Originally Posted by
jwlunt
Guys, my Xootr chain keeps jumping the front ring... normally when accelerating and changing up a gear. I'm no mechanic (no kidding), but the bike is well oiled and otherwise riding like a dream. I wondered if my chain was loosening (it's about a year old on a three year old Swift and has just done the London winter) so I pulled the back wheel back a little: no difference. The pesky chain jams itself between the ring and the chain guard with is messy and v annoying. I need to get this fixed in time for the Smithfield Nocturne! Any ideas from the mechanically superior?
Thanks!
Jonathan
Yes I know the reason.
When you shift to a smaller cog in the back, a sideways wave is set up in the chain when it suddenly jumps to the next smaller cog. This sideways wave propagates to the front and is sometimes enough to make the chain climb on the teeth and off to the outside.
As the chain wears, it gains more sideways play and is able to propagate the wave better, so older chains suffer more.
The only effective cure is to arrest that wave before it gets to the chainring. Obviously a front derailer does the job well enough if adjusted properly.
A chain guard works if it is close enough to the chainring. My Birdy's chainguard was not mounted close enough for the job and it would still occasionally climb off. So for the Birdy, I mounted a flat chainguard as close as I could to the chainring without it scraping the chain when crosschaining. Since then I have never again had a front derailment. Sometimes when I shift I hear a "ting!" as the chain hits the chainguard and I think, "That's another chain drop saved."
So your solution is to see if you can reduce the spacing between th chainring and the guard as much as possible. If there are no spacers to be removed, you could have a go at gently bending the chainguard support prongs such that the guard ends up closer to the chain. This assumes the guard is aluminium. But work carefully as otherwise it ends up wobbling and it will be like a splinter in your mind.
On my Swift I mounted a little plastic tube around the chain to arrest the wave. I have seen other solutions in the form of a little bracket which holds a U-formed shape over the chain just before it reaches the chainring.
Changing to a bigger cog in the back is a much more gradual affair so does not result in a sudden sideways movement which sets up that wave.