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arms weighed
a total of 6.49 lbs.

In a competition where
the weight to power ratio
matters significantly, FSAE teams always strive to design and engineer a car that is as light as possible yet still
structurally sound and that delivers
maximum power to the wheels.
By using carbon fiber tubes to replace the 16
steel tubes the total weight of the A?arms
can be reduced by at least 50% and the overall stiffness of the tubes
can be maintained or even increased.

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While the
A?arms act as structural members of the suspension, the push rod, also shown in figure 2?1, acts as the spring?damper component of the car’s
suspension. The A?arms,
push rod, and sprung and unsprung masses of the suspension system
all need to be carefully designed so that
optimal car performance is reached on the race track.

 

3.0
Carbon Fiber Tubes

 

                3.1
Types of carbon fiber

 

Carbon fiber
tubes are most
commonly manufactured using
two methods: Pultruding and roll?wrapping.

Pultruded – Pultrude Carbon Rods and Tubes are
manufactured for maximum rigidity with minimum mass. Pultrusion orients the
fibers lengthwise down the shaft for maximum structural rigidity. Carbon fiber
contributes to the rigidity, while minimizing weight. Carbon is 70% lighter
than steel, 40% lighter than aluminum, and three times the stiffness of either
for the same weight.

The Carbon fiber has a negative coefficient
of thermal expansion, meaning it expands as the temperature lowers. The resin
matrix, on the other hand, has a positive coefficient. The net result is
virtually no expansion or contraction of the composite over a wide range of
temperatures.

 

 

 

Figure 3­1:
Pultruded tube (left) and roll­wrapped tube (right)

 

 

Flexural Modulus

18.5 msi / 127 GPa

Fiber Volume

60%

Thermal Expansion Coefficient

-0.1 ppm/cm3 / -0.2 ppm/°C

Glass Transition Temperature

100° C

Matrix Material

Bisphenol Epoxy Vinyl Ester

 

Roll?wrapped

Roll-wrapping involves the applying of resin pre-impregnated
composite fiber cloth (Pre-Preg) around a mandrel. The outer diameter of the
mandrel thus determines the inner diameter of the final tube. The mandrel and
cloth are then spiral wrapped with a consolidation tape under tension to hold
the laminate in place during the curing phase. After curing, the mandrel is
extracted to leave the tube ready for machining or finishing as necessary.

                3.2 Advantages of carbon fiber

 

The Carbon fiber A-arm would increase the
performance at least in three different areas: Lower weight, higher strength
and higher stiffness.

·        
Lower Weight.

From
an aerodynamic perspective, it is the general consensus that the lower the
weight, the higher the performance. In the field of racing, where speed is
worshipped, weight can be considered an evil force or enemy. As a high
performance racing car, Formula SAE cars also require light weight to achieve
high acceleration and speed. Lower weight also comes with the benefit of fuel
efficiency. The weight of the car depends in part on the density of the
materials being used. Carbon fiber is a material that consists of fibers about
5-10 ?m in diameter and are composed mostly of carbon atoms. The atomic
structure of carbon fiber is similar to that of graphite which consists of
sheets of carbon atoms arranged in a hexagonal pattern. The difference lies in
the interlocking of these sheets. In graphite, the sheets are stacked parallel
to one another. The intermolecular forces between the sheets are Van der Waals
forces; that is why graphite is soft and brittle. On the other hand, carbon
fiber is strong bu still very light–weight which is one of the biggest
advantages of using carbon fiber in A-arms. The density of carbon fiber is
almost five times less than that of steel as well.

·        
High Strength

The
formula SAE car requires a high strength structural frame to withstand the
forces caused by acceleration, braking and turning.  In order to fabricate qualified A-arms, it is
very important that the A-arm meets the required criteria for strength and
stress. There are two types of stresses: the one most relevant for the A-arm design
is the tensile stress, which is the maximum stress that a material can
withstand while being stretched or pulled before breaking. This can be
calculated by the formula, F/A, where F is the tensile force, and A is the
fixed cross sectional area. In carbon fiber, the carbon atoms are bonded
together in microscopic crystals, which make it extremely strong. Carbon fiber
materials are classified by the tensile modulus of the fiber. The strongest
carbon fiber has a tensile modules of 500 million to 1 billion kPa, which is
much higher than steel. Typically, steel only has a tensile modulus of about
200 million kPa. In other words, the strongest carbon fibers are ten times
stronger than steel, which means the carbon fiber can withstand ten times
larger force. The other form of stress or strength is the compressive strength,
c, which is the resistance of a material to breaking under compression, and can
be calculated by the formula, P/A, where P is the compression and A is the area
of the cross section. The carbon fiber also has a very high compressive
strength, which strongly withstands deformation.

·        
High Stiffness

Stiffness
is the measure of rigidity of an object; in other words, the extent to which it
resists deformation in response to an applied force. Stiffness is also an
important aspect of concern for the Formula SAE car design because the
excessive deflection or bending may affect the control systems and the
acceleration mechanics drastically. During high speed motion, if the A-arm is
not stiff enough, the FSAE car will have high roll in a turn, which could
potentially cause loss of control on the wheels. Its high stiffness could be
the ultimate advantage for the team on the track.

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