Among the numerous physical forces acting on a road cyclist, rolling resistance plays a crucial role. Alongside the rider's weight and air resistance, rolling resistance constitutes the third-largest force generating resistance in road cycling. Rolling resistance can account for approximately 9% of the total resistance. Therefore, it is worthwhile to take a closer look at rolling resistance for optimization.
What Causes Rolling Resistance?
Rolling resistance corresponds to the energy lost during the rolling of the tire. It results from the elastic deformation of the tires during contact with the road. This deformation generates friction, hindering the bike's movement and dissipating as heat. Additional factors include the friction between the inner tube and the tire and the road surface's texture. On uneven roads, the rider-bike system is slightly lifted, consuming energy and slowing down the ride.
Factors Influencing Rolling Resistance
Rolling resistance is influenced by the road surface, tire pressure, and tire construction (diameter, width, wall thickness, structure, and tread).
Road Surface and Tire Pressure
The road surface's condition plays a significant role in choosing the tire and optimal tire pressure. A smooth, freshly paved surface generally offers less resistance than a cracked or bumpy road.
To minimize rolling resistance on a flat surface, theoretically, a higher tire pressure is needed to reduce tire deformation and the contact area with the road. However, on uneven surfaces, excessive tire pressure diminishes the tire's ability to absorb shocks, leading to more significant upward movement of the road bike and increased overall resistance.
The graph below compares theoretical rolling resistance (blue line), measured under laboratory conditions on a rolling resistance test stand, with the actual resistance measured on a real road (green line).
It is essential to adjust tire pressure to find the optimal balance between tire pressure, road surface conditions, and rider weight. Generally, a finer road surface allows for higher tire pressure. Rider weight is also crucial: the greater the weight, the higher the optimal tire pressure.
Tire Selection
In the road cycling domain, various tire systems exist: tires with inner tubes, tubeless, and tubular tires.
Tires with Inner Tubes: Classic clincher tires with inner tubes are widespread, easy to handle, and offer a wide range of models. When using classic tires, it's crucial to ensure that the inner tube used is not too heavy and causes minimal friction. For instance, latex tubes reduce friction between the inner tube and the tire, and modern thermoplastic polyurethane (TPU) tubes weigh only about 35g.
Tubeless: Tubeless tires eliminate the inner tube, reducing weight and minimizing the risk of punctures. Additionally, tubeless tires eliminate the friction between the inner tube and the tire, resulting in lower rolling resistance. Tubeless tires are becoming increasingly popular, also because they can be ridden with lower air pressure, providing a more comfortable riding experience.
Tubular Tires: Tubular tires offer excellent road grip and are popular among professionals. However, they require special rims, and their installation is done using adhesive or tape. Repairing damage on the field is comparatively challenging. Rims for tubular tires are slightly lighter, as less material is used for the rim sidewall.
Wide Tires Roll Easier than Narrow Ones
This statement often raises scepticism, but the explanation lies in the suspension behaviour. Tires with a smaller diameter exhibit higher rolling resistance at the same air pressure because the deformation of the tires, due to the smaller air volume, becomes more significant. At the same air pressure, the narrow tire sags more and thus has to overcome more material deformation.
The shape of the tire's contact area with the road is another factor. Each tire flattens slightly at the bottom under load, creating a level contact area. At the same air pressure, both wide and narrow tires have an equal-sized contact area. While the wide tire tends to compress in width, the narrow tire has a narrower but longer contact area in the direction of travel. The long flattened section acts as a lever arm against the tire's rolling movement. The wide tire is less affected by flattening in the longitudinal direction, making it roll more easily.
The tire's construction also influences rolling resistance. The less material used, the lower the deformation. Additionally, higher material flexibility (such as with a softer rubber compound) results in less energy loss through deformation, allowing a lighter tire to accelerate more quickly.
Rolling Resistance vs. Aerodynamics
One might think that wider tires should be used. However, there is a turning point determined by the aerodynamics of the tire. Wider tires have poorer aerodynamic properties, which is particularly noticeable at high speeds, starting around 30-35 km/h. Therefore, in professional road cycling, slightly narrower tires are still used, even though wider tires would be better from a rolling resistance perspective. However, when combined with air resistance, narrower tires provide better overall performance at high speeds.
Puncture Resistance and Longevity
Last but not least, puncture resistance should be mentioned. Extremely low rolling resistance tires can be very prone to punctures, making them less practical for everyday use. Softer rubber compounds, which have lower rolling resistance, usually have more wear and therefore have a shorter lifespan.
Conclusion
Overall, these aspects are part of a complex interplay that influences not only rolling resistance but the overall performance in road cycling. Understanding these factors allows road cyclists to adjust their tire selection and tire pressure according to anticipated road conditions, aiming for optimal performance.
The rolling resistances of various tires measured under laboratory conditions can be compared on the website https://www.bicyclerollingresistance.com/. Here are some examples of tires that we also use on our Vélobsessive road bikes:
