do you need any "Spring assist" to launch the motor into contact with the tyre. ??
Yes it uses a balance spring to make the motor neutrally buoyant. you can see the spring in the first picture a couple of posts ago.
When the motor is in a hanging position like this setup and gravity forcing the motor away from the tire, the spring is wound up in a direction that neutralizes the motors weight. The end stop is then adjusted so that the motor just clears the tire. With the over the tire config as per my seat mount setup, the spring need to wound to in the opposite direction to ensure it stays clear of the tire. In both cases the torque reaction of the motor on start-up then give enough movement to have the motor touch tire for launch.
you do know that you can get 26" x 23mm "road race " tires to fit MTB rims ?
much cheaper than a full 700c wheel conversion.
Really? Smallest tire I have seen that fits a 26" rim is 32mm and even that was a suspect fit.
Either way, for best efficiency, the 700C rim is going to be the best option simply due to the larger diameter. Totally agree though that a 700C conversion is not a cheap option.