Changing 8 speed Shimano Sora Rear Derailleur

richard36

My derailleur needs replacing. A direct replacement doesn't seem to be available. I was going to go for a 9 speed (read somewhere they are compatible) and then I noticed that the Shimano 105 RD 5701 rear derailleur is compatible with 8,9 and 10 speed drivetrains. However I don't fully understand the spiel about maximum low sprocket, total capacity etc, and would therefore appreciate some help.

I have a triple chainset which is 52-42-30 and the cassette is 12-25. Here is what Wiggle for example says about the short cage and medium cage. Which of these 2, if any, will fit my bike?

Many thanks

Top Features of the Shimano 105 RD-5701 Rear Derailleur

Short Cage
Large 11 tooth pulleys increase service life and reduce noise
Super light shift action
Compatible with 8, 9, and 10-speed drivetrains
Maximum low sprocket 30T
Maximum front difference 16T, total capacity 34T
Average weight 221 grams

Medium Cage
10-speed 105 wide link rear dérailleur medium cage - GS
Advanced Wide Link design increases rigidity and shifting performance
Super stiff for fast and accurate changes across the cassette
Large 11 tooth pulleys increase service life and reduce noise
Super light shift action
Maximum low sprocket 32T with double chainset
Maximum front difference 22T, total capacity 40T
Average weight 232 grams

dj58

With a triple crankset you would generally use a GS cage derailler. Would also give you the option to go to a 32T low sprocket if required. This is the chart for 10 speed 4700 Tiagra same principle as for 5700 triple.
http://productinfo.shimano.com/#/lc/1.7 ... ?tocst=254

ugo.santalucia

To play it safe, I would go for a MTBike 9 speed derailleur, this one is perfect for your 8 speed system and will shift any ratio. It says it is compatible with 28-34 as a biggest sprocket but I am sure it will be absolutely fine with your 25 T too

http://www.chainreactioncycles.com/shim ... lsrc=aw.ds

richard36

Not knowing anything about this I'm not sure why a MTB derailleur is the better option but I'm sure you know why . If I did want to go for the 105 5701 derailleur would I go for the medium cage?

I assume the MTB derailleur would be compatible with my shifters or does that not matter?

Thanks

ugo.santalucia

A MTB derailleur is a better option because you can use MTBike cassettes with bigger sprockets, which in 8 speed are cheap as chips, whereas the road version are increasingly difficult to source...

Call it future-proofing if you like

A 9 speed MTBike derailleur will work fine with your shifters

richard36

Okay. Thanks.