Gear problem

[mike killay]

The rear derailleur mechanism on my bike occasionally seems to want to change down momentarily from top(8) to 7th gear and then goes back to 8th with a bang.
I have done all the adjustments with no luck.
What I do notice is that the mechanism is not in line at all with the chain. The chain line is true fore and aft except for the slight 'off' angle when in top or bottom. The mechanism however seems to point to a point about 6 inches to the left of the front wheel.
Probably it has had a bash at some time. I could try twisting it back into line, but as it is a cheap Shimano Acera, I thought of replacing it with something better, because the problem might just be wear and not damage.
Any advice?
Mike

 

[Streethawk]

Before you blame the derailleur, check if the derailleur hanger on the frame has been bent. These are designed to bend if they take a knock, to save writing off your frame.

The other possibility is that your cable tension is just a little too high, try slackening it off a little at the adjuster and see how it goes. A too tight cable will prevent the derailleur from getting to it's end point and could well cause the issue you describe.

 

[D C]

I would replace it as trying to get one of these right when it's worn drives you mad.
I've just binned one which had a weak spring and it just couldn't be adjusted to change correctly.
Careful if you try to twist it that you don't bend the frame where it's connected.

 

[mike killay]

I have tried all adjustments.
I am reluctant to try bending with no real idea as to how far to bend.
I think that the Acera is the cheapest so probably wears out quickly

 

[awol]

I had a similar problem on my newly built rear wheel's first test ride, everything looked fine. To cure it I packed the frame out 1.5mm that side with a washer and problem solved.

 

[RobF]

Park Tool's DAG2 is a derailleur alignment gauge.

Finding someone who has one and knows how to use it might be difficult.

Park Tool Co. » DAG-2 : Derailleur Hanger Alignment Gauge : Frame & Fork Tools