Design Tips: Parts Finder
We find that we have many questions from entrants asking about where various components can be found. We hope the following notes will be of assistance.
Chassis:
Originally a lot of Formula 24 cars were built up from old go-kart chassis and bike frames. Although this enabled cars to be put together quickly there were some major disadvantages.
Firstly, the go-kart frames were found to be very heavy and, as they were made to incorporate go-kart running gear and wheels, a lot of difficult modifications were needed to reduce the weight to anything near an acceptable level. Wheels had to be changed from the wide slick tyres, to narrow tyres for improved rolling resistance.
Using bike parts can be a good way and many cars have large parts of their design based upon bicycle frames and drives. Bike drives offer the option of gearing which many teams have used, some successfully, some not. Experience has shown that the use of a steering system based upon bicycle front forks is impractical.
In general try to design your chassis around some light materials that you can get easily. Start with aluminium and try anything from greenhouse frames, to ladders, to standard tube. Wood can be very useful for making a relatively light and strong chassis.
Bodywork:
Many materials have been used and can be used, from high-tech carbon fibre to discarded corrugated plastic. Aerodynamics are clearly a key part in designing a quick car - look at some of the designs around now for tips. Also visit the University of Southampton website (via the links page) which has an area devoted to Greenpower aerodynamics.
Drive System:
Several options present themselves. Some can be built up from second-hand parts and some can be purchased relatively cheaply.
The most commonly used systems are bicycle derived - lots of different sizes of sprocket and chain are available. The real challenge offered by this route is the gear ratio availability. If you use large drive wheels then a high gear ratio is needed and this can prove a problem. BMX type chain and sprockets are the best option.
If you want to use chain but also want a good range of sprocket options then it is much better to use industrial chain available from any engineering factor offering bearings, gearboxes and drive systems. We recommend nothing larger than 3/8th chain. A wide number of gearing ratios will be able to be used as sprockets are generally available in one tooth increments across a range from 10 to over 100. Remember how small the motor is and keep the drive system weight to a minimum. Go-kart sprockets and chains can be ideal.
Toothed drive belts are used on two of the Greenpower kit cars, and are a reasonably good drive system. They are quiet, easy to work with and there are a wide variety of tooth options available. 'V' belt drives have been used but the extra tension needed to ensure a good efficient drive can causes unnecessary friction.
Wheels:
Bicycle wheels are the most obvious choice and many sizes are freely available both new and used. Beware of old ones unless you have plenty of spares! Remember that cornering forces in a car are very different to those on a bicycle, where the wheel is supported on both sides. The larger the diameter of the wheel, the bigger these forces.
To summarise, if bike wheels are to be used, go for sizes between 12 and 20 inches. Also do not trust standard spindles - go for extra strong ones. Don't ever rely on old tyres as they won't last a race. Keep some spares too.
Other wheel options should reflect the need to keep rolling resistance down and road stability up. Wide tyres are out, especially go-kart wheels. The rolling resistance is too high and you do not need the extra rubber on the road for grip.
We stongly advise against the use of wheels with plastic spokes. Look at the choices made by other teams - you will notice that they all have narrow tyres for minimal rolling resistance.
Brakes:
Important and an area to look at carefully. The selection of drive and wheel type will determine your policy on braking. With bike wheels it may seem simple but ensure that the brakes you select are up to the standard needed. The new generation of bike disc brakes (particularly the mechanical, not hydraulic types) are an excellent option and should be considered.
For other wheel types look at braking the axle rather than the wheels. High quality drum brakes are also available in some bicycle hubs.