Joe K

Hi guys, my chain fell off today a few times, and I figured out that when I shift the front derailleur too quickly, the chain falls off the side of the larger cog.

I have an old Suntour GPX friction DT shifter so I thought it could be just a cable adjustment, but I'm still having problems.

It looks like it's not catching correctly, and then falls off. Here's a picture:



Any suggestions?

JiveTurkey

At first thought it sounds like a loose high-limit screw on the front derailleur. Put the bike in the highest gear (largest front chainring and smallest rear cog). Tighten H screw while pedaling (backwards OK) until it begins to rub, then back it off till rubbing stops.

But why is the chain not seated on the chainring teeth in the picture?

Randallissimo

Looks like the "old" chain is stretched out (worn) and is too long to fit the cogs in the sprockets. You probably need a new chain. And that probably means you need new rear sprockets, either a new cassette or freewheel, depending how old.

Joe K

Thanks guys, I thought maybe the chain was stretched out too much. I attached the photo because it looks like the inability to catch the chain is what's causing the chain to fall off. It doesn't rub against the derailleur once it hits that cog.

Sekine

Yeah, it should, your high limit screw is too far out, like post #2, pedal, turn the high limit in until it rubs, and then back off just a little, in your large (front) / small (rear) combination there should be less than 2mm gap between the outside of the derailleur and the chain.

gregstandt

Probably an improperly maintained chain. Check this out https://sheldonbrown.com/chainclean.html.

z415

Oh, how I shall miss these....