Derailleur Restoration - Page 1 of 10
Unless your bike building hobby allows for an unlimited budget, you will probably end up salvaging parts from various used or worn out bicycles. Even a beat up ten dollar yard sale bike will yield at least a few good parts and components, and with a little work you can often restore many components to a like-new state. Some of the components such as levers, cranks, wheels, and derailleurs may only need a good buffing and re-greasing, and as you know, these parts can be quite expensive to purchase new. I almost never purchase new components for a project, especially when it is in the prototype stage, but I have learned how to bring old and tattered parts back to life. In the 25+ years of my bike building adventures, I have never purchased a new derailleur, but have amassed a good collection from scrap bikes. Let's have a quick look at what it takes to bring some life back to a typical yard sale bike rear derailleur.
There are two distinct types of rear derailleurs - "hangers" and "frame mounted". A Hanging type derailleur includes a slotted plate that matches the slot on the rear dropout, allowing the rear axle nut to hold it in place. As shown here, this plate matches the shape of the rear dropout slot, and is held tight to the frame by the axle nut washer. This type of derailleur mounting system has been around forever, and is more popular on lower end bicycles and older bicycles, which are the type you will often find in your scavenging.
A bolted on rear derailleur, like the one shown here, does not include a slotted dropout plate, it simply has a mounting bolt. This type of derailleur requires a mating hole on the frame, which will be positioned just under the right rear dropout. Functionally, both derailleur types function the same way, and only differ in the fact that the hanging type includes the mounting plate in its design. When making a trike or a quad, the hanging type may be easier to work with since you can cut and weld the hanger plate for adaptation to a new mounting system. You can hack a bolt on frame mounted type as well, but will have to chop the mating mounting plate section from the frame.
Here is a frame mounted derailleur, showing how the main bolt is fastened directly to the frame. This type of derailleur is usually found on higher quality cycles, and is a more mature design that the hanger type. A frame mounted derailleur requires a mounting tab installed on the frame as part of the right rear dropout.
Another distinct difference you will see between rear derailleurs as you begin to build up your parts collection, is the spacing between the two idler wheels. This idler wheel spacing will vary greatly between models and brands, but typically, one would classify a derailleur as either a long cage or short cage. Because the two idler wheels act as a tensioning system to pick up the return chain slack, in theory - the longer the cage, the more chain that can be pulled back. So to simplify this even more, a short cage derailleur is usually used on a road bike, and a long cage will be used on a mountain or touring bike, since it will have a much larger range of gears. A larger "range" of gears means more difference between the diameter of the front and rear chain rings. I almost never use a short cage rear derailleur, because I like to have a full range of gears, which makes hill climbing easier, and downhill sprinting more fun. The derailleur shown in this image is on the extreme side of the short cage variety. This dude is probably from an old 1980's ten speed.
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