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Dynohubs are nifty electrical generators built into special bicycle hubs. They were made by Sturmey-Archer in England for several decades, and as of 2010, a new version has been introduced by Sturmey-Archer in Taiwan, though only in combination with a drum brake. (Generator hubs are also currently made by Schmidt and Shimano. Strictly speaking, those are not "Dynohubs" because "Dynohub" remains a Sturmey-Archer trademark.)
[The classic Dynohub's power output is 1.8 watts at 6 volts, while that of other bicycle generators including the new Dynohubs is 2.4 or 3.0 watts. The headlamp sold with the classic Dynohub was carefully designed to make the best use of its low power. A classic Dynohub will produce a dim output with the usual 2.4 watt headlamp and 0.6 watt taillamp of other bicycle generator systems.
Advances in magnet technology now allow hub generators to be smaller and lighter, yet more powerful. Lights also are more efficient, with halogen bulb or light-emitting diodes, so that a modern generator system can put out several times as much light as a classic Dynohub system. -- John Allen, June 2010]
Generator hubs are fairly heavy -- the new ones less so, thanks to rare-earth magnets and aluminum shells -- but they are totally silent and have no moving parts nor mechanical friction. They work by revolving a ring-shaped multi-pole magnet attached inside an over-sized hub shell around a stationary armature (coil) attached to the axle.
Dynohubs are AC devices. Like other bicycle generators, they will fry bulbs if you go too fast -- unless used with modern lights that include solid-state voltage regulators. Dynohub systems put out useful amounts of light at lower speeds than tire-driven generators, because the alternating-current frequency is low. The bulb has time to heat up on the peak of each pulse of current. At low speeds, a Dynohub-powered light pulses visibly, but a light powered by a tire-driven generator just goes dim.
As the speed of any bicycle generator increases, so does the frequency of the AC. This is convenient, because the inductance of the generator's armature coil passes AC less efficiently as the frequency increases, and this substantially counters the tendency of the voltage to increase with speed. This inductive effect makes it possible for the generator to be used at a wider range of speeds than would otherwise be possible. High speed cyclists, however, still have to worry about cooking bulbs, unless using a modern lighting system with a voltage regulator.
[Sheldon, writing sometime around the year 2000, goes on to describe a primitive voltage regulator which he constructed a number of years before that. If you are going to build one, note that the Dynohub's rated output of 0.3 ampere requires a nicad or nickel metal hydride battery with a capacity of 10 ampere-hours or more (typically, a stack of 5 high-capacity nickel metal hydride D cells -- one example) to avoid overcharging with his setup in the "day" position.
A nickel-cadmium or nickel-metal hydride battery can also over-discharge, suffering permanent damage as the stronger cells drive power backwards through the weaker ones ("polarity reversal"). A"smart" system shuts off charging when the battery is full, and stops discharge at a safe level. Modern generator systems with a stay-on when stopped ("standlight") feature use an LED light and a capacitor for storage, to avoid all these issues.
Front- and side-facing reflectors are nearly useless when the bicycle is moving, but they are effective when it is stopped, and are another effective backup to a generator system -- John Allen]
I used to have a Dynohub on a tandem, and the bulb consumption was unacceptable. I solved the problem (and some others) by running the Dyno's output through a full-wave bridge rectifier and then hooking the DC in parallel with a 6 volt (5 x 1.2v cell) nickel cadmium battery. This not only provided light when I was stopped, the Dyno would re-charge the nicads, and, when we went so fast that the voltage rose above 6 volts, the low internal resistance of the nicads sucked up the excess, gaining a bit of extra charge and saving the bulb.
The rectified output of the Dynohub was always connected to the lights. There was no way to turn the light off while you were in motion. It would have been easy enough to rig a switch for that purpose, but I didn't see the need. The Dynohub has _very_ low drag.
I had a three way switch connecting the battery pack to the lights. In the "night" position, the nicads were in parallel with the rectified output of the Dynohub, as described above.
In the "day" position, the nicads were connected to the lights and Dyno through a diode (rectifier). This would allow the Dyno to charge the battery when it was going fast enough, but would not pass electricity the other way, so that the battery would not drain running the lights.
The "park" position completely disconnected the battery, because diodes are not perfect, and there is a slight drain that would discharge the nicads over a period of time.
Dynohubs are suitable for this type of set up because they do not use the bicycle frame as a ground. If you use a full-wave rectifier, you must keep the AC circuit seperated from the DC circuit. Since most bicycle generator setups are made to use the frame as one of the wires in the circuit, this is a problem. To use full-wave rectification, either the generator or the lamps must be kept insulated from the frame. Dynohubs are already insulated from the frame, that is why they have two screw terminals and use twin-lead wiring.
A "rectifier" is a device for converting AC (Alternating Current) into DC (Direct Current). The simplest kind of rectifier is a "diode", sort of a one-way valve for electricity. If you hook up a diode in series (as part of the circuit wire) with an AC source, it will only pass current half the time, when the AC is in its compatible direction. What comes through is pulsating DC. The problem with using a simple diode is that you are throwing away half of the electricity.
There is a simple circuit using four diodes, called a "bridge rectifier" that gets around this. It effectively flips the polarity around every half-cycle (an oversimplification, I know) and turns the AC into DC with negligible loss. You could solder four diodes together in the proper configuration, as I did, but these days you can just buy a ready-made "full-wave bridge rectifier". [Considering that the Dynohub operates at low voltages, the loss is significant when using silicon diodes -- 1.4 volts. If you use germanium diodes, it is only 0.4 volts. Fancier circuitry can get the loss much lower yet, but this is not a do-it-yourself project unless you are a serious electronics hacker. -- John Allen.]
The Sturmey-Archer Dynohub is truly named:-
The ultra - modern generator, which is covered by world patents, has no mechanical losses whatever and electrically is highly efficient, giving an output of 2 watts at 6 volts. [The bulbs used were rated at 1.8 watts -- John Allen] Using the existing wheel bearings and having no troublesome contact brushes it is entirely without mechanical friction or wearing parts, so that the effort to propel it is negligible. Being gearless it is absolutely silent, and its position in the hub protects it from damage. Voltage regulation is remarkably good, giving a good light at low speeds without an undue rise in voltage at the higher speeds, so guarding against the burning-out of bulbs.
Chris Hayes was kind enough to type up the instructions for dismantling and re-assembly of GH6 Dynohub from "The Secret is Fully Enclosed" the Sturmey Archer Catalogue for 1956.
[The Dynohub's bearing cups are integral with the hub shell, and can't be replaced. The bearing cones look like those for the AW hub, but they aren ot the same, as the axle diameter is smaller -- John Allen]
SPECIAL NOTE. - GH6 hubs prior to 1952 had the adjusting cone on the dynamo side. This cone is extended to pass through the armature body and is flatted at the outer end to take (K428) notched adjuster washer, by means of which the cone may be turned. Dismantling instructions from 1 to 5 remain exactly the same as for the current model. For paragraph 6 read 'Unscrew the dynamo-side cone and lift the ball cage out of the hub shell. The spindle may now be pulled out from the other side, together with the fixed cone.' All further comments apply equally to all GH6 hubs.
Proceed as follows (see notes at end if hub was supplied before April 1952):
|Raleigh Parts Threading/Interchangeability|
|Three-Speed Parts from Harris Cyclery|
|Sturmey-Archer Brochures 1935-38
You'll need a fast connection for this.
|Evolution of the Raleigh Sports|
|The Raleigh Twenty|
|Some of My Raleighs:
Competition | International | Robin Hood | Superbe Roadster
|New! Sturmey-Archer Heritage Site|
|Martin Hanczyc's roadster pages.|
|Tony Hadland's Sturmey-Archer Pages|
|Tony Hadland on the history of Raleigh|
|Articles by Sheldon Brown and others|