The standard road-racing gearing system until 1938 was the double-sided hub, usually "fixed and free." That is, the hub was threaded for a freewheel on one side and a fixed gear on the other. The cogs were chosen so that the cyclist had a relatively high fixed gear for the flatter parts of the course and a relatively lower free wheel for the climbs and descents. I have not heard of systems in which one could put two cogs on one side, although, in an engineering sense, hubs could probably be designed to accept such a system. For some events that did not involve such steep descents, the riders preferred a double fixed wheel, with fixed on each side. The cyclist dismounted, loosened the wing nuts that held the rear wheel, turned the wheel around, remounted the chain, and tightened the nuts again. The nuts were also used, probably more frequently, when changing flats.
In those days the riders were responsible for all their own support, and carried their spare tires slung over their shoulders in a figure-8 pattern. When I started racing in 1949, that was still the usual way, although derailleurs were used in all the hilly races, such as those in N.California and in Massachusetts, the two areas in which I raced.
Tullio Campagnolo did not invent the derailleur. Derailleurs had been used by tourists since about 1905. The typical derailleur until the late 1930s had its jockey wheel mounted on a bracket brazed to the chainstay. On my first custom frame, from Drysdale in New York, built in 1949, I specified such a derailleur, a Cycle Oppy, partly because it used 1/8 inch chain when 2-mm chain was hard to find in the USA. Many of my cycling friends used similar derailleurs, although that was the year that the Campagnolo derailleur became standard for racers. The disadvantage of such derailleurs was that the jockey wheel had to be sufficiently far forward to clear the largest sprocket. Therefore, it was a long distance from the small sprockets, so that changes between the high gears were slow and required considerable overshoot. During those years, derailleurs were not generally allowed in road races because derailleurs required freewheeling, and mixing riders with fixed and free wheels produced problems on the turns, when fixed-gear riders were limited by pedal scrape on the turns while free-wheeled riders were not. However, there was also a series of special races for derailleur-equipped bicycles, typically hill climbs, that were sponsored, at least in part, by the derailleur manufacturers.
By 1938, Simplex had perfected the next generation of derailleurs whose jockey wheel moved both in and out, to change gear, and forwards and backwards to follow the change in sprocket sizes. The Simplex used a cage very like those often used today, where the cage pivot is behind and below the shaft for the jockey wheel. Therefore, as the amount of free chain increases as smaller sprockets are selected, the jockey wheel rises to remain close to the sprockets. However, on these derailleurs the lateral movement was produced by the cage pivot shaft sliding laterally, lengthwise, inside a bushing held by the "fixed" part of the derailleur. I put fixed in quotes, because Simplex had two pivots, one at the derailleur tang and another at the cage pivot, which worked in tandem to take up the slack and whose tension springs were individually adjustable. The lateral movement was powered by the cage pivot spring, that both took up slack and pushed the cage toward the large sprockets. That spring was opposed by a pull chain that ran inside the hollow pivot shaft and out through the radiused boss on the outside the fixed arm, through a cable housing stop on the fixed arm, and then up to the shift lever. The problem with this design was that the bushing was really too short for its required diameter and stroke. The least bit of dirt in the sliding fit and shifting went to hell. The derailleurs were fitted with a spiral expanding dust cover (that many people thought was the actual spring), but that did not keep out all the debris that flew around. Cleaning and regreasing frequently was highly desirable. I fitted Simplex 8-speed gearing on the bicycle that had come with the Cyclo Oppy, once I had decided that replacement parts, including 2-mm chain, had become more readily available in the USA. Cyclo, which had some financial relationship with Simplex, copied the Simplex principle with the Cycle Benelux series, as did several other makers in time. By 1938, the Simplex design had become so accepted that it was allowed in the Tour de France.
After WW II, Campagnolo invented two derailleurs. The second was the modern parallelogram movement, 1949, that replaced the sliding bushing. This was later modified by Suntour, who placed the parallelogram at an angle that approximated the angle formed by the sprocket cluster and thereby allowed the jockey cage to be again pivoted on the same pivot as the cage. However, few of you have ever seen Campagnolo's first derailleur, produced in 1947. I have raced against a rider using one, and a few years ago one showed up completely unused and was built into a racing frame by a local framemaker as a historical exhibit. This derailleur contained no system for taking up the slack chain. The chain went direct from chainwheel to sprocket in all gears. How did he do it? What's more, how did the rider use it? All the slack chain was taken up by movement of the rear wheel in and out of the frame. To ensure that the wheel continued to run true, the axle had a small spur gear around each end, and the upper surface of the rear dropout was machined with rack teeth to match. Therefore, the axle rolled in and out of the dropout in perfect alignment because both sides had to move the same amount. The bicycle was fitted with two derailleur control rods that extended up the right seat stay from the hub. One rod released and clamped the quick-release hub, the other carried a fork that pushed the chain laterally to line up with the desired sprocket. To change gear, the rider first loosened the rear hub. Then he shifted the chain to the desired sprocket by coordinated pedaling and movement of the fork. As the chain shifted from sprocket to sprocket, the axle rolled forwards or backwards under his weight (no forceful pedaling here -- probably the chain had to be pedaled backward, because I remember that the fork was on the upper side of the chainstay). When the chain was on the correct sprocket, the rider reclamped the hub and resumed forceful pedaling.
Along with the parallelogram rear derailleur, Campagnolo invented the parallelogram front derailleur. The combination proved unconquerable. For decades, most racing bicycles (and all bicycles where price was no object) were "all Campy." Before that, the typical front derailleur was just a fork pivoted on a pivot that extended forwards from the seat tube, with its upper arm extending up to a handle. The rider reached between his knees and flipped the fork over. I used such a front derailleur with my first racing derailleur setup. The Campagnolo front derailleur used a shift cable, so the "all Campy" bike had two shift levers mounted at the top of the down tube, another reason why racers adopted the setup. Simplex put out a front shifter with cable drive to a sliding bushing to compete, but these disappeared as less satisfactory than the parallelogram movement.
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