ABSTRACT 3 mm Ultimate Tensile Strength 400 N/mm2

 

 

 

 

 

ABSTRACT

This report compiles the overall strategies undertaken by the team in
finalizing the design, fabrication and testing of the buggy. The objective of the design team was to satisfy these functions while
meeting the SAE’s rules and regulations with special considerations given to
safety of the driver, ease of manufacturing, cost, weight and overall
performance.

 

INTRODUCTION

This report interprets the methodology followed by Team Invicta to design,
fabricate and test an All-Terrain Vehicle that will compete in the Baja 2018. This event mainly focuses on demonstrating the passion of
the students on their core domain in design, fabrication
of their very own model. The primary concern is safety so the buggy should be
durable and easy to control and must also be able to negotiate rough terrain in all weather conditions.

 

VEHICLE DESIGN

Design focus – The design focuses with rigid frame suitable to withstand the rugged
terrain with an enduring suspension design and a versatile drive train which
best suits an All-Terrain Vehicle. With an easy access to a pipe bending
machine the team was able to use
more continuous members and thereby reducing the number of welds. Similar
robust and durable designs were adopted in transmission and brake system. In
order to prevail the commonly occurring design failure modes the design is analyzed
and a Design Verification Plan and Report (DVPR) is made to ensure it.

 

FRAME DESIGN

Material selection: Material selection is one of the key factors in designing
the frame of the ATV as it is the measure of safety, reliability and strength of
the roll cage. We made a

 

 VEHICLE NUMBER
– 28

TEAM INVICTA BAJA SAE (2018)

DESIGN REPORT

                          
Copyright @ 2018 SAE
International

 

 

Comparison over the frame materials which were widely used
for fabricating the roll cage.

Material

AISI 1018

AISI 4130

DUPLEX 2205

Outside Diameter

25.4 mm

25.4 mm

25.4 mm

Wall Thickness

3 mm

3 mm

3 mm

Ultimate Tensile Strength

400 N/mm2

525.13 N/mm2

620
N/mm2

Yield Strength

370 N/mm2

435.68 N/mm2

450 N/mm2

Density

7.87 g/cm3

7.85
g/cm3

7.805 g/cm3

Comparing the above-mentioned roll cage materials American Iron and Steel Institute
(AISI) 4130 seems to have a marginal yield strength for off-road derivatives. DUPLEX
2205 provides a very good yield strength in comparison with other
availabilities. Economically AISI 4130 saturates all the necessary requirements.
The density being the same for all materials, it is beneficiary to opt AISI
4130. The final roll cage was fabricated after making the Mild Steel Prototype and thereby making the
necessary modifications in AISI 4130 steel. AISI 4130 is a low-alloy steel
containing chromium and molybdenum as major constituents. It is also known as
Chromoly. The steel has good strength and toughness, weldability and
machinability.

 

WELDING

Introduction: We opted for Gas Tungsten
Arc Welding (GTAW), which provides superior weld quality and are generally free of defects. It has greater
control over the weld than competing processes such as shielded metal arc
welding and gas metal arc welding and it allows for stronger, higher quality
welds.  TIG reduces the spatter that
occurs with arc welding processes and can be used with or without filler metal
as required for the specific applications.

Preheat Treatment
Process:
Preheating drives moisture and other contaminants off the joints; moisture, lubricants
and other contaminants which are the sources of hydrogen. It also serves to
reduce the rate at which the metal cools down from the welding temperature to TPH. This is so whether
preheating is above or below Ms.  Cooling rate reductions will lead to a general
reduction in residual stress magnitudes, and also allow more time for hydrogen removal.

 

SUSPENSION:

 

Objective: An off-road suspension must be
engineered such that it ensures the durability of the vehicle and also
safeguards the driver against the shocks and vibrations. It should support the
entire load on the vehicle under all circumstances.

 

Design: The design process was initiated with scrutinising
the motion ratio parameter. Motion ratio is considered the critical parameter
in designing the ATV suspension because it determines how much the wheel
travels when hit by bumps. The mounting points on the front and rear suspension
were virtually foreseen via Solidworks and was analyzed for the desired load
conditions in ANSYS and the parameters were verified to be safe.

 

1. Front Suspension: The front suspension is   of equal
double wishbone arrangement. The center of gravity is at 600 mm from the lower under
seat member to reduce  body roll. The
upright of the system is manufactured by CNC and is Symmetric. It has  good strength to absorb loads. The knuckle
also provides a location to mount the brake calipers. In order to compensate
for dive-effects during aggressive cornering, the camber angle of the front
suspension has been set to 0° at ride height. In addition to that, the camber
angle has been set to decrease when the shock absorber compresses during turns.
As the suspension is double wishbone, front and rear arms are made from AISI
4130 material for high strength and stability.

 

2. Rear Suspension: customised arm is used  for rear suspension for better stability. Also
the loads are shared on the 3 mountings which will reduce the stress concentration.
As like front suspension, the rear upright is also a single manufactured piece
which provides for the connection of  A-arms and callipers.

 

3. Shock Absorbers: The front shock absorbers
were mounted on the lower arm to resist the impact forces. The  shock absorbers have a stiffness of 15 N/mm at
the front and 13 N/mm at the rear. Because of  uneven high momentary loads at the front
suspension, the stiffness of the shock absorbers are kept high compared with
the rear. The rear shock absorbers were mounted on upper arms for just
withstanding the self-weight of the buggy.

 

STEERING SYSTEM

 

Objective: The objective of the  steering is to determine the desired course of
the vehicle and to withstand high stress in off terrain condition. The
mechanism should reduce the steering effort and to provide good response among
the road, driver and buggy.

 

Design: Rack and pinion steering mechanism is chosen as it provides high degree of
feedback while being light in terms of weight. The rack and pinion  does two things.
It converts the rotational motion of the steering wheel into the linear motion
which is indeed to turn the wheels. It provides a gear reduction, making it
easier to turn the wheels. The mechanism travels from
one end to other  (12 cm) in 1.75 turns
of the pinion.

 

1. Steering ratio: If
there are many teeths per inch, the rack will be easy to turn (lower steering
effort), but the handling will be slow. If there are  few threads per inch, handling will be quick,
but steering effort will be high. In order to
minimize the steering effort, 8:1 ratio is chosen so that for every 2-degree
rotation of steering wheel tires will be turned by 1 degree.

 

2. Tie rods: The material we chose is mild steel to
which ball joints are attached on both sides. Models are studied for correct tie
rod length and to study bump steer effects (toe, camber change).

 

3. Correct steering angle: While taking turns, the
condition of perfect rolling is achieved if the axes of the front wheels when
produced meet the rear axis at one point. This is the instantaneous centre of
the vehicle. The inner wheel deflects by a greater angle than the outer wheel.
Larger the steering angle, the smaller is the turning circle. The steering
angle of the inner wheel can have a maximum value of about 41 degrees

 

 

 

Steering ratio

8:1

Rack travel

12 cm

Front track width

 

Wheel base

 

Inner ball joint center distance

160 mm

Outer ball joint center distance

560 mm

Rack length

279 mm

Turning radius

3900 mm

Toe In

Motion Ratio

0.71

Scrub Radius

55.7 mm

 

Transmission:

 

Objective: The effective utilization of the
available torque results in the greater performance of the buggy irrespective
of the off-road conditions. This factor pertaining to the proper gear reduction
helps in the needs of the vehicle in the competition. We favoured for the
manual transaxle over CVT because of the following reasons

· Gives better acceleration

· Lighter and economical

· Slippage losses are less
in manual gearbox

·   Heat generated in manual transmission is less due to the time gap between
the shifts.

 

ENGINE: . As given by the rulebook, the engine
is said to develop 10HP and maximum torque of 14.5 Nm. The governor is set to  maximum of 3800 rpm while the idling speed is
to be 1750rpm.

 

Design Methodology: The rugged operation of
the buggy in off-road conditions may cause frequent misalignments of the chain
drive. The usage of CVT reduces transmission efficiency as a result of slippage
between the belt and pulley. Hence our design methodology comprises of the
direct transaxle coupling.

 

1. Transaxle Design Details   :The
market survey with the available transaxles  vehicles in Indian market showed the existing
models: Mahindra Alfa Passenger, Piaggio Ape Mini Truck, Jeeto goods truck and Mahindra
Gio Mini Truck, Eicher polaris.

1.1 Maximum Gear Ratio = Radius of tyre,

 r = 12 inch =
0.31m

Mass of vehicle, m = 380 Kg (laden)

fr = rolling resistance coefficient = 0.035

? = Maximum Angle of Inclination = 45°

? = Efficiency of Manual Gear Box = 0.82

Hence, Maximum Gear Ratio =

380*9.81*0.31*(0.035*cos45+sin45)/(14.2*0.82) = 72.63

 

1.2The Smallest gear ratio of the gearbox is given by the equation:

 Minimum Gear Ratio
=

N = Speed of engine (RPM) = 3800

v = Speed of vehicle (kmph) = 60

Therefore, Smallest Gear
Ratio =

(3.6 X (3.14/30)
X3800X0.31) / (60) = 7.39

 

1.3 Mahindra Alfa gear box overall gear ratios

Since the desired range of gear ratios was close enough to that of the Mahindra Alfa gear box, hence we opted for it.

 

1st gear ratio

4.6

2nd gear ratio

2.733

3rd gear ratio

1.6667

4th gear ratio

1.074

 

Net gear reduction: Gear ratio * clutch gear reduction * final drive reduction:
=4.6*2.6*2.38=28.465

 

 

2. Speed and Acceleration Calculations:

· Max Speed of Engine=3600RPM

· Max Torque of Engine=14.2
N.m

· Diameter of the
wheel=0.635 m

· Mass of the vehicle= 380
kg (approx.)

· Considering forward 1st gear

 Speed = 3600/28.465 = 126.47 RPM =14.78 km/hr

 

Acceleration calculation-

Torque=14.2*28.465=404.203 N.m

A=F/M =T/Mr = 404.203/ (380*0.31)= 3.43m/s2

.

 

Gear

Net reduction

Max speed (km/hr)

Max acceleration (m/s2)

1

28.465

14.78

3.43

2

16.91

24.88

2.04

3

10.31

40.81

1.24

4

6.64

63.3

0.8

 

 

 

 

 

BRAKE SYSTEM

           

Objective: The purpose of the braking system is
to increase the safety of the vehicle as well as the driver by dynamically
decelerating all four wheels on both paved and unpaved surfaces.

 

Design: Our
hydraulic disc brake system is controlled by a single pedal in line with master
cylinder and booster. The caliper is of double piston type. Our brake system is
diagonal split (X split). In this system
the master cylinder has two outlets which are connected to two brake lines
carrying brake fluid. One of these lines connects the brake calipers of the
front left and rear right wheels, while the other one connects the front right
and the rear left tire. As it connects wheels in diagonal position and brake
lines somewhat form an X, they are called as X split brake system or Diagonal
brake system. The advantage of this over the normal front, rear split system is
that whenever if one brake line fails, you still have one of your brakes on the
driving wheel working. Where else in front, rear split suppose you have a front
wheel drive system and front rear split brake line. Now if the front brake line
fails, you cannot handle the drive wheels.

 

Front disc diameter (mm)

180

Rear disc diameter (mm)

180

Caliper piston diameter

25.4

Master cylinder bore
Diameter (mm)

19.05

Master cylinder stroke length (mm)

16

Pedal ratio

4:1

Coefficient of friction of brake

0.3

Weight of the car with driver (kg)

380

Brake biasing

60:40

Stopping distance (m)

11.16

 

 

Brake configuration:

Ø The pedal S&
balance bar are with a pedal ratio of 4:1 for maximum power
multiplication.

Ø Brake booster is
installed to improve the efficiency of the brake simultaneously reducing the
braking distance.

Ø Armoured with
steel, the brake lines run through the length of the car. It was chosen due to
the flexibility, strength and ability to maintain high line pressure values,
yet the main cause remains the same, i.e., to withstand sudden jerks and to
avoid undesirable tear-off especially during suspension travel.

Ø The reliability of
our braking system is improved by mounting disk and calipers on each wheel. On
comparing with conventional drum brakes, the calipers are small in size,
giving acceptable values of clearance, whilst maintaining good braking
capability.

Ø All four calipers
are floating type because of the fact that they are very compact to easily pack
on the vehicle and they reduce the  temperature generation in the brake fluid due
to only one contact surface between piston and brake pad. In addition to that, they have fewer leak points as compared to fixed
piston types.

 

SAFETY

 

Objective: SAE BAJA primarily focusses on the
safety of the driver and the vehicle. Accidents and mishaps are unforeseen
circumstances that could easy immobilise the vehicle. Lack of imprudence and
negligence in the safety considerations leads to fatal injuries. Hence the design
considerations are devise. This clinches the inviolability in the design.

 

Mountings and Equipment:

Fire extinguisher:

·        
As per the regulations of
the rule book, the primary extinguisher is clamped to the drake kit, mounted on
the rear roll hoop using metal fasteners.

·        
An extinguisher with a
UL-rating of 10BC is used to provide maximised clearance between the base fire
and the track worker.

 

5-Point Seat Belt:

·        
 

Impact attenuator

·        
The
crumble zone is provided to absorb the energy during collision and thereby
reducing the impact force acting on the structure, vehicle and passenger. Thus the
succeeding zones of the buggy are safeguarded.

·        
As
a measure of safety the crumble zone is given a additional safety  named attenuator.

·        
The
impact attenuator is made of polyurethane. This material has a wide molecular
structural variability providing good possibility of ambient curing.

 

Roll bar paddings

·        
These
are generally use in motorsports to provide safety to specific areas especially
to the places where constant contact with the roll cage is seen.

·        
We
have provided paddings to RHO and FBM members since it often comes in contact
with the driver during popping in and out of the buggy.

 

 

 

 

ELECTRICAL SYSTEM

 

 Certain
electrical components have been installed in the ATV to ensure its safety. Two
kill switches have been mounted with easy accessibility, which instantly kill
the engine. Brake lights, reverse light and reverse alarm

pertaining to SAE standards have been used. A transponder is being used
which relays the number of laps completed to a timing device on the track. All
electrical components are powered from safely secured 9V batteries.       

 

 

CONCLUSION:

 

The final product manufactured  is a
result of collaborative multi disciplinary 
design. Material selection for
each and every component remained a priority for the team considering an
optimum strength to weight ratio, durability, cost effectiveness and
feasibility. Before initiating the actual fabrication, real time conditions
were simulated using various FEA packages like Solidworks, ANSYS, and critical
parts of ATV were analyzed for safety and optimization issues. The parts were
manufactured in techniques which can be considered suitable for mass production
of this model, if introduced in market. Apart from manufacturing issues, the
team also has to focus on other aspects like managing funds and working with a
budget plan, achieving targets within deadlines, project management, marketing
and sales, making its members competent automobile engineers and tech savvy, in
short attempting to make the team members ready for real time industry
experience.