"Multiple-gear ratios provide mechanical flexibility, but it is also important to extend the range of the rider, and a useful aid for this can be riding a ‘fixed’ gear. This is a single sprocket without a freewheel, so that when the back wheel turns, so do the cranks. The only way to stop pedalling is to stop the bike.
Riding fixed is a traditional European method for winter training. At the end of the road-racing season the multigear bike is put away, and replaced with a fixed-gear machine. Sometimes the bike is a track model made for the job, but often it is an old, stripped-down road bike that won’t be too deeply offended by the grunge and grime of winter. The classic gear ratio is 63 inches, which is low and easy to spin.
The bike is used for everything. With only one gear ratio, the rider is forced to learn how to spin the cranks at blinding speeds. There’s no other way to make the bike move, or stay with it on downhills. The rider becomes progressively more supple, fluid, and fast. In the spring, on returning to the multi-gear bike, the rider is a tiger." -RICHARD BALLANTINE