Shimano question

[Gruff]

Anyone know if a Shimano RX100 8 speed rear mech is any good & can it be used on an MTB as it says it came off a tourer with triple chainset :?:
I've never heard of it

http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... K:MEWAX:IT

 

[one-eyed_jim]

RX100 was just below 105 in Shimano's road range in the mid nineties - about where Tiagra sits today. As long as the pivots are still tight, you've not done badly.

It'll work (and index) as an mtb mech, but you might be limited in the size of the biggest rear sprocket it'll stretch to. That depends on the dimensions of your derailleur hanger too, so suck it and see, but you're probably looking at 28-30t max, rather than 32-34t.

 

[taffy]

IIRC a short cage will not go above a 28T. I remeber trying to coax a 105 rear mech to work on a 13 -13 some 17 years ago an it wouldn't have it.

That one looks like a mid length (ask the seller though) to me so should cover up to a 32 (maybe a 34)

 

[one-eyed_jim]

IIRC a short cage will not go above a 28T. I remeber trying to coax a 105 rear mech to work on a 13 -13 some 17 years ago an it wouldn't have it.

That one looks like a mid length (ask the seller though) to me so should cover up to a 32 (maybe a 34)

The cage length doesn't change the maximum sprocket size the derailleur will handle - it only affects the total chain wrap - so the short (SS) or medium (GS) versions will handle the same maximum sprocket.

As I wrote above, the precise figure depends on your frame - specifically the dropout and derailleur hanger. I've run a short-cage 105 with a 32t sprocket on one bike while it would only just clear 28t on another.

The derailleur pictured in the auction is the GS (road triple or mid-cage) model.

 

[Gruff]

Thanks for the advice guys......I think maybe i'll just wait & get my hands on a good old XT or LX.

 

[Steve Kish]

I've got one - it's fine and works just as well as my LX one. I've used this with a 11-30 with no problems at all.

 

[taffy]

I just always assumed that a short cage wouldn't go above a 28 tooth (and from my few attempts at getting a short cage to work with a wide ratio cassette)

93 XT RD-M735 short cage

 

[one-eyed_jim]

There are a few different issues here.

First, the Shimano-rated maximum is always conservative. What Shimano say a component will do (often within the context of assumptions about other component choices) and what it will actually do can often differ quite widely. If Shimano state that a derailleur will reach a 28t sprocket, they're not really stating a maximum. They're guaranteeing a minimum performance limit within the context of a system of compatible parts.

The only difference between a short-cage XT derailleur and the long-cage model is (obviously) the length of the cage (all the other parts of the derailleur are identical) and the cage length only determines the total chain wrap. The geometry of the parallelogram (identical for all cage lengths) determines the maximum sprocket. Of course, if the maximum wrap is exceeded, the derailleur may not be able to make use of its potential full cassette range.

The MAVIC short-cage derailleur on my Stumpjumper is rated 26t. With careful adjustment it just clears a 32t:

http://www.retrobike.co.uk/forum/viewtopic.php?t=53086

If I had three chainrings it would still clear the 32t sprocket, but I'd need a longer cage to wrap up the extra chain.

 

[Steve Kish]

Strangely enough, mine is a mid-sized cage with distance of 75mm between the jockey centres. Compares with my Deore long of 90mm and Campag short of 55mm.

I never take any notice of what's marked on the mechs. Fitting it establishes whether or not it works.

 

[dbmtb]

Chain length can be an issue if you are running a triple with a small granny.

I can't get 12-28 to run on 24-34-44 without chain being too long or too short. Shifts fine but you need to be careful not to cross chain.