Re: Potential (fast) Road Bike build
I think there are practical reasons why you don't see road bikes with motors very often.
One is the skinny tires and light rims do badly on potholes.
No suspension. The faster you go- the worse the ride gets.
If you plan to go 40 mph - you need real good brakes. Car drivers are not aware of bikes going fast. They will pull out in front of you or a signal light changes in your path and you will slam into someone if you can't stop fast.
I swithched over to a mountain bike after diving off the road to avoid danger on my road bike. You want every possible advantage riding a powered bike.