Just Imformational

Single-speed Cycling
Until 1937, when they were first permitted to use derailleur gears, those giants of men who competed in the Tour de France – and in all other races up to a similar year – rode bikes with just a single gear. Initially, in the case of the Tour until about 1906, they were not permitted to use freewheels, thus rode fixed-gear or fixed-wheel. Whenever the rear wheel was rotating, so the pedals were rotating too. The advent of the freewheel at least allowed the rider some respite from constant pedalling, and helped average speeds increase. However many races were won at very respectable speeds despite the absence of derailleur gears: most riders mounted sprockets of different size on each side of the rear wheel, and could thus change gear by removing the wheel and flipping it over to use the other gear. Later some put double sprockets on one or both sides, and changed gear by manually shifting the chain from one to another. This often required a chain tensioning device, which can be seen in some contemporary photographs.
There are plenty of good reasons for still riding single-speed bikes even today. They are much simpler, cheaper, and can easily be as light as much more expensive geared bikes. Moreover they are excellent for training, as you have a simple choice: pedal in the single gear, or walk! You cannot progressively chicken out on a hill and crawl your way down to 'granny' gear. When travelling at higher speeds, you must learn to pedal smoothly at very high cadences of 120 per minute or more, and this builds what is termed souplesse in your pedalling style. Traditional European training methods for pro cyclists normally put them back on a fixed-wheel or single-speed bike when they returned to training in the New Year, and used that to improve souplesse before switching back to a normal road bike after 1000 to 2000 miles. Being so simple, single-speed bikes are ideal for bad weather training, as they can be cleaned and maintained very quickly. Some pros still recommend single-speed or fixed-wheel bikes for certain types of event, notably hill-climbs, although no-one would attempt more general or multi-stage road racing on such a machine now.
Riding single-speed and fixed-wheel can normally be done on the same bike, but they are quite different experiences. Fixed-wheel has been described as an almost spiritual experience, in which you forge an intimate link with your bike, the ultimately pure bike ride. However it can be dangerous: 'fixies' tend to dispense with brakes, as you can brake your rear wheel by slowing the pedals down. In traffic and many real road situations, this can prove risky. When riding fixed-wheel, your pedals are constantly rotating, so you cannot position them to avoid groundstrike when cornering. Finally, if you ride fixed on hilly routes, descents can force extremely high cadences, and the slightest mistake such as a desire to freewheel can be catastrophic. If you want a similar experience but without the spiritual depth and concomitant risks, ride single-speed rather than fixed-wheel.