Note: This is NOT a set of step-by-step instructions for a specific design. We have built quite a few cycle powered generators, each one is different, based on what bits we had available. This guide is intended to give you pointers and pass on some of what we have learnt, but ultimately it is your ingenuity and the materials and tools available to you that will dictate the design.

The basic requirements;

Old bikes vs. exercise bikes.
Unwanted bikes can be very cheap (ie. free) and easy to obtain. They have good bearings so little power is wasted overcoming friction and they often have large back wheels that make good pulleys (you will see why that could be important latter). However, you will need to make some form of stand to make the bike stay upright and stationary while pedaled. Further more, it makes sense environmentally and ethically that, if the bike works or can be made to work, it is better being used as a bike (thus replacing more environmentally destructive forms of transport). There are always people who want working bikes, there are charities that send thousands of serviceable bikes to the third world every year.

Exercise bikes are an interesting option. Loads of people have bought exercise bikes thinking they would actually use them but the novelty soon wears off and they become a waste of space. The free ads papers, boot sales and recycling centres are full of them and many end up in the tips. This means that they can be cheap to obtain. We have never paid more than five pounds for one and it is of course possible to get one for nothing. You won’t have to build a stand because they already have one and they are easily adjustable to fit different people. Additionally, the fly wheel will prove useful. But beware - not all exercise bikes are good for making cycle generators. Many have no bearings and many really cheap ones have no fly wheel but rely on friction on the crank to provide the ‘exercise’. There is very little scope for using such a device.

Alternators vs. Permanent magnet motors
The world is full of an increasing number of cars - built quick and cheap, used and abused and then sent to the breakers yard. There are therefore loads of old alternators waiting to provide a regulated 13.4 volts DC to whoever can rotate them fast enough. You acquire them very cheaply (or free) and they even come with brackets and a pulley already fitted. Alternators provide an obvious source of power and so many people have used them in home made cycle generators or wind power systems. However alternators require very high rotation speeds which mean you need to provide very high gearing ratios. Further more, they need ‘exciting’ and are designed to provide up to 500 watts while a person is likely to manage something more like 60 watts. This means that you need to do provide some simple electronic wizardry to start the alternator generating any power at all and also to prevent it from demanding excessive amounts of power from the cyclist.

Permanent magnet motors are another option. They come in an amazing variety of shapes and sizes. We have used a Whirlpool washing machine motor with great success. We have also used motors out of old computer magnetic tape spools. You could use electric fan motors from cars. They can produce a couple of amps at 12 volts so using two of those with their shafts joined could be worth a try. Because of the variety of motors available their specifications will obviously also vary significantly (ie. the rotational speeds required, the output in terms of watts, the voltage range produced etc.). One thing you can be sure of is that the output of a generator built with one of these will need to be regulated in some way for most applications. The regulation could be in the form of a voltage regulating circuit or simply a large capacity battery placed in line to be charged (which should cause the output voltage to drop to little over the battery voltage).

  Drive and gearing:

Shafts, cogs, wheels, chains, and belts.

The pedals of your machine must somehow turn your chosen generator at a suitable speed. Your average cyclist will find it comfortable to pedal at around 60rpm (ie. one rotation of the pedals every second) and it would not be unreasonable to base your calculations on that figure. If you happen to have found a generator that produces your required output (eg .5amps worth of 12volts) at just 50 RPM then you could find a way to link the two without gearing. However the chances are you will need to step up the speed significantly.

Gearing could consist of a gearbox full of cogs. These are not easy to build but there are small gearboxes available to those with the initiative to find them. For example we found a hand cranked grinding disk with 20:1 gearing. You could use a simple rubber wheel on the shaft of your generator which is pressed against the rim of your bike's wheel/flywheel. The difference in circumference of the two wheels provides the ratio of the gearing you obtain. We used this method fairly successfully using a homemade wheel made out of wood and plastic hose but found that under heavy load slipping could acquire and we were concerned that vibrations would reduce the life of the bearings and brushes in the motor.

With enough cogs and chains from old bikes you could multiply your gearing up to the desired level but it would be noisy and inefficient. Unless the gearing you require is between 3:1 and 9:! you will probably have to use another method.

If you are using an alternator than it will need at least 1,800 RPM before it produces output. This would require a gearing ratio of about 30:1 When you consider that the highest ratio on your average bike provides about 3:1 (eg. 52 teeth to 14 teeth) then you see the potential problem with using alternators. The usual way to solve the problem is to use the back wheel of the bike as a massive pulley (assuming it is a racing bike with a 26 or 27 inch wheel) You then use a large fan belt to drive the (usually 3 inch) pulley on the alternator which means you provided an additional gearing ratio of about 9:1 and a total ratio of about 27:1. Belts big enough are not easy to find. They can be purchased fairly easily and if you are on a tight budget you can improvise with old stockings etc. If you wish to build a generator on these principles then there is a booklet available from the Campaign For Real Events which describes in detail how to do it.

We have successfully used a belt from the fly wheel of an exercise bike to a homemade pulley fitted to the motor. The flywheels tend to be about 14 inch diameter which isn't very big but the motors we have used require far lower speeds than alternators so the gearing ratio has proved more than adequate. We found suitable belts in old washing machines.

Where to get stuff: