Shifters and Derailleur Cable and Housing
The job of shifters is very simple: to tighten and loosen the cable attached to the derailleur, causing the derailleur to shift gears.
Way back in the old days, bikes had "friction" shifters--shifters that were not indexed, that is shifters that did not click once for each gear shifted. With friction shifters, you have to feel around for your gear.
All modern shifters are indexed. Indexed shifters have tiny ratchets and spring loaded pawls that are set at precise intervals, so one click of the shifter moves the cable exactly enough to shift the derailleur one gear. Many shifters can be moved one click or several, allowing rapid shifting through several gears.
Friction Shifters
It is easy to adjust derailleurs (links to rear derailleur adjust and front derailleur adjust) for friction shifters. The cable tension does not have to be exact. The most frequent issues unique to friction shifters are:
1. The derailleur will not go in to the gear that requires the most cable tension. On the front derailleur, this is the big ring. On the rear derailleur, this is the big cog. Usually, this is because the cable has stretched. Shift to the small ring or cog, loosen the cable bolt on the derailleur, pull the cable tight, and snug-up the bolt.
2. The shifter won't hold the shift. You move the shifter, but it goes back to its original position. Most friction shifters are in effect bolted together. The top of the shifter can usually be twisted clockwise to tighten its action--or counterclockwise to loosen it. If it won't hold a shift, tighten the shifter.
Indexed Shifters
Adjusting indexed shifters is more complex than friction shifters, because the cable tension must be exact.
Cable tension is adjusted either by loosening the cable attachment bolt on the derailleur and retightening it, or by working the barrel adjusters. Study the cable line on your bike, from the derailleur to the shifter, and identify all the barrel adjusters. You may find barrel adjusters:
on the rear derailleur, except Sram
on the shifter, except most road shifters
on the frame, especially at the down tube cable housing stop on road bikes.
Cable tension is tightened by twisting a barrel adjuster counterclockwise, and loosened by twisting clockwise.
Whenever you disconnect the cable from a derailleur, you should return all barrel adjusters to their best position: tighten the barrel adjuster all the way clockwise, then loosen it one revolution counterclockwise. This gives you plenty of room to tighten the cable (the most frequent adjustment), but a little room to loosen it.
Never loosen a barrel adjuster anywhere close to as far as it goes. If you disconnect a barrel adjuster while it is under pressure from the cable, you are likely to strip the female threads.
To test whether your indexed shifter is working, disconnect the cable from the derailleur. Manipulate the shifter while holding the cable in your hand. If the cable moves for every shift, the shifter is working. It is extremely rare that a shifter fails to move the cable the right distance--it it moves at all it is very probably moving the right distance.
If the shifter will not click for every gear it should have, probably either the springs that return the pawls to their original position are gummed up by aging lubricant, the ratchets are stripped, or the internal parts are out of place. Very rarely can an index shifter be taken apart and repaired. Try it, but don't bet on it. Most shop mechanics won't try. However, if the problem is that the springs and pawls are gummed up with old, dry lube, the shifter can often be salvaged by soaking it with a degreaser or Teflon bike lube. Spray degreaser in every opening you can find in the shifter, push and pull the levers to work it in, and let it soak for a day or 2.
In our shop, if we have a miscreant indexed shifter, we will soak it with a degreaser. If that works, great. If it does not, we replace the shifter.
Lubricating Shifters
Shifters should be lightly lubricated with a bicycle specific Teflon lube, and then wiped clean. Squeeze a little lube in the openings, work the shifter to spread the lube, then wipe it off. How often to lube depends on your riding conditions and frequency, but don't overdo it. Lube buildup can be a problem.
Don't use a dry lube. Dry lube is basically alcohol and wax, and you will get a waxy buildup that will ruin your shifter.
Remove and Replace Shifters
Tools needed:
work stand
hex wrenches, usually 5mm
small Phillips head screw driver
cable and housing cutter
The procedure to remove and replace shifters:
1. Cut off the cable tip at the derailleur.
2. Detach the cable from the derailleur cable attachment bolt.
3. Memorize the cable and housing routing.
4. Remove the cable and housing from the frame.
5. On a road bike, remove the bar tape and bar end plug. On a flat bar bike, remove the grips. If you will reuse the grips, lift the edge of the grip with a screw driver and squirt some hair spray, WD 40, or water under the grip for lubrication, and pull it off.
6. If necessary, remove the brake lever from the bar.
7. Unbolt the old shifter, usually with a 5 mm hex wrench twisted counterclockwise, and remove it.
8. Install the new shifter. New shifters usually come with the cable already inserted. You can simply run the new cable through your old housing, but lube the housing first.
9. Install the cable and housing. Remember to install a cable tip.
10. Follow the same final procedures for installing a new rear derailleur or front derailleur.
Derailleur Cable and Housing
Poor shifting is often caused by bad derailleur cable or housing.
Inspect the cable line. Replace any cable that is rusted, that is frayed at the end, or that has a kink that interferes with its movement.
Inspect the housing. Replace any housing that is rusted, that the plastic covering is frayed, or that has a kink.
Use good quality cable. It is ridiculous to use anything less than stainless steel cable. Teflon coated and other fancy cable are available, but it is questionable that anything works any better than stainless steel.
Always install a cable tip on the end of any cable. It's also way cool to solder the cable end.
Use a good quality housing. Derailleur cable housing is different from brake cable housing. Derailleur housing has supporting metal wires running the length of the housing, under the plastic coating. Brake housing has supporting metal wires circling the cable. Brake cable housing is not stiff enough longitudinally for derailleur purposes.
Derailleur cable housing comes in 4 mm and 5 mm exterior sizes. Some frames, especially road frames, require 4 mm housing.
At housing stops on the frame, use a ferrule over the end of the housing. If you don't, the support wires will pull out.
Fancy housing is available and is useful in nasty weather or if you have intractable problems with cable compression. Jagwire makes a nice compression resistant cable/housing set.
Cable Housing Lubrication
Lubricating cable and housing is simple.
1. Disconnect the cable from the derailleur.
2. Pull the housing from the housing stops on the frame.
3. Squirt some Teflon based lube in to the housing, and move the housing up and down the cable to spread the lube.
4. Replace the cable on the derailleur.
5. Install a cable tip.
Cable and Housing Replacement
The only hard part of replacing a derailleur cable is removing it from the shifter. Here are some tips:
1. On most friction shifters, you can see the end of the cable in the shifter in either the highest or lowest shifter setting. If you don't see it, shift until you find it. Just pull it out.
2. On most road indexed shifters, you access the cable end either from a slot on the outside of the shifter toward the front or from a slot accessible by depressing the brake lever and looking into the opening. Disconnect the cable from the derailleur and push on it, while exploring the little slot.
3. Most Shimano Rapidfire type shifters (push--pull shifters) have a small port that can be unscrewed with a small Phillips head screw driver. But on some, you must remove either the top or bottom plate. Look for the access port screw first--removing the top or bottom plate is a pain. (Those extremely tiny screws tend to get lost.)
4. Most Shimano twist type shifters have access under an easily removed, obvious plate on the top of the shifter near the cable entry.
5. Some Gripshift, especially old ones, must be disassembled to access the cable. Some models of Gripshift are a terrible pain to replace the cable, so try to avoid it.
6. Shifters usually need to be in either the highest or lowest gear to access the cable. If you have trouble locating the cable end, shift to another gear and look again.
To remove and replace a derailleur cable, simply take one out and put another back in. Unless the housing is damaged, it is often reasonable to keep the old housing. Remember to install a cable tip.
To replace the housing, use the original housing as a template to measure the new housing. Reroute the cable and housing the original way. You always want to use enough housing to allow free movement of the cable through it, and to allow the handlebar full range of motion without moving the cable. But you don't want to use much more housing than is necessary, because it will make your shifting less responsive.
When cutting cable and housing, always use a bicycle specific cable cutter. Cable cutters from the hardware store are not fine enough tools--they will crush the cable and housing.
Shifter Compatibility
Friction shifters are compatible with almost any drivetrain. The only issue is whether the shifter has enough range of motion to move the derailleur over all the gears, and this is only a question on more expensive bikes. It will not be a problem on 5 gear cogsets.
Front indexed shifters are specifically designed for either 2 or 3 gears.
Rear indexed shifters are designed to work on a specific number of gears, from 5 to 10.
Although it is best to replace a shifter with the same brand as the derailleur, many shifters will work with different brand derailleurs. Sram and Shimano components are often mixed. However, some Sram shifters have a 1:1 ratio and only work with the corresponding Sram derailleur.
When buying a new shifter, it is best to ask if it is compatible with your specific derailleur and cogset or crankset.