Which rear mech size?

[youngoldbloke]

Interesting. It is (and was - when I changed my cassette) ridiculously difficult to get definitive information - I was just looking at http://www.campagnolo.com/repository/documenti/en/10s_Rear_derailleur.pdf - from 2010. See p20 Recommended Combinations where it clearly states double/compact chainsets + short cage mechs: all 10s Campagnolo sprocket sets. Given the date I assume that must include 29T cassettes.

 

[derrick]

I changed for a medium cage in the end. And the Centaur comes in 12-27.

Well at least it gives you room for a bit of movement if you need to go to a 29+.

 

[Pedrosanchezo]

Sorry for the thread revival guys but thought better to post here than start a new thread.

I have a Campag chorus compact with 12-25 cassette - 2008. Bike comes with standard Chorus short cage derailleur.
I am looking to have a changeable option, for when the need arises, to have more spin friendly gears. The two cassette options that appeal are the 13-29 and 12-30. Both Centaur as Chorus + prices are extortionate.

I am aware that Campag state that a short cage (55mm) will take upto a 26T officially but have heard reports that other manage fine with a 27 & 29. Bearing in mind that many off the peg bikes come fitted with 50/34 - 12-25 = 29 wrap value. 2 more than the stated 27 for a short cage!
The questions are as follows:

Will either cassette work with short cage derailleur if a new chain is correctly lengthened and fitted?
Separate cages can be bought, made from Carbon, could a medium cage be fitted to a short derailleur? Just thinking out box here.
If not is a med derailleur required? Coupled with a 12-30 cassette and new chain? This would mean changing cassette, derailleur and chain for long climbing days and switching back for most other times.

Just asking as i would prefer to not spend silly money for what really is the use of a few granny gears in select events where the 25 will make life harder than necessary. That being said i'd rather spend more money now than end up with more serious problems as a result of trying to wing it!

Final option is to go with a 12-27 but not sure the extra gear will be noticeable. Events in question are the likes of Fred Whitton, where spinning is desirable if not damn near necessary.

Thanks in advance.

 

[RecordAceFromNew]

Sorry for the thread revival guys but thought better to post here than start a new thread.

I have a Campag chorus compact with 12-25 cassette - 2008. Bike comes with standard Chorus short cage derailleur.
I am looking to have a changeable option, for when the need arises, to have more spin friendly gears. The two cassette options that appeal are the 13-29 and 12-30. Both Centaur as Chorus + prices are extortionate.

I am aware that Campag state that a short cage (55mm) will take upto a 26T officially but have heard reports that other manage fine with a 27 & 29. Bearing in mind that many off the peg bikes come fitted with 50/34 - 12-25 = 29 wrap value. 2 more than the stated 27 for a short cage!
The questions are as follows:

Will either cassette work with short cage derailleur if a new chain is correctly lengthened and fitted?
Separate cages can be bought, made from Carbon, could a medium cage be fitted to a short derailleur? Just thinking out box here.
If not is a med derailleur required? Coupled with a 12-30 cassette and new chain? This would mean changing cassette, derailleur and chain for long climbing days and switching back for most other times.

Just asking as i would prefer to not spend silly money for what really is the use of a few granny gears in select events where the 25 will make life harder than necessary. That being said i'd rather spend more money now than end up with more serious problems as a result of trying to wing it!

Final option is to go with a 12-27 but not sure the extra gear will be noticeable. Events in question are the likes of Fred Whitton, where spinning is desirable if not damn near necessary.

Thanks in advance.

I think those after market plastic carbon cage plates are only the inside halves - the outboard one is nearly always a complicated piece of engineering with pivot designed to undergo some stress.

The other thing is I don't see why "changing cassette, derailleur and chain for long climbing days and switching back for most other times" is necessary unless you are interested in saving 50 grams - any set up that can handle a large large sprocket will work fine with another cassette/wheel with a smaller large sprocket - otherwise it would not be able to handle your downhill gears.

It is impossible to give you firm assurance whether a short mech is going to be able to handle a 12-29/30 cassette, not least because it depends on your frame in two aspects corresponding to the two mech limits, but you can perhaps do some measurements/calculations. In additional to conservatism Campag rear mechs' max sprocket size limit is designed to avoid clash between the large sprocket and top jockey based on a minimum axle to hanger c2c distance of 24mm. If yours is greater then you have a greater limit. In any case you can find out what existing can handle by measuring the clearance between your 25T and the top jockey - for every 2mm it can take one more T, and adjusting the H screw on the cage can help.

Regarding the wrap limit a key influencing factor for how much you can push is the chainstay length. Because a chain loop can only come in lengths of exact inches you can use the same e.g. 55" (110 links) chain when the minimum allowable wrap between large large is 54.1" or 54.9", while the former would have wasted more rear mech wrap capacity than the latter. What that means is you can use a chain length calculator which tells you the theoretical minimum wrap length to do some what-ifs to get an idea how lucky you are for different cassettes (noting again the H screw will have an impact).

Personally, I would rather have a lower gear than I think I would ever need, but ymmv.