Front Rollover Impact 2.14
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FABRICATION
6.1
3D Model Refinement
We decided to use our 3D model for fabrication purposes as well. For this, we needed to refine our roll cage model so that the area of the welds is accurately depicted in the model.
Figure 6.1 3D Model Refinement For this, we used the Trim/Extend command in Solidworks extensively to obtain correct end profiles for the tubes.
6.2
Templates
To print templates of the tube end profiles, we isolated the tubes into different files and flattened them out to produce a sheet metal part. Figure 6.1 shows the isolated tube and Figure 6.2 shows the associated template.
Figure 6.2 Notched Tube
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Figure 6.3 Tube Template This template is used to roll onto the tube, so that the required notch profile is marked.
6.3
Tube Cutting & Notching
Tubes were cut and notched using an angle cutter. While this is not a perfect tool for notching the tubes, it provided sufficient accuracy for our welds. For templates that had extreme angle cuts, we made used milling to produce the notches. Using the templates got us very accurate tube cuts, and so the end result was very similar to our 3D model.
6.4
MIG Welding
Gas Metal Arc Welding (or MIG welding) was used to weld the rollcage together. This was preferred as it leads to clean welds without any slag, unlike stick welding (SMAW). It also provides better strength compared to SMAW as experienced by previous year Baja teams. Following are the specifications of the welding machine: • • • •
Filler: Copper coated MS wire (AWS 5.18 ER 70S6), 1.2mm dia Shielding Gas: CO2 Gas flow rate: 15 Ar std litre/min (in a semi-open environment) DCEP, ~23V
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DESIGN VERIFICATION AND VALIDATION
7.1
Assumptions
The purpose of the roll cage is to provide a minimal 3D space around the driver. The functions of this 3D space are: 1. to provide safety to the driver in various scenarios 2. to provide good ergonomic reach to all operations needed for the driver 3. to serve as a frame onto which sub-systems of the vehicle are to be integrated securely mounted and properly shielded For the end-user, that is, a buyer of the product, the expectations from the roll cage are the same as its functions, and so provision of these functions is the target for the manufacturer. It is assumed that the following three considerations will respectively make sure that the three functions of the roll cage as enumerated above will be successfully provided: 1. compliance with all rules specified regarding roll cage design, and implementation of fundamental design theories and practices 2. sizing up of critical distances and angles in the cockpit and provision of adjustability 3. a holistic approach towards roll cage design as an integral part of the vehicle Checking if the designed roll cage is in line with these considerations, i.e. checking if the design meets the target specifications forms a part of the design verification.
7.2
Verification Procedures
Our procedure for design verification consisted of the following:
7.3
Performing several design reviews at different stages of design Checking of roll cage dimensions as per rulebook and design targets with the help of templates and/or working standards of measurement Checking for sufficient clearance between driver and cockpit as per rulebook with special driver equipment and helmet put on Inspection of welds to ensure continuous welds, and to detect significant presence of any cracks, craters, or undercuts Destructive testing of a weld sample similar to those in the roll cage, to demonstrate higher strength of the weld than of the material Inspection of body panels and firewall to ensure no presence of large gaps between the panels and frame members
Plans for Validation
The design validation, however, is different from design verification. Validation involves actual testing of the product to see if the product performs its functions as expected by the end-user. Design validation of a roll cage is therefore done by real-world testing after manufacture of the whole vehicle.
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Validation for the design target of driver safety is proposed as follows:
An exit time test for the driver to ensure the driver is able to egress out of the vehicle as soon as possible and within a specified time, which can be vital in case of emergency situations Since physical impact testing on the actual roll cage is not viable, dependence on the imperfect finite element analysis software is inevitable. Although, the constraints and the loading conditions specified in the software can be far from real-world values, they often give good enough approximations, especially of worst-case scenarios. Crash tests like full frontal impact, off-set frontal impact, rear impact, side impact, and rollover loading are performed to determine effects on roll cage structure integrity, and thereby threats to driver safety.
Validation for the design target of optimal driver ergonomics is proposed as follows:
Use of the largest and the smallest drivers of the team to make sure all sub-assemblies like steering wheel, pedals, gear rod, emergency brake, and other dashboard controls are within ergonomic reach in case of both drivers, along with comfortable seating postures.
Validation for the design target of integration with other sub-systems is proposed as follows:
Aggressive vehicle driving at an off-road course to confirm secure mounting of subassemblies like engine, transmission, steering rack, battery, fuel tank etc Water wading test to spot major gaps between panels and frame members, and to identify areas where splash protection needs to be given
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CONCLUSION
8.1
Summary
Roll cage design is vital to performance of the all-terrain vehicle and influences every other subsystem design. We followed a computer-aided approach in nearly all design processes from start to end to aid in accuracy. The final success of the design can only be assessed after completion of the Baja event in February 2012.
8.2
Recommended Enhancements for the Future
Some areas where improvements can be made by future teams are:
Consider other steel grades for lowering the roll cage weight (current design is 44 kg; including rear will make it to over 70kg) Using better constraints as seen by the roll cage in the real world, possibly using transient, dynamic FEA Using standard tube notchers instead of angle cutters for better fit and welds Extensive training for welders in order to produce quality welds
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APPENDIX A: BAJA SAEINDIA 2012 RULES
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26
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APPENDIX B: EXAMPLE FEA FORCE CALCULATION FRONTAL IMPACT 1 Initial velocity of our car Initial velocity of other car Mass of our car Mass of other car Coefficient of restitution Time of impact
Final velocity of our car (va)
Final velocity of other car (vb)
ua ub ma mb cr t
13.33 0 400 400 0.1 0.15
ub - ua cr*mb*(ub-ua) ma*ua mb*ub ma + mb
-13.33 -533.2 5332 0 800
4798.8
ua - ub cr*ma*(ua - ub) ma*ua mb*ub ma + mb
13.33 533.2 5332 0 800
5865.2
1/2 * ma * ua^2 1/2 * mb * ub^2
800
7.3315
800
35537.78 35537.78 0
KE lost / energy absorbed by crush
17591.2 1/2 * ma * va^2 1/2 * mb * vb^2
Impulse force on our car
5.9985
ma*va ma*ua
7196.4 17946.58 10750.18 2399.4 5332
-2932.6
-19550.7
In slinch in / s^2
-4391.08
On each corner point
-1097.77
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