Gears changing themselves

fastercyclist Posts: 396
edited March 2010 in Workshop
As title says. I've noticed it's often worst in certain sprockets on my rear cassette. In particular there's about 2-3 sprockets where it's guaranteed to happen. Sometimes it does a "full" gear change, stays there then changes back. Other time I hear a strange noise as it just engages a different gear then quickly goes back to where it was before.

It's really fatiguing me cycling in gears avoiding these ones (mainly because there in the middle of the cassette!) nevermind pissing me off like hell on climbs as my foot suddenly "flies through" its pedalling motion due to random gear change.

I personally got the rear mech on my hitlist of suspects causing this problem. Are these observations consistent with a bent rear mech ?

The other thing I wonder if it is if I need to adjust one of the screws in the rear mech perhaps?

I'd hate to bend the rear mech to try and get it "right" - whatever "right" is - only for it to of been a screw issue. Don't want to go to bike shop because I really cannot afford it (unemployed).

I have noticed that as I go into gears with more teeth on the cassette, the position of the rear mech vis-a-vis the downward vertical changes and it angles more towards my spokes. Is this normal?

Suspicion on the bent rear mech because I had to unbend it before when it got caught in spokes during a chain chew up (you really don't want to know).
The British Empire never died, it just moved to the Velodrome


  • Slow Downcp
    Slow Downcp Posts: 3,041
    Could be a worn chain, or more likely cassette. How many miles have you done on them?

    Could just be that the cable needs adjusting slightly.
    Carlsberg don't make cycle clothing, but if they did it would probably still not be as good as Assos
  • Most rear mechs have a third screw — called the B-tension or B-angle screw — which adjusts the clearance between the top jockey wheel and cassette sprockets. Using a Phillips screwdriver, adjust it so that the distance is 5-6mm. This ensures that when you put the chain on it’ll be close enough for quick shifting, yet clear enough that the shifts are still clean.

    Could this be it?

    I've done around 1500-2000 miles roughly.
    The British Empire never died, it just moved to the Velodrome
  • John.T
    John.T Posts: 3,698
    The 2 main culprits for this are dirty or worn cables, especially the rear outer bit, or a bent mech hanger. The rear outer cable can wear through the liner quite quickly unless it is kept clean and this causes drag on the cable. You will need to get the hanger checked properly by a good LBS. It may be an indexing problem but these tend to affect most gears.
    A wrongly adjusted B screw will either give slow gear changes or noise as the jockey rattles on the cassette. It will not give phantom changes.
    I assume you are on a road bike as this problem also occures with worn suspension bearings on MTBs.
  • keef66
    keef66 Posts: 13,123
    If you've stuffed the mech into the spokes I'd suspect a bent mech hanger at the very least. Have the LBS check the mech hanger alignment

    Failing that you need to replace the cable + housing.
  • oldwelshman
    oldwelshman Posts: 4,733
    I presume you have the correct tension on the gear cable as that is the most common cause of the problem you describe before doing some of the above.