Originally Posted by
pierce
... the frame geometry of a road bike is designed around you being bent over... for a flat bar bike with a more upright riding position, the tube lengths and fork angle/rake are likely all wrong.
Flat bar shifters and brake levers aren't very compatible with the stuff on road bikes either
The tube length issue depends on how you want to ride. If you have a bad back and want an upright position, a drop bar frame is perfect to reduce reach. You can use an upright stem like an Ergotec High Charisma and bars, like Ergotec Moon Cruiser 31.8 bars, to address issues of stack if the bike came with a cut steerer tube.
Most groupsets up to Tiagra have flat bar shifters. For 105 you can use (as of late 2020) SL-RS700 shifters. Refer to bikes like the Merida Speeder (
https://www.merida-bikes.com/en/bike/2051/speeder-400) or the Giant Fastroad to see how they're being shifted with Shimano flat bar levers or check out Microshift (
https://www.microshift.com/en/).