When riding on the two smallest chain rings, there will be much more slack chain to contend with than when riding on the two largest chain rings, so a strong spring built into the rear derailleur must pull back on this chain to keep it from flopping around, hitting the rear tire or the ground. The return chain is the lower part of a bicycle chain and it is never under any tension because it does not drive the transmission. Only the top side of a bicycle chain does any real work.
Although there are quite a few small moving parts in a rear derailleur, it’s actually not complicated to install, setup and maintain once you know the basics.
Figure 1 - A standard rear derailleur
A typical bicycle rear derailleur is shown in Figure 1. Like most bicycle components, quality and price can vary greatly, with a typical steel departments store cycle rear derailleur priced about $20, compared to a carbon fiber body brand name rear derailleur which typically costs hundreds of dollars.
Being a garage hacker, I often use only the low quality steel rear derailleurs I find on salvaged cycles, and to this day have never seen one become defective or fail if setup properly, so spending and extra $500 to shave off 10 ounces of steel does not seem worth it, in my opinion. The lower quality steer body derailleurs are also very easy to repair, and modify for trike use since they can be cut up and welded.
Figure 2 - Setting the optimal chain length
A rear derailleur is shown in Figure 2, picking up the return side of the chain. Notice how the two small chain idler pulley wheels are almost in line with each other as indicated by the black arrow. This alignment is a good starting point when trying to calculate the best length for a chain on a custom built cycle that includes a rear derailleur. By placing the chain on the largest front chain ring and the middle rear chain ring, the length of the chain will be close to optimal when the two derailleur pulleys are in the position shown in Figure 2.
A chain that is too long will flop, possibly striking the rear tire or even the ground, and a chain that is two short will pull the rear derailleur too far forward when on the largest chain rings, causing severe shifting problems. Some experimentation is always necessary when installing a rear derailleur and new chain on a custom built cycle.
Figure 3 - The high and low adjuster screws