aka: Dr. Cannondale
Join Date: Jul 2009
Bikes: I'd rather not count how many, thank you
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The old chainrings may shift OK, but 9-10 speed ones will be better due to the OD chain dimensions. You may also run into issues with the cage on the 8 speed RD being too wide to neatly manipulate the chain over the cogs.
My general rule of thumb is that Tricolor is awesome up to 8 speeds, but not so much beyond that (I've built about 20 bikes with 6400 series stuff). The crisper shift feel of components designed around 9 speeds and thinner chains is worth the price of entry, if you ask me. Even 5500/5600 105 components, which are dirt cheap these days, will outperform 6400 when you go beyond 8.
Others, of course, may disagree. Popcorn, anyone?
Hard at work in the Secret Underground Laboratory...