A 50/39 would make sense for me. I have an 12-25 10 speed Ultegra in the rear. The small ring 39 is fine for me as I rarely go beyond the 23 in the rear on the hills. But I'm a little bit of a masher and like to ride in the large ring in the front.At the speeds I ride I find myself in the larger rings in the back especially up the milder hills and rarly go fast enough for the smallest cog. A 50 would keep me more offen in the middle of the back and make it easier to go up more difficult hills without going to the small chain ring in the front. Can I just replace the large front chain ring??