Chain length and derailleur adjustment

My bike has gears in the front and in the back. In the front, they're called "chainrings", and they're part of the crankset. In the back, they're called "cogs", and they're part of the cassette. I wrote about installing them in a previous post.

Because I have gears in two places, I needed two "derailleurs". These are the parts that move the chain around to different gears when I press on the shift levers. The front derailleur moves the chain side to side to switch between my three front chainrings. The rear derailleur does the same with the cogs, and it also takes up extra slack in the chain as you switch between gears of different sizes.

Before installing the rear derailleur, I needed to check that the derailleur hanger was angled correctly. The derailleur hanger is the part of the frame that the rear derailleur mounts to, and it must be angled flat and parallel to the rear wheel. Sometimes, this angle is slightly off when the frame is manufactured. A misaligned derailleur hanger will make shifting adjustments very difficult down the line.

To check alignment and adjust, I used this derailleur hanger tool at Camden Cycles open shop. The tool screws into the derailleur hanger, and then you spin it around at different angles and make sure it remains the same distance from the wheel rim.

My derailleur hanger was just slightly misaligned, so I used this tool to gently bend the frame into the correct position. Then, the derailleur simply screwed onto the frame.

The front derailler clamps onto the frame. It must be placed so that it floats 1-2 mm above the chainrings as it moves, and stays parallel to the side of the frame.

Once both derailleurs were mounted on the frame, it was time to install the chain. By default, chains normally come slightly too long. To determine chain length, I rolled my chain onto the largest chainring and largest cog, and then determined the amount of overlap at the end. The correct length should overlap by about 2 chain links (I learned this from Sheldon Brown). I needed to remove four chain links to get the right length. I used a chain breaking tool to remove the extra links.

Next, I threaded the chain through both derailleurs, and joined the ends with the master link that came with the chain. This master link simply clicks together, making for easy installation.

Now onto derailleur adjustment! I've opted for a shifting style called "friction shifting", which is simpler to adjust and maintain. Instead of setting stops that click between each gear, I move my shift lever until I feel the chain move. This means I only need to set the upper and lower limits for each derailleur, with no fussy adjustments in between.

Both derailleurs have a high and low screw. These screws are like bumpers in a bowling lane: they control how far the derailleur can move in each direction. Before attaching the cable to the derailleurs, I adjusted these screws. In the image below, the high/low screws on the rear derailleur are the black ones on the bottom:

The "high" screw (marked with an H) controls the highest end of the gear, which is the smallest cog. The "low" screw (marked with an L) controls the lowest gear, which is the largest cog. When there is no tension on the cable, the derailleur is at the high gear position, so I adjusted this screw first. Tightening the screw reduces the distance the derailleur can travel, and loosening the screw increases the distance the derailleur can travel. Once the high screw was in a good spot, I manually pressed the derailleur with my left hand and spun the crank arms with my right hand, moving the chain into the lowest gear position. If the gear chain didn't quite make it to the largest (lowest) gear, I loosened the low screw. If it fell off of the cassette and landed between the cassette and the spokes, I tightened the screw.

The adjustment for the front derailleur was similar. I set the low limit screw first, and then pressed the derailleur with my hand to set the high limit screw.

Once the limit screws were in a good spot, I attached the cables to each derailleur. A good rule of thumb is to pull the cable as tight as possible with your hands, then clamp it to the derailleur. Once I did that, I tested out the shifting range with the shift levers. The first time I tested this, my rear derailleur didn't quite make it to the lowest gear, so I tightened the cable slightly. Fortunately, my shift cables have a barrel adjuster too, so I didn't need to undo the cable end. Once the cables were attached, I needed to make some more small tweaks, and then it was ready to go! I probably should have trimmed the cables a little shorter, but I can always do that later.

Both shift and brake cables will stretch slightly as they're used, so I will likely need to make another adjustment after riding on it for a few weeks.

The build is almost complete!