rear derailleur adjustment?

[skydog]

I took my '82 Conti out today for it's second trip. When the chain is on the smallest 2 sprockets in back and I backpedal, the chain either throws or goes extremely slack. Obviously something is out of adjustment and needs looked at but I can't seem to pinpoint where. It doesn't do it on the larger sprockets, and the front derailleur position doesn't seem to make a difference.

[acg_schwinn]

I'm thinking either a weak derailleur tensioner spring or a gunked-up chain, or some combination of the two.

When you back-pedal, the derailleur gets drawn out tight, and slack forms in the top chain run instead of its normal slack in the lower. If the chain does not fold itself freely around the tight radius of the small back sprocket, it can start skipping over the tops of the teeth instead, and things can get all loosey-goosey.

I think I'd make sure that the chain is properly lubed with no seized-up or stiff links, and maybe even move the lower derailleur tension spring onto the next highest hook (if so equipped).

[bargainguy]

Derailleur systems were not designed to be backpedaled excessively, and there is excess wear caused by this.  The only time I backpedal is to flip up a pedal to engage a toe clip or cleat.  

There is no guide for keeping the chain on the sprockets when backpedaling.  I would only be concerned if you were encountering problems with forward chain motion, and that's where I'd want to start with adjustments.  So if you're having forward chain motion issues:

1.  Lack of tension on the rear derailleur cable.  If you have a barrel adjuster on the rear derailleur, try tightening slightly.

2.  Worn chain, too much side-to-side play.

3.  Worn freewheel.  The smallest cogs get the biggest wear because there are fewer teeth to handle the load.

4.  Misaligned rear derailleur.  Are the pulleys parallel to the sprockets?  One of the best investments I ever made was the Park DAG-2 alignment tool on the advice of my LBS after I saw them aligning the RD on brand new bikes.

5.  Inaccurate B-screw adjustment -- don't know if your Conti rear derailleur has this.  There should be some space between the upper jockey pulley and the sprocket; if they're right up together, will cause shifting problems.  Tightening the B-screw moves the derailleur down and away and gives the space needed for the chain to work properly.

Don

[skydog]

I know; it's a bad habit. No issues pedaling forward though.

[JenniferC]

Your Conti should have a GT 510 rear derailleur. It has adjustments for low and high but if your not having problems with the chain jumping off while shifting and pedaling forward I wouldn't change the adjustments. I would however check your chain for excessive wear. I did a complete post on checking chain wear in this thread http://www.schwinnbikeforum.com/index.php?topic=5481.0. I would also spray your derailleur wheels with a lubricant like CRC 336 or similar to clean out any built up gunk and free things up a bit. As Don says though pedaling backwards is not something that derailleurs like to do.