9 speed mech, 10 speed cassette

secretsam

Have an almost unused Sora rear mech, and some perfectly fine Tiagra 4500 STIs. My inclination is that the rear mech is "dumb" and it's the shifter that's "smart", so the rear mech should cover a 10 speed cassette just fine.
Or am I missing something? (and being a tightwad who won't stump £30 for a new rear mech!)

jpj84

Tiagra 4700 had a different pull ratio, irritatingly - was 4600 10 speed? That should work, as would 105 5700, DA 7900, ultegra 6700... in fact any shimano 10 speed shifters other than 4700.

Edit - just read that back and it's not dead clear...

4700 Tiagra shifters won't index, 4600 will.

secretsam

jpj84 said:

Tiagra 4700 had a different pull ratio, irritatingly - was 4600 10 speed? That should work, as would 105 5700, DA 7900, ultegra 6700... in fact any shimano 10 speed shifters other than 4700.

Edit - just read that back and it's not dead clear...

4700 Tiagra shifters won't index, 4600 will.

Apologies - yes, I found out about the 4700 horror show! The shifters are pre-4600 (4500?) with the 'washing line' cables sticking out of the side.

The Sora is an older generation (one before current) - but sounds like it works, I did try a dummy set up with the wheel in and no chain, certainly has the ability to cover the block, it's the pull ratio thingy that's the issue.