Troubleshooting by Symptom

Most common cause first. 1. Worn chain stretching past wear limit → measure chain (3.1). A chain at 0.5%+ skips on cogs that have worn to match the old chain. Replace chain; cassette may follow. 2. Bent derailleur hanger → check alignment (3.16). Even a 2 mm deviation makes some gears unindexable. Single most underdiagnosed cause. 3. Cable tension out of spec → re-index rear derailleur (3.15 step 7+). 4. Worn jockey wheels → derailleur cage pulleys eventually develop sharp teeth. Replace as a pair. 5. Worn cassette only on a few cogs → if you ride mostly in 2–3 gears, those wear faster. Visible "shark fin" tooth shape confirms.

1. Front derailleur limits set wrong → reset H/L limits (3.14). 2. Worn chainring teeth (especially small ring on 2x setups) → visible tooth wear; replace ring. 3. Stretched chain (1x setups: chain that's too long jumps off in rough terrain) → measure (3.1) and re-size. 4. Missing/worn chain retention → narrow-wide chainring teeth wear smooth on 1x; consider chain guide.

1. Dirty drivetrain → drivetrain deep clean (12.3). 2. Dry chain → lubricate (12.4). 3. Worn chain rolling on worn cassette → measure chain; replace as a set if needed. 4. Damaged BB bearings → with the chain off the front ring, spin the cranks; should be silky and silent. Grinding/notching = BB needs replacement (3.12/3.13). 5. Crank arm loose → check pinch bolts (Hollowtech) or torque cap (5–7 Nm preload, then bolts to spec).

This always indicates a torque-loaded interface. 1. Loose chainring bolts → torque to spec (typically 8–10 Nm; check spec). 2. Loose pedal → re-grease and torque (35 Nm typical). Both pedals. 3. Loose crank arm → check Hollowtech pinch bolts, or square taper bolt. 4. Cleat loose on shoe → check cleat bolts on both shoes. 5. Saddle rail clamp loose → check seatpost head bolts.

1. Cable contamination/corrosion → especially after winter. Replace cable and housing (3.17). 2. Cable end frayed inside housing → visible at pinch bolt; replace cable. 3. Crushed housing end → cut a fresh end with proper cable cutter. 4. Worn shifter mechanism (mechanical only) → replace shifter unit.

1. Pad contamination (degreaser, sealant, finger oil, chain lube overspray) → clean rotor with isopropyl alcohol; if pads are contaminated, replace them. Once contaminated, pads usually can't be saved. 2. Glazed pads → light sand on fine sandpaper, re-bed (4.2). New pads also need bedding. 3. Toed-in incorrectly (rim brakes) → set up so the front of the pad contacts the rim 1 mm before the rear. 4. Loose caliper mounting bolts → re-align caliper (4.7), torque to spec. 5. Bent rotor causing intermittent contact → true rotor (4.3).

1. Air in hydraulic system → bleed the brake (4.4 Shimano / 4.5 SRAM). 2. Worn pads → measure pad thickness; replace at or below 1 mm as a conservative app threshold, or earlier/later according to the brake manufacturer's stated minimum (4.1). 3. Pistons sticking → push pistons back, clean exposed area with isopropyl, work them in/out by lever pumping with the pad spreader in. 4. Master cylinder seal failure (rare) → lever rebuild kit or replacement.

1. Caliper not centered over rotor → align (4.7). 2. Bent rotor → true (4.3). 3. Pads stuck out (after fitting new pads without pushing pistons fully back) → remove pads, push pistons back with pad spreader, refit. 4. Wheel not seated in dropouts → loosen axle, settle wheel, re-torque. 5. Loose hub bearings allowing wheel to wobble laterally → check hub adjustment.

Stop riding immediately. Walk the bike home. 1. Hose has leaked dry → check for fluid trace anywhere on the hose, lever, or caliper. Repair source, then bleed. 2. Pad fully worn through to backing plate → caliper now pushes piston past the seal; needs rebuild. Replace pads first; re-bleed. 3. Total cable failure (mechanical brakes) → replace cable.

1. Bent rotor → true (4.3). 2. Heat-warped rotor (after a long descent) → cools down, may stay warped; true or replace. 3. Inconsistent pad transfer (uneven black film on rotor) → re-bed (4.2) after cleaning rotor.

1. Small puncture → on tubeless: top up sealant (5.5), spin to redistribute, often self-heals. On tubed: find leak by listening or submerging tube (5.3). 2. Valve core loose → tighten with valve core tool. Common on tubeless setups. 3. Bead seating slightly off (tubeless) → unseat, clean bead, re-seat with proper inflator. 4. Rim tape failure (tubeless) → leaks past spoke holes; re-tape rim (5.4 prep stage). 5. Pinhole in tube at the seam → replace tube.

1. Out of true → lateral truing (5.7). 2. Loose hub bearings → cone wrench adjustment (5.10) or cartridge bearing replacement (5.11). 3. Loose axle / quick release → snug to spec. 4. Cracked rim or bent spokes → inspect closely; severely damaged wheels should be retired.

1. Out of round (radial truing needed) → radial truing (5.8). 2. Tire mounted unevenly → deflate, work tire bead around the rim, re-inflate. 3. Damaged rim (dent from impact) → may not be repairable; rim or wheel replacement.

1. Bead not seated properly → unseat tire, clean bead and rim seat, use proper inflator (5.4). 2. Tape damaged (especially after recent tire swaps) → re-tape (5.4 prep). 3. Major sidewall cut → plug (5.6) or new tire. 4. Sealant dried out → too old; clean out and replace (5.5).

1. Wrong sag setting → measure and adjust (8.1). Most common cause. 2. Compression damping wrong → start at manufacturer's middle setting, adjust 2 clicks at a time. 3. Stiction from dirty seals → at minimum wipe stanchions clean before each ride; if persistent, lower leg service (8.2). 4. Wrong air pressure for rider weight → start from manufacturer's chart for your weight, then sag-tune.

1. Dirty stanchions / dust seals → clean and lubricate stanchions; consider lower leg service (8.2). 2. Old oil / dry foam rings → lower leg service (8.2). 3. Damaged stanchion coating → if you see scratches in the anodizing, the fork needs servicing professionally.

1. Insufficient air pressure → increase 5–10 PSI per iteration. 2. No bottom-out token / volume spacer → add one (air spring service 8.3). 3. Damper failure → if changes to air pressure don't help, damper may need professional rebuild.

1. Rebound too fast → slow it 2 clicks and re-test. 2. Compression too firm → open compression damping. 3. Tire pressure too high → lower tire PSI before chasing suspension settings.

1. Loose headset preload → tighten top cap until no play, then secure stem (6.1 step 5). 2. Worn headset bearings → replace bearings (6.3). 3. Loose stem clamp on the steerer → torque to spec. 4. Cracked headset cup or frame → inspect carefully; serious if confirmed.

1. Indexed bearings (worn from impact loading) → bearing replacement (6.3). 2. Contamination / rust in bearings → new bearings. 3. Misaligned crown race → re-seat (6.4).

1. Dry interface between bar and stem → degrease, apply carbon paste (carbon) or light grease (alu), re-torque. 2. Stem clamp bolts unevenly torqued → loosen all, retorque in criss-cross pattern. 3. Steerer-stem interface dry → strip, clean, light grease (NOT on carbon steerer — paste only).

1. Chain too long → check sizing (3.5). 2. Worn jockey wheels → replace. 3. Insufficient B-tension → adjust (3.15 step 5).

1. B-tension too tight → loosen by 1/4 turn at a time. 2. Chain too short for the cog selection → re-size.

1. Worn or missing chainstay protector → replace neoprene/rubber protector. 2. Chain too long → re-size. 3. Clutch derailleur disengaged (if you have a clutch RD) → check clutch lever / button position. ---