Guitar Ted's How-To: Rear Derailleur set-Up Basics - Part 2

Welcome to Part 2 of how to set up a rear derailleur from scratch on your bike. Again, this applies to the way I would take a new rear derailleur for a mechanical system and set it up to work properly. This would also work for anyone swapping on a used rear derailleur as well. While this doesn't cover electronic rear derailleurs, many of the same basics still apply. 

Now, if you missed the basics of rear derailleurs, that post is HERE and a link to yesterday's post is HERE. I will assume you have read and understood that previous material. 

Keep in mind that if you have any hesitation at all in doing this task, stop and seek the help of a professional bicycle mechanic. Getting any of this wrong can lead to serious injury, property damage, and even death. So, unless you are confident in your mechanical skills and unless you own the proper tools, don't attempt this! 

Okay, with that out of the way, I will remind you that this post only will deal with how to set up the lower limit screw.  How to set up the "B Tension" screw is best done while doing a tune on the cable and rear shifting, so that will come later. . See that link for the earlier steps in this process which MUST be completed BEFORE you do these steps. Now, on with the show.....

The "LOW LIMIT" screw does not come into play until the rear derailleur is at full extension. This happens in your lowest gear on the cassette, or the "biggest cog" if you think of things in that manner. Its job is to prevent the rear derailleur's travel inboard to the point that the chain goes over the top of that last cog and into the spokes. It also aligns that upper jockey wheel on the rear derailleur so it is perfectly in-line with the last cog of the cassette. 

Above you can see #1 which is the "LOW" screw and where it hits the inner portion of the parallelogram of this Shimano XT rear derailleur at point #2. Remember from yesterday's post that turning that "LOW" screw clockwise prevents travel inboard and turning it counter-clockwise allows that travel to continue further inboard toward the spokes. This assumes that your cable tension is correct, that you have the correct, operational shifter, and nothing is damaged or bent on a complete system. But for this job, we aren't worried about that just yet. 

Now, for a quick diversion...... Since setting the "LOW" limit screw is best done with a chain on, I am going to show you a couple of hacks I use to make chain installation a bit easier. Both of these work best if you have a repair stand, by the way. 

First, you want to get the derailleur in the position I have shown to the left here. This will probably mean that your bike will have its front wheel on the ground and the back one straight, or almost so, above it in the work stand. What you want is to make a straight path downward for the chain toward the ground through the derailleur cage. 

This allows you to use gravity to your advantage, instead of fighting gravity - if you were trying to put the chain through the rear derailleur with the bike in a horizontal position. This technique I am sharing makes the job of threading a chain through a derailleur far easier. But again- it works best with a repair stand. Don't have one? Well, I would guess that if you have read this far, you want to do some of your own work on your bicycles. The best thing you can add to your tool stash is a repair stand. Get one!

Drop the chain in from the top, using gravity to feed it through the cage.

Okay, now on with the show... Tip: Make certain that you have cut your chain to length (use the old one as a guide if you can), and make sure you thread the chain in the right orientation. 10, 11, and 12 speed chains will likely have either a top side or bottom, or a right and left orientaion. Check your chain manufacturer's tech spec guide or ask a mechanic if you are not sure about this. Most 9 speed, 8 speed, and 7 speed chains are not affected by this.

Using gravity, you should be able to drop the chain in from the top, through the cage, and past the upper jockey wheel of the derailleur, (which is the lower one as this is oriented here in the image) , and then pass through at least half the chain. You can even thread the chain through a front derailleur at this time, if that applies to your bike. Then adjust your stand back so the bike sits horizontally again. 

Hack #2: Use a bungee cord to hold the cage forward like this to relieve chain tension for ease of quick link attachment.

Ever had trouble connecting those Shimano quick links? Read on...

Now for Hack #3- This applies mostly to Shimano quick links, but some SRAM 7 & 8 speed links are stubborn to go into place as well. If you have had trouble connecting a quick link like this, here's a quick way to solve the problem. First, carefully attach the link with the chain tension relieved as I have shown with the bungee. Try to get the link as tight as you can first, as I have shown immediately above with this Shimano 11 speed link. 

Then, carefully, slowly pedal the bike forward so the quick link is in the "top run" of the chain as I have shown immediately above here. Get the quick link about midway from the cassette cog and the chain wheel. Then, grab a rubber mallet, (you can use your hand, but it usually hurts if you do!), and apply the rear brake firmly with one hand and then strike a pedal (Usually should be the right side since it is closet to you)  sharply with the rubber mallet, as if you were going to pedal forward. This will snap the link into place. Done! 

Now- On to setting initial "LOW" limit for the derailleur. You'll see why I like to have the chain on for this in a minute.... (Here is a link to yesterday's post for reference if needed)

With the rear derailleur on the smallest cog on the cassette, place your hand as I have shown here on the rear derailleur. Use your left hand, as you'll need your right hand to pedal with. Pedaling slooooowly, push with your thumb on the derailleur's knuckle in the direction shown. (Towards the inboard side of the bike). 

The derailleur should start walking the chain up the cassette toward the biggest cog. You may find that with a new rear derailleur that you can only get so far, then you cannot push the derailleur any further, but you are two, maybe three cogs from the largest cassette cog. Okay, if that is the case, you need to turn the "LOW" limit screw counter-clockwise. Always make small, incremental adjustments. I'd turn it maybe a quarter turn at a time, and recheck how far the derailleur can be pushed up. Repeat the process until you can get the chain to drop into the last gear on the cassette, but no further!

Now, lets say that you can push that chain up and it wants to go over the last cog if you keep pushing. (This is why you want to pedal SLOWLY and be very careful) If that seems to be the case, the "LOW" limit screw needs to be turned clockwise, again a quarter turn at a time, until you cannot push the chain off the last cassette cog. 

Showing the technique at the extreme end of the derailleur's travel here.

Keep in mind that you do not need to use excessive force, or much force at all to manage this technique. The carefully applied force and slow movement of the drive train as you pedal will yield the best results here. 

Now if you've found that point where you can get the chain to drop in the last, largest cog on the cassette and not have the chain go over it, you've got the "LOW" limit in the ballpark. You should be okay as far as a safe system now. I'd recommend doing a finer adjustment if necessary when you get to adjusting the cable for shifting, but that will be for my next post in this series. I'll also show how to set up the "B" tension screw for optimal shifting when I get to how to tune the rear derailleur as well.

At this point, your basic set-up of the rear derailleur is complete. Stay tuned for more "How-To" posts soon.