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