Here's my problem.

I have an 8 y/o bike I tried to keep going this summer while I intern for as cheap as possible. I put in a new bottom bracket, new chain, and new cassette. The things that were absolutely warped.

Everything was fine since March when I did the replacements until this past week when I started mt. biking.

When I apply pressure (standing or sitting) and the chain is in the middle chaining, it jumps off and goes down to the bottom bracket. This will happen with any middle and rear gear combo. It will not jump off if I am coasting or pedaling easily. It also does not jump off on the 1st or 3rd chain rings.

I tried doing adjustments from square one on both front and rear derailleurs three times now. The chain rings do not seem to be bent, there isn't any major wear on the teeth, and the cranks are tightly installed.

Besides taking it to the LBS and probably having to replace something (which I will not do-- spent too much money already), I don't know what else to try.

Any suggestions?

Sounds like a worn out middle chain ring.

+1

No doubt about it, the teeth on your middle ring have had it. The new, unstretched, chain is what brought out the problem. Replacing the chainring is a must do. bk

Most people spend the lions share of their time on the middle ring, so it makes sense that if your BB, chain, and cassette were"absolutely warped", then the middle ring is probably toast as well.