Building an electric racing car

A small team of school kids and myself have built and raced an electric racing car. This is how we did it.

Step 1: The Chassis

We built this car to enter a series of national races here in the Uk - Details can be found at http://www.greenpower.co.uk. To some degree this controlled how we built our car and what we used as the race series has rules that must be followed.The chassis was a simple "Ladder" design made from 19mm aluminium square section tubing. This is easy to work and light as well as moderately low cost.The various sections were joined with aluminium plates and pop rivets. We followed a Greenpower design as the basis for our car but made numerous changes along the way.

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Step 2: The steering

A simple lever steering system was chosen as i did not want the kids to have a "spear" pointed at their chest when they drove the car. As it turned out this was simple to get used to especially by drivers who have not yet driven cars and so have not preconceived ideas of how it should be. The front wheels have a simple bent 2 mm mild steel king pin arrangement with a 19 mm bolt though for the king pin. With plenty of grease this works well. the steering rods have trackrod ends easily available from bearing suppliers to allow a certain amount of give in the system so no complicated alignment was required just set the wheels straight ahead. We decided not to apply any toe in or wheel camber. the wheels are a 12.5 inch by 2.25 inch wheel chair wheel. Again, aluminium for lightness and ease of obtaining new tires.

Step 3: The brakes

Although during a race the brakes are rarely used - the motor provides a lot of drag if you turn it off, they are necessary sometimes. We borrowed a bike disk brake set but found it very difficult to set up and easy to get out of alignment so will look to revert to a more simple system using a normal bike caliper brake over the winter closed season. The brake levers are on the steering arms and operate the rear wheel brakes independently - this isn't a problem even at speed.

Step 4: The motor and drive system

The rules stipulate a 24 volt motor - the supplied motor is a wheel chair motor style without the attached slow speed gearbox. It operates at 20 amps at 2000 RPM. With the gearing we have used it gives us a top speed at present of about 25 to 30 Mph - Not super fast but when your bum is only 2 inches from the ground!! At a later date we will raise the gearing ratio slightly to up the top speed to 30 to 40 MPH provided we can do thig without over stressing the motor. Cycle gears are also an option but may insert a complex system into something that needs reliability.

Step 5: The motor control/electrical system

This is VERY simple - just on and off. A 70 amp 24 volt automotive relay is switched by the red button on the steering lever and this in turn switches current through to the motor. Although this system is very simple it is enough and reliable. Pulsing the switch on and off provided a crude speed control although for most of the race the motor is on if going in a straight line and off is the wheels are turned. In line with the motor is a 70 amp thermal cut out to protect the motor from over current - I strongly recommend you fit this - the motor costs £100 new so protection seems advisable. The motor runs at about 80 deg C in normal use but if stressed, e.g. pulling away from a slow spped too often, driving the motor when the wheels are turned, gearing up the drive too highly in the hope of attaining supersonic top speeds the motor temperature can ramp up alarmingly to the point of breakdown of the epoxy glue holding the windings in at about 200 deg C. Some teams use water cooling as the motor is water proof, some like us use fins to provide air cooling.

Step 6: The body

We wanted a racing car lookalike, a light body, and a gesture towards aerodynamics. Our body is made from paper, laminated with wood glue this dries to almost wood stiffness with little weight. getting a good finish however has proved difficult and we will look at a thin fiber glass alternative. The general shape works well but the top cover should be closer to the driver with a small aero screen and the rear should be rounded to provide a smooth exit path for the air flowing over the body.

Step 7: Batteries

These are standard lead acid car batteries. 2 per car one each side just in front of the rear wheels. They sit on the chassis and are held down with a simple bar and threaded rod arrangement to allow ease of changing. We are allowed 4 batteries in total and can get about 60 to 100 miles out of them at about 20 Mph. In our initial race we did 67.5 miles at an average of 18 Mph but we changed drivers 6 times during the race. Our peak speed was 26.5 Mph. Safety is paramount in any racing sport so full 4 point seat harness was fitted and a full face crash helmet worn at all times when racing. That's the nutshell overview of our project there are many alternatives to our design some exotic some very simple. You pay your money and take your choice and have fun.