There is surprising a lot of physics that goes into a simple motorized bike. Even something simple on it like the weight of the tires. They are very important and in many cases can be the difference between you flying or stalling. This is because of leverage on the bike and the wheel is basically just a round lever. That means that the bigger the wheel and the more weight, than much more power is needed to move that wheel. This could be needing a bigger motor as a whole or may just slow the bike down. Another factor is the friction that goes into these bikes. Friction drive can have both advantages and disadvantages such as ease of installation. However it may not be able to be used in rain or wet conditions and the tires may wear done much faster. That is where the designers of all these bikes have to decide the ratios they want or friction between all these factors that come into play.