Don't overthink it.
Most of the time, the bike will just be the way to get from one scene to the next. Not even a need for a roll. If they are pressed for time, you could ask for an endurance roll, to see if they can keep up a good speed, in order to get to their destination in time. A general rule for fate is: if failure isn't interesting, don't roll.
What kind of scenes would you see that would be actually set on a bike? A chase? Just use drive as an attack roll and athletics to dodge. Or maybe even Athletics to attack as well, might fit more with riding a bike. Kicking someone could be done simply by the fists skill, like anything else. In a chase, falling off the bike would likely be a taken out result.
Declaring an unlocked bike in the street should be no problem, if you are in a city that has already been established as having tons of bikes everywhere.
