you may not know it but there are a lot of physics that go into motorbikes, such as sprockets chains weight spindles and the power of the engine. what good is a powerful engine, if you cant use it to its full potential?
friction drive vehicles:
friction drive can have advantages and disadvantages such as ease of installation and setup but just as well, it can't be used in rain or wet roads and the tires wear down much faster.
In direct friction drive the only thing that matters is the spindle size not the tire size! the bigger the spindle the faster you will go but the less torque you will have, the smaller the spindle the slower you will go but you will have more torque.
chain/belt drive vehicles:
In chain drive vehicles everything matters, the clutch the sprockets and the tire size. if you have a small front(clutch) sprocket and a big(tire) sprocket then you will have more torque than if it was the other way around but this all gets more complicated when you add a jackshaft into the equation. what i like to do to figure out speed is use percentages, for example,
B being secondary sprocket size A being clutch sprocket size.
this is how you find a percentage of your sprocket setup. to use this you would find your engines max rpms, (4 strokes typically run about 3600 rpms governed) (weed wackers/chainsaws usually get 10,000 rpms)now say you had a 13 tooth clutch sprocket and a 60 tooth secondary sprocket the percentage would be: 21.666%. now you multiply the max rpms by this percentage and that would be (in my case) 780 rpms would be transferred to the rear wheel or what ever the secondary sprocket was attached to .
a great website for the clutch/secondary sprocket setup is: http://www.diygokarts.com/speed-calculator.html
weight of the tires, they are very important they can make the difference of you flying or stalling. you know they say that taking 10 pounds off a truck tire is like taking 100 pounds off of the whole truck. this is because of leverage. the wheel is basically a round lever. and the bigger that wheel and the more weight, the more power is needed to move that wheel. so it is important to try to use lighter weight wheels