DIY bike generator

Fed up of continually spending money on buying batteries to power the lights on my bike, I have taken up building a generator that will power the lights (and anything else if it can extract enough leg power).

Phase I

The first incarnation of this project was a 30-minute bodge, I strapped two hard disk magnets to the spokes of the rear wheel, added some glue and strapped a solenoid to the bike frame to extract the mysterious magnetic energy as the magnets passed it. The wires from the solenoid were then fed to a bridge rectifier and 2200uF capacitor respectively, and to provide smooth DC. This was not a complete failure, at a moderate speed I was able to extract an astonishing 6mW of power (barely enough to light an LED). Unsatisfied with this result, I decided that some engineering was required.

[Pictures to follow when I find the data cable for my phone]

Phase II

The initial though here was that more magnets = more power, so I cannibalized several old hard disks and extracted their magnets, this included 1 quantum bigfoot drive, for which I have been reliably informed that hordes of angry villagers are going to arrive on my doorstep carrying torches and gallows, and shortly thereafter I shall be burning in the 9th level of hell. However, aside from this footnote, I noticed the nice big platters (2 of them) of the bigfoot drive, and this inspired me - I could use these platters to form a hub, with the many magnets sandwiched between the two platters in a circular pattern.

The magnets were removed from their metal retainers. This was no easy task as they seemed to be fixed with some sort of crazy glue that defied the laws of physics. some I could only remove by using a vice to place enough force on one corner of the magnet to twist and sheer the magnet off, others I had to jam in a vice and use a rather large pair of liars to literally peel the 3mm thick steel backing away from the magnet. One magnet did not survive and broke up when I tried to lever it off with a screwdriver. I also cut my finger and lost a few drops of blood.

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The bigfoot drive

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The central spacer was glued to the platter. Note that I had previously keyed the glue side for better stickyness (in other words I deliberately scratched the fuck out of it).

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The magnets sitting on the platter. These were arranged so that the poles alternated.

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The magnets being glued in place.

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All magnets glued in place. Neatness isn't a necessity here, as long as the magnetic focal points are roughly equally spaced.

Fixing the hub to the wheel

Time for some engineering. The hole in the center of the platters is just big enough to fit over the hub of the rear wheel, but a little too big. I will also want it a few mm out from the base of the hub because I want to build a custom electromagnet that will fit around both sides (and form a nice closed magnetic field). Spacers will be required here, but not tonight because it's midnight and I can't fire up any heavy machinery without waking the neighbors.

coil design

The design of the coil is fairly simple, a ferrous former will be anchored to the bike frame to focus as much of the magnetic juice through it as possible. A coil in the center will convert the magnetic force into usable electrical energy. The main design considerations for this are that the former will need to leave enough space to allow the rear wheel to spin without catching on the former. Also the former must be rigid enough to resist warping from the magnets (they are seriously strong), otherwise the hub will make horrible scratchy sounds and slow down my cycling.

Circuit design

The output from the coil will be AC, so this needs to be rectified and smoothed before it can be used. The output voltage will also vary considerably at different speeds, and this too needs to be regulated to ensure that lights are not blown.