Originally Posted by
chase.
this part is true. friction force is determined by the relevant coefficient of friction and the surface area (contact patch) involved; on pavement or other smooth surface your maximum contact patch will be on a fat slick with medium air pressure. knobbies prevent the full surface of the tire from touching the ground, thereby reducing available frictional force.
Good point. Do knobs drag in a way that produces more friction though? I would expect them to have a higher coefficient of friction than the slick rubber; I realize it's the same compound, but it seems like the knobs (being solid rubber and not a pressurized shell) would be gummier and thus drag more.
With that said, I had no trouble skidding on my 82 gear inches with 700x32 treaded tires, so there's more to it than just tires and gear inches.