I have a LeMond Zurich (all steel frame; I think they're now part steel,
part carbon fiber). This is the third year I've been riding it. It has
Shimano Ultegra components and a triple front chainring crankset. I've
pretty much always have a hard time dialing in the front shifting.
Recently, my front crank on the drive side has been coming loose. I even
had to use blue loctite to try to stop the front crank from coming loose.
Because it was loose, I would look down sometimes and thought I saw the
front chainrings wobbling relative to the frame. I chalked this up to
perhaps a worn bottom bracket/crank connection.
I purchased a new bottom bracket and new (Truvative) cranks to try to fix
this problem. After I installed everything, I was having a bear of a time
getting the front end to shift correctly. Specifically, when in the
middle chainring and the large cog (or second to largest cog) on the rear
cassette (12x27, 9 speed), the chain would jump to the bottom chainring.
The chain and the rear cassette are new. (I had previously ordered a new
wheel.) The chainrings have shift pins, which my old -- but not original
-- chainrings on my Ultegra crankset did not. What looks like what was
happening was the cranks wobble just enough to cause the chain to jump
from the middle to the small chainring when in the large cogs of the rear
cassette.
Now, I've confirmed this -- the front chainrings really are wobbling. I
put cardboard up against the downtube and put a line on it. The large
chainring wobbles inside the line and outside the line with each complete
turn of the cranks. The largest travel (farthest from the downtube)
appears to coincide with the drive side crank; the smallest at the other
end (when the non-drive side crank would be aligned with the paper). The
amount of wobble is small; about 1/16 inch, but it's enough to affect
shifting. I've confirmed this by comparing the distance from the
non-drive side crank to the downtube and the distance from the drive-side
crank to the downtube (former smaller than latter).
What could cause chainring wobble? The crankset and bottom bracket are
both new. Could the bottom bracket be crooked relative to the downtube?
I do not see any metal breaks or anything other type of broken frame parts
-- the frame looks darn new.
Thanks.
--
Bob M
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