Originally Posted by RedRockRider

Biggest problem I can foresee is that the frame might be too small. If a bike fits properly with drop bars, when you convert to flat bar, you end up with the primary hand position several inches closer to the seat; i.e., the default "hoods" position is generally a few inches forward of the front of your stem. With a flat bar, the default hand position in in line with the front of your stem. And this distance is often too far to account for with a longer stem, i.e., going from the 100mm stem that is often on road bikes, to even a 130 or 140mm stem, only closes about 1/2 of the distance difference. And if you have an angled stem to raise the bar position, you get even less forward correction, resulting in a cramped cockpit.

Ask me how I know, lol.....

Discovered this the hard way several years ago building a carbon flat bar road bike with a frame and parts off of ebay. Succeeded in building a sub 19lb. bike with very good components, but I didn't correct for frame size, buying the same size frame I would ride with a drop bar, and the cockpit was hopelessly cramped. I put a monstrosity 140mm stem on it and rode it for awhile, but finally admitted defeat and took it apart, sold the frame, and used the components for future builds.