Smooth Shifting by: Pete Dzirkalis
Even a new bike can shift poorly if it's not set up correctly. I'm going to limit this article to rear shifting. I'll follow up with front shifting later.
After assembling thousands of bikes over the years, I can tell you that many new bikes often take just as much of a beating in getting to the shop as they do out on the trail.
Oftentimes, derailleur hangers can be bent, cables can be kinked, and chains may have tight links. So whether you have a shiny new wonderbike, or a trusty old steed with lots of miles, there are some things you should look for before making a diagnosis on why the bike isn't shifting the way it should.
Lets Get Started
First off, the bike should be clean so you can see what's going on. Sometimes out in the field, this is impossible, but a clean bike makes it easier to see problems. Also, a workstand to hold the bike up securely is a must. Once you use one, you'll wonder how you ever got by without one. A few basic tools like some hex wrenches, an adjustable wrench, a splined cassette tool, chain lube, and a rag may be needed.
Let's look closer at the rear shifting of the bike. Get the bike up on workstand and look things over. Is the chain rusty? Do any of the teeth on the gears look like they have damage? Do the derailleur pulleys look like they are parallel with the gears? Grab the rear cogs with your fingers and give them a wiggle. Does anything feel loose? After this quick inspection, you can check how the bike shifts while "pedaling" it on the workstand.
Turn the pedal with your left hand and shift through the gears with your right hand. Does the chain move from gear to gear as you push the shifter? Does the shifter feel hard to push? Does the chain skip around? Maybe all of the above? An experienced mechanic can take all this in at a glance, but if you're new to wrenching, take your time here and look and listen for things that don't seem right.
Now That We Know Whats Going On
Ok, now that we have a basic idea of the condition of the bike, we can start tuning. The first thing to check is the rear derailleur alignment. Look at the bike from the rear. The derailleur pulleys should be parallel with the gears above. What if they're not? This is where that clunky old adjustable wrench can help. There is a shop tool to align derailleur hangers, but most people probably don't have the luxury of having one in their toolbox. If the derailleur is bent in or out of parallel from the gears, you may need to align the hanger (the part of the frame where the derailleur is attached).
To do this, unbolt the derailleur from the frame and use the adjustable wrench to bend the hanger back into alignment. Keep the wheel in the frame if possible for extra support when you do this, and use a gentle hand. Re-attach the rear derailleur and see how you did. Hopefully everything is parallel--if not--try again until you get it close. If the hanger is severely bent, it may be easier to remove it from the bike and put it in a sturdy vice to bend it.
If you break the hanger, you'll have to buck up and buy a new one. They are generally specific to brands and some models and cost about $20 or so. If the hanger is straight, and you remount the derailleur and it's still off--the derailleur might be bent as well. Sometimes you can compensate for a bent derailleur, by bending the hanger one way or another, but you may have to replace the rear derailleur. Your average Shimano Deore or Sram X7 derailleur is in the $50 range, so it may be worth trying a little bending before you buy a new one.
Need to check the Cogs
How about those cogs? The tightness of the rear cassette (the cluster of cogs) on a new bike is sometimes overlooked by an assembler, or they can work loose over time. Remove the rear wheel and quick release and fit the a splined cassette tool (a hex shaped chunk of steel with a round toothed pattern on one side) into the lockring on the end of the cassette, hold it in place by re-installing your quick release through it, set your adjustable wrench on the splined tool, and give it a turn clockwise to make sure the gears are tight on the wheel. Loose cogs can really throw off the shifting.
Now that the derailleur is running true and the gears are tight, we can check out the high-limit adjustment. I also like to call this the starting point of the derailleur. Pedal the bike and shift into the smallest cog on the rear--don't worry if it doesn't go all the way down into that gear for now. Unbolt the cable from the derailleur--this is usually done with a 5mm hex. Pedal the bike again. It may drop down into that gear now. If it doesn't go into the small gear, you may need to loosen (turn counter-clockwise) the high limit screw (the screw marked H) slightly and pedal the bike until it drops onto the little cog.
If the chain is already on the small cog, your high adjustment may be ok, but look at the alignment from the back of the bike. If the pulleys are to the right of the small cog--or the chain is wanting to jump off toward the frame, you may need to tighten the H screw slightly by turning it clockwise. The pulleys should be aligned with the small cog, or just a shade to the right of it to get the right starting point.
You can check your low (L screw) adjustment now as well while the cable is detached. Pedal the bike and push on the parallelogram (linkage) of the derailleur with your fingers to manually move the derailleur and chain toward the larger cogs. You should be able to get the derailleur all the way up to the big cog. If you can't--loosen (turn counter-clockwise) the L screw and try again. If the chain goes over the last cog--tighten (turn clockwise) the L screw until the chain does not go into the spokes.
Re-attaching the Cable
Before you re-attach the cable, turn your barrel adjusters (the thing that looks like a mini black faucet handle where the cable meets the derailleur and shifter) in-(clockwise) all the way and then back them out a turn or two to leave a little room for adjustment later. Most Sram derailleurs and the new Shimano Shadow derailleurs don't have a barrel adjuster, so you will have to do this up at the shifter mechanism on the handlebar. Make sure the inner cable is pulled tight and that the housing is seated before re-attaching it with the 5mm hex.
Pedal the bike and try shifting to a larger cog one at a time and shift back down. If the derailleur is not moving up to the bigger cogs, try turning your barrel adjuster(s) out (counter-clockwise) a quarter or half turn at a time and pedal the bike again until the chain moves up to the next cog. If the chain rubs on the next larger cog, or moves past the next cog with only one press of the shifter, you may have the cable too tight. In that case, turn the adjuster back in (clockwise) until the chain rides smoothly on the cog after a shift.
If the chain is shifting up smoothly to a larger cog, but not returning to the small cogs, your cable is sticky and not allowing the spring in the derailleur to return properly. This can also be caused by sticky or corroded pivots in the derailleur linkage, or a buildup of gunk on any moving parts of the derailleur. In this case, the cable and derailleur may need to be cleaned and lubed or replaced. Starting with new or well-cleaned and lubed cables makes derailleur tuning much easier. The effort to push the shifter is also much lighter with properly fitted-and-lubed cables as well.
The Final Check
For a final check, shift the chain up to the biggest cog in the rear gear cluster. If cable tension is set right, the chain shouldn't go over into the spokes. If the chain goes into the spokes, your cable tension may be too tight, so you may need to turn the barrel adjuster in (clockwise) a touch. Most shifters have a little extra play or an extra click past the last gear. When the chain is in the biggest cog and you press on the shifter slightly, it should feel like the shift lever shouldn't move any more. If the shifter still moves, and/or the chain goes into the spokes, your low-limit screw (L) may be to loose. tighten it gently until you feel a hint of resistance. This should keep the chain from going over the big cog into the spokes.
Make sure you lube the chain and all the derailleur pivot points and pulleys. Also pedal the bike gently to see if there are any tight links in the chain--apply a little extra lube to these spots and try to work them back and forth to free up any stickiness. Make sure the teeth on the front chainrings are not bent, too.
A test ride will reveal the truth. If the drivetrain is solid and well-lubed, you should be fine. If you are getting skipping under a load without shifting the bike (like when climbing up hill) this may be a sign of drivetrain wear, and no amount of derailleur adjustment will cure that. Check your adjustments again, but if you still get skipping you may need a new chain and gears.
I hope this helps. Happy wrenching!