ACME Racing Design Report 2009
"The WS09 Design Report. The written section is complete as it was handed into the FSAE competition staff. The pictures however are a work in progress as the parts are photographed or finalised. Please enjoy this section of the webpage and check back soon to be sure to catch the updates as they happen."
In 2009 ACME Racing will be presenting the fifth car (WS09) from the University of New South Wales at the Australian Defence Force Academy. WS09 has been designed with several overarching principles that the team has determined to be critical to a successful FSAE car. These principles are reliability, simplicity, maintainability and manufacturability. Reliability has been at the forefront of all design briefs as ACME Racing has suffered the consequences of trying to compete with a fundamentally unreliable vehicle.
The design process for WS09 commenced with a review of the strengths and weaknesses of the teams’ previous vehicles followed by a check of the 2009 rules to understand the implications of the rule changes. The 2008 FSAE-A competition results were studied in order to set performance bench marks for WS09, these included the ability to maintain steady state cornering at 1.6g, brake and 1.6g and accelerate at 0.9g.
The major vehicle parameters were set using a steady state simulation which utilises the FSAE Tyre Testing Consortium data. For a given lateral acceleration and corner radius the steering angle is iterated in a sub loop which iterates heading angle to solve the sum of the moments about the centre of gravity equal to zero. Whilst ACME Racing understands the limitations of the tyre data with regards to absolute accuracy it was used to visualise trends so the team could set realistic vehicle parameters including front and rear track and wheel base. Targets were also set at this stage for longitudinal weight distribution and centre of gravity height. The team intends to validate the steady state simulation during the vehicle test and evaluation phase.
WS09 utilises a space frame chassis, steel weldment beam axle suspension front and rear powered by a four cylinder Suzuki GSX-R 600 engine.
Chassis
The decision to use a steel space frame chassis was based upon the ability to manufacture in house with existing skills and equipment as well as the inherent damage tolerance and repairability of such a structure. The chassis integrates with a custom designed structural sump plate to reduce the number of chassis tubes required. The main roll hoop supports integrate with the rear bulkhead which performs several functions including mounting the final drive, rear engine lugs, watts linkage and accusump mount. The impact attenuator is a nested aluminium weldment frustra designed using non linear FEA and verified using a dropped mass; instrumented with an accelerometer.
The chassis design aids maintainability by allowing easy removal of the engine by removing the rear bulk head and lowering the engine.
Suspension
The cockpit template rule changes forced ACME Racing back to the drawing board regarding suspension design. The wider cockpit required a revision of the previous concept; which was a short/long arm independent suspension geometry. This required the function of the suspension be reviewed from first principles. The requirements decided upon were; wheel attitude control, vehicle body motion control and the ability to maintain contact between the tyre and the ground, over undulating terrain (typical of FSAE event venues). Beam axles were decided upon over independent suspension due to ability to maintain accurate camber angles in both braking and cornering. The inability of beam axles to absorb single wheel bumps without detrimentally effecting vehicle handling was considered to be a shortfall of the concept however, the stiff anti roll springs required to maintain camber angles to the same degree as the beam axles was a mitigating factor. The beam axle was also favoured for design simplicity and ease of manufacture. This was due to its ability to be made from sheet metal weldments requiring minimal machining.
The front suspension uses a peg and slot arrangement for kinematic control and features shim adjustable camber (0 to 2 degrees), adjustable castor (3 degrees plus or minus 1.5 degrees), adjustable toe and adjustable ackerman (by replacement parts). The uprights are machined from 7075-T651 alloy with floating disc brakes mounted on the inboard side to reduce scrub radius whilst maintaining a steering axis inclination of 0 degrees. The chosen tyre is a Hoosier 18x10 7.5, the decision to move to the smaller wheel size was based on stiffness and reduced rotational inertia with the added benefit of providing a cost reduction over comparative 13 inch rim based designs. The wheels are a three piece design using Keiser spun aluminium rims with a bespoke CNC machined wheel centre. The wheel centre has been designed to efficiently distribute material to deliver high cornering stiffness whilst also meeting braking strength requirements. A four stud wheel retention mechanism was chosen over centre lock as there is no requirement to remove the wheels in under 60 seconds. The steering rack is attached to the unsprung mass in an effort to minimise bump steer. A large amount of bump steer was considered to have a greater effect on vehicle handling than the required increase in the unsprung mass. To overcome some of the packaging and integration problems inside the 10 inch wheel, steering passes through a bell crank on each side of the vehicle.
The rear suspension is a de dion twist axle, kinematically controlled with two trailing arms and a watt’s linkage. The watt’s link was chosen over the peg and slot used on the front beam as it provided improved structural packaging. To ensure the rear suspension was not kinematically over-constrained a slot was cut down the length of tubular beam to significantly reduce the torsional stiffness. As a result, the beam requires 2 kg (determined using FEA) of vertical force to move through the entire range of motion in the roll mode. Consequently the amount of antiroll stiffness required in the rear will be reduced.
For the first iteration the roll centres were designed to be non adjustable to allow ease of manufacture and simplify the design.
Brakes
WS09 uses outboard brakes front and rear mounted on the inner side of the uprights. The brake discs are floating, both axially and radially, on hollow steel bobbins. The decision to move to dual outboard rear callipers, rather than the previous singular inboard system, was aimed at producing improved braking feel and achievable brake bias in the rear. In addition, the use of a single inboard brake disc would have required modification to the Drexler differential housing.
Stainless steel hard brake line is used as much as possible to reduce the amount of compliance in the braking system. To allow easy engine removal, the rear brake line uses dry break connections.
Engine
The Suzuki GSX-R 600 engine is installed in almost stock condition with no internal modifications other than the relocation of the oil bypass valve. The intake manifold was designed using past experience in conjunction with tuned pipe theory and CFD analysis. Previous designs had used side feed log manifolds resulting in vastly different mass flow across the four cylinders, this led to the stipulation of a design requirement to achieve equal length flow paths. Furthermore, the intake runner lengths were chosen for peak power at 8600 RPM.
The engine is tuned using throttle position and rpm in conjunction with compensation tables for atmospheric conditions.
The exhaust was also designed using tuned pipe theory and matched to the intake manifold. It is a four into one design with a lambda sensor downstream of the collector. The silencer is mounted in the left side pod to assist in achieving a low centre of gravity height and low yaw moment of inertia.
The cooling system was designed to aid low centre of gravity and low yaw moment of inertia, whilst also being positioned to take advantage of the induced air flow. To aid in cooling during slow driving the radiator is ducted with a MoTeC controlled thermofan. To improve the integration of the cooling system tubing, the mechanical water pump was removed in favour of parallel electric water pumps to achieve the desired flow rate. To reduce the likelihood of air pockets forming in the radiator, as a result of the radiator being mounted below the fill point, a bleed line runs back to the header tank.
Several engine rebuilds and subsequent inspection of the crankshaft bearings indicated that the engines were suffering from poor lubrication, this was confirmed by lateral acceleration tests conducted whilst monitoring oil pressure. To reconcile this shortfall an Accusump oil accumulator was chosen in favour of a dry sump system. This solution required less modification to the engine and contained fewer moving parts, thus increasing reliability. The team recognises that the level of prevention provided by an Accusump is finite by nature, however through development of procedures during the test and evaluation phase this will be mitigated.
The gear shift is actuated by a cylindrical solenoid directly driving the sequential shift shaft. The electric shifter allows gear shifts to occur, with ignition cut, in 0.1 seconds. The electric shifter was chosen over an electro-pneumatic system as it provides a weight saving and significant integration improvement. More over, the removal of the pneumatically based previous design allowed for further reduction in the complexity of the system. The shifter has been modified to reduce weight by replacing the steel housing with aluminium and adding a spherical mounting.
Final Drive
The power is transferred to the differential through a chain drive. The gear ratios were chosen using lap time simulation, comparing shift latency and final drive ratio to determine the fastest lap time. The chain drive was chosen for ease of parts replacement, ease of ratio adjustment and general simplicity. The rear sprocket is mounted to a custom designed sprocket flange that integrates with the purchased Drexler clutch plate limited slip differential. The final drive was designed to allow the comparison of three differential options including; Drexler LSD, Torsen and a solid spool.
The drive shafts are hollow quench and tempered 4340 with shot peened splines. The inboard Tripod joints sit in custom designed housings machined from heat treated 709M steel. The tripod housings have been truncated to reduce the outer diameter, whilst this reduces the angularity possible it allows the outboard tripod housing to sit within the upright as an integrated tripod housing/hub. The hub is machined from 7075-T651 with wire cut steel liners to take contact stresses.
Electronics
WS09 engine management is achieved by a MoTeC M400 engine control unit. Traction control and launch control are the only activated driver aids. Ignition cut is activated for gear shifts to occur without having to lift the throttle pedal.
A MoTeC Sports Dash Logger is used to log suspension position, wheel speed, lateral, longitudinal and vertical accelerations, yaw rate, brake pressure, steering wheel position.
Ergonomics/Driver Controls
WS09 has a conventional cockpit with a seatback angle of 35 degrees from the vertical. The steering wheel is inclined at 15 degrees from the vertical; which has been chosen from an iterative design approach and is deemed by the majority of drivers to be the most comfortable. Gear shifting is achieved by activation of paddles mounted to the steering wheel so they rotate with the wheel.
The pedal box features three pedals; throttle, brake and clutch from right to left. The pedal box is adjustable over 150mm to cater for the 5th percentile female up to the 95th percentile male. The clutch is cable actuated as it provided a weight saving over hydraulic actuation. The pedals are a hollow monocoque design to provide a stiff yet light weight structure. The brake pedal was designed for an emergency braking force of 3000N with a safety factor of 2.5. Brake bias adjustment is provided through a pedal box mounted balance bar utilising spherical mounted master cylinders for improved efficiency and reduced play within the assembly.
A MoTeC shift light is mounted above the steering wheel to inform drivers of the correct shift rpm and to notify the driver of any predetermined warning conditions (over temperature etc).
