Calculation & Order Drivetrain

Naturally, the drivetrain is one of the, if not the most, important parts of a vehicle. As we were designing an Electric Vehicle, our drivetrain will consist of:

Batteries -> Motor Controller -> Electric Motor -> Gear Reduction System -> Wheels

The batteries are already provided to us (3* A123 System 12V Max 40A + 1 optional if we so desire), but there are still many unknown variables in this open ended problem about which parts to purchase. Thus, our team decided to “lock-in” some variables that are easier to solve and reverse-engineer the rest of the system.

Firstly, the wheels:

Preliminary designs of our EV necessitated a larger than usual wheel. Our vehicle has 1 front wheel, 1 rear wheel, and 2 smaller castor wheels. We decided on a 12.5 inch wheel diameter for both the front and  rear wheel, after doing some basic modelling work.

Secondly, the Gear Reduction:

As we probably do not have the luxury of buying/designing/machining a multi-gear system, our vehicle will only have one gear ratio, and we have to balance torque VS speed. Since this variable is easy to modify (by simply switching out gears and sprockets), we gave it a general ball park figure of 1:8.

Lastly, the Motor Controller/Motor Combo:

It made more sense to look at the controller and the motor as a system, as an expensive controller will be a waste on a cheap controller, and a cheap controller cannot power an expensive motor.

To decide on the power output of our motor, we did some rough calculations given the rest of the drive train variables we decided on above.

Big Table Of Variables
Mass of EV + Rider	100 kg
Gear Reduction	1:8
Wheel Diameter	12.5 inches
Force required to accelerate at 5m/s	500N
Force required to accelerate at 2m/s	200N
Drag	~21.93N
Rolling Friction	~10N
Power Required at Max Speed	31.93 * 10 = ~320W
With 1000W	500N Torque * 2m/s Speed

Although the power required to cruise at max speed is about 320W, our team decided to go for a 1000W Motor + Controller combination. This is because:

1.     Our budget has enough leeway to purchase a more powerful motor.

2.     A more powerful motor will allow for a more powerful acceleration, at 1000W our cart accelerates at 5m/s² when travelling at 2m/s, and 2m/s² when travelling at 5m/s.

3.     We want to win the race (the underlying motive)

With a proper gear ratio, we can ensure that the maximum speed of the cart does not exceed 9.7m/s easily, which is the regulation.

Prototype~

After settling on one design for our Electric Vehicle, we needed to make a rapid prototype to examine how the steering might work for our proposed design. With limited access to the workshop (as we hadn’t been safety-trained yet), we fashioned together a quick prototype using washers for wheels, screws and bolts for axels, and foam board for the main body.

 Wheels and axels in position.

Fully constructed prototype. Only the steering mechanism needed to be tested, so things like seats and motors were left unrepresented.

Testing of the steering mechanism. The real EV would not be constructed to turn in this manner, but cutting these grooves allowed us to simulate a steering system quickly and simply.

Overall, this prototype demonstrated a capability to make turns, and while of limited applicability to the full sized EV, gave us the confidence we required to proceed with the design.