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Our primary goals in suspension - steering - brakes system are to:
Make the system as easy to tune or adjust as possible to accommodate changes in racing environments and situations.
Maximizing suspension response to keep contact between tires and track surfaces.
Reduce unsprung mass as much as possible while maintaining good structural integrity.
Wheelbase & Track width
Wheelbase & Track width
The wheelbase of cars competing in a dynamic event that rated in our interest score range is 1535 to 1575 mm, the front track width of every team is the same at 1200 mm, and the rear track is in the range of 1150 to 1200 mm.
The shorter wheelbase provides more agility, while the longer wheelbase provides more ride quality (Ref). So, a shorter wheelbase would benefit us in competition because our race car wants agility more than ride quality.
A shorter track width at the front and wider track width at the rear induce more understeer. On the other hand, a wider front and a shorter rear track increase the oversteer tendency of the car (Ref). To balance these, we should set the front and rear track widths at the same number.
Therefore, the wheelbase and track width values we will set for the design specs must be chosen in that range. We choose the wheelbase value of 1560 mm and put the front and rear track width at 1200 mm. The specified value may be changed as appropriate during the design process.
Suspension
Suspension
To complete a circle of the skidpad event in 4.9 s, The suspension component must provide enough lateral acceleration to complete the event in time. In practical terms, our car might not perform a lateral acceleration of 1.53G as we calculated because of the limitation of tires, change in track condition, and other situations. Therefore, the maximum lateral acceleration values we will set for the design specs will be assumed at 1.60G. We will use this parameter in the later design process (such as the strength of the mounting points).
Suspension control arm (A-Arm) material
Suspension control arm (A-Arm) material
As observed in the TSAE Auto Challenge, many of the teams in the challenge chose structural steel to produce their suspension control arms. This is because this material is very inexpensive compared to other materials and can be used for various purposes. That's why we also use this material in the suspension control arm.
Suspension Type
Suspension Type
Teams in FSAE competition mostly use Double wishbone suspension configuration due to its good handling, more suspension geometry options, and it's also easier to tune or adjust the suspension setup. Therefore, for these reasons and the time window in the design process, we choose the Double wishbone suspension configuration too.
Spring & Damper layout
Spring & Damper layout
In the FSAE competition, most competitors use the rocker arm and aluminum rod to transfer vertical loads from each wheel to its spring and damper instead of directly transferring them. This allows engineers more freedom in designing wheel travel without worrying about spring and damper travel limitations.
There are two types of spring & damper layouts found in FSAE competitors, push-rod, which is easier to tune the spring & damper, and pull-rod, which provides a lower center of gravity due to its spring & damper location. We decide to choose the push-rod layout. The first reason is that the push-rod layout is way simpler to design and analyze its mechanism so that we can do it in a limited time. The second reason is that the spring & damper location in the push-rod layout sits higher in the chassis, making the system easier to assemble, and the spring & damper can also be tuned or adjusted easily.
Brake
Brake
From Benchmarking
The Type of brake that most of the team use is The Disc brake type, There are 4 different kinds of brake rotor profiles i.e., solid, drilled, slotted, drilled, and slotted. From the literature review in the title of design and analysis of brake rotor for formula student vehicle, The results showed that the slotted drilled disc produces greater heat flux than the others and can withstand the force applied to it because it is within the allowed limits and has a smaller mass than the others, So we choose Drilled and Slotted Disc design to improve the performance of our car.
Brake Calipers of formula cars must be designed as lightweight as possible and yet very stiff. When the vehicle brakes down, the load of the wheels will transfer to the front wheels of the car. If want all 4 wheels to brake at the same time must create more braking force on the front wheels so that we can increase the caliper piston area which is why most cars have more pot calipers at the front or can operate by using the same size of the caliper and use a balance bar to convert force to master cylinder. So first step we choose 1 pot caliper in the front and 1 pot caliper in the rear to improve brake efficiency and reduce the weight of the braking system and design braking system calculation.
From Regulations
(T.3.1.2) The braking system must be capable of locking all four wheels(Recommended locking at the same time) ----> We must have Balanced Bar to adjust Brake proportion for the change in weight transfer.
(T.3.1.3) Hydraulic braking System that consists of two independent circuits.
(T.3.1.12) The brake pedal shall be designed to withstand a force of 2000 N.
From Literature Reviews
The Maximum rate of deceleration performance is based on the overall weight of the vehicle and the maximum tire adhesion.
A Slotted drilled disc produces greater heat flux and can withstand the force applied to it because it is within the allowed limits and has a smaller mass than the others.
The Dimension of the disc brake is based on the size of the wheel to install All the components of the Disc brake.
Powertrain
Powertrain
Torque-Weight ratio
Torque-Weight ratio
In order to complete acceleration event in 4.23 s, The car need enough torque to accelerate and gain speed to complete the race in time. The minimum torque is calculated by using equation of motion and equation of force to know acceleration and force
but in practical term there's some limitation such as air resistance, regulation( 80kW limitation which make torque no longer constant after it reach the limit but it became constant power instead, so we must devide it into 2 stage and use different equation.
กราฟ Traction limit
กราฟ Traction limit
Acceleration
Distance
When :
traction limit back 100%
Friction coefficient = 1.5
Mass total = 310kg
h = 1.1m
Wheel base = 1.55 m
Roling resistance = 45.6156 N
C.G height = 25%
A=0.75 m^2
Cd=0.6 Pair=1.204
Motor Layout
Motor Layout
There are three types of motor 1.single motors 2.double motor 3. in wheel motor
Single motor is one motor locate at back of the driver
Pros The best Packaging
Con Heavywiring due to high current load , Harder to distributed torque
Dual motor are twin motor locate at the back separate left and right
Pros True torque vectoring to the rear wheel can distribute torque when conering ,Lighter motor wiring weight
Con More space to mount to the frame
In wheel motor are located motor in each wheel hub
Pros The best acceleration , 4wheel true torque vectering(The best conering )
Con More unsprungmass , Need more motor wire protection
We did not choose the single motor choice because performance and cornering are lower than dual motor . In wheel motor is the best perfomance and cornering but have two main problem 1. More unsprung cause to poor response 2. It limit the choice of the suspension design . From the reason above ,so we will choose the dual motor disign
Regenerative Braking
Regenerative Braking
Formula E Student use 100 kW Motor Power, but use only 31.8 kW Regenerative Power which can regen back only 14.2% (890 Wh) of Battery Capacity (6.3 kWh) from 22 km Endurance event in simulation. (Ref)
Only one Research of Student Formula is found
(More Regenerative Power ==> More Recovery Energy ==> Less Battery Capacity ==> Less Weight)
Round-trip efficiency is the percentage of electricity put into storage that is later retrieved
Round Trip Efficiency = (Energy Recovered)/(Energy Input)x100
Battery Li-ion has Round Trip Efficiency in the range of 80-95%
Battery Ni-MH has Round Trip Efficiency around 70%
Battery Pb-Acid has Round Trip Efficiency in the range of 70-80%
Capacitor has Round Trip Efficiency in the range of 60-70%
Supercapacitor has Round Trip Efficiency in the range of 95-98% (Ref)
(More Round Trip Efficiency ==> More Recovery Energy ==> Less Battery Capacity ==> Less Weight)
Compare to common EV
Compare to common EV
The power of regenerative braking is low compared to the motor power and the battery capacity, which is average of 22% motor power and mostly around 0.8C. Because EVs have the controller to limit the power of regenerative braking to save the battery life cycle. It differs from the formula student race, which does not concern about the battery life cycle. The formula students’ regulations do not limit the maximum regenerative power, but due to small battery capacity, achieving high regenerative power will result in high C rate and decrease the battery lifespan.
Battery Capacity
Battery Capacity
Indicator: Net Energy*
Overall Range = 22 km** (approximate) [D.12.2.1]
Our Team uses Net Energy from FSE22 Efficiency Scoring Results to define Battery size.
From the results, The maximum energy used is around 6 kWh so we will use 7.67 kWh from the maximum first because we don't know an Actual Motor Specification and Regenerative Performance.
Then when we got Actual Motor Specification, we can simulate energy consumption with Regenerative Braking to get more accurate energy consumption. So maybe we can reduce Battery Capacity to make vehicle weight lower.
In short, we will use 7.67 KWh and reduce it when we know the energy consumption
*Net energy is the energy used at the end of the event compared to before the event start.
**We have to approximate the range because we don't have actual racing track information.
Structure
Structure
In the design specification aspect, there are three specifications for the structural system.
Weight
Chassis Stiffness
Aerodynamic Effects
Weight
Weight
The targeted total weight of a space frame race car is 235 kg, which is determined from the average of the spacce frame cars competing in the skid pad, acceleration, and autocross in similarly expected ranks. The reason that we didn't use the lightest weight because some systems may be designed heavier than expected or some parts may have been changed to improve performance which may cause the total weight being greater than the lightest weight from the benchmarked weight.
We calculated the space frame weight from the total weight – the weight of the suspension, powertrain, aero part, driver seat, pedal box, steering assembly and impact attenuater.
The powertrain system’s weight is 33% of the total weight excluding cooling part.
The suspension system's weight is 19% of the total weight including rims, tires, braking system,and spring-damper unit.
The aero and body part's weight weight is 15% of the total weight
The aero and body parts is calculated from the part created by Chula formula student club.
Note These ratios got from the car that has the lightest calculated frame weight.
Many parts in each system cannot be found. Then, the calculated weights aren’t the true space frame weight. The lightest calculated weight is 55 kg, which is an excessive mass. this being the case, a target of a 20% weight reduction, while not only compromising strength but actually enhancing it, was set. So, we aim was stronger frame with much less weight.
THE WEIGHT TARGET: 45 kg. (MAX)
TEAM Ref.
Hamburg(Ref.)
Chemnitz TU (Ref.)
Berlin TU (Ref.)
Manheim UAS (Ref.)
Zurich UAS (Ref.)
Trento U (Ref.)
Chassis stiffness
Chassis stiffness
To perform vehicle dynamics as excellent as designed, the allowance motions of the vehicle should perform on the suspension system only. Furthermore, the structure should be as rigid as possible.
the general stiffness has 3 types.
bending stiffness
torsional stiffness referenced from MEEG 402-010 Chassis Design Report a good torsional stiffness also has adequate bending stiffness.
extensional stiffness (do not happen in formula student racing case)
We will only consider torsional stiffness. So, referenced from Chassis Torsional Rigidity Analysis for a Formula SAE Racecar show that the lightest chassis possible that has a torsional stiffness greater than 1200 Nm/deg.
Aerodynamic Effects
Aerodynamic Effects
In order to get a good performance, the vehicle has too aerodynamically efficient. The capability of the aerodynamic engineer is to compromise between downforce and drag.
From the research that formula student without aerodynamic Device attachment, the drag coefficient is around 0.5 - 0.7. therefore, we aim drag coefficient(nosecone) equal to 0.4 to keep our drag coefficient in that range.
Seat and belt joints
Seat and belt joints
In accident which has an impact that immediately stop the vehicle ,the seat and belt joints have to be strong enough in order to withstand 40g impact force without any failure of the joint to stay attached with the primary structure.
From the regulation , the seat joints have to withstand 40g force and each belt joints have to withstand 15,000 N force and have the spape that able to attach seat belt.
so we use excel to calculate the minimum force after considerate limitation. The minimum force is 2440 N, assume that the car use 0.3 radius tire, The minimum torque is 732 Nm and minimum Torque-weight ratio is 2.66 Nm/kg (Assume : distance = 75m, Weight = 275kg, Cd=0.6, Area=0.75m^2, Rwheel=0.3m)
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