I am the same height as you. 1.68 short! The real problem is that around 1.68 or 1.7 is the common point to go from small to medium size frame. I had a 17.5 Trek that was properly set up and had serious back and knee pain. I kept on thinking the problem was that the bike was too small. Big and expensive mistake! I now have a 16 inch with a 90mm stem and 170mm crank. The effective top tube has reduced from 585mm to 555mm. This seems small but I now can ride my bike for as long as I like without any pain what so ever. Good luck man, there's nothing worse than back pain!
