Miles wrote:You can get a greater specific torque with an iron core. That's what is needed for the Challenge.
I looked at it from a different angle... In my opinion you need to convert
electrical power efficiently into mechanical power, independent of whether
you want to race uphill or just ride around on the flat. Core laminations help
with efficiency but only in a certain rpm range (too low rpm -> cogging,
too high rpm -> eddy current losses). No iron core gives a lower specific
torque but this can be solved by chosing a different gear ratio so I wouldn't
say this is a disadvantage of having no iron core.
For me in the end the efficiency with no core was good enough, the disadvantages
of the iron core outweighed its advantages.