24-08-2012, 01:06 PM
Union College SAE Baja Vehicle Design Report
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ABSTRACT
An SAE Baja vehicle is a single-seat, all-terrain vehicle
powered by a ten horsepower Briggs & Stratton engine.
Undergraduate students at Union College from multiple
academic fields collaborated to design and manufacture
a safe, high-performance, cost-efficient Baja vehicle to
serve as a prototype for mass production. The students
utilized and refined both financial procedures and
engineering analyses to complete this objective while
conforming to the prescribed SAE rules.
INTRODUCTION
The Union College SAE Baja vehicle was designed as a
prototype for manufacture by an outdoor recreation firm.
The ideal vehicle is safe, simple and inexpensive: safe
for its occupant to be protected during use, simple for a
novice rider to operate and maintain, and inexpensive to
allow for general production and purchase. Additionally,
the vehicle should be attractive to potential buyers in
both its visual appearance and performance. These
characteristics were considered in design of the
following major vehicle systems: frame, drivetrain,
flotation, suspension, steering, and braking.
VEHICLE DESIGN FRAME –
Objective - The purpose of the frame is to provide a safe
environment for the occupant while supporting other
vehicle systems. Several steps were taken to ensure
this objective was met. Tungsten Inert Gas (TIG) electric
arc welding was used to guarantee solid joints and a
rigid foundation to support the main components of the
vehicle. In addition, extensive finite element stress
analysis proved the car would remain intact and protect
the driver under the most strenuous of crashes. The
frame was designed to comfortably accommodate a sixfoot,
three-inch tall driver.
Overview of the Design
In order to conform to the SAE
frame requirements, major redesign was decided upon
rather than alteration of the existing frame. For the 2008
season, several positive modifications were made to the
frame, including a shortened wheelbase, a seven-inch
front-end rake, and new cockpit layout. The front wheels
were moved six inches back, reducing the carʼs turning
radius and improving the weight distribution, which had
previously been heavily rear biased. In the 2009 season,
the team has redesigned the rear section of the frame to
accommodate new rear suspension and drivetrain
components.
DRIVETRAIN
Objective – The drivetrain for this yearʼs car has been
radically overhauled to improve overall car performance
and correct vulnerabilities of previous designs. Past
drivetrain designs have been double-reduction, openchain
designs, based around a CVT, chains, sprockets
and a jackshaft mounted in an open sub-frame. The
system benefited from simplicity and low cost, at the risk
of premature failure due to misalignment and direct
exposure to water and dirt. Adhering to recommended
wrap angles and center distances for chain drives
requires the system to occupy a large volume, thereby
increasing frame weight. Reducing the number of chains
in the transmission was a logical choice to
simultaneously address issues of reliability, performance
and size.
FLOTATION AND PROPULSION–
Objective - The goal of the flotation system is to provide
the buoyant force necessary to keep the vehicle and
driver afloat in an aquatic environment. This must hold
true at both a horizontal orientation and up to at least a
30-degree roll situation. While the car is floating, the rear
wheels function as the means of propulsion, with the
treads of the tires acting as paddle wheels. In the past,
it was believed that fenders close to the tires were
necessary for propulsion. We learned last year that thick
mud can easily render these fenders not only useless,
but harmful to the drivetrain. We have therefore decided
against fabricating fenders tight to the tire. This is
covered more carefully in the flotation section
TIRE SELECTION–
The rear tires are a 25” outer diameter by 10”
wide ITP Mud Lite model mounted on aluminum rims.
They are the lightest six-ply rated tires available which
give the durability needed to withstand rugged terrain.
The large outer diameter of the tire provides an increase
in ground clearance which is vital in traversing off-road
conditions. The treads provide ample water propulsion
when mounted correctly, which means the tires are used
in their proper orientation for optimum traction on rough
terrain. These tires were also chosen for their ability to
perform in mud, a clear requirement when off-road in
anything but a very dry terrain. The front tires are Maxxis
Razr tires mounted on 10” aluminum rims. Aluminum
wheels were chosen for their durability with minimal
weight. Minimizing the amount of unsprung weight on the
car is a major goal when designing suspension
components and saving weight anywhere possible
allows for added strength elsewhere. The Razr features
a wide tread pattern to reduce tread squirm and the
sipes tighten up under acceleration and braking forces.
These sipes are small cracks in the tread, designed to
move water away from under the tire quickly. Under a
sliding force, they lock together and prevent the treads
from shifting. These front tires have a less aggressive
tread pattern than the rear tires.
CONCLUSION -
The Union College Mini Baja vehicle has been designed
to appeal to customers and manufacturers by effectively
meeting the initial objectives and offering a safe,
affordable recreational vehicle to fill an otherwise empty
market segment. The current frame yields high factors of
safety and drive comfort. The drivetrain was improved
upon to further optimize the vehicleʼs performance and
enhance reliability and ease of maintenance. The
flotation system has been revised and lightened to
effectively traverse water without sacrificing land
maneuverability. The steering systemʼs design yields a
responsive and controllable car with no noticeable bump
steer. The braking system affords maximum overall
braking force on the front and rear wheels. The
suspension has been designed to provide ten inches of
ground clearance with the flotation, ample for nearly any
off-road terrain. The tires were selected to run on nearly
any terrain. The resulting vehicle is safe, attractive,
reliable, economical and fun to drive.