42t expander sprocket on Tiagra

[Ootini]

Hi all,

This is (at the moment) a purely theoretical question, is it possible to add a 40t or 42t expander sprocket to a standard Tiagra 12-25 cassette? My CX bike is running a 50/34 compact double chainset with the Tiagra cassette. Which is fine for pottering around, but when the bike is loaded up with touring kit hills can become an issue. I'd rather not have to faff fitting a triple crankset so figured the expander sprocket might be an easier way to do it.
Would I need to change the rear cage / mech?

 

[TheJDog]

you'd definitely have to change the rear mech for a 40 or a 42. Standard Shimano goes only to 30T (maybe 28 or maybe 32, but definitely not 40).

 

[PpPete]

You can push the standard Shimano road derailleurs to work with a 11-32 cassette, maybe even an 11-34 with the latest ones. You'd need an MTB rear mech to go further than that.

 

[Cubist]

You'll definitely need a long cage MTB mech. Even then, not all MTB mechs work with a 42T cog, as the B tension screw won't allow the cage to shift properly. You can bodge some by adding a longer tension screw, but shifting is dodgy.

Drop these guys a line. They may be able to tell you how to go about it.
http://int.oneupcomponents.com/

 

[outlash]

Put the cross chainrings back on the bike?

tony.

 

[Cubist]

Is you Tiagra the 10 speed version? For the same money as an expander you could fit a Tiagra GS rear mech and a Deore 11-32 or even an 11-34 cassette which would probably work OK. You could fit an 11-36 cassette with a Deore Long Cage 10 speed mech, but I don't know whether your shifters would throw the same as the Dynasis shifters required to run a 10 speed MTB mech.

 

[User33236]

Put the cross chainrings back on the bike?

tony.

CX bikes usually come with 46/36 so moving back to this from 50/34 isnt going to help the OP on hills whilst touring.

 

[KneesUp]

You can get a 42-32-22 triple at Decathlon for £20. Just saying (also forgetting you'd probably need new shifters until just now)

 

[outlash]

CX bikes usually come with 46/36 so moving back to this from 50/34 isnt going to help the OP on hills whilst touring.

So smaller chainrings aren't easier for hills than larger ones? The rings on my crosser must be deceiving me....

Tony.

 

[User33236]

So smaller chainrings aren't easier for hills than larger ones? The rings on my crosser must be deceiving me....

Tony.

The lowest gear available on a standard 46/36 CX with the 12-28 cassette the OP has will be 36 front 28 rear. Using a 50/34 compact it will 34 front 28 rear. The CX therefore has a LARGER small ring than a compact.

Overall at it stands the OP has a wider range of gearing, both high and low ends, than he would have moving to CX rings.

 

[Ootini]

I think I'm going to go the easy route (well not quite) and just throw a 12-30/32 cassette on it and see how it goes with the existing rear mech. Maybe a 40t expander was a bit optimistic. Hopefully the 5 extra teeth on the back should make a bit of a difference.

 

[Ootini]

Put the cross chainrings back on the bike?

tony.

That would give me a very narrow range of gears, plus the chainset's gone.