Originally Posted by VictorKnox

I'm an inch taller than you with the same inseam, but I'm on a larger frame, 60cm. (60cm top tube with 100mm stem.) I was having the same issues as you, dealing with lower back pain. At first I thought it was a result of being too stretched out, but raising and/or shortening the reach did not help. I also realized that when sitting on the bike just coasting, I didn't feel too stretched, but the back pain persisted even on short rides. What finally worked for me was moving the saddle forward. A LOT. Something like 2cm or more. Note that I did not do this to shorten the reach, I did it to change my position in relation to the pedals. I'm no expert, but I feel like before with the saddle where it was, when I pushed on the pedals, I had to push my legs forward, which rolled my hips back, and arched my back. I used to have to consciously think about rolling my hips forward and keeping my back straight/flat. With the new forward position, my hips and back naturally fall in to this position, and I have NO pain in my back. Plus, I'm actually thinking I need to now buy a longer stem since I shortened the reach quite a bit when I moved the saddle.

I hope you find something that works, I know how frustrating it is to deal with fit issues.