Recently I bought a used Tommasini road bike 8 or so years old with Campy Chorus 8 speed (13-26) and 53-42 chainrings. For easier hill climbing I changed the 42 to a 39t chainring last night. The front shifts well, but now I am having rear problems: on the smaller rear cogs (#5 and smaller) the cage flexes so much that the chain rubs against the bottom/front of the cage. Also when on the largest rear cog the cage sort of bumps against the cog. Before changing the ring I the only problem I notice was a very slight rattle while on the largest cog. I notice now that the derailer body does not spring all the way backward on its own, and if I pull it back by hand all this works much better (tricky to do that while I'm riding though ). I'm thinking that either I have to shorten the chain (but not too short for the big chainwheel?), or that the spring which is supposed to pull the derailer body backward is weak, or both? Any suggestions would be greatly appreciated - thanks.

I presume that your problem is only on the 39T? Or does it occur on the big ring?

What rear dérailleur? Bet it's the short cage. You may have to change to a medium cage.

Chain length should be determined by the big-big combo, so no change should be necessary. Still, you can't be sure that it was sized correctly when it was installed. Re-measure.

Correct, it's only while using the 39T - everything seems to work with the big ring. Yes I think I have the small cage - maybe changing to a medium would fix this as you say. But should the spring be pushing the derailer back further? - it only goes part way back to the set screw unless I push it by hand. Thanks!

Could be drag caused by the position of the cage. Check the chain length, and just to be sure, the hanger alignment.

Also, have you checked the cable/housing? There could be some drag.

This part of your explaination sounds like the B adjustment screw needs to be adjusted. This is the screw just behind the RD hanger and determines the distance the upper pulley rides from the cogs. Since pulling the RD back solves part of the problem this adjustment does the same thing as holding the RD farther back. I"m not sure this your only issue but it could be one of them.

Actually I think that the screw behind the hanger keeps the RD from pivoting too far backwards; my problem seems to be that it isn't swinging back as far as the stop on that screw. Making me suspect that the spring where the RD attached to the frame is weak as that spring pushes the RD backward. Has anyone had experience replacing this spring, and/or know where to find one (Campy Chorus 8 speed)? I wonder of you have to not just put in the spring, but also wind it up some how to get tension on it?

Try tightening "B" nut..the adjusting screw in rear of rear derailer... it will increqase distance between top derailer roller and cog ... to take up slack smaller front ring is causing..without shortening needed chain lentgh for large front ring.

I assume the first part of your problem ("the cage flexes so much that the chain rubs against the bottom/front of the cage") refers to the chain rubbing against the front der cage. That would be expected, since the chain now leaves the der lower than before. You will have to re-adjust the trim position (by increasing FD cable tension) to compensate, and will probably find that the smallest rear cog you can cross-chain to is more limiting than before, since the chain is running lower in the FD fork.

PROBLEM SOLVED! by my local bike shop by shortening the chain, no other adjustments were needed. Actually I got a new chain as they didn't have the right locking link for my Campy 8-speed chain; the chain had some wear anyway. Thanks very much to all for your help & advice. - Michael