Shimano cassette and rear mech compatibility?

Lagavulin

Currently running a standard 53/39 with a 12-25 cassette. There's not many hills around me but the ones that do exist are fairly skyward and I do occasionally find myself searching for a gear that doesn't exist.

My new bike will almost certainly have a 50/34 Compact and 12-25 cassette. With this in mind I think my Allez could do with an easier gear or two. Rather than switch to a Compact or Triple and the expense that incurs, if I went to a 12-27 block at the back could I get away with the existing short-cage rear mech or is that long-cage territory?

If it does mean replacing the rear derailleur is it a fairly simple job or is there a lot of faff involved?

JWSurrey

I can only go on what my 105 bike comes with....
Firstly, if you have a a 34t chainring at the front, then you're already using a compact.
The largest Shimano branded cassette is the 12-27, and mine has the longer cage Shimano derailleur.

If you'd gone for Campagnolo, you would be able to use a short cage with the 12-27.
Campag's medium cage is only required for the larger standard cassette: 13-29, with the compact.

Changing the derailleur is very easy.
Firstly, you may find you need a slightly longer chain.
Sheldonbrown.com has details on how to calculate the required new length - though you could probably get away with it as it is - just test it out.

Other than unbolting the old, and bolting on the new, having greased or threadlocked the hanger thread to ensure it will come off next time you need to maintain it, It's just a case of reattaching the cable and adjusting the top/bottom stops, plus the tension screw.

Park Tool is another good website to reference for maintenance instructions.

robbarker

Shimano short cage rear mechs are fine up to 27 teeth.

http://cycle.shimano-eu.com/catalog/cyc ... 6690738430

You're supposed to fit a gs cage version if you run a compact, although in practice the short cage one does work.

Compact chainsets are not the answer to life, the universe and everything that some people seem to think. They are flavour of the month at the moment but do come with downsides - you find yourself running crazy chainlines a lot of the time and the big difference in front cogs can interrupt your rythym. 53/39 and 12-27 is a nice combination too and there is an argument for a triple if that doesn't give you enough gears.

I'm seriously thinking of changing my 50/34 back to the standard Ultegra 53/39, despite the hills round here..

aracer

JWSurrey wrote:

If you'd gone for Campagnolo, you would be able to use a short cage with the 12-27.

...if Campag actually made a 12-27 (is the one major ratio missing from their range IMHO) - therefore the comparison is unfair, since I suspect if they did offer one they might also mandate a medium cage, noting that there's only 1 tooth less capacity required than for the 13-29.

In theory based on the specs, the short cage Ultegra doesn't have enough capacity for 50-34 and 12-27 - that's a total of 31, and the spec is only 29. However it certainly won't be a problem as long as you don't mind not using the 12 and 13 with the 34, and those are gears you shouldn't really be using anyway (and if you do want to use them they'll probably work just with the chain a bit slack) - that's assuming you size the chain to allow the 50/27 to work, which is recommended. Of course according to the specs the short won't work with 50-34 at all anyway, but I'm not convinced that is really the case, as plenty of bikes are specced that way.

Lagavulin

Thanks very much for the replies.

Forgive me, can't believe I didn't state what groupset I'm currently using! :oops:

New bike will be Campag Centaur (50/34 and 12/25) but I'm not really concerned about the gearing on that as yet (current lowest is 42.1" so 36.7" appeals).

My current bike is 9-speed Tiagra (53/39) but with a 105 rear mech (circa 2005/2006).
Current stead will become crap weather/winter bike so a little concerned that when its time to use it it'll be over-geared.

andy_wrx

You need a long cage mech with a triple because of the difference at the front, not just at the back
i.e. (whilst it crosses the chainline up badly so you shouldn't use it, but people will...) with a 52/42/30 triple and a 12-25 cassette it needs to cope with extremes of 52x25 and 30x12
- there's a lot of difference between these, a chain long enough to cope with 52x25 will slop about everywhere on 30x12, so you need a long cage mech to take up that much slack

Many people run 50/34 compacts with 12-27 casettes and use short cage rear mech, and that's extremes of 50x27 and 34x12
(and using a compact I reckon people probably do run those extremes a lot more often then you would ever do with a triple)

So answering your original question, if you fitted a 12-27 cassette in place of 12-25 on your 53/39, your existing short cage rear mech will be fine with the lesser extremes of 53x27 and 39x12.

You should lengthen the chain - it would be too short if you did use 53x27
- but hey, doesn't the chain need its regular replacement anyway ?

You do also seem to be querying however whether a 27 cassette will give you a low enough gear...
If it doesn't, you'll have to take one of the fit-compact/fit-triple/fit-MTB cassette&mech routes as described in 3 or 4 other threads I've read here just this week !

aracer

andy_wrx wrote:

You should lengthen the chain - it would be too short if you did use 53x27

Not necessarily - depends how you sized the original chain. If you use Shimano's standard formula for doubles (rear mech pulleys vertical when in top gear) you get a longer chain than by the wrap around big-big method for a 12-23 or 12-25 cassette.

robbarker

Lagavulin wrote:

Thanks very much for the replies.

Forgive me, can't believe I didn't state what groupset I'm currently using! :oops:

.

The thread title gave us a bit of a steer..... :-)

aracer

Though some of us got slightly distracted by the mention of compact when he hasn't got that on the bike in question - and of course a 12-27 with a standard chainset is within spec for a short cage mech (indeed that's what I ran for sportives last year, so it definitely works).

robbarker

53/39 and 12-27 is even an officail Mr. Shimano approved combo with a short cage mech.

I've definitley decided to go back to a "normal" chainset this year - I've put the 53,39 on this afternoon (and built a wheel, but that's another story) and ordered a 12-27 cassette from Parker's.

The compact backlash starts here!