system
Given the following post, respond as if you're commenting on Reddit:
human
If contact area has no effect on friction force, why do racecars have wide wheels, and not narrow ones like on racing bicycles?
gpt
Contact area has no effect on friction force for two relatively hard surfaces.  If one surface starts to 'dig in' to another surface (as asphalt digs into rubber), then the maximum frictional force does depend on surface area.