Guitar Ted's How-To: Rear Derailleur Basics

Back again with another post on "How-To" and this time it will help you to understand rear derailleur basics. 

The last How-To post was about tubeless rim taping and can be found HERE. There is a link from that post to another How-To on tubeless tire mounting, by the way. 

Okay, this post will NOT describe every rear derailleur situation, but it WILL describe how almost every rear derailleur on modern bikes works, even electronic ones, to a degree. All rear derailleurs owe there basics to how these devices evolved up to the era of indexed shifting which dawned in the 1980's. 

While variations on the theme have been produced, almost every rear derailleur since the 1980's has basics that are the same in function. That's what this post will help you to understand, and when you understand these basic principles, your understanding of adjustments and care for rear derailleurs should be enhanced. 

Or let's hope so! 

Okay, on with the show here.....

Rear Derailleur Basics: Anti-clockwise from the upper left.....

1. "B" tension screw. This can be a Phillips (It isn't really, but for sake of simplicity here...) head screw or a hex head screw. It determines where in space that the upper jockey wheel (See #7) tracks the cassette or freewheel cogs. Needs only to be set once- then forget it!
2. Outside (or HIGH) limit screw. Can be a Phillips, (again- it isn't really, but..), or a hex head screw. On Shimano rear derailleurs it is almost always the upper/higher screw. On SRAM it is usually the lower one. On SRAM rear derailleurs with screws set side-by-side it is the innermost to the rear wheel screw. Generally marked with an "H", but not always. The function of this limit screw is to keep the derailleur from traveling too far outward and just enough to place the upper jockey wheel underneath the smallest cassette/freewheel cog in the highest gear setting. Set once and forget it!
3. Inner (or LOW) limit screw. Similar to #2 only the opposite is true with regard to which screw it is and what it does. This one is really important to get right or your chain will go over the top of the cassette/freewheel and into your spokes, and likely will destroy the rear derailleur in the process. Limit the travel of the rear derailleur so that this does not happen, but so that the chain will shift into the lowest gear. Set once and forget it!
4. Rear cable adjuster. This tensions the derailleur cable so that the shifter will operate the derailleur correctly. (Not applicable to electronically shifted derailleurs, of course) This should only be adjusted when the derailleur is in its resting position, (almost always in the highest gear, but there are exceptions), and should be adjusted so that when the shifter is activated, the chain moves immediately into the second smallest cog. Counter-clockwise rotation of the adjuster tightens the cable, and clockwise loosens it. The adjustment procedure will be covered in a different "How-To" post. NOTE: Not all rear derailleurs have this. Most notably some SRAM rear derailleurs. 
5. Rear derailleur cable attachment: Not applicable to electronically shifted systems. The big thing here is to make sure you thread the cable past the fixing bolt in the proper manner or the rear derailleur will not operate correctly. SRAM rear derailleurs and some newer Shimano rear derailleurs require a specific, circuitous routing, so be careful when replacing a cable! This matters on ALL rear derailleurs, by the way. 
6. Chain routing: When replacing a chain, be extra careful to route the chain through the rear derailleur properly. It is easy to get this wrong which can lead to noises, poor shifting, and permanent rear derailleur damage. 
7. Jockey Wheels: The arrow points to the lower one here, but there is an upper one as well. Both need to be cleaned and lubricated regularly. Failure to do this will result in excess friction, requiring more watts of work from the rider. Failing to clean and lubricate jockey wheels can also lead to premature wear, poor shifting, noises, and more. If you dare to remove a jockey wheel keep in mind that the upper one cannot be swapped for the lower and vice versa. They have specific design and functions. Also: It is imperative that when you replace the axle bolts that they be treated with thread locking compound to prevent them from backing out while riding and possibly causing a crash. 
8. Rear derailleur attachment bolt. Most often a 6mm hex head, but it can be a Torx bolt on some higher end rear derailleurs. Once attached you generally don't have to ever mess with this bolt.

Even oddball rear derailleurs like this one from Ingid use these same basic mechanical principles.

Okay, so this isn't really a "how-to" so much as it is a "how it is". The thing is, once I do get to the article on how to adjust these, I don't want you to ask me "Hey Ted, What does that screw do?". You should know now because you read this article. 

As you may have guessed the next "How-To" will address setting up and adjusting the rear derailleur. Stay tuned...