It starts with a click. Then a crunch. You’re grinding up a 6% grade, lungs screaming, and you push the lever for that easier gear, but instead of a smooth transition, you get a rhythmic clack-clack-clack that makes you want to hurl your bike into the nearest drainage ditch. Honestly, nothing ruins a ride faster than a drivetrain that refuses to cooperate. Most people assume their derailleur is broken or that they need a $200 trip to the local shop, but usually, it's just physics acting up.
Learning how to fix bicycle gears isn't some dark art reserved for grizzled mechanics in grease-stained aprons. It’s mostly about managing tension. Think of your gear system like a tug-of-war between a high-tension steel cable and a stiff spring. If the cable is too loose, the spring wins. If the cable is too tight, the shifter can't let go. You're just the mediator.
The Ghost in the Machine: Cable Stretch and Indexing
Most shifting issues come down to "indexing." This is the alignment between your shifter’s clicks and the actual movement of the derailleur. If you've got a brand-new bike or just replaced your cables, you'll notice the shifting goes wonky after about 100 miles. People call this "cable stretch." Technically, the steel isn't stretching; the housing ferrules are just settling into the frame stops and the cable weave is tightening.
To fix this, look at where the cable enters the rear derailleur. There’s a plastic or metal knob there called a barrel adjuster. It’s the hero of the story. If your chain is hesitant to move into a larger cog (up the cassette toward the spokes), you need more tension. Turn the barrel adjuster counter-clockwise. Just a quarter turn. Seriously, don't go cranking it like a garden hose. If the chain won't drop down to the smaller cogs, you have too much tension. Turn it clockwise.
I’ve seen riders spend hours messing with limit screws when all they needed was a half-turn of the barrel adjuster. Don't be that person. Limit screws—those two little screws marked 'H' and 'L'—rarely ever go out of adjustment once they're set. If you start turning them without knowing why, you're likely to send your chain flying into your spokes, which is a great way to turn a $10 fix into a $400 wheel rebuild.
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Why Your Derailleur Hanger is Probably Crooked
Here is a secret: if you’ve dropped your bike on its right side, or if it fell over in the garage, your gears are likely toast until you fix the hanger. The derailleur hanger is a small, intentional "weak point" made of soft aluminum that connects your expensive derailleur to your frame. It's designed to bend or snap so your frame doesn't.
If that hanger is even two millimeters out of alignment, you can spend a lifetime learning how to fix bicycle gears and you will still fail. The shifting will be perfect in the middle of the cassette but jumpy at the top or bottom. You can check this by standing behind the bike and looking at the "pulley wheels" (the two small gears on the derailleur arm). They should be perfectly vertical and parallel to the cogs. If they’re slanted inward toward the wheel, you’ve got a bent hanger.
Professional shops use a tool called a DAG (Derailleur Alignment Gauge). It’s a long metal rod that threads into the hanger and measures the distance to the rim at four points. If you're a DIYer, you can sometimes "eyeball" it with a large Allen key, but honestly, just buy the hanger alignment tool if you plan on riding for the next decade. It pays for itself in two uses.
The Grime Factor: Cleanliness is Next to Godliness
You cannot tune a dirty drivetrain. Period.
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If your chain is covered in a thick, black paste of grit and old lube, it won't sit properly on the gear teeth. This creates "ghost shifting" where the chain hops between gears because it's literally too fat with grease to fit. Before you touch a single adjustment screw, get a degreaser and a stiff brush. Scrub that cassette until the silver shines. Wipe the chain until it doesn't leave a black streak on your hand.
- Degrease: Use something like Citrus-based cleaner or simple dish soap.
- Dry: This is the step everyone skips. A wet chain won't hold lube.
- Lube: One drop per roller. Not a waterfall of oil.
- Wipe: After ten minutes, wipe the excess off. The lube belongs inside the rollers, not on the outside where it attracts dirt.
High and Low: The Truth About Limit Screws
Limit screws are the "fences" of your gear system. They stop the derailleur from moving too far left or too far right.
- The High Limit (H): This stops the chain from falling off the smallest cog and grinding into the frame. With the cable disconnected, the derailleur's natural spring tension should hold it directly under the smallest cog. If it's noisy, turn the H screw.
- The Low Limit (L): This is the "save your life" screw. It stops the chain from overshifting past the largest cog and into your spokes. Shift into your biggest gear. Push the derailleur body with your thumb toward the spokes. If it can move past the cog, tighten that L screw immediately.
Park Tool’s Calvin Jones, basically the patron saint of bike mechanics, always emphasizes that these screws are "positional," not "indexed." They don't care what gear you're in; they only care about the absolute physical boundaries of the derailleur's travel.
The "B-Gap" Mystery
Ever felt a weird vibration when you're in your easiest gear? That’s likely your B-limit screw. This screw adjusts the height of the derailleur's upper pulley wheel relative to the cassette. If the pulley is too close, it’ll physically rub against the gear teeth. If it's too far away, the shifting will feel slow and "lazy" because there’s too much unsupported chain between the derailleur and the cog.
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Modern 1x11 and 1x12 systems (like SRAM Eagle or Shimano XT) are incredibly sensitive to this. Most come with a little plastic gauge to set the gap. Usually, you want about 5-15mm of space. It’s a small detail, but it’s the difference between a bike that shifts like a Swiss watch and one that feels like a tractor.
When Parts Just Give Up
Sometimes, you can't fix it. Chains wear out. They "stretch" (again, the pins and rollers wear down, making the chain longer). Use a chain checker tool. If your chain is at 0.75% wear, replace it. If you wait until 1.0%, you've likely ruined your cassette too, because a worn chain will eat into the teeth of the gears, reshaping them into "shark fins."
Also, check your housing. If there’s a kink in the cable housing or if the inner wire is frayed, no amount of barrel adjustment will help. Friction is the enemy. If your shifts feel "heavy" at the lever, pull the cable out. If it looks rusty or feels gritty, toss it. A new cable and housing set costs $20 and makes an old bike feel brand new.
Actionable Steps for a Perfect Tune-Up
Stop guessing and start following a sequence. Mechanics use a sequence because it eliminates variables.
- Clean and Inspect: Scrub the drivetrain and check for a bent derailleur hanger or frayed cables.
- Reset Tension: Shift to the smallest cog. Turn your barrel adjuster all the way in (clockwise), then back it out two full turns to give yourself room to move in both directions.
- Set Limits: Ensure the H and L screws are keeping the chain on the cassette without it falling off the edges.
- Index: Click the shifter once. If the chain doesn't move to the next gear, add tension (counter-clockwise) until it jumps up perfectly.
- Stress Test: Run through the entire range while pedaling. If it hesitates going up, add a click of tension. If it hesitates going down, remove a click.
Fixing your own gears gives you a weird sense of power. You're no longer at the mercy of a shop's two-week turnaround time. You can head out for a Saturday morning ride knowing that if things start clicking, you've got the muscle memory to reach down, tweak a barrel adjuster, and get back to the silence of a well-oiled machine.
Now, go put your bike in a work stand—or just hang it by the seat from a sturdy tree branch—and start pedaling. Listen to what the metal is telling you. If it's grinding, it's out of alignment. If it's silent, you've won.