关于推荐第十届中国公路青年科技奖候选人的通知

Question

My Specialised Hardrock seems to have developed a problem with the rear derailleur. In gear 7/8 out of 24 (low gear on front/ highest on rear!) the chain is catching the pulley. When I push the derailleur with my hands it goes to what is clearly the correct position by the notches which stop it moving further but when I let it go it moves further back, the tension appears to be fine within the axle. Is this likely repairable?? Don’t want to remove as other gears still working! Bike is only used for going out with kids on cycles really.

Answer 1

What You're Describing

You’re in the smallest chainring up front and one of the smallest cogs on the back, which puts the rear derailleur in its most extended position. In that setup, the derailleur’s B-knuckle (the pivot by the mounting bolt) is what controls how far the derailleur cage sits from the cassette.

If you’re saying the jockey wheel (pulley) is catching the cassette or chain, and the derailleur "springs back too far" when you let it go, then:

The B-tension screw might be set too shallow (or has backed out).

The B-pivot spring inside the derailleur could be tired or partially seized.

There’s a chance the hanger is slightly bent, causing weird alignment under load.

What to Check (Without Removing Anything)

Look for the B-tension screw — that’s the little screw on the derailleur body that rests against the derailleur hanger tab. It adjusts how close the top jockey wheel sits to the cassette.

Get the bike in the workstand or flip it upside down.

Shift into the smallest front and smallest rear combo.

Turn the B-screw clockwise to increase the gap between the upper pulley and the cassette cog — usually you want ~5–6mm clearance.

Even half a turn can make a big difference.

Manually check spring tension:

If the derailleur springs back too far when you let go, the internal spring might be sticking or worn — but if it still shifts fine across most gears, it’s probably not dead yet.

You can give the derailleur a little spray with a light lube (like GT85 or light oil) around the B-knuckle and pivots to free up any stickiness. Avoid soaking it — just a few targeted drops.

Check alignment of the derailleur cage:

Look from the back — is the cage sitting square with the cassette?

If it’s angled or looks slightly twisted, you might have a bent derailleur hanger, which is super common after even a light knock. That can cause the cage to sit too close and rub under tension.

A bent hanger would need to be realigned with a proper tool or replaced — but this isn't something you need to do unless the shifting is noticeably off or the cage is way out of line.

Is it Repairable?

Yes — most likely it’s just an adjustment, not a failure. The derailleur itself is probably fine, especially since you said all other gears are working. These mid-level Shimano derailleurs can last years with minimal fuss.

If the B-screw is fully wound in and you're still having issues, you can try adding a slightly longer screw (they're just M4 machine screws usually) to give yourself more range — a trick many mechanics use when dealing with older or stretched springs.

Final Advice

Try the B-screw adjustment first — low-risk and often solves the issue.

Lube the pivots while you're there — just don’t overdo it.

Keep an eye on chain tension and pulley wear, but if the bike shifts well otherwise, it’s not urgent.

No need to remove the derailleur if everything else is working — sounds like a minor tune-up job.

You’re doing the right thing by catching it early. Keep it clean, adjust that screw, and you’ll probably have it sorted in under 10 minutes.