Direct drive hub mounted motors are constrained by low rpm operation and power delivery is limited to how much brute amperage you can force through them. They'r

Direct drive hub mounted motors are constrained by low rpm operation and power delivery is limited to how much brute amperage you can force through them. They're often designed with a large diameter to compensate for the low rpm issues, which increases weight. Some bicycle-scale hub motors have built-in gear reduction, which reduces the available motor size and adds a barrier between the motor and air cooling. They're more efficient and lighter than direct drive motors, but heat retention limits constant power output. Watt-for-watt of power output, frame mounted motors can be much smaller and lighter. They can have excellent cooling (including liquid coolant circulated to a front mounted radiator), gear reduction allows them to operate at efficient speeds, they can be designed to target only efficiency rather than compromising to match hub size, and can be scaled fairly arbitrarily.