Figure 1 - A standard rear derailleur

A bicycle rear derailleur has several jobs. The main one is to shift the chain between the multiple chain rings on the rear freewheel. The rear derailleur must also pick up any excess slack in the return side of the chain, which will vary depending on which set of chain rings (front and rear) the chain is currently wrapped around.

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.

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

High and low limiting screws (shown in Figure 3) must be properly adjusted to allow the full range of motion without having the chain become shifted right off the last chain ring. Adjustment of the min and max setting is accomplished by turning the adjuster screws with a Phillips screwdriver until the shifting range of motion is set properly. Having the cycle up on a stand so you can turn the cranks while making the adjustment is the best way to set the min and max screws properly. A chain that is shifted beyond the smallest or largest rear chain ring can be a real annoyance, as it could jam up so tight against the frame that you may have to remove the rear wheel to get it back out. Setting the adjuster screws so that the chain is on the middle rear ring is also a great way to test your cycle without having to install all of the cables initially.

Figure 4 - The derailleur range of motion

Figure 4 demonstrates the movement that the rear derailleur must make to force the chain from one chain ring to the next. Unlike the front derailleur which moves the drive side of the chain entering the top of front chain ring, the rear derailleur forces the return chain as it enters the bottom teeth of the rear chain ring. Both derailleurs require that you pedal in the clockwise rotation for shifting to occur properly. Shifting gears while not pedaling can cause a chain to jump over too many chain rings and become jammed, or pull the cable away from the fastening arm, causing a shifting problem.

Figure 5 - Adjusting the min and max setting

To set up your rear derailleur properly, jack up the rear wheel and try all of the gears, ensuring that the smallest chain ring (Figure 5) and largest ring can be reached. Newer shifting hardware is “indexed”, meaning that the shifting hardware is matched to the derailleur so that one “click” translates into one shift.

Older shifters are simple levers that allow you to move the derailleur position in either direction until you have shifted gears. The older systems are much easier to setup and maintain, but do make an annoying ratcheting sound as you move between gears. Both types need to have the limiting screws set properly to avoid shifting beyond the smallest or largest chain ring.

Figure 6 - Removing the shifter cable

The shifting cable shown in Figure 6 is responsible for pulling the derailleur body in one direction against the built in spring that will push it back in the other direction as the cable relaxes. Some older indexed shifting systems use a dual push and pull cable setup, but these are fairly rare now due to being a real pain to setup and maintain.

Also shown in Figure 6 is the small bolt and nut that secures the end of the cable to the derailleur arm. This bolt may have a small hole to allow the cable to be “threaded” into it, or it may simply squash the end of the cable by clamping down on it.

Figure 7 - Removing the dropout bolt

There are two common styles of rear derailleur mounting systems: one that includes a slotted plate to match the rear dropouts, and one that bolts directly to a threaded tab on the rear dropout as shown in Figure 7. Both derailleur types operate exactly the same way, but the tabbed style derailleur can only be mounted to a frame that includes the threaded hole on the dropout. To remove a rear derailleur, turn the mounting bolt or axle bolt in the counter clockwise direction to loosen it. The derailleur will be freed from the frame, but trapped by the chain that is held between the pulley plates.

Figure 8 - A derailleur with a dropout slot

A typical rear derailleur with a slotted dropout style mounting system is shown in Figure 8. The slot in the dropout matches the slot in the frame, allowing the axle and nut to hold the derailleur in place.

Figure 9 - Removing the lower chain idler

To free the rear derailleur from the chain, it will be necessary to either open a link in the chain or remove the lower idler pulley. The easiest method is to remove the lower idler pulley by loosening the bolt as shown in Figure 9, allowing the small plastic idler pulley to be removed from the two plates, freeing the chain.

Figure 10 - One of the chain idler pulleys

Figure 10 shows the small plastic idler pulley removed from the rear derailleur body, allowing the chain to be freed.

Figure 11 - The brass bushing and bolt

The idler pulley is secured between the two side plates by the bolt shown in Figure 11. Also shown are the two washers and brass bushing that provide the bearing surface, allowing the pulley to spin with minimal friction. The space under the two washers is filled with grease so that the pulleys do not squeak when they are moving.

Figure 12 - Removal of the second idler pulley

When both idler pulley bolts are removed, the side plates will be separated as shown in Figure 12. One plate is attached to the spring loaded pivot on the derailleur body. You will likely not need to take apart a rear derailleur beyond this point unless it needs serious repair or you are going to make modifications, which is often the case when creating some kind of custom transmission system for a new home built cycle.

Figure 13 - Popping off the plastic dust cap

To continue dismantling a rear derailleur, pop the plastic caps off of the body by prying them up with a small screwdriver as shown in Figure 13. Under both plastic caps are the axles and springs that allow the derailleur bodies to move and keep the chain under constant tension.

Figure 14 - Removal of the snap ring

Once the plastic dust caps are out of the way, use a small screwdriver blade to pop off the snap rings that secure the derailleur body to the axles as shown in Figure 14. Because the spring is under a good deal of tension, the parts my pop away from each other, so be prepared!

Figure 15 - The derailleur cage is removed

Once the snap ring has been removed, the other half of the pulley cage will come away from the derailleur body as shown in Figure 15. The spring that keeps the chain pulled back may also fall out or be affixed to the body as it is in the rear derailleur shown in Figure 15. The end of the spring will fit into a small hold in the cage next to the axle, and this is how it creates the chain tension.

Figure 16 - The spring that keeps the chain tension

Figure 16 shows a closer look at the spring and the small hole it fits into after removing the rubber dirt protector from around the spring. The little knob shown on the cage body to the left of the axle is a stopper to limit the range of motion of the axle when forced back by the spring.

Figure 17 - Removing the connecting arm

At the other and of the rear derailleur are another dust cap, snap ring, axle and spring as shown in Figure 17. By using two springs and axles, the rear derailleur is allowed to keep chain tension as it travels up and down the rear freehub from ring to ring. On rear derailleurs that do not have the slotted dropout, the bolt that holds the derailleur to the frame is also the axle.

Figure 18 - A rear derailleur disassembled

The rear derailleur is shown in Figure 18 pulled apart about as far as can be done without actually breaking a rivet. Nothing else can be removed from here without brute force or by cutting part of the steel body.

Figure 19 - A rear derailleur on a delta trike

Unless you plan to use some type of internally geared rear hub, you will need a standard rear derailleur on your vehicle to have the ability to change gears in the rear transmission. A standard bicycle rear derailleur will do its duty without any issues, as long as it is installed in its usual position under the freewheel and set up properly.

Figure 19 shows a typical rear derailleur hung under a freewheel that is mounted to an axle on the rear of the Kyoto Cruiser Delta trike. This configuration may look odd at first, but it is no different than that of a standard bicycle, so the rear derailleur operated exactly the same way.

Figure 20 - Making a custom connecting arm

On a delta trike, there is no rear dropout, so a little creative thinking (hacking) will be needed to get the rear derailleur installed in the correct position to traverse the chain rings on the rear freewheel. Figure 20 shows one of many simple methods that I have used to fasten a rear derailleur to the back of a trike or quadcycle. Made of steel, the lower quality rear derailleurs are very easy to cut, modify and weld, not to mention the parts are usually interchangeable so they are very easy to repair.

Figure 21 - A derailleur in a mid drive system

A rear derailleur does not always need to live at the rear of a cycle as shown in Figure 21. On our Atomic Zombie LodeRunner Tandem Cargo Trike, one of the two rear derailleurs is being used in a mid-drive system that allows both riders to have their own independently shifted gear systems.

Once again, the system is not typical, but the position of the rear derailleur in relation to the freewheel is always the same, showing that it is possible to place a rear derailleur into practically any type of cycle as long as the rules are followed.

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