-
ENGINEERING DESIGN AT THE UNIVERSITY OF MELBOURNEDEPARTMENT OF
MECHANICAL AND MANUFACTURING ENGINEERING
FORMULA SAE STEERING AND SUSPENSION DESIGN
Kenneth Chan, Ania Romanowski, Sam Steward, Terence Yuen and Ken
Brown
Keywords: manufacturing sector, automotive design, Formula SAE,
suspension & steering,upright, stub axle, design, racing, car
handling
1 Problem Definition
Each year, the Society of Automotive Engineers Australia runs a
competition in whichuniversity students design and build a formula
race car to be manufactured in small quantityfor the consumer
market, known as Formula SAE. The teams compete against each other
indesign, performance, manufacturability (including cost) and
aesthetics. Melbourne Universityhas joined with Kangan Batman TAFE
for the third consecutive year to form MUKBMotorsports, and
together will design and manufacture the 2002 vehicle.
Overall, the MUKB objectives involve producing a vehicle
which:
conforms to all SAE-A rules and regulations
will attract and meet the needs of the weekend autocross racer
as our consumer target
is light-weight, high in strength and has superior performance
in all areas includingacceleration, suspension, handling, fuel
economy and safety.
minimises the cost of manufacture, fabrication and assemblyThe
steering and suspension team is primarily responsible for the
dynamic and handlingcharacteristics of the vehicle. Our goals,
whilst reflecting those of the whole team, alsoinclude designing a
system which:
provides superior ride, handling and manoeuvrability
characteristics
is easily manufacturable and comprises the minimum number of
parts possible
is compatible with other vehicle systems and easily integrates
all components
provides adjustability for different track situations
is structurally sound, has minimal compliance and will not fail
during competition
-
2 Solution Proposals
2.1 Dynamics
Suspension type
Independent double wishbone suspension was chosen for the front
suspension because of theversatility available to the designer and
its common use in open wheel racing.
The inboard pullrod activation configuration was chosen as it
best met the overall suspensionobjectives of low C.O.G and light
unsprung mass. Having the activation rod (pullrod) intension
represents a weight-saving advantage.
Configuration
The objectives of the suspension configuration are:
to achieve a roll centre for the front and rear suspension at
heights of about 30 and 50 mmabove the ground respectively
to achieve a design in which the roll centres don't migrate
vertically or laterally but remainconstant ensuring the car behaves
predictably
to achieve a camber change rate of less than 1 / body roll
to achieve a lateral tyre scrub of less than 1mm/G
The resultant design can be seen in figure 1. It makes use of
the Short-Long Arm (SLA, shortupper arms and long lower arms)
configuration that allows easy adjustment of the roll centreby the
designer. The achieved results of roll centre height, migration,
camber change and tyrescrub were found to meet the objectives of
the front view configuration and are shown in table1.
Table 1. Front view configuration characteristics
Car end Roll centreheight (mm)
Verticalmigration
(mm)
Lateralmigration
(mm)
Camber change rate(deg/deg body roll)
Lateral tyrescrub (mm)
Front 30 1 50 0.35 0Rear 50 1 26 0.5 0
-
Final Suspension Geometry design
Figure 1. Final suspension geometry
2.2 Suspension components
A-arm design
The 2001 suspension failure was due to fracture of the rod end
in the A-arm. Therefore, in2002 the arm end has been designed using
a rigid arm end instead of an adjustable rod end.The best solution
proved to be a strong bearing house as the arm end.
The A-arm can be divided into different components:
1. A-arm tube
The A-arms are manufactured from circular Chrome-Moly tubing.
Two tube sizes wereselected based on yield and buckling
analysis:
Table 1. A-arm tube specifications
Diameter Wall thickness Application 35thou Upper A-arm
7/8 35thou Lower A-arm
-
The A-arms have a safety factor of 3 in yield and will withstand
more than three times theexpected load in buckling. The choice of
hollow circular tubing represents a significantweight saving.
2. Bearings
Spherical bearings were chosen for their ability to withstand
large loads and rigidity,whilst allowing a full range of movement
for the arms.
Table 2. Selected spherical bearings
Part No. Outsidediameter
Bolt size weight
GE10C 19 mm 10 mm 0.012kgGEH10C 22 mm 10 mm 0.02kg
3. Bearing housingBearing housings were designed from solid
steel bars in two sizes for the selected bearings.The bearings will
be connected into them using eutectic brazed joint to provide
superiorstrength.
Suspension Mounting Brackets
The mounting of the suspension arms to the chassis proved to be
a challenging designproblem. Due to the difficulty of welding
aluminium, the bracket either had to be bolted orglued. Bolting
would require holes in the chassis tubing, weakening the structure
and creatingstress concentrations, so a bonded bracket was
designed.
The brackets are constructed from a bent sheet of steel with a
spacer to increase rigidity. Theyare angled toward the suspension
arms to reduce bending in the brackets.
-
Spring and Damper Selection
An analysis of the Fox Vanilla RC shock absorbers used in
2001found they were:
bump and rebound damping adjustable
lightweight
small
reliable
serviceable
These features were found to meet the requirements of the damper
again this year and thuswere selected for the 2002 car. The
significant cost of reliable dampers and opportunity toreuse these
was seen a good cost saving.
2.3 Steering
The steering system must work with the suspension to provide
good handling characteristicsas well as enabling the driver to:
manoeuvre the car around the tightest hairpins maintain good
control of the vehicle operate the system without excessive force
or work
The 2002 Formula SAE vehicle will have front wheel steering.
Steering rack
The steering rack and pinion is taken from last year's car for
the following reasons: Specifically manufactured for Formula SAE
High quality materials Lack of unwanted movements Expensive to
replace Includes steering stops as specified by FSAE rules
-
Front suspension and steering specifications
The following specifications were chosen to achieve the set
aims.
Table 3.Front suspension and steering specifications
Track 1220 mmCamber 0Castor 2 30
Steering axis offset 0 mmTrail 11 mm
Steering axis inclination 5Scrub radius 37 mm
Toe in 4 mmSteering angle - inner wheel 30Steering angle - outer
wheel 26
Steering ratio 5.6Number of turns from lock to lock 0.89
Turning circle 8.5 mAckerman point 1823 mm (approx) behind front
wheels
The design represents a reduction in steering effort of
approximately 40% whilst allowing fora tighter turning circle of
less than 9m.
2.4 Wheel mounting system
Wheels and tyres
It was decided that the 2001 wheel and tyres satisfied the
requirements of the 2002 vehicle.
-
Stub axles and uprights
The wheel mounting assembly must mount the suspension and
steering arms in the correctlocation, withstand all forces applied
through the wheels and suspension members and be alight as
possible.
The wheel assembly comprises of the following components:
1. stub axles2. uprights3. hubs4. wheel bearings5. brackets for
suspension arms6. calliper and brake rotor mountings (front
only)
The number of parts and overall manufacturing effort and cost
have been reduced bydesigning a single symmetrical system. The
assembly involves- front and rear- identical livestub axles and
symmetrical, identical uprights, with the front including a welded
bracket forthe calliper and rotor. Hollow shell uprights provide
maximum rigidity, while reducing theunsprung mass by 20% from 2001.
Hubs are machined directly onto the hollow stub axle forgreater
rigidity, less mass (shaving 40% from 2001) and to simplify
assembly. Single rowdeep-groove ball bearings allow the stub axle
to rotate within the upright. A selection ofremovable brackets
allows for suspension mounting in any location, and a range is
providedto facilitate camber, toe and castor adjustment.
-
3 Current Status
The next step for MUKB Motorsports involves manufacture and
assembly of the vehicle.Whilst our designs are primarily complete,
we envisage several minor changes during thisstage to facilitate
the manufacturing process.
Testing of the suspension brackets will verify the choice of
glue as the bonding method, andcontinued FEA on the uprights will
ensure their rigidity before final assembly.
The final race meet in December will hopefully prove the
superior ride, handling andmanoeuvrability of the 2002 MUKB vehicle
and allow the team to showcase our unique andinnovative
designs.
The steering and suspension design has enjoyed considerable
praise from the other teammembers. The significant weight savings
to the unsprung mass and the superior strengthproperties have led
to the expectation that the 2002 MUKB Formula SAE car will exceed
allprevious performances.
Problem DefinitionSolution ProposalsDynamicsSuspension
typeConfigurationFinal Suspension Geometry design
Suspension componentsA-arm designSuspension Mounting
BracketsSpring and Damper Selection
SteeringSteering rackFront suspension and steering
specifications
Wheel mounting systemWheels and tyresStub axles and uprights
Current Status