Electric car components have been available for a while now. A friend converted his first electric car 10 years ago then changed it to lithium batteries 6 years ago. The technology is now becoming more readily available, costs are coming down and performance is increasing and this trend is set to continue.
Why build your own? Because you can and it is great fun to build a very light, simple car reasonably inexpensively. It is a heap of fun to drive and has excellent performance because of the light weight. Buggies, gokarts, grass roots racers and kit cars such as Lotus 7 clubman style car that is still going strong since the 60's have spurned their own industries. Electric cars bring new opportunities for a fresh look at homegrown performance.
Concept and Design
This Instructable provides a summary of a basic layout for an EV platform that suits a wide range of applications and can be easily tuned with different size motors, batteries, gearing and size. It demonstrates a simple and compact system with a low centre of gravity that is strong, stiff and straightforward to build. The Instructable does not go into the design and fabrication of bodywork, I will leave this to others and your imagination. It is pretty easy to see that this rolling chassis is very flexible in the bodywork it could accommodate, but keeping the body light will maximise performance and range.
Key Design Parameters
When designing a new car platform from scratch, there are a lot of choices. A lot of thought and design effort has gone into keeping the design as simple, light weight and very easy to build - simpler than a Locost or clubman style car.
I will get straight to the point here and outline some of the key design features and why.
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.
Driving the Car - With one gear and heaps of torque (300Nm from the twin motors) Driving is simple and effortless and the car rapidly gains speed and without a body the sensation of speed is greatly exaggerated.