Bicycle Mechanics - RD-6400 can't shift into low gear with large chainring

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pbrant
09-30-11, 04:48 AM
Hi all,

I recently purchased a new old bike off of eBay. Drivetrain is all Shimano 600/Ultegra in what appears to be good used condition (at least not obviously worn to my untrained eye).

The RD needed adjustment though and although it now shifts quickly and accurately with the chain on the smaller (42 tooth) chainring, I can't shift into low gear with the chain on the outer (52 tooth) chainring. The chain just kind of sits up on top of the sprocket without meshing into it. I know this gear combination isn't recommended, but it still should work, right?

Similarly, I can't shift from the inner to outer chainring if I'm already in either of the two lowest gears.

Chain is lubricated and not excessively worn (< 0.75% per chain wear indicator).

Could the chain just be too short?

Thanks,

Pete

MNRon
09-30-11, 05:22 AM
Yes, the chain is probably too short.
pbrant
09-30-11, 06:29 AM
Thanks much. I figured as much, but wanted the opinion of somebody more experienced than myself before plunking down for a new chain. Thanks again.

rogerstg
10-01-11, 06:50 AM
Thanks much. I figured as much, but wanted the opinion of somebody more experienced than myself before plunking down for a new chain. Thanks again.

You really don't need an opinion, you could simply look at it. Is it stretching the RD tight when approaching those gear combinations?
pbrant
10-03-11, 06:03 AM
Well, right, good point. I already did that and it seems pretty tight, but not completely taut either. At the other end of the spectrum, it doesn't seem the RD could take up the slack on more than an extra link or two which left me ever so slightly doubtful the problem really is the chain. In any case, chains aren't exactly expensive and one will arriving shortly. I'll give that a whirl and report back if that doesn't work. Thanks for the response.