Tech Q.

[Jon H]

The short one's short, the long one's long, and medium one's in between.

Short one's OK for racing gears (double chainset & cassette up to 27). If you run a triple chainset you'll need a medium or long mech depending on what combination of rings & sprockets you've got. If you look at the techy bit of the Shimano website you should find a figure for the maximum teeth difference each mech will cope with.

[Jon H]

DA is a bit lighter, a bit shinier, and more expensive. In terms of performance it would be very hard to tell any difference really.

[Andrew G]

Think you need a medium/long one Sean as you run a compact. Might be wrong though .

The reason for the difference in length is to take up the extra slack in the chain depending which chainring & sprocket combo you happen to be on at the time.

[Wal]

You can run a short cage with compact or triple chainrings - as long as you avoid the extreme crossover gears i.e large/large or small/small. Depending on your chain length, either the derailleur won't be able to take up the slack on the small/small gears or the chain will be too short for the large/large gears. The crossover gears should be avoided anyway because they put lateral strain on the chain, accelerating its wear rate.

Some folks are happy to risk accidental miss-shifts and subsequent drivetrain damage in trade off for the aforementioned crisp shifting and lighter weight, while most prefer to play it safe and ensure their rear mech can cope with all possible gear combinations.

I've seen MTBs with triple chainrings and 27+ tooth rear cassettes running road rear derailleurs. In fact, I seem to remember noticing a Dura Ace rear mech on Sylvain's MTB...

Does the Ultegra rear mech have sealed pulleys? The DA does...

[Elliot M]

[quote="Wal"]You can run a short cage with compact or triple chainrings - as long as you avoid the extreme crossover gears i.e large/large or small/small. Depending on your chain length, either the derailleur won't be able to take up the slack on the small/small gears or the chain will be too short for the large/large gears. ...

[Elliot M]

actually, I think Shimano road derailleurs only come in short (SS) and medium (GS).

MTB only in medium (GS) and long (SGS)...

[Andrew G]

[quote="seanieh.Piano_Condor©"]Well, I got an OE DA medium cage rear mech. Cheers for the tips guys.

[Elliot M]

...which is the capacity of all Shimano short rear derailleurs...hmmm, might not change my existing long rear derailleur to a short one when I get my compact chainset then, in case i want to put a bigger cassette on it.

With a long cage mech you have a 37t capacity which would be enough for a 1:1 lowest gear -- 50-34 / 13-34 (which I think do exist, although designed for MTBs)