My girlfriend has an oldish Cannondale CAAD8 bike with Tiagra 9 speed triple. The rear derailleur is the RD-4500 GS with a 12-26 cassette. The front is a 50-39-30. She'd like to get a lower bottom gear as we plan on touring up some intimidating hills this summer... The issue is that the RD-4500 doesn't support larger than a 27 tooth sprocket at the back.
Would we be able to switch the RD-4500 for the Sora RD-R3000 GS? My hope is that it's 9 speed, so should be compatible. It has a much wider size (supporting up to a 34T at the back, total capacity of 43), whilst still supporting front chainring difference of 20.
I would then throw on a 11-34 cassette or something like that, changing the chain at the same time.
Is she likely to run into any issues? Is current-Sora worse or better than 15-year-old Tiagra in terms of smoothness etc. (though of course she'll lose out on some smoothness by widening the cassette)?
Is there anything else I should be checking - or any other good ways to achieve a similar range of gears? (She doesn't care too much about the fastest gears, rarely finds herself in them so if you have any options that involve sacrificing a bit of room at that end please go for it.) We're thinking about this option because at least she won't have to upgrade to 10 speed and change all the shifters, etc.