|MadSci Network: Physics|
Lukas, your question is a good one, and requires more than one answer. First, let us think of a hard surface, such as a sidewalk. You would not slide very easily over it, would you? But if you were on a bicycle, you would be very easily moved. This is because the bicycle uses a large wheel diameter, and a small axle diameter to almost eliminate any sliding motion.
With a wheel, most of the sliding occurs at the small axle. If the axle is ten times smaller than the wheel, then the friction is reduced by a factor of ten, because of this geometry. In addition, the axle is hard and smooth, and usually is lubricated with an oil or grease. This further reduces friction. These two factors, wheel/axle geometry and axle bearing lubrication, can reduce friction by a factor of a hundred or more.
But wheels have rolling friction, and this sometimes can be large. Have you tried to ride a bicycle on soft grass, sand, or mud? If so, you know about how rolling friction can be large. Because of this, off-road bicycles have wide tires and use much lower pressures. Racing bicycles that are driven on paved roads have narrow tires, and use very high inflation pressures. You may have noticed how cars made for maximum fuel economy have hard, skinny tires, while dune buggies have very wide, soft tires. The best rolling surface for a wheel changes with the surface it rolls upon, its load, and its speed.
You should know a third factor about wheel friction. Small wheels have greater friction, when rolling over uneven surfaces. Big wheels easily 'bridge' cracks and other low spots as they roll. Small wheels, as on roller skates, drag when rolling over sidewalk cracks. Roller blades cleverly solved this problem by placing several wheels in a straight line.
To recap this for you, Lukas, wheels reduce friction by:
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