Tiagra groupset, largest (smallest?) usable cassette?

[nickAKA]

Morning folks,
My Btwin bike has a typical 10 speed Tiagra 4700 groupset with compact crank. I'm thinking of putting an alternative cassette on a spare rear wheel for the very hilly stuff; The current setup is an 11-28 cassette, I've got an 11-32 cassette which I'm hoping I can use... the bloke in the LBS reckons this won't work (he thinks max I'd get away with on a compact crank is a 30 which aren't that easy to come by plus buying another cassette - http://www.wiggle.co.uk/miche-primato-10-speed-cassette-shimano-1/).
Bearing in mind "bloke in LBS" has been wrong about (lots of) things I've asked him, is there a hard & fast rule on compatibility of cassettes / groupsets?

TIA

EDIT - crank a red herring? Rear mech / chain length the issue?

ANOTHER EDIT - does this suggest compatible? http://productinfo.shimano.com/#/com/2.0?cid=C-544&acid=C-256

 

[si_c]

It'll depend on your rear derailleur cage. If you have the GS (Long Cage version) it'll take up to a 34t with the possible need to make an appropriate adjustment of the B-tension screw. If you have the SS cage, then he's probably right, you would likely get away with a 30t, despite the shimano docs saying 28t max. The same holds for the long cage too though btw, you'll likely get away with a 36t sprocket instead of the 34t max.

A relatively easy way to tell which derailleur you have is to look at the cage, if there is an inch or more of space between the teeth of the jockey wheels then you have the long cage. Otherwise you have the short cage.

A long cage derailleur is about £25 or so anyway, so not the end of the world if you replace your derailleur.

 

[jiberjaber]

FWIW I'm running an 11-36 10spd on my Tiagra RD (50x34 front)

 

[nickAKA]

FWIW I'm running an 11-36 10spd on my Tiagra RD (50x34 front)

I assume that's the long cage then?

 

[DCLane]

Long cage derailleur and/or a derailleur hanger extender: https://www.sjscycles.co.uk/gear-spares/aluminum-rear-derailleur-hanger-extender/

The latter solves your problems. WolfTool's is £22 but there are cheaper alternatives like the one above.

My son runs a Tiagra 11-36 setup on his cross bike using a long cage derailleur but we're looking at an 11-40 with the above adaptor.

 

[nickAKA]

Long cage derailleur and/or a derailleur hanger extender: https://www.sjscycles.co.uk/gear-spares/aluminum-rear-derailleur-hanger-extender/

The latter solves your problems. WolfTool's is £22 but there are cheaper alternatives like the one above.

My son runs a Tiagra 11-36 setup on his cross bike using a long cage derailleur but we're looking at an 11-40 with the above adaptor.

Out of stock (typical)... Got them on ebay but, y'know, ebay...

 

[jiberjaber]

I assume that's the long cage then?

Yes, it's on a 2015 CdF 20 so a Shimano Tiagra RD-4601-GS

 

[youngoldbloke]

Out of stock (typical)... Got them on ebay but, y'know, ebay...

I bought one from the far east on ebay not long ago, no problem.

 

[nickAKA]

Back

I bought one from the far east on ebay not long ago, no problem.

Back in stock today, ordered one from SJS but might pick up a spare off ebay to see how they compare - odds on they're identical!

Checked the RD, looks like it's the long cage (big gap between the jockey wheels) so I should be covered whatever.

Many thanks for your help all!