You see shiny marks, cut plastic or metal dust where the chain touches the chain guard, the slider on the swingarm, the chain case or the frame, and you may hear a buzzing or scraping. A slider is meant to take light contact; the problem starts when the chain is too slack, the slider wears through, the guard is bent, or the chain runs crooked because the wheel is out of line or a sprocket is bent.
Is it safe to ride?
Ride with care. A chain cutting into a steel swingarm weakens it, and a chain running crooked can jump off.
Quick checks first
- Check the chain slack.
- Look at the slider near the swingarm pivot: is there plastic left?
- Look at the guard and chain case for dents and loose bolts.
- Compare the adjuster marks on both sides.
- Marks on both sides of the swingarm help to align the wheel, but they can be slightly off. Look along the chain from behind to see that it runs straight, or use an alignment tool.
Diagnose it step by step
1Is the chain slack above the specified range? (K-Light 125: 20 to 40 mm, bike upright.)
- Yes: a loose chain slaps the slider and guard. Check and adjust chain slack.
- No: go to step 2.
2Is the slider worn through, split, or missing, so the chain touches the swingarm metal?
- Yes: Check chain guides, sliders and guards. If the chain has cut a deep groove in the swingarm, have the swingarm inspected.
- No: go to step 3.
3Is the chain guard, chain case or a panel bent or loose so it touches the chain?
- Yes: straighten or refit it and tighten its bolts (Check chain guides, sliders and guards).
- No: go to step 4.
4Are the adjuster marks different on each side, or does the chain look crooked when you sight along it from behind?
- Yes: the wheel is out of line. Realign it: Check and adjust chain slack.
- No: go to step 5.
5Does the rear sprocket wobble as the wheel turns, or are its teeth worn on one side?
- Yes: Inspect sprockets.
- No: go to step 6.
6Does the swingarm or rear wheel move sideways when you push it?
- Yes: Check the swingarm pivot; see Swingarm moves sideways or has play.
- No: go to step 7.
7Was a new chain or sprocket fitted recently?
- Yes: a wider chain or a sprocket with a different offset can rub. Have the parts checked against the original sizes: Replace the chain and sprockets.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Chain too slack | Slack above range | Check and adjust chain slack |
| Worn or missing slider | Plastic gone, metal marked | Check chain guides, sliders and guards |
| Bent or loose guard | Guard touches chain | Check chain guides, sliders and guards |
| Wheel misaligned | Marks unequal | Check and adjust chain slack |
| Bent sprocket or swingarm play | Wobble or sideways movement | Inspect sprockets, Check the swingarm pivot |
| Wrong replacement parts | Started after new chain or sprocket | Replace the chain and sprockets |
Reference values (K-Light 125)
| Item | Value | Source |
|---|---|---|
| Chain slack, bike upright | 20 to 40 mm | K-Light 125 manual, PDF p. 61 |