Campag centaur short cage: how low can I go?

barrybridges

Apologies if I've asked this question before in a slightly different variation, but:

I've got campag centaur (2010) short-cage rear mech.

Currently running compact with 34/26 as my lowest gear.

If I want to replace the cassette for something that gives me a lower gear, will a 12-30 work:

http://www.wiggle.co.uk/campagnolo-cent ... -cassette/

Or is 30 teeth too many?

If that doesn't work, will a 13-29 work:

http://www.totalcycling.com/a-z/cassett ... ntaur.html

I basically want to get the lowest gear I can that works smoothly and doesn't require much fannying about or bodging. If the 29 works well but the 30 is a bit iffy, I'm happy with the 29.
FYI, the bike is a Wilier Izoard - not sure if this makes a difference in terms of chainstay length.

Paulroth

I have a Campa Chorus short cage that happily runs a 13-29 cassette.
(In fact, having bought the 13-29 cassette, I then mixed it with my original 12-26 cassette to make a 12-29 which runs well)

g00se

I have a 2010 short cage Centaur with a compact front and it'll take a 13-29 and just take a Miche 12-29 in terms of capacity, even though it's over the 2010 max capacity spec (the 2011+ models are fine with 29). Your mileage may vary as ideal chain length varies depending upon the chainstay length - and mech hanger length. I would think a 30 would be pushing it for my 2010 mech though.

gfk_velo

You need to be careful in what you define as a 2010 rear gear - as 2011 product (11s shape. N.B. NOT geometry) started to appear in the market as original equipment on built bikes & through other sources in July / August 2010.

A *true* 2010 rear gear has the older shape and geometry and an overall capacity of 30T, a max sprocket size of 26T, a min sprocket size of 11T and a maximum difference between top and bottom gear of 16T.(N.B. Chain stay length has no effect on gear capacity, only on overall length of the chain).

This would mean that 26T is the maximum size rear sprocket that could normally be accommodated on a dropout / hanger that conforms to Campagnolo's specification. Some hangers might allow a slightly bigger sprocket but care needs to be taken with chain length and wraparound. If chain length is insufficient for the "big to big" combination to be obtainable and it is selected, failure of the rear gear and hanger are inevitable, and damage to the chain usually also occurs. In many cases, the rear gear will swing into the rear wheel and the knock-on effects are usually very expensive.

Again, although in some cases it is possible as a result of the design of the hanger to force a larger sprocket and maintain wrap-around, this over-strains the upper pivot return spring (which can snap) and can also result in the failure of the upper pivot bushing - we see this quite frequently at the Service Centre.

Campagnolo in their technical manual for 2010 state clearly (page 45, section 2.2):

Double or CT crankset + short cage rear derailleur: all Campagnolo sprocket sets EXCEPT 13-29 (capitals are mine - as at the time the 12-27 didn't exist this isn't mentioned but is implicit in the spec given in section 1.2)
Double or CT crankset + medium cage rear derailleur: all Campagnolo sprocket sets.

Dropout spec can be found in the 2010 technical documentation, page 46, section 3.2

The PDF of the relevant technical manual is here:
http://www.campagnolo.com/repository/do ... -07-09.pdf

HTH
Graeme FK
Velotech Cycling Ltd - Campagnolo Mian UK Service Centre | Velotech Industry-Standard Technical Training

g00se

gfk_velo wrote:

Again, although in some cases it is possible as a result of the design of the hanger to force a larger sprocket and maintain wrap-around, this over-strains the upper pivot return spring (which can snap) and can also result in the failure of the upper pivot bushing - we see this quite frequently at the Service Centre.

Didn't know that, thanks for posting it. I assumed that if the hanger was long enough, it would be OK. I'm still going to risk using the 29 on the Cheshire Cat though - if you see a photo of someone face-planted half way up Mow Cop, you know who it is .

gfk_velo wrote:

(N.B. Chain stay length has no effect on gear capacity, only on overall length of the chain).

I'd like to politely disagree here - so please correct me if I've misunderstood the following:

In an ideal world, you would split a chain so that on the big-big combination, the rear derailleur would be at the maximum safest extension. However, for this to be the case, the chain would have to come together right dead on a link. So really, the chain length would have to finish on a full inch with no fraction. If the chain was just the smallest fraction of an inch shy of this, then you would need to add a complete link to keep the derailleur within it's limits.

So, if the chain does not measure to an exact inch, you have to add a full link of chain and are loosing up to two teeth in the rear mech's effective capacity.

As the chain length is a function of the chainstay length, then so is the capacity. See chain sizing by equation:

http://www.parktool.com/blog/repair-hel ... gth-sizing

My understanding - but only by hearsay - was that the Campag specified capacities were assuming the worst case chainstay/ideal-chain length, so if you are lucky, you can get a couple more teeth out of the setup.