Road resistance, more generally called rolling resistance, in bicycle tires is the friction generated as the tire moves along the ground. As the tire touches the road, it flattens slightly, then returns to its curved shape when it leaves the ground. The change in tire shape causes the friction. A variety of factors affect the degree of rolling resistance a tire experiences. However, tires experience other types of resistance, though much less significant ones, such as air resistance, bicycle weight and chain friction.
At the same pressure, wide and narrow tires have the same area of tire in contact with the ground; wide tires flatten over their width while narrow tires flatten over their length. The longer flattened area of the narrow tire yields proportionally less roundness in the tire and thus more rolling resistance. In the wide tire, a smaller proportion of the tire circumference flattens against the ground, making the tire rounder and easier to roll. However, narrow tires should be inflated to higher pressures than wide tires, and do not function well at wide tire pressures, so to a certain degree, this comparison is moot. The ride may be more bumpy on a narrow tire, but there is less air resistance against the tire and the lighter mass of the wheels makes the bicycle more agile and easier to accelerate.
According to Schwalbe North America, a tire manufacturer, a smaller tire diameter will have greater rolling resistance. Tire deformation on a small tire is larger as a proportion of the tire size; therefore, the tire is less round when ridden. However, Gilbert Anderson of North Road Bicycle Imports and expert cyclist Jobst Brandt assert the lightweight strength of a smaller tire, such as a 20-inch, 24-inch or 26-inch tire, with thinner tread and lighter tubes can give lower road resistance. Gilbert Anderson states that a well tuned road suspension system on a bicycle with smaller tires can offset rolling resistance issues.
Bicycle mechanic and expert Sheldon Brown points out that a thin and soft rubber tire is more flexible and provides less rolling resistance. A flexible tire loses less energy through tire deformation against the ground. However, soft rubber tires are also more fragile and will tend to wear out sooner.
The higher the air pressure in the tire, the less the tire deforms against the pavement and the less you experience rolling resistance. However, tires that are overinflated make your ride bumpier, increase the danger of tire blowouts and reduce traction. Sheldon Brown advises that although road resistance decreases with higher pressure, modern high-quality tires have so little resistance that any benefits are outweighed by the downsides.
Smooth or slick tires will generally experience less road resistance than tires with knobby tread. According to expert cyclist Jobst Brandt, thick tread with wide gaps creates more resistance on the road, because the rubber of the tire bulges into the gaps between the tread when the tire hits the road. In off-road situations, however, the increased traction makes treaded tires much more efficient. Inverted tread tires will have less rolling resistance than regular tread tires, but more than slick tires.