Adjusting Your Bike’s Gears (Derailleur Indexing)

Tools Needed: A bicycle multi-tool or a set of hex (Allen) keys (commonly 4mm or 5mm for derailleur clamp bolt), a Phillips or flat screwdriver (for derailleur limit screws), and a bike stand or helper to lift the wheel. Patience and a good eye are also important!

Step-by-Step Guide:

1. Diagnose the Issue: If your gears are slipping, skipping, or won’t shift into certain cogs, it’s likely your rear derailleur indexing is off​. First, make sure the drivetrain is clean and lubricated (a dirty or rusty chain can also cause poor shifting). Inspect the derailleur hanger (the small piece attaching the derailleur to the frame) – if it’s visibly bent, the bike may need the hanger straightened or replaced for proper shifting​. Also check that your shift cables are not frayed and move freely; cable stretch or drag can cause mis-shifts​.

2. Set High Gear Limit (H): Shift the chain to the highest gear in the back (smallest sprocket) and the big chainring in front​. From behind, the chain should run in a straight line from that smallest rear cog down through the derailleur pulleys​. Locate the derailleur’s two small limit screws, usually labeled “H” (high) and “L” (low). Turn the H-screw until the guide pulley (upper jockey wheel) sits directly below the smallest cog​. Turning the H-screw clockwise moves the pulley inward (left), and counter-clockwise moves it outward (right). The goal is alignment: no rubbing or outward slack – this prevents the chain from shifting past the smallest cog.

3. Attach Cable and Adjust Tension: If your cable was very loose, you may need to reset it. Shift to the smallest rear cog, and relax the shifter (no tension). Loosen the cable pinch bolt on the derailleur. Ensure the shifter’s barrel adjuster (the knurled knob where the cable enters) is turned about halfway out, so you have room to fine-tune either direction later​. Pull the cable snug (not super tight) and re-clamp it. Now shift up one gear to the second-smallest cog while turning the pedals. If the chain hesitates to climb up to the next cog, increase cable tension by turning the barrel adjuster counter-clockwise (a quarter-turn at a time)​. If it overshoots or seems eager to jump two cogs, decrease tension (turn barrel clockwise). Continue fine-tuning: each click of the shifter should cause the chain to cleanly move up or down one gear.

4. Set Low Gear Limit (L): Shift the chain to the lowest gear (largest sprocket in back) and the small chainring in front​. The guide pulley should line up under the largest rear cog. Adjust the L-screw to position it correctly. This screw stops the derailleur from moving too far inward (which could throw the chain into the spokes). As with H, turn L-screw gently until alignment is achieved. You want the chain to just make it onto the biggest cog without throwing the chain past it.

5. Check B-Screw (Chain Gap): Most modern derailleurs have a “B-tension” screw that controls the gap between the top pulley and the cogs. Shift to the largest rear cog and examine this gap. If the pulley is brushing the cogs or the chain is nearly binding, tighten the B-screw to create more space. If the gap is overly large (more than ~6-10 mm depending on derailleur spec), you can loosen the B-screw slightly. The ideal gap is typically ~5-6 mm for road derailleurs (check manufacturer spec if needed)​. Note: B-screw is usually pre-set and may not need adjustment unless a new derailleur or cassette size change.

6. Test All Gears: Shift through the full range of gears while turning the pedals. Each shift should be crisp. If some shifts hesitate, give a slight tweak to the barrel adjuster (usually 1/8 turn can make a difference). Often, once the first few gears are dialed in, the rest will follow suit​. Ensure the chain doesn’t try to go past the lowest or highest cog (if it does, readjust the limit screws). A properly indexed derailleur means one click = one gear change​.

7. Front Derailleur (if applicable): Many gravel/road bikes are 2x. If your front shifting is off, similar principles apply: adjust the limit screws for the front derailleur to prevent the chain falling off inside or outside, and set the cable tension so it shifts smoothly between the two (or three) rings. Front “L” limit should stop the cage just above the chain in the smallest ring (no chain rub), and “H” limit stops just shy of the chain in the big ring​. Adjust cable tension so shifts are quick without throwing the chain​. (Front indexing is usually simpler once set – most issues come from cable tension or limit screw misalignment.)

Common Mistakes: Mixing up the H and L screws – turning the wrong one can make things worse, so adjust only one at a time and re-check alignment​. Over-tensioning the cable – too tight and the derailleur may overshift or not drop back to smaller cogs​. Forgetting to check the derailleur hanger – if it’s bent, you might never get perfect shifting until it’s straightened​. Not pedaling during adjustments: remember, you must be turning the pedals whenever you shift gears or test alignment, otherwise the chain can’t move. Finally, be patient – making large twists on the barrel adjuster or screws can overshoot the correct setting; it’s usually a game of small tweaks. If you hear constant rattling in a gear, stop and fine-tune that barrel adjuster until it’s silent (rattling often means the derailleur is between indexed positions).

Quick Tips: Work in good light so you can eyeball the pulley-to-cog alignment from behind – a straight chain line in the highest and lowest gears is key​. If new to this, turn the barrel adjuster only a quarter-turn at a time, then test shift; small increments help zero in on the perfect tension​. It’s a good idea to check rear indexing after transporting your bike or a crash – a knock can misalign the hanger. If in doubt, a simple trick is to reset: shift to highest gear, undo the cable, and start fresh with steps above. And remember, indexing bike gears is a skill even beginners can learn – with practice, you’ll tune your drivetrain to “silent and smooth-running” in no time​!