Join Date: Apr 2002
Location: Montréal (Québec)
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Just to clarify Retro Grouch's great info:
Road derailleurs vs Mountain derailleurs
When you shift across with a road derailleurs, the pulleys tend to stay "level", i.e. almost at the same distance from the hub. They work best with close-range cassettes. If one were to use a road derailleur on a 11-32 cassette, the top pulley would be far from the 11 and 13 cogs, making imprecise shifts, and would litterally walk on the 32, making it very noisy. Indeed, depending on the exact geometry of the rear end, the road derailleur probably won't work well for a cog larger than 30 and may crash into the large cog, break and...
On the other hand, when you shift across, the top pulley moves up and down, so it's closer to the axle at the top end and further away from the axle at the bottom end. So it's perfect for a 11-32 cassette because it means that the top pulley travels always at the same distance from the cogs. On the other hand, if one were to use a mountain derailleur with a 12-23 cassette, the pulley would be very far from the cassette at the "23" end, meaning sloppy shifts.
As to why having the pulleys too far away from the cogs result in sloppy shifts, think of handling a dog with a leash. The shorter the leash, the better you can control the dog.
Short cage vs long cage
A short cage derailleur has a "better grip" on the chain, so the chain starts to move more rapidly than with a long cage derailleur. With modern systems, it may make a difference if you like to shift aggressively during hill climbing or sprinting while standing on the pedals. Otherwise, it's a non-issue, so buy a long cage derailleur because it allows more flexible gearing. But...
A long cage derailleur is a bit low and hangs low on small-wheel bikes. I have a long-cage LX derailleur on a Piccolo (20" wheel) because I had it at home, and it's very easy to have the chain drag about 1" off the ground. While I don't see it as an issue on a bicycle with a 26" or 700c rear wheel, it definitely is an issue with small-wheel folding bikes or recumbent bikes.