01
Front · inside
Between the small/only ring and frame
Inspect contact first. On multi-ring systems, cage position, chainline, cable state, and the exact inner-limit procedure are separate checks.
BBR · Location-first drivetrain diagnostic
Bike chain keeps falling off? Map where it lands first
A chain inside the front ring, behind the cassette, outside a derailleur cage, or physically separated is not the same repair. Record the event before reseating, adjusting, or buying.
A falling chain is an incident to route, not a single diagnosis. Start with the moment it happens, then work through the drivetrain, limit, alignment, and wear checks in order.
Chain Exit Map
0 / 4 observations
Map where it left before you turn anything
Four non-identifying observations separate a clean drop, jam, spoke-side event, broken chain, front shift, rear limit, tension/chainline, and compatibility path. No universal screw direction or turn count is generated.
Map incomplete
Answer all four observations
The result will not infer a screw, part, chain guide, quick link, or test ride from a symptom alone.
Landing position
“It fell off” hides the evidence that determines limits, tension, compatibility, damage, or connector work.
Six exits, six different first decisions
02
Front · outside
Beyond the large/only ring
Do not assume the opposite screw and repeat. Confirm the ring, chain, cage or 1x retention system before adjustment.
03
Rear · spoke side
Beyond the largest rear sprocket
Stop the wheel. Inspect spokes, hanger, cage, chain, wheel seating, and the exact low-limit state before another shift.
04
Rear · frame side
Beyond the smallest rear sprocket
Inspect frame contact, wheel/axle seating, hanger/cage alignment, outer limit, cable state, and model-specific procedure.
05
Pulley / cage
Outside the intended derailleur path
Treat a bent, loose, damaged, or misrouted cage/hanger as a stop condition—not an indexing experiment.
06
Physically separated
Open connector, displaced pin, cracked/twisted plate
Retain the parts. Identify the chain family, connector, reuse rule, length, and joining procedure before choosing a tool.
Architecture first
A shared symptom does not create a shared adjustment procedure.
Do not transfer derailleur logic across four systems
| Layout | Controls | Boundary |
|---|---|---|
| Single-speed / internal gear | Wheel alignment, chain tension through a full rotation, tight spots, sprocket/chainring alignment, axle or tensioner procedure | Rear-derailleur limit and clutch instructions do not apply. |
| 1x derailleur | Chain length, clutch/tension, chainline, ring profile, chain compatibility, wear, freehub behavior, existing retention | A guide or narrow-wide ring is not the first diagnosis. |
| 2x / 3x | Front cage position, mount, cable state, chainline, chain/ring condition, inner/outer limits plus the rear system | Front-inside and front-outside drops are different observations. |
| E-bike | Exact power-down/service mode, motor-area contact, chain load rating, chainline, sensor and drive-system instructions | Motor power is not a workshop test method. |
Stop conditions
A chain event can damage more than the chain
- The chain is jammed against a frame, chainring, guard, motor, cassette, wheel, or spokes.
- A plate, pin, rivet, connector, tooth, pulley, cage, hanger, spoke, axle, dropout, or frame area is damaged or displaced.
- The event followed a crash, fall, strike, visibly bent part, or new unexplained loss of drive.
- The chain repeatedly enters the same unsafe location or the observation requires road speed, standing load, or motor power.
- A rear spoke-side event occurred; do not deliberately reproduce the shift near the spokes.
- The drivetrain, chain, connector, component change, or exact next procedure cannot be identified.
current specifications
The exact component record still wins
- Park Tool rear-derailleur adjustment — Hanger visual gate, limit purpose and model-aware sequence.
- Park Tool front-derailleur adjustment — Height/rotation before limit and cable state; not a universal screw turn.
- Park Tool front advanced troubleshooting — Chainline, cage position and recurring shift failures.
- Park Tool CT-5 current product record — Supported/excluded chain categories and peening boundary.
- K-EDGE current chain-catcher record — Mount, hardware and current compatibility language.
This is a practical decision guide. The site did not reproduce a chain drop, adjust a derailleur, install a catcher, repair a broken chain, test a tool, or road-test a result.