There are two systems of gearing in common use on bicycles:
* Derailleur gears
* Internal hub gears
Although derailleurs are simpler in principle, hub gears were developed first, before the turn of the 20th century. An ancestor of the classic Sturmey-Archer 3 speed hub, used on hundreds of thousands of bicycles worldwide, was working perfectly by 1905. Derailleurs have the advantage of being much easier to manufacture than hubs, and use much cheaper materials. No powder metallurgy or gear-tooth machining required. And when they are new, derailleurs are really very good. Indexed shifting means that they select perfectly and they have a well-organised increment of gear ratios.
However, for the age of bikes that we might be interested in restoring, the derailleur is not likely to be in good condition. No doubt it will be worn and sloppy. Indexed shifting is rather a recent development. Before this time, it was a case of move the lever and wait for the clack clack clack noises to stop. Now, when the pivots of a rear derailleur have play or the spring is weak, it can be very difficult to select all the gears, particularly the highest. There is not much you can do to repair the derailleur; it is best just to get a new one.
Hub gears are a totally different kettle of fish. Even old hubs made during the war can be cleaned up and will run as well as the day they were made. Sturmey-Archer has a fascinating heritage and many of the parts required for even very old hubs are still available new. You can cannibalilse one hub to provide parts for another. There are many different older hubs with four and five speeds, not just three, and there are close and wide ratio hubs, hubs with drum brakes and hubs with built in dynamo generators. The mechanisms range from being very straightforward to intricate, complex and ingenious. Sturmey-Archer Heritage is a brilliant site with a huge number of nostalgic brochures, catalogues and technical drawings.
Here are a selection of old hubs that have been collected by my brother and me over the years. It amazes me that we have so many. They came from abandoned bicycles and deliberate hub hunting at Household Waste Recycling Centres. Also many were cannibalized from old Moultons that may well have been past restoration. Many Moulton Club friends have given me hubs, often in exchange for re-built wheels and other parts. I often take the most horribly filthy hub I can find, then scrape the hard coating oil off, clean the shell with paraffin and polish it with Solvol Autosol. These black hubs usually come up a treat, protected by the coating of oily grime. And inside, the mechanism is often in excellent condition. The real enemy of the ancient Sturmey-Archer is a lack of lubrication.
Hopefully you are convinced that it is much more fun to explore the wonderful world of old hubs rather than buy your way out of a derailleur problem. Don't worry; no springs are going to bounce out of the internals when you take them apart. You will be able to re-assemble it all after it is all in bits. It isn't like a clock. And let us not forget our German friends, Mr. Fichtel and Mr. Sachs, who contributed the magical Duomatic and bulletproof Torpedo three speed to the worthy cause of epicycology.