The Botmotion EV platform suits a wide range of applications, motors and batteries. It demonstrates a light weight, simple and compact system with a low centre of gravity that is strong, stiff and straightforward to build.
The basic design philosophy for the platform is a simple, light weight and very easy to build car that will demonstrate performance potential of electric cars. Key design features include.
Drive – Rear wheel drive, one electric motor powers each rear wheel. Eliminates the need for a differential and CV joints.
Motors – AC Induction. Have good torque over wide speed range. Simple and robust with a motor controller for each motor. Mounted inboard.
Batteries – Large lithium cells. I used 45 Lithium cells for a total of 148V and 100Ah. This needs to be matched to the motors and controllers. This is a relatively small pack compared to production EV’s vehicles but is ample for a car that is light and is not used for long range driving. Keeping the battery pack size down helps keep down the vehicle weight and cost. My large lithium cells are good for a peak current draw 3 to 5 times the rated hourly figure above (3C to 5C). Lithium polymer cells are available that have a higher energy density and will do much higher peak currents than this and they are commonly used in model cars and planes, but at present the large lithium cells are a lot more economical for larger packs and the 3C peak current is not a major limitation unless you need a high peak demand such as for drag racing.
Chassis – Folded aluminium box. The batteries are contained in the box which also handles all the vehicle loads. This is the key to a simple, light and very easy to construct car. It provides a high level of strength and stiffness from a very simple and light structure.
Suspension, Steering and Brakes – Double wishbones were used and they are the best choice for a number of reasons including lower height for maximum flexibility in body design, height adjustable again for flexibility in body styles and optimum handling performance. There are numerous vehicles that can be used to source suspension and steering components. I used parts from a Mazda MX5 (Miata) which has front and rear wishbone suspension and rear wheel drive so all the parts could be obtained one source. It also has 4 wheel disc brakes and a straightforward steering rack. Using mass produced parts helps streamline the project, keeps costs down and ensures that these important items are robust and reliable.
Gearbox – Nil. The electric motors have such a wide torque range that they will operate effectively with one fixed gear. I use a toothed drive belt at a ratio between 1:3 and 1:5 for smooth quiet and maintenance free transmission. A chain drive would also be ideal and would be lighter and cheaper but a little noisier.
Weight – The weight of the EV platform including motors and batteries is approx 500kg. Major components of the weight come from the batteries (150kg), wheels and suspension (140kg) and motors (118kg).
Vehicle Platform – A vehicle platform is basically rolling chassis with drivetrain installed. It is drivable and just needs some bodywork to complete the package. I avoid any body styling discussion in this Instructable and rather present a very flexible platform that will suit a range of body styles