Consider a single proton moving at velocity "v" from left to right through the uniform magnetic field. The force on the proton will be perpendicular to both the direction of motion and the direction of the magnetic field. This means that the proton will initially experience a force that is "up" in your drawing.
Now, as an extension of this example, assume that you have some type of "container" for a group of protons, and you orient this container in a "vertical" direction in your drawing, and move the container at velocity "v" from left to right through the uniform magnetic field. All of the protons in the container will feel an upwards force on them, and they will tend to accumulate at the top of the container.
As a further extension of this example, the spokes of the bicycle wheel are normally composed of a metal, which typically has one free electron per atom available to move within the metal. In reality, this makes the wheel's spokes the containers for the charged particles, which are actually electrons rather than protons, and those electrons moving through a magnetic field will also feel a force on them. Despite the fact that the bicycle wheel is turning in a circle, and despite the fact that the magnetic flux experienced by the bicycle wheel is constant, the electrons in the spokes are always moving perpendicular to the stated magnetic field, so they always feel a magnetic force on them. This means that electrons will move towards the center of the wheel, and there will be a net positive charge on the wheel's periphery, resulting in a voltage gradient between the rim of the wheel and the center of the wheel.