We were inspired by DIY wind turbine generators to design bike dynamo. They used a dual axial flux generator consists of powerful magnets placed on either side of a fixed ironless stator winding, like a sandwich creating a very strong magnetic flux between them, and thus inducing an electric energy in the stator winding. Since the stator windind core is non magnetic(air) there is no additional resitance (cogging torque) for rider. Here are details about this type of generator [1].

Two rotating iron parts that hold magnets is made by laser cutter with steel thickness 4mm.  Stator holder with coils and magnet separators are made from acrylic same way. Production drawings are attached in file section.
In our design we used rectangular magnets and round coils. Isn't an optimal desing but round colis are easy to made. For better power performance coils must have also rectangular shape as magnets. All the coils has same winding direction and are conected serial-parallel as shown in the Figure 1. The reason why we did this is to reduce the voltage and increase the output current. If we used coils of thicker wire with a lower number of turns we could connect them only in series

Figure 1

Our generator is not actually a dynamo, which creates direct current (DC), because produce alternating current (AC) whitch is convert to DC by rectifier and capacitor. Block digram is shown in the Figure 2 below. Since the output voltage dependent on the wheel speed we used DC-DC step down converter to get constant voltage 5V. Converter is adjutstable so you can choose other output voltage as well.

Figure 2

Here are some power measurements on Table 1 done with variable resistive load. Output current value was measured when the voltage reach 5V. For every speed was value of resistor different. We decided to choose five voltage output thus is compatible with most of power banks. We don't recommend attaching the sensitive devices (phone,gps) directly to the converter output as it may damage them or if you stop riding lights turn off :-))

Table 1
Speed  [km/h]5202530
I  [A]0.350.60.851
Power  [W]1.7534.255

[1] https://www.semanticscholar.org/paper/Design-and-Testing-of-a-Permanent-Magnet-Axial-Flux-Price-Batzel/60b14f0e54edbf4b638026bfd08007abaea9de13?navId=extracted