Guitar Ted's How-To: Rear Derailleur Set-Up Basics - Part 1

 Hey Everyone! This is a follow-up on the series on "How-To" posts and this time I am continuing on with rear derailleurs. The basics on parts of a rear derailleur were discussed in THIS POST. Make sure you are familiar with the parts of a derailleur first as these following posts today and tomorrow will cover basic set-up and adjustments. Following these posts I will get into how to adjust shifting with the cable adjustment feature found on many rear derailleurs- but not all. Stay tuned for more on that coming soon. 

Today's post will get into how one would start to set up a brand new, or new-to-the-bike, rear derailleur. There are certain things you will want to do in a certain order that will make your life a lot easier when replacing one of these mechanisms. Also- while I will not discuss electronic rear derailleurs here, many of these basic principals still apply. Always refer to manufacturer's instructions, or take it to a bicycle technician if in doubt. Getting today's lesson wrong (or tomorrow's) can result in damaged parts and possibly a crash and injury/death. (No kidding!) So, if you have any questions at all about your understanding of these basics, get help! It's not worth it to take a chance here. 

Okay, with that out of the way, I would start this process with the chain off. You should have the rear wheel installed with whatever cassette you are going to use. The rear derailleur can then be mounted to the hangar of the frame (replaceable hangar in most cases, probably) and then we can get started.

First, let's familiarize ourselves with the important parts of today's discussion in regard to setting up a rear derailleur. I'll refer to the image above: Working from Left to Right...

1. The cable fixing bolt, where the cable attaches. To get here, you may have to follow a circuitous path (SRAM) or specific leverage arms may have to be engaged. These are specific to individual derailleurs and may not apply in your case as well. Check manufacturer's manuals on your specific derailleur or consult an expert if in doubt. (This part won't be covered today or tomorrow)
   
2. The "HIGH" limit screw on a Shimano rear mech is almost always the upper of two screws, or as in this case with a GRX rear derailleur, a pair of 2mm hex head bolts. These are used ONLY ONCE and then you should not have to touch them again. 
3. The "LOW" limit screw. Both the "HIGH" and the "LOW" limit screws do what they say- They LIMIT derailleur travel. One on the "high"- or smallest cog of the cassette side, and the "low" is for the lowest gear/largest cassette cog side. These LIMIT the travel of the rear derailleur so your chain does not go off into the spokes or off on the other end into the frame. NOTE- These screws DO NOT AFFECT HOW YOUR BICYCLE SHIFTS, other than in one important way, which I will get to in my follow-up post. 
4. The "B Tension" screw, or as in this case, a 2mm hex head screw. This is the adjustment for the upper jockey wheel's relationship to the cassette. This will be discussed in more detail when I get to the end of this post.

Now, if you have the rear derailleur mounted, with no chain on, and no cable attached, you are ready to set the derailleur's travel limits using the limit screws. First, determine what type of screw/bolt you are looking at. Most of the time these will be JIS/Phillips type screws which have what looks like a "cross" slotted head. Other times you may see Torx/flat blade screwdriver heads. Other times you may see these 2mm hex head bolts, as with the example above. 

Also worth noting is that the "HIGH" and the "LOW" limit screws won't always be arranged in the same manner. Refer to the following images for more...

A typical SRAM MTB set up with the HIGH and LOW screws reversed from the typical Shimano positions.

Another SRAM set-up where the screws are not marked.. Note the interface tool type.

What if, as in the last image above, you don't have any markings to know which is "HIGH" and which screw controls the "LOW" limit? Well, you can visually inspect the rear derailleur for clues. 

Most rear derailleurs (NOT ALL) have a "resting" position. This is the state of the mechanism when there is no cable attached. The rear derailleur is typically in its "collapsed" state, which is when the HIGH limit screw will be in contact with some part of the rear derailleur. Visual inspection should lead you to identify which screw is the HIGH one. That leaves the other screw as being the LOW limit. See the following images...

#1: This screw rests against the derailleur at point indicated by #2 when the rear derailleur has no cable attached. So, this is the "HIGH" limit screw on this derailleur. 

On this SRAM rear derailleur you can see the limit screws protruding from the inside of the parallelogram part of the rear derailleur. The nearest to to foreground is the "HIGH" limit, as can be seen by the indicator on the parallelogram's outer side. But if that marking weren't there, you'd be able to see that this screw would be in contact with the inner part of the front knuckle of the derailleur if it were at rest. I have the derailleur partially actuated for this view. 

Now that you know which screw is which, and how to determine that, you can move on to setting the limits of derailleur travel. Typically, you'd set the "HIGH" limit first, as this is the critical 'starting' point for shifting. If the start point is off, the indexing of your shifter won't place the rear derailleur in the exact position it needs to be in to do its job. So, in a way, this particular screw is crucial to the operation of your shifter, but once it is set, you forget it!

As with either limit screw, clockwise rotation of the screw results in limitation of movement to the "outer-side" of the system. So, in the case of the "HIGH" limit screw, clockwise rotation sets a limit for movement further inboard as you turn that screw. Counter-clockwise rotation of the screw results in setting the limit further outboard, so the derailleur will move away from the center of the bicycle as you make this adjustment. 

Ideally, you want the upper jockey wheel of your rear dérailleur to sit directly underneath the smallest cassette cog when the rear derailleur is at rest. Adjust that limit screw until you see that relationship, and that is your "starting point" to adjusting the shifting. Now when the cable is adjusted correctly, (more on that later on in this series), your shifter will index correctly with the cassette and the rear derailleur will "derail" the chain properly to the next lowest gear, and when coming back again, it won't hesitate or overshoot the last cog and go into the frame. 

Okay, that's enough for today! Tomorrow I will cover the trick I use to set the low limit screw. The "B tension" screw discussion will come later. Also, in a following post, I will get into how to connect the rear cable, how to address cable tension for your rear derailleur, what tricks I use, and that should be all you'll need to set up and adjust most rear derailleurs out there. 

Tomorrow: Rear Derailleur Set-Up Basics - Part 2