Bicycle Mechanics - Rear derailer has a mind of its own

It seems as if my rear derailer on my Trek 2200 has a mind of its own. It seems to shift into any gear under any pressure from major pedaling. It willl be fine for a while and then all of a sudden it will just shift to any other gear. I tried just about everything but to no avail. The only other thing I can think of is that the chain may not be right. Any other suggestions?

When I am faced with such enigmas I try to start from scratch. Disconnect the shift cable, take off the chain, get my book and start with step one. Often this will finally correct the problem. I find that after a few months of occasional tweaking, sometimes the various adjustments seem to be working against each other. Starting from scratch seems to get things back in line. Make sure the derailleur is clean and all pivots lubed and moving freely. Which way does it seem to shift at these times, to a smaller or larger cog? Also check the cogs you use most. IF the teeth on those cogs are worn, say half the width of teeth on less used cogs, you may need to replace some cogs. Look at a number of the teeth as some teeth are narrower than others by design.
Regards,
Raymond

Your problem sounds like it is shifter related.

The problem you described was common with the old friction shifters on 10-speeds when the friction cups we toast or if the tightner bolt came loose.

Did you lubricate your shifter with, for example, WD-40? This could cause it to slip and give you troubles.

Rainman is correct to start a methodical check, but start with the shifters and work your way down the cable to see all the connections.

If that's not it, check for a bad chain link. If you have a bad link that is froze up, it will cause the chain to skip over some gears.

Originally posted by RainmanP
Also check the cogs you use most. IF the teeth on those cogs are worn, say half the width of teeth on less used cogs, you may need to replace some cogs. Look at a number of the teeth as some teeth are narrower than others by design.

I recently had to replace my chainring because of worn teeth. At first I couldn't figure out why it was slipping out of gear under pressure. I had never worn a chainring out, so I didn't know that was even a possibility.

As Rainman states, I would also say that there is a good chance that your cogs need a looking at.

The chain would be my first suspect, other than that it could be the derailleur's been knocked very slightly out of alignment with the cassette (I don't know whether a slightly out of true wheel could put the cassette slightly out of alignment with a perfectly straight derailleur....). The real long shot would be your cables (I recall reading about someone who shifted when he went round corners and stretched the cable at the bars).

Richard

It also sounds like the pivots are wearing out on your rear der. With the R der in one of the middle cogs in back push in toward the spokes and pull out away. Sometimes the slop is so bad the der cannot consistantly go to where the cable/shifter is telling it to go.

As an after thought, check your jockey pulley that the bolt isn't loose (this pulley does float, make sure you're not losing it completely). I hope the problem is a simple fix.

Thanks all for the advice. This weekend I will start from scratch and do what Rainman says. I really do not think it is the shifter but will give it a look. I am hoping that it is just something loose or something kinked in the wrong direction. I will give the derailer a good lubin and check the cables and clean and lube the chain too. Hopefulley this will fix the problem.

P.S., It seems to happen mostly when I am in the lower gears on the rear cassette. It will jump down most of the time but every once in a while it will jump up.

Thanks for the help!

The big question is does the derailleur make a better decision about gearing than the rider ;)

Good luck in tracking it down

Richard

If you decide to strip your dearaileur, keep the top and bottom pully separate, the top one has float.

Originally posted by MichaelW
If you decide to strip your dearaileur, keep the top and bottom pully separate, the top one has float.

I wonder if that's part of the problem, Michael. The "float" is needed to make indexing work. Could not a worn jockey pulley, i.e., one with excessive float, permit erratic shifting? I still like an old friction derailleur -- instead of having a mind of its own, it has no mind at all, unless the shifter itself is either too loose or too lubed.

A bent derailleur will cause this shifting because the cage is flexing as you apply more pressure.

This would be fairly obvious though.

You can get this from dropping the bike on the ground.
The most common way it happens is when the chain comes off and wraps around the wheel, dragging the derailleur up and twisting it.
This usually only happens on old bikes with totally shot chains or chainrings though.

Thanks everyone for the suggestions and help. I found out this past weekend that it is the chain. The chain has a bad link that is bent outwards. It kept catching on the derailer and making it skip. So a new chain is on order. Thanks for the help!