Originally Posted by
cango
There is only 1.6mm difference between the road and mountain bar grip area diameter. This is dumb except to make sure you can't move parts back and forth.
You know what else stops you moving parts back and forth? The shape of brake and shift levers. They’re not designed to go on both. But by all means go ahead and install a pair of 105 levers on your flat bars, and do let us all know how great they would be if not for the bar diameter.
Also, this is why lights, computer mounts, accessories etc all come with extra spacers; so you can install them on mountain or road bikes.