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Punjab Engineering College SAE Mini BAJA ASIA 2010 Preliminary Design Report Harjinder S. Brar, Puneet Pal S. Sarabha, Harish Puri 4th year Mechanical
Kamal Garg, Rohan Garg, Mohit Garg, Harmanpreet S. Maan, Prince Malhotra 3rd year Mechanical
ABSTRACT The objective of SAE BAJA ASIA 2010 competition is to simulate real world engineering design projects and their related challenges. The aim of TEAM RPM from Punjab Engineering College is to win the BAJA 2010 by making not only the best performing vehicle but also the rugged and economical vehicle that will comply all the SAE BAJA design requirements. To achieve our goal the vehicle has been divided into subcomponents and each member is assigned a specific subcomponent. The team is focused to, design the vehicle by keeping in mind the SAE BAJA requirements, driver’s comfort and safety, and to increase the performance and driveability.
INTRODUCTION SAE BAJA ASIA event is organized by SAE INDIA. The teams are given the challenging task to design and fabricate a single seat, off-road, rugged, recreational and fun to drive, vehicle which will intended for sale to weekend off road enthusiasts. The goal of TEAM RPM is to make the best performing car that will comply all the SAE BAJA requirements. To achieve our goal the has been divide into various groups and each group is assigned a specific component of the vehicle (Chassis, Suspension, Wheel Assembly, Steering and Brakes, Power transmission). For designing, analysis and optimization of the vehicle components various software like CATIA (design and analysis), ANSYS (analysis and simulation), Optimum K (suspension design), Suspension Analyzer (for suspension analysis), ADAMS (vehicle dynamics), LOTUS (steering and suspension design) are available. In previous year the team mainly focused on custom designing rather than using some standard parts. But this year the team has decided to use some standard parts. For example: in previous the axle was custom made and there was too much play in the splines after
race due to wear . So we have decided to use standard axle of Maruti 800 because its not possible to manufacture a single component with accurate precision. The team has done a detailed study of our previous vehicle and we have found our shortcomings. As a whole, the main objective of the team is to reduce the weight of the vehicle, augment the performance and minimize the power loss. Each part is being designed using CATIA, OPTIMUM K, LOTUS, ANSYS software by keeping in mind these objectives. A detailed analysis is being done on each part using CATIA and ANSYS software to remove the unnecessary and extra material. An iterative process is being used for the same. Benchmarking will be done for selecting each component. A special attention will be given to manufacturing process to improve the quality of final product.
MAIN SECTION CHASSIS STRUCTURAL DESIGN The PEC BAJA chassis is designed by keeping in mind some objectives like SAE BAJA requirements, driver’s comfort and safety, compact size, weight reduction, minimum stress concentration and a competing design. The chassis team did a detailed analysis of our previous year’s chassis and we have successfully reduced the length till firewall by 280mm approx. A special consideration is given to safety of the occupants, ease of manufacturing, cost, quality, weight, and overall attractiveness. Other design factors included durability and maintainability of the frame Material Selection
The material used in the vehicle must meet the requirements set by SAE. And as the vehicle will be used for racing so weight is a crucial factor and must be considered. The proper balance of fulfilling the design requirements and reducing the weight is crucial to a successful design. We are allowed to use only steel members in our frame. The available materials that fulfill the requirements are AISI 1018, 1020, 1026 and 4130. Benchmarking was done to select the material by comparing various properties of each material. Material
Modulus of Elasticity (ksi)
Yield Strength (ksi)
Elongation at break
AISI 1018
29,700
53.7
15%
AISI 1020
29,700
42.7
36%
AISI 1026
29,700
60.12
15%
AISI 4130
29,700
63.1
25.50%
Fig 1 Stress analysis of chassis
As we know the increase in Yield Strength affects the bending strength of a material. And bending strength is not only affected by cross sectional moment of inertia but also by radius of material. Thus CHROMOLY i.e. AISI 4130 having maximum yield strength will allow the usage of large diameter tubing with smaller wall thickness. This will in turn reduce the weight of our chassis. Also 4130 Chromoly is more ductile than other materials so it will deform more before its ultimate failure. So considering the above said factors we have chosen chromoly 4130 pipes to be used for our chassis. FEA Analysis In order to maximize the strength and durability of chassis and minimize the weight ANSYS was used to model and analyze the chassis under predetermined loading conditions. Using this analysis the dimension of the tubing was found to be 1.25” OD and .065” wall thickness. This tubing was modeled as line structure in ANSYS and the material properties of chromoly i.e. Young’s modulus= 205GPa and Poisson Ratio= .3, were applied on that line model. First of all chassis without truss was analyzed in ANSYS. results for stress and displacement analysis are shown below. The vehicle was analyzed for frontal impact. Taking the worst conditions the stress in various parts was calculated. Results for stress and displacement after front impact are shown below. After the front impact analysis weak points and deformed points in chassis were found and after calculations the required truss were added wherever necessary
Fig 2: Displacement after front impact
SUSPENSION DESIGN The sole purpose of the suspension is to reduce shock loads acting on the car, while providing optimal wheel contact when operating under dynamic conditions. When designing a suspension there are a number of factors that influence the behavior of the suspension and a lot of these factors also interact among each other in one way or another. Along with these factors (shown in the fig) various constraints put forward by compromises like chassis design, transmission and engine placement etc play a huge role in the design of suspension. Having the experience of two cars our aim is to work on the shortcomings of our previous designs and thrive towards augmenting the design this time around. So we have already chosen to go with independent double- wishbone arm type suspension system along with gas shockers as this type is preferable in the case of rough terrain because they provide better resistance to steering vibrations and reduce un-sprung mass.
To objectives of this year’s suspension are: Wheel Base
Longer wheel base-softer springs. Shorter wheel base-smaller turning radius for same steering input.
Influences vehicle cornering & tendency to roll.
Track width
S U S P E N S I O N
King pin axis& scrub radius
Larger track width-less lateral load transfer but more lateral movement of the vehicle is needed to avoid obstacles.
Larger the king pin inclinationmore the car is raisedleading to increase in the steering moment at the steering wheel.
Castor & trail
Inst Centre & Roll Centre
Tie rod location
The roll centre establishes the force coupling point between the sprung and the unsprung masses of the car. When the car corners the centrifugal force acting on the centre of gravity can be translated to the roll centre and down to the tires where the reactive lateral forces are built up.
Both influence the amount of load transfer b/w the front &rear during accelerat ion & braking and load transfer from curve inner to curve outer wheels during cornerin g. So both should be kept small.
Scrub radius is kept small so that the car is easier to handle &reduces the risk of sudden loss of traction.
Has +ve effects during cornering but too much castor causes weight transfer that will have an over steering effect.
The higher the roll centre is the smaller the rolling moment around the roll centre.
If roll center must be raised up from the ground, it is influenced the most by the angle of the plane made up by the lower Aarms in the front view.
Minimizing bump steer (change in toe angle due to wheel travel)-locating tie rod in same plane as either A-arms.
•
Maximizing the wheel travel
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Maximize ride height for through roughest terrain.
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Maximizing traction and surface contact.
•
Minimizing camber gain and bump steer.
•
Adjustable roll center height.
easy
navigation
To achieve these objectives we are working on various software like LOTUS, SUSPENSION, OPTIMUM K, and CATIA for designing and analysis purpose. Considering the above said criterion, last year’s analysis and various constraints put forth by other parts of the vehicle we conclude the following: Characteristics
Optimum range
Last year’s values
This year expected value
Wheel Base
60”
68”
60”
Track Width
Max 64”
56”
54”
King Pin Inclination
0°-10°
6°
9°
Scrub Radius
0-10 mm
15mm
<10mm
Castor Angle
3°-7°
6° in front
6° in front
Roll Height
Front 0-11”
Front 10.6” Rear 13.5”
Should be within optimum range
Center
Rear 0-13”
Center of Gravity height
Depends on roll moment
21”
19”
Wheel Travel
Front 10”
Front 9”
Front 12”
Rear 8”
Rear 7”
10-
Rear 8-10”
STEERING SYSTEM The primary objective for steering team was to decrease steering effort, decrease the amount of steering wheel travel and increase the steering responsiveness. To achieve this objective first of all a bench marking was done between all possible steering systems on a scale of 0-9.
Steering System
Cost
Weig ht
Availa bility
Efficie ncy
Total
Rack and Pinion Cam and Lever
7
8
8
5
28
6
6
7
6
25
Worm and Sector Recircula ting Ball
5
5
5
6
21
5
6
4
8
23
Worm and Roller
7
5
6
7
25
Fig 4: Graph between steering angle vs Turning angle
Rack and Pinion Steering System is chosen after benchmarking. After the detailed analysis of previous vehicle we found that in previous year’s steering design the driver had to turn the steering wheel by 450° to bring the wheels from center to lock. So the driver had to remove his hand from steering wheel while turning from center to lock. Our goal is t reduce this angle by achieving 7:1 steering ratio by calculating Ackerman length.
Ergonomics of the driver will be considered for both steering wheel position and its rotation for maximum wheel angle. In the last year’s design there was a bend in the steering column and it created an obstruction in driver’s legs but this year to increase the leg space for driver we have kept the steering column straight upto pinion. This year the rack and pinion will be placed below the cockpit base. A special mounting is provided in chassis for the same to increase the free space in cockpit.2: Angle between steering angle vs Turning angle
A rack and pinion with rack travel .1889mm per degree rotation of steering wheel is chosen. Rack travel at different steering angles is calculated and different turning angles are calculated using simulation on CATIA corresponding to different steering angles.
Fig 5: CATIA design of steering system
Fig 3: Turning angle at max steering angle
For all wheels to pivot about a common point, the inner wheel must turn at a sharper angle than the outer wheel. In reality, the tires must slip to generate lateral forces, so the outer tire should be steered at slightly higher angles than inner wheels which is predicted by Ackerman geometry. This year we aim to improve the Ackerman geometry to 120% from previous year’s 90%.
Bump and roll steer will be minimized by adjusting distance between inner tie rod pivots, through analysis it is found 14.7”. Location of the outer end of tie rod will be fixed by spindle geometry, thus only inner pivot location can be adjusted and so bump and roll steer.
Overview of Wheel Assembly Design:
WHEEL ASSEMBLY Small and light weight with simple design & easy to manufacture with ample strength.
The wheel assembly has a very important contribution towards vehicle’s weight. So to achieve our main objective of reducing the overall weight of our vehicle we have to reduce the weight of wheel assembly. We have a detailed study of previous year’s wheel assembly. In previous year’s vehicle the wheel assembly was custom made. But this year we have decided to use Maruti 800 standard axle because in previous year’s vehicle the axle was custom made. And there was too much play in the splines due to spline wear out. To avoid this kind of situation we are using standard axle of Maruti 800. The knuckle will be custom designed and an iterative analysis will be done on knuckle to remove the unnecessary material make it lighter. In previous year’s vehicle AISI 1018 material was used for wheel assembly but this year as our aim is to reduce weight so a benchmarking was done between available materials for our wheel assembly. Param eters
Hardn ess (w=.21 )
UTS (w= .21)
TYS (w= .21)
Machi ning (w= . 21)
Dens ity (w= . 17)
Utilit y (∑p* w)
Materi al (steel alloy) 1018
(p*w)
(p*w )
(p*w )
(p*w)
(p*w)
(p*w )
.5943
.571
.615
.932
1.36
4.07
1020
.498
.456
.564
.576
1.36
3.45
1026
1.89
1.89
1.89
1.8
1.53
9
Knuckle
Knuckle and Hub Material
Hub
Wheel Assembly
Spherical Rod end
Rim & Tyre
Lighter & easily available with low machining cost , AISI 1026 steel alloy has the higher utility than AISI 1020 & AISI 1018.
Wheel Hub is designed to accommodate brake assembly which gives efficient & compact packaging without compromising hub’s strength.
Thus we finally achieved our objective s of strength, compact ness, weight reductio n, easy assembl y and disasse mbly, with minimize d stresses
Standard spherical rod end can adjust camber angle and make shock loads ineffective to a greater extent.
25” Polaris tire which provides better grip on off road surfaces.
Axle Axle should have no wear out of splines & have sufficient strength to handle the load
As the AISI 1026 has the highest utility so it has been selected for the knuckle and hub material. Brake System
The purpose of the braking system is to increase the maneuverability by locking all the four wheels in a time of less than a second. This year our aim is to reduce the weight of wheel assembly so after studying various available options and to get effective braking we have decided to use hydraulic disc brakes on all the four wheels. And Honda Aviator’s disc and caliper has been chosen for the same. Hydraulic disc brakes have high performance, easy replacement, and comparatively less weight than other available options. Fig 6: Exploded view of wheel assembly
In previous year’s design we used hydraulic disc brakes with two different master cylinders for brake fluid, one
for front wheels and one rear wheels. The main drawback of this design is that if any of the 2 cylinders stops working the respective brakes will fail. To overcome this drawback we have decided to use dual master cylinder. In dual master cylinder there are two chambers but they have the common circuit of brake fluid for all the four wheels. So if any one chamber fails we can still apply the brakes to all the four wheels.
depending on the RPM at which they spin and the amount of torque required.
POWER TRAIN Engine and Transmission
Fig: 8
Design
Fig 7: Layout of Power Train
As we are all provided with the same LOMBARDINI LGA 340 engine so a lot of emphasis has to be placed on the design of power train. This year our objective is to harness the power of 10HP engine and efficiently deliver this power to the tires for peak performance. In design of the drive train the optimization of several desired characteristics are being kept in mind including towing capacity, acceleration, top speed, and durability. As it is known internal combustion engine have a power band (range of speeds) at which the engine produces the maximum torque. Below or above this power band the engine does not provide enough torque to overcome the resistance torque and accelerate the vehicle. Since with such a small amount of power we do not have much room within our power band for the performance we need. Therefore we either have many gear speeds or we use a CVT (Continuously Variable Transmission). Continuously variable transmission (CVT) belt drive is a device that is much smoother than a conventional transmission and also has the ability to harness peak engine power during operation. The CVT transmits power from the engine to drive train in place of a conventional clutch dependent multi-gear transmission that requires constant shifting to change reduction ratios. It consists of two variable pitch pulleys, the drive and driven, that semi-dependently change their ratios
The goal of the design for this year’s driveline is to eliminate as many losses, in transfer of power from engine to the wheels, as possible. To accomplish this goal the drive train consist of CVT, two speed reverse gear box with reverse and a chain driven open differential. As the engine reaches its governed rpm limit 3600 rpm, the gear reduction across the CVT have been determined to be 0.76:1 and thus serving as an "overdrive" for the car. At low engine speeds the CVT produces a reduction of 3.83:1 providing necessary torque. The CVT is assisted by a two speed with reverse gear box. The two speed gear box enables the vehicle to achieve both high torque and high speed. With the cockpit mounted shift lever in low gear the driver has a torque available for towing heavy load, climbing steep gradients and driving through mud and sand. The high gear is designed for top speed and acceleration. The final gear reduction for the drive train comes from sprocket sizes on the gearbox output shaft and the driven sprocket located on axle. Several combinations of drive and driven sprockets were analyzed. The analysis was performed with the goal of maximizing the relationship between vehicle top speed and torque output. A low friction O-ring 520 series motorcycle chain will be used to power the axle from the output shaft of the transmission. The yield strength of the chain is listed from the manufacturer to be 8100lb. Estimated forces on the chain will not exceed forces greater than 3800lb, providing a 2.1 factor of safety for the chain. Constant Velocity (CV) joints will be considered for the half shafts
because of their ability to transmit torque through a higher range of suspension articulation. These features will create a vehicle that utilizes all of its power in a smooth, quick transition from rest to top speed, and insures minimal maintenance.
CONCLUSION This project has provided the students a platform to explore their designing and technical skills. TEAM RPM is considering every possible aspect while designing every single part. And we are very sure that the approaches used by us will bring innovation to our vehicle. Major innovations would include: •
Light weight chassis.
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Improved steering system.
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Shorter wheel base and track width.
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CVT coupled to high-low-reverse transmission.
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Double-A suspension arms in both rear and front.
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Custom designed and CNC manufactured hub and knuckle.
We hope that this year’s vehicle will outperform the previous year’s vehicle in every aspect.
REFERENCES 1) Fundamentals Of Vehicle Dynamics by Thomas D. Gillespie 2) Book By Carol Smith – Tune to win on vehicle dynamics 3) An Introduction to Modern Vehicle Design by Julian Happian Smith 4) MAN – Vehicle Calculations 5) The Automotive Chassis : Engineering 6) Vehicle Body Layout and Analysis by John Fenton
ABBREVIATIONS OD: Outer Diameter UTS: Ultimate Tensile Strength TYS: Tensile Yield Strength
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Fig 12:
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