Chassis design for baja sae

Chassis Design for Baja SAE
CETPA INFOTECH PVT. LTD.
Designed & Arranged by:-
Bhola Patel
Design Engineer at CETPA INFOTECH
ABSTRACT
This paper acts as a mock design report
describing the overall chassis design
that I developed for the Baja SAE
competition. It is a basic design, acting
primarily as an artifact to help further
my academic writing skills.
INTRODUCTION
Baja SAE is an intercollegiate
competition sponsored by the Society of
Automotive Engineers (SAE). The over-
arching goal is to simulate the real world
engineering and design conditions by
having each team competing for their
design to be manufactured by a fake
firm. Thus, each team must design, build,
test and promote their design to be not
only the best but also operate within the
given rule specifications. The vehicle
itself is a single seat, all-terrain buggy
that is powered by a 10-hp Briggs &
Stratton single cylinder engine. In order
to compete in the dynamic events, the
vehicle must first pass the technical and
safety inspection. This is where the car
is inspected to insure that it is safe to
operate.
After the vehicle has passed the
initial inspection, it can be scored. The
scoring is broken up into two parts,
static and dynamic. The static events
consist of the design report, the cost
analysis and the sales presentation. The
Dynamic events are acceleration,
traction, land maneuverability, rock
crawl, suspension and an endurance
race. Teams are scored with respect to
the standard guidelines, and the team
with the most points wins. The
incorporation of the static events, not
only prevent the team with the most
money and therefore material quality
from winning outright, but they also
encourage good engineering practice
which is also a goal of this competition.
This paper covers the ideas
behind my design and its over-all
methodology.
1 Chassis - Overview
A chassis at its most basic, acts as
the skeleton for the entire vehicle. It
provides a rigid connection between the
front and rear suspension, creates
structural support for the other
necessary systems and provides
protection for the driver. There are a
variety of different frame designs to
choose from; however a tubular space
frame is most commonly used due to its
inherent structural properties, and ease

of fabrication and modification. This
style of chassis consists of a series of
tubes connected together in different
ways to form a coherent structure. An
example of this style of frame can be
seen in figure 1.
2 Design Considerations
Before an actual design can be
put down on paper, the design
parameters necessary to the design
must first be addressed. Several
parameters for instance could be total
weight, material, mounting points, etc.
Typically these design parameters are
selected after the suspension, engine,
drive train and the other major
components of the vehicle have already
been laid out. Then, the parameters can
be chosen such that the frame is
designed exactly to the specifications
needed, which saves time, money and
weight. For my project, since I did not
have the entire vehicle designed, I chose
frame stiffness as my primary design
consideration since stiffness
incorporates many other design
considerations into it and allows me to
showcase the design process, which is a
primary goal of this paper.
Frame Stiffness
The suspension is designed to keep all of
the tires on the ground when subjected
to the dynamic loads of the vehicle. To
do this, the frame is usually assumed to
be a rigid structure[2]. Because this
assumption is made, if the fame is in
actuality, not a rigid structure, un-
anticipated forces and degraded
suspension performance can occur. This
can lead to either lost vehicle
performance in the best case and total
frame failure in the worst. That being
said, most frame designs walk the fine
line between stiff enough to handle the
loads yet light enough to deliver good
performance[3]. This is where most of
the time spent on a design goes.
Furthermore, due to the frame
requirements set down by the SAE rules
committee, most teams agree that if the
frame is stiff enough to withstand design
loads then it will not fail if crashed[3].
There are two main types of stiffness
when it comes to frames: torsional and
bending. Bending stiffness, which
corresponds to the frame tubes between
the front and rear suspension bending,
is not as critical within this vehicle
design as torsional is. This is because
midpoint bending of the tubes does not
affect suspension performance as
critically as torsional loads do[2].
Torsional stiffness – Torsional stiffness
is the resistance of the frame to
“twisting” loads. These torsional loads
develop primarily due to the fact that
there are four suspension attachment
points, two on each side of the vehicle.
Since the suspension loads react the
loads on the opposite sides of the
vehicle, moments are created that twist
the frame resulting in torsional loads.
When the frame twists, it creates
Figure 1. Tubular Space Frame

misalignments within the suspension
which can cause the problems
mentioned earlier.
There are several ways to increase
torsional stiffness. One method involves
adding copious amounts of material to
the frame structure. This strategy is
usually detrimental to the over-all
performance because it also increases
the weight. Another method is called
triangulation which incorporates the
polar moment of inertia [3].
Triangulation – It is well known that the
triangle is one of the strongest principle
design structures and because of this, it
is beneficial create a frame of
interconnecting triangular sections. A
good example of this triangulation can
be seen in figure 2.
Notice how most of the frame members
connect and combine to form triangles.
This significantly strengthens the frame
without adding lots of weight. This
strength to weight ratio is key when
designing a race chassis. Furthermore,
when using triangulation, the nodes
where members connect become
extremely strong and stiff, making them
excellent mounting points for large force
producing components such as the
suspension and engine[3]. Thus, the
frame should be designed in such a way
that the main triangulation nodes for the
chassis also be the attachment points for
the suspension.
Performing triangulation in this way
also creates a coherent load path within
the chassis. The load path corresponds
to the path that the force and
subsequent stress follows through a
structure. If there is not a coherent load
path within a structure and forces are
being reacted in middle of long tubes
then the structure will fail because the
load is only being reacted by one
element, which by itself if very weak. By
having a coherent load path, then the
force and stress can be spread out over
many interconnected members which
recduces the force and stress felt by any
individual member. Thus the possiblity
of a member failure goes down and
lighter materials can be used to react
those loads.
Since frame members consist of thin
walled tubes, triangulation is very
important. This is because tubes of this
type perfrom very well in compression
and tension but not in bending [3]. This
means inorder to avoid bending in the
main structural members, those
members must be kept short. And the
best way to create a coherent structure
from short members is through
triangulation.
Polar Moment of Inertia – Torsional
stiffness of a frame is also heavily
influenced by the concept of polar
moment of inertia. This moment of
inertia corresponds to an object’s ability
to resist torsion. It is commonly used to
determine shear stress acting in beams
under torsion and in power calculations
for turbines and the like. In the case of
the chassis, only the basic concept is
used to enhance torsional stiffness.
From the definition of polar
moment of inertia, it is understood that
Figure 2: UM-Rolla 1996 FSAE Chassis[3]

the farther structural material is away
from the axis of twist, the stiffer will be
in bending and torsion [3].
3 My Design
Figure 3: Isometric view
Figure 4: Front View
Figure 5: Side View
Figure 6: Birds-eye View
Figures 3-6 are different views of the
chassis that I designed. To create this
chassis, I used the 3D-modelling
software called SolidWorks. It belongs to
the class of programs known as
computer aided design (CAD) suites and
is commonly used across all disciplines
in industry. As can be clearly seen in the
figures, my chassis is a tubular space
frame consisting of a mix between 1.25”
and 1” tubular steel members. I debated
between what alloy of steel (4340 or
4130)to use, which becomes important
later in the analyisis process. I reached
my decision by weighing the strength,
cost and machinability for each material
against one another. After that, I decided
to go with the 4130 alloy as it is cheaper
and easier to machine than 4340. It is a
little weaker but that being said, with an
ultimate strength of 97.2 ksi, it is more
that adaquate for this design.

Other aspects that drove my
design were the required structural
members as dictated by by the rules,
weight, and over-all frame dimensions.
The required members can be
seen in figure 7 and consist of the main
support hoop (MSH), the roll over bars
(ROB), side support bars (SSB) and rear
support bars (RSB)[ref].
The rules require only the members
mentioned above and then stipulate that
they can be connected in any way to
from a coherent structure with adequate
strength.
Inorder to keep the weight down,
I tried to minimize the total number of
members in the design. SolidWorks
actually has a tool in it that will calculate
the combined mass of all of the solid
bodies in the model, but I was unable to
use it. This is because I had to create the
model using solid tubes instead of
hollow ones due to computational and
time constraints. However, most teams
aim for a chassis weight of around 50 to
60 pounds [1].
The over-all dimensions of my
design were also a driving factor. The
rules stipulate that the courses for the
dynamic competition are designed for a
maximum vehichle size of 64 in. at the
widest point and 108 in. at the
longest[1]. The longer and wider the car
is, the more stable it is. However, it is
more a function of aspect ratio versus
outright dimensions. As in, how long the
car is with respect to how wide it is. If
the car is too long, then it will tend to
“push” through corners, causing a poor
turning radius and loss of speed.
However, too short of a car will be really
“twitchy” in steering at high speeds
which can result in a roll over.
Furthermore, since this is an off road
competion, a longer wheel base would
be beneficial in the rock-crawling
competion for an extened reach, but
during the hill-climb portion, a longer
wheel base can cause the front end to lift
off the ground earlier than a shorter one
would. It is all a matter of compromises
and suspension design. Due to the lack
of suspension in my project, I chose a
wheel base of 97 in. and a width of 62 in.
This is one parameter that would need
to be investigate in much more detail if
the chassis were to be built.
I chose to keep the design wide in
order to maximize my polar moment of
inertia. As seen in figure 4 (reproduced
below),
Figure 7: Required Members
Figure 4: Front view

the chassis members extend farther out
past the central axis of the design. This
helps increase the torsional stiffness of
my design. Having the chassis widest
where the driver is sitting also makes
driver ingress and egress easier and
safer, which is one part of the technical
inspection.
I tried to triangulate as much as
possible too. As can be seen in figures 3-
6, most of the structural members form
triangles with the surrounding elements.
Also, at the extreme four corners of the
chassis, there are points where more
than 3 members attach into a node.
These points are very strong and have a
good load path to the rest of the
structure. I designed these nodes as the
suspension attachment points, where
most of the force will be transmitted to
the chassis.
4 Analysis
When I use the term analysis in
this context, I mean to analyze the stress
and deflection within the members of
the chassis, under reponse to structural
loads. To do this, I used a computer
program called ANSYS. This is a finite
element analysis (FEA) program that
allows the user to analyze virtually any
complex geometry.
FEA is a type of analysis where
the solid geometry created in a CAD
program is “chopped” up into a bunch
(typically on the order of 100,000)
pieces and each of those pieces is
analyzed with respect to the type of
analysis being done. This is a very
powerful tool, however for large models
like the chassis, this analyis mode takes
a lot of computational power, which
equates to lots of times. In order for me
to be able to solve a model in under 20
minutes, I had to use soild bars in my
model. If I were to use hollow tubes with
a thickness of .045 in, ANSYS would have
to represent the model with elements no
bigger than .045 in. Thus in such a big
model, there would be a very large
number of pieces, probably close to 106,
which would take almost 2 hours to run.
My goals for the analyis I performed
were to get a rough sense of the primary
load path through the chassis and to see
if there where any stress “hot-spots”.
For this analysis, solids bars will
generate similar results as what one
could expect from hollow bars. This is
because the applied load will travel
along the same path whether the bars
are solid or not. Furthermore, if there
are any stress concentrations in the
solid bar model then they will still be
there and exagerated in the hollow
model and must be remedied any ways.
Because of this and the time constraint I
determined that a solid bar model would
perform as needed.
The Setups
I decided to analyze my frame based
upon several different loading schemes,
vertical loading, side-impact and front
impact. I decided not to perform a
torsional stiffness analysis because the
use of solid bars does have a large
impact on this type of analysis. The solid
bars have too much inertial stiffness
which leads to errouneous results when
subjected to torsional loads. This would
be an analysis that needs to be
completed farther down the design path,
once it becomes prudent to invest the
time needed to perform the analysis.

Vertical Loading - For this
analysis setup, I rigidly fixed the model
in ANSYS at the four suspension points
and loaded the chassis at the center of
gravity (CG) with a large 700 lbf.
This setup is fairly realistic of a dynamic
loading situation during vehicle
operation, as the force at the CG would
be reacted equally by the four
suspension points. The results can be
seen in figure 9.
Figure 9 shows the deformation of the
chassis as well but keep in mind that the
scale is on the order of 10-3, which is not
even visible to the naked eye. What is
seen from this stress distribution is that
the vertical loading is reacted by the
majority of the structural members
within the chassis. This is indicative of a
good load path and transfer within the
members, which is what I wanted to see.
Also, there are no bright red spots in the
model which indicates that there are no
large stress concentrations which is also
good to see.
Side Impact – For this analysis, I kept the
chassis rigidly connected at the four
corners again and applied a force to the
side support member. This can be seen
in figure 10.
The stress distribution can be seen in
figure 11.
Once again, there are no stress
concentrations and although the
Figure 8: Vertical Loading
Figure 9: Vertical Loading Stress Distribution
Figure 10: Side Impact
Figure 11: Side Impact Stress Distribution

impacted side of the chassis distributes
the load pretty well, I would like to see
the load transmited to the opposite side
of the chassis a little better. Also, this
style of analyis with the chassis rigidly
fixed, represents a much harsher impact
than what would ever be possible in a
real impact. That is because in a real
impact, the suspension would give and
the car would slide over the ground,
lessing the impulse force of the collision,
but this analysis gives a good idea of
what load transmission to expect in the
event of a side impact.
Front Impact – The chassis is still
rigidly fixed in this analysis and the load
is applied to the front of the chassis. See
figure 12.
As seen from the stress distrubtion in
figure 13, there is almost no load
transmitted to the rear of the chassis.
Upon review, I think this is due to the
fact that there is only a comined loading
of 600 lbf, which is probably not enough
for a solid bar model. Also, the front
rigid supports should be removed if the
analysis were to be run again as well
since they are holding the chassis in one
place, which would not be the case in a
real front impact. The results are still
useful and it can be seen that all the
structural members attached to the
node are being loaded almost equally.
This is very good to see and indicates
that there is adaquate structural support
for a frontal impact.
Conclusion
Frame design, as most design problems
are, comes down to a series of tradeoffs
between various competing aspects.
With a chassis, the main aspects are
stiffness, weight and cost. There are
several ways to maximize these trade-
offs as discussed earlier and this is
Figure 12: Front Impact
Figure 13: Front Impact Stress Distribution

essentially what the design process
consists of.
I feel that my design incorperated the
concepts of triangulation and polar
moment of inertia into a coherent
chassis design that is representative of a
first cut. The actual design process, is an
iterative effort and once the analysis
results are in, the design can be tweaked
and updated to accommodate the
discovered weaknesses. This is where
my design currently sits. It has been
created and analyzed and the next step
would be to update the design to
address the issues found in the analysis.
However, within the scope of this
project, I feel I have suceeded in
designing a basic chassis that satifies the
design requirements of the Baja SAE
competion and allowed me to explore
and write about the design challenges
and process.

Chassis design for baja sae

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