ok, I'm confused! I have a chainset of 30/39/50 and a cassette of 12-28. I got the triple as I'm struggling with hills due to knee surgery, any advice appreciated!
Your inner chain ring the 30 you could change to a 28 or possible 26, you will need to find what
B.C.D. (Bolt centre Diameter) its a bit awkward to measure, so measure edge bolt to edge bolt or centr to centre Shelden explains better than I do. The BCD is usually 74mm for the inner (mine was odd so I had to get a new chainset).
Your current set up of 30 front 28 back gives you a ratio of 1;1.07 if you change to a 28 front it gives you a 1:1 ratio and a 26 1:0.93 which is pretty low you can go lower by changing the rear cassette for something with either 32 or 34 teeth the 34 giving you a really low ratio of 1:0.76, I found this to low though it will get you up anything just about though.
But you need to make sure your rear mech can cope with 34, some cant my sora couldnt so I changed it for an acera, when I am better I will probably revert to more road oriented gears, though my next project is to convert from 7 to 8 gears then I can go 13-15-17-19-21-24-28-34 or 13-15-17-19-21-23-26-30 and up the inner ring from the current 26 to maybe 28.
In all situations you will need to check that both front and back derailleur's can travel over the required number of teeth, i.e. the front 14 teeth if you change to a 26, it should do 12 if you go to 28.
Since your back is already 28 (12-28 = 16th) it probably will do 32 or possible 34, so your total max difference would be 26-50 on the front = 24 and 12-34 =22 on the back giving you a total of 46 which I think is getting a little much, but I am not very sure on all the specifications on all modern stuff.
