Subject: How does wheel size affect skateboard performance?

We were trying to figure out whether large diameter wheels would be more 
efficient than small diameter wheels on our skateboards.  By efficient, we 
mean take less energy to go the same distance, all other variables being 
constant (e.g., length and weight of skateboard and wheels and the human on 
the skateboard).  We had the same question about the width of the wheels, 
i.e., whether thin wheels would be more efficient than wide wheels.  The 
frictional force should be (?) the same (=coefficient of friction x normal 
force), but speed bikers always use thing wheels, and we presume the same 
would be true for skateboard wheels.  Why?

Re: How does wheel size affect skateboard performance?