This didn't happen before....

[Sillyoldman]

I have just replaced my worn chain (0.5% indicator on the tool). 1000 miles from new. Chain set and cassette look like new.
New chain is the same length as the one that came off.
Gear changes are silky smooth and all is well with the world except.....twice the chain dropped off the small chainring (compact) and it has never done that before. On both occasions I was in the middle of the cassette (9 speed) when I dropped onto the front chainring and the chain came off.
The front derailleur works perfectly when I have it on the stand and I can't replicate the fault.
Checked for stiff links, all ok. I have not changed or adjusted anything else.
Was I just unlucky and maybe hit a bump as I shifted, would this cause it? I am at a loss as to what to do next. Any ideas?

 

[RecordAceFromNew]

You could check to see if the chain is no more than 1mm clear from the inside of the front mech cage when it is on small front large back. If it is more turn L limit screw clockwise until it is. If chain continues to drop after that a Deda dog fang or similar will help.

 

[wakou]

Yes, you need a tiny adjustment to the limit screw, A dog-fang will not 'help' exactly, but it prevents possible serious damage if it happens again. Drop the chain to the small front ring, and carefully turn the limit screw in until you feel resistance, you will then see the front derailleur moving slightly in response to further turning of the screw, just screw it in a tiny amount from there. You might need then to take up a tiny bit of slack in the cable, using the adjuster......

 

[Sillyoldman]

cheers both will give that a go. Think I will try a dog-fang for extra peace of mind

 

[02GF74]

my theory, and I am not saying it is the right one, is that a new chain will bend less than a worn chain - so if you think of the chain line - small ring on front and middle of cassette - you are putting the chain at an angle (ie. it would be straight if it was on large sprocket on back) ... since the new chain wants to be as straight as possible, it leaps off the little chain ring.

a turn on the derailleur adjustment screw will fix it.