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I have damaged my rear derailer and are looking for a replacement, aside from replacing it with the exact same model, what do I look for in terms of compatibility.

I note that there are long medium and short cage sizes, what is the difference?

Also, there are 7, 8 and 9 speed versions.

Some models I have seen have large plastic pulleys and look like they are definitely not for a mountain bike. What is this about?

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In addition to the above, consider getting a 'shadow' derailleur. These don't stick out so far and are less likely to get damaged. !enter image description here You also need to get the right 'top normal' or 'low normal' type. 'Top normal' is the sort where the spring pushes the mech to select the small cog, 'low normal' is where the default position, i.e. no cable present, is the big cog. If you don't get the right type then it will not work with your lever properly. The 'shadow' mechs can be obtained for £30 or so, that is for a quality Shimano Deore one –  ʍǝɥʇɐɯ Jul 31 '11 at 16:47
    
As for the large 15t plastic pulleys, these are for the low end systems that have a huge cog on them - MegaRange sprockets. The big jockey wheels take up the slack chain without relying on just spring tension alone. They are quieter and last longer than regular jockey wheels. MegaRange gears are good, the idea is that you still get your hill-climbing easy gears plus a good selection of close gears for when riding on the flat. Equal spacing between the gears is not as useful for most riders as what MegaRange offers. –  ʍǝɥʇɐɯ Jul 31 '11 at 16:58
    
Top normal or Low normal is not a compatibility issue. It's just a preference about which direction you want it to shift. –  zenbike Jul 31 '11 at 19:11
    
I've tried replacing derailleurs and cassettes and had 'mystery' incompatabilities that even my LBS couldn't explain. I'd suggest getting your LBS to advise and fit because that way they absorb the risk that the parts won't be compatible. Otherwise you might be stuck with a derailleur that is no use at all. –  Mac Aug 1 '11 at 3:54
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@Mac: I'd be interested in hearing more about that situation. Can you post it as a question, for us to solve the incompatibility? –  zenbike Aug 1 '11 at 8:01
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2 Answers

A derailleur for your mountain bike should match the specifications on the original derailleur. That is, while it doesn't need to be the same model of derailleur, it does need to be the same brand, in almost every case.

It also needs to match the number of gears, or have more intended. Meaning a 7 speed will fit for 5 and 6 speed bikes. An 8 speed will work for 7 and 8 speed. A 9 speed will work for 8 and 9 speed, and sometimes 7 speed (but no promises.) A 10 speed mountain derailleur has more limitations. It may work with 8 or 9 speed, or it may not, depending on model and brand, and other variations. It is the newest, and least tested, with non 10 speed systems.

In addition, the cage length, either short, medium, or long, should be the same as the original. If you don't know what yours is, then long is the safest bet, but you may also need to change the length of your chain. The difference between long and short cages is the amount of chain the derailleur is able to absorb, which in turn determines the size of the gears you can run, particularly as relates to the front chainrings.

The large plastic pulley derailleurs you have seen are called Mega Range derailleurs. They are generally used for department store quality bikes, or hybrids, and are definitely not intended for a mountain bike. They give a very easy climbing gear, but are low quality, and often shift very poorly.

I hope that helps. If you need more info, feel free to ask.

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Sounds like the exact same model is the least risky solution. –  kmm Jul 31 '11 at 15:53
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Getting the exact same model is not quite that easy. The same model will have all of the same options for cage length as every other model in the series. Your best option is to take it to a local bike shop, tell them what you want, and ask them what options are available. You should be able to get an upgrade for a relatively minor cost differential. –  zenbike Jul 31 '11 at 15:58
    
What bike, model year, and derailleur model do you have? I can probably give you better info, if you can tell me that... –  zenbike Jul 31 '11 at 16:00
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There was another thread on pretty much the same subject about 2 weeks back, but I didn't find it after a weak attempt at a search.

As zenbike indicates you need to match the # of speeds and the "cage length", though simply picking a derailer that claims to be, say, "long cage" isn't a reliable technique for sizing cage length. Rather, you need to check the tooth "capacity", which is the difference in # of teeth between the largest two cogs (front and rear) and the smallest two. This specifies (indirectly) the amount of chain that the derailer must "absorb" when going from the "tightest" cog combo to the "loosest". (Also check the specs for the largest and smallest rear cogs that the derailer is good for.)

With a true mountain bike, one that's used off-road and in weeds and brush, you want to avoid an overly long derailer that will hang down too close to the ground and thus be more subject to damage or picking up weeks -- pick the one that fits.

(Unfortunately the old Suntour triple design is no longer made -- it packed a large tooth capacity into a short arm.)

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Assuming you stay with the same brand, tooth capacities will be equal with the same cage lengths in a particular brand. Or at least compatible. No need to worry about it. –  zenbike Jul 31 '11 at 19:13
    
Assuming you can find the exact same model. Not necessarily the case for a 5-10 year old bike. –  Daniel R Hicks Jul 31 '11 at 19:42
    
True, but newer versions get more capacity, not less, so there should be no issue with that, assuming you stick withthe same brand and gear count. –  zenbike Jul 31 '11 at 20:13
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