Bicycle Mechanics - front mech dropping chain onto BB

Hi,

I keep getting this recurring problem where my chain keeps dropping onto the BB when shifting from the middle to the lower chainring. It only happens occasionaly (always on the toughest climbs though!).

Adjusting the mech any further to the right causes me problems shifting to the ring in the first place.

Any ideas ??

B.T.W. I've set the height of the derailleur as well as its angle (parellel to the chainrings right?)

Claude

Claude,
Well first off what shifters are you using? What model der. is it? A couple of things come to mind. One is the height of the cage over the front chainring. it should only be about 3 to 4 mm. If you say the limit screw is adjusted enough and it still throws it then you may want to check the middle ring to see if it is bent or the granny gear also. If you are riding a road bike the same applies but not with a granny gear unless you have a triple. If you rings are bent and you can remove them try to get them straight. If this does not work then something we did was to slightly bend the front inside part of the cage in toward the outer part of the cage. You can do this with a pair of neddle nose pliers or a small adjustable wrench. Another thing is to shift down to the granny or smaller ring before you get to the climb. This will assure you do not shift under stress. The next thing I would do and this may not solve it but it never hurts to do it is replace the cable and housing. Hope this helps.

Claude,

if your front derailleur is perfectly set and your chain still flips of, maybe this is a option.

http://gvtc.com/~ngear/

(its called jump stop?)

Cheers,

Mark b.

One more thing to check, and this is almost too crazy to mention, is "is your BB possibly loose?"

Grab a crank and give it a good pull and good shake. Does it wiggle at all? If yes, this could be giving enough play for the chain to bang against the front derailure and throw the chain off of the sprocket.

After all the above mentioned fixes, your problem is probably trying to shift under to mush pressure.
This will cause your chain to overthrow, try to shift down to low enough gear before you start putting heavy pressure on your chain.
Steve33

Yeah thanks for the agreement Pete.