Pat said:Is this something I can fix by myself? I looked at the middle chainring and
do not see any obvious flaws, but something is out of order....
In general, you can not get all of your cogs to work
with all of your chainrings.
E.g., on my bike, I know not to use the leftmost cog with
the right chainring, nor vice-versa. The primary reason is
the excessive angular stress on the chain where it meets
the gears will tend to stretch and weaken it, essentially
by bending it back and forth until it breaks; plus, the
chain will wear off the teeth of the gears. And I suppose
it can induce mystical shifting.
Sounds like all that's happening is that the angle between
the chain and the chainring is large enough to cause the
chain to disengage that ring and, luckily, engage the
next one. The tensioner on the rear derailleur takes
up the angle in the rear, and keeps the chain on the
selected cog, or it would slide down the casette, too.
Basically, you should just stop using that cog with
that chainring.
Chart the gear ratios for all of your chainrings and
cogs to determine where the overlaps lie. You probably
don't need to use the combinations that have large angles
between them.
E.g., on my bike (currently set up with 42-52 front and
14-16-18-20-22-24 rear) I have use for only 8 of the 12
available ratios:
42 52
24 X
22 X X
20 X X
18 X
16 X
14 X
IIRC, Sheldon Brown has a gearing calculator.
http://www.sheldonbrown.com/gears/
--Blair
"IRC."
