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I bought a 2013 Cannondale Quick 6 Hybrid for my daily commute almost exactly a year ago, and I'm looking to give it some upgrades. It currently has a 7-speed Shimano Altus derailleur, and I was hoping to replace it with something a bit more "road"-y, like the Shimano Claris.

However, the Shimano Claris is an 8-speed derailleur while both my cassette and shifter are 7-speed. Also, I don't see anything saying if the Claris would work with the rapid-fire shifters (also Shimano) that I have on my bike right now. The chain is certainly 8-speed compatible, but that's about as much as I can be certain of at this point. If I wanted to switch out my derailleur to the 8-speed, what other parts and/or changes would I have to buy or make in order to make it work?

Thanks for all the help in advance!

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Rear derailleur's speed markings are generally for marketing purposes - if you're under 10 speed in Shimano, you can swap between road and mountain rear derailleurs without problems provided they have enough capacity for the rear cog combination you're using - the cable pull is the same, so road and mountain shifters can't tell the difference (often in fact, you'll see touring bikes or road bikes with big rear cogs with Shimano Deore derailleurs since the mountain derailleurs can have bigger cogs in the back at the cost of a few grams of extra weight and looking less cool). The speed number you can effectively ignore, since the amount the derailleur moves is determined by the amount the shifters pull the cable (in theory, the cage might be thinner or something, but in practice this doesn't really matter).

You also won't notice a difference in performance or anything if you just stuck a Claris rear derailleur on. It would just be a waste of money.

If you want 8 speeds, you need to buy an 8 speed cassette (Shimano compatible) and an 8 speed Shimano shifter for the rear. You can keep the existing derailleur - you just need to adjust it a bit. You'll need some special tools (cassette remover, cable cable cutter, big crescent wrench) in addition to hex keys in order to do this as well. Swap the shifter and put the cassette on, adjust the derailleur and you've got an 8 speed setup.

Personally, I think you should just stick with 7 speeds as it is sufficient for commuting (the extra speed doesn't really help - its just a bit easier to find 8 speed parts these days, so thats pretty much the only reason to go from 7 to 8). Spend your money on beer or something instead. The Cannondale Quick 6 is a relatively cheap hybrid (~500 dollars new), so if you want something faster, you're probably better off getting a different bike with probably more aggressive gearing and geometry (say a flat bar road bike like the Giant Escape or something if you want to stick with flat bars).

  • 7 and 8 speed cassettes have different thickness, so you can't put a 8 speed cassette on 7speed freehub. (as I see, it's not a freewheel, but a cassette). – Alexander Sep 20 '14 at 18:00
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It currently has a 7-speed Shimano Altus derailleur

The RD-M310 is actually a 7/8 speed derailleur. Kind of between the bottom-line Shimano Tourney line, designated as 7 speed, and the Acera 8 speed (RD-M360). Still, just this morning I saw a Tourney on someone's 8 speed commuting bike (one chainring front, 11-34T 8 in the back).

I can't think of any reason why your shifters wouldn't work with the Claris RD-2400. The derailleur doesn't see the shifter, just the cable movement. The derailleur's ratio is the same 1.7 as your Altus.

Make a careful selection between the "SS" (short cage) and "GS" (medium cage) version of the Claris. If you have three rings in the front (which I see from googling is standard on that Cannondale hybrid), you probably want the "GS", which can better pick up the chain slack if you shift to the small ring, and a small cog at the same time.

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You will be fine with an 8-speed cassette, as long as you also think about compatible shifters.

As regards the chain width, 6-, 7- and 8-speed are all interchangeable. 9-speed and above, the chain becomes narrower to cater for the extra sprockets on the cassette.

As exemplified by @Batman's answer, there is an amount of scepticism regarding how much this narrowing matters, in terms of the smooth running of the drivetrain.

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