My wife has a worn 9 speed 12-27 cassette and worn chain, so when replacing I would like to use a cassette with a 30T if possible so was looking for advice on cassette and Derailleur replacement.
Currently I think she has a Tiagra RD-4400 derailleur which has a max rear sprocket of 27T with 105 9-speed shifters and a 52-30 chain-ring.
So I was looking at HG50-9 11-30 cassette which would make capacity (30-11) + (52-30) = 41T.
I found Shimano 105 5701 10 Speed Rear derailleur which can take 30T on back, but capacity is 40T, so is there flex in capacity for extra 1 tooth of 41T.
I did look for a 12-30 as she doesn't need the 11T sprocket and this would make capacity 40T, but could only find IRD 12-30 cassette which is £75 compared to £20 for Shimano 11-30.
Are then any downsides to changing cassette - would there be much difference in how smooth gear changes are having a wider gear range, or because Shimano 105 5701 is both newer (I think Tiagra RD-4400 is 10-20 year old technology) and 105 is above Tiagra, would it actually be smoother?
Any advice/comments appreciated.