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I have a bike with an 8-speed cassette and would like to switch to 9-speed cassette.

Can I keep my rear derailleur? which is a Shimano Alivio one? I understand the indexing might mess up this, but I figure the indexing is handled by the shifters, the the derailleur itself!

Also, Saint Sheldon seems to confirm it is possible (, is that really reliably so?

Apart from that, I know I will need a new chain to handle the 9 speeds, is there a consequence on the crankset? or do cranksets support all chains width?

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up vote 1 down vote accepted

You're right, indexing is handled by the shifters. So if you replace the 8 speed to cassette with a 9-speed cassette you should be able to use the same derailleur. But you will have to upgrade your shifters. The only caveat with the derailleur is that it has to be long enough to accommodate the amount of chain slack generated by going from the largest gear to the smallest gear. This becomes a problem when trying to use a road derailleur with a mountain bike, which usually has a much bigger range of gears.

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Thanks! Actually the situation is that I need to change my chain and my sprockets and on the same time I have a project to replace flat handlebars with drops so I am changing my levers (and brakes, and stem) and the brifters that I prefer have a 9-speed indexing so I figured to replace the cassette (which needs replacing anyway) by a 9-speed one. The derailleur length is not an issue, both my current worn cassette and my future one have the same range but the new one will have closer steps so I should be fine. So thanks!!! – Nic Mar 1 '13 at 21:21

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