Some tires include steel in their construction, usually in the bead. This can lightly attract the Zero's magnets the whole way around the rim, and can occasionally make it annoying to set the magnet distances, since if the magnets get too close, they will try to jump all the way to the rim. In our experience, they're typically included as the OEM tire on many road or hybrid bikes. PRO Tip: Run a magnet all the way around the rim of your tire. If it is lightly attracted the entire way around, you most likely have a steel-beaded tire
In other cases, wheels can include steel pins leftover from the manufacturing process. These can cary in their size, but typically make the wheel incompatible because of the amount of attraction the magnets feel to the big chunk of steel. When riding, the magnet arrays will tend to pulse each time the steel pin goes by, and may change their position annoyingly. PRO Tip: If you run a magnet around the rim and it is strongly attracted at one point (usually opposite the filler nozzle), then you have a steel pin.