The Rotary Racer 7 is the current car produced by Chipping Sodbury School's Rotary Racer Greenpower team. The team has been entering Greenpower races form many years, although the team members have changed as older pupils leave the school and new pupils and parents join the team.
The Rotary Racer 7 car's design was started in October 2006. There where a lot of new members in the team and it was thought best to create a new car from scratch rather than develop the existing car. The main reason for this was to allow the pupils and parents to experience all of the processes in order to design, build and race a Greenpower car.
The existing Rotary Racer car had been very successful, winning a number of races and coming 8th in the national championship. So we had quite a challenge if we wanted to produce a better car! We had a lot of experience of Greenpower racing from the older team members and other teams within the school all of which which helped in shaping the teams ideas. We had a number of design meetings, quite boring for the pupils !, to try and get our ideas in order. Some of our original ideas were:
We managed to get some time in the wind tunnel at the University of the West of England. So a number of model car designs were made by pupils and parents out of foam to test in the wind tunnel. We tested many things including how much drag our external motor had as well as the steering tubes etc. After looking at the results of this, and considering the ease of producing the design we choose one to build.
The cars chassis was built in a similar manner to the previous cars. It was made using a space frame made from 25mm x 1.6mm aluminum tube connected using plastic bungs with aluminum triangular plates used to strengthen some corners. This is the same basic method as used in the standard Goblin Greenpower cars. . It is relatively easy for the pupils and parents to do. The cars base was made using 6mm plywood with an additional layer of 6mm plywood at strategic points. The car was covered with 1.6mm ply and Boatcoat. Boatcoat is a plastic film designed to cover yachts for transport. It is stuck on with double sided tape and gaffer tape and then shrunk using hot air paint stripper guns. This construction technique produces quite a light car which is good for low rolling resistance. The cars weight is about 48 Kg. With batteries and driver the all up weight is about: 125 Kg, depending on how many bacon baps the driver has had !
The nose code was made of layered foam and sculpted by the boys and dads to create a suitable aerodynamic shape.
The steering assembly was taken from the old car. It consists of two cycle head sets welded to a cross tube.
The wheels were taken from the old car. They are 20inch cycle wheels made up for us at the local cycle shop, Terrys Cycles. The wheels are filled in with thin ply and use good quality Hope hubs. We use thin, high pressure tyres installed to give low rolling resistance and low aerodynamic drag.
The rear axle was going to be a simple aluminum bar as per the existing car. Unfortunately, later in the cars development, we noticed the car was very nose light now that the batteries were further back. We had not considered the center of gravity very well ! So we made up a rear sub-frame from MIG welded steel square tubing to allow us to use stub axles further back and either side of the batteries. This allowed us to keep our idea of loading the batteries on from the rear.
The battery installation system is quite unique! It is a bit like a roll-on roll-off ferry. The back of the car hinges upwards and allows the set of two batteries to be easily slid out and in. Kitchen door hinges were modified for this purpose. A single plug and socket is used to connect the batteries for ease and speed. The battery packs were made of two batteries connected using a piece of ply and packing tape. This rear battery loading system certainly helped our pit stops. The lads had them down to 31 seconds at Castle Combe !
The motor is simply mounted in the airflow on top of the car behind the driver for effective cooling and employs a simple fixed gear chain drive to a single wheel.
The old car had no electronics, just a simple on/off switch. Rotary Racer 7 however, has a Car Computer and Electronic Speed control. The Car Computer was developed by a parent with the pupils helping. A prototype design was built using the standard PicAxe computer modules that the school used for teaching purposes. The idea of this was to make the system so that the pupils could use the module in the future. After trialling the system in some races, it was decided to actually design a board with all of the components we needed to simplify the overall system and make it more reliable. A 2 layer PCB was designed, the lads did some of the PCB tracking, and sent to a PCB company for manufacture. A number of boards were made for the teams at Chipping Sodbury School. The Lads and Lasses of the teams made up the boards. A separate Motor Speed controller was designed and built to be driven from the Car Computer. This employs high current MOSFET transistors as the PWM (Pulse Width Modulation) output. The design for this was challenging as the motor could draw up to 130 Amps. The PCB for this was made in the school using the schools PCB fabrication facilities.
The software and electronics are being continually updated but the current system performs the following functions on the car:
We are using the CarComputer/MotorSpeed controller combination instead of having a gear change mechanism to keep the car running at optimum speed for a race. We over gear the car a bit, by choosing a suitable fixed gear ratio and then let the Car Computer manage the motor speed to keep the current averaging at the 25 Amp level. The idea is to gear the car so that the motor speed controller runs the motor at about 90% of full voltage normally. This provides a +-10% power range. In effect this provides a similar ability to having a continuously variable gearbox over a 20% gearing range. This is quite suitable for the nature of a Greenpower car.