In this blog post, we explore the nature of drag that limits bicycle speed and the possibilities when it is overcome.
What is the maximum speed a bicycle can achieve? Can a bicycle powered by human leg strength truly be faster than a motorized car? To get straight to the point, under certain conditions, a bicycle can indeed be faster. The current flat-ground speed record for a bicycle is 132 km/h, set by a Canadian cyclist at the WHPSC 2008. This record was achieved on a velomobile, a type of bicycle. A velomobile is essentially a recumbent bicycle—ridden in a reclined position—enclosed in a streamlined body. At first glance, it resembles a thumb, with only the head section protruding upward for visibility. The bicycle wheels protrude only slightly outward, minimizing the frontal air resistance typically encountered by conventional bicycles during riding.
However, evaluating a bicycle’s performance based solely on speed records is challenging. A bicycle’s true performance hinges on how efficiently it can ride under various conditions and how harmoniously the rider and machine move together. These factors play a particularly crucial role in bicycle racing. Racing is not merely a battle of speed; it is a comprehensive sport requiring strategy, endurance, and the ability to adapt to situations. For instance, on steep mountain terrain, stability and control become far more important factors than speed.
So, how fast could an ordinary person ride a bicycle? If the same bicycle is used and external factors are identical, the speed will be determined by the rider’s leg strength. So, what if the same person rides the same bicycle? The result would obviously be the same, rendering the question meaningless. But what if the same person uses the same bicycle, yet the shape of the wheels differs? The outcome is staggering. If you’ve ever seen a racing bicycle, you’ve likely noticed wheels without spokes. If you’re reading this, you’re probably wondering, ‘Why use wheels without spokes? Can the influence of spokes really be that significant?’ The reason relates to drag force.
Drag force is one of the aerodynamic elements and one of the largest external resistances a bicycle faces. For example, when running a 100m dash, the headwind makes it difficult to run forward—that’s drag force at work. The drag force discussed here is closely related to air resistance. On a bicycle, the wheel is a crucial component that propels the bike forward using friction with the ground. When considering the aerodynamics of a moving bicycle, one might assume the wheel’s area is so small compared to the total frontal area that it has little impact. However, during motion, the wheel is the most dynamically active component. Unlike when stationary, it significantly influences the surrounding airflow. While values vary by bicycle, for a typical bicycle traveling at 48 km/h, the wheel’s drag is measured at approximately 35N. This accounts for about 10-20% of the bicycle’s total drag, making it the second largest contributor after the rider.
While the rider’s contribution can be significantly reduced through posture correction, the wheel’s contribution can only be addressed through aerodynamic spoke design. So, how can this be designed? G.S. Tew and A.T. Sayer (1997) published a paper on the effects of commercially available bicycle spokes. They conducted experiments using wheels with six different spoke configurations and discovered a very interesting point. For disc wheels (flat wheels without spokes), when wind blows from the front, they showed a drag reduction effect of about 30% compared to other wheels. However, when wind blows from the side, their drag was greater than that of other wheels. This can be attributed to the disk wheel’s inherent characteristic of not allowing side winds to pass through, causing it to directly absorb the force.
However, for bicycles traveling at 80 km/h, the impact of side winds is not significant enough, which is why disk wheels are used on racing bicycles. It’s similar to how, when running a 100m dash, even if there’s a strong crosswind, you primarily feel the headwind when actually sprinting. Some readers might now question: Why do some racing bikes use a spoked wheel on the front and a disc wheel only on the rear? This is because the front wheel is responsible for changing direction.
In bicycle racing, where frequent direction changes occur, a disc wheel on the front increases the impact of crosswinds, leading to higher drag. To prevent this, spoked wheels are used on the front. Efforts to design aerodynamic wheels that reduce drag continue to this day.
I believe that someday, wheels will be researched and developed that generate minimal drag from both the front and sides. This research is not limited to bicycles alone. Reducing drag is an essential challenge for other modes of transportation like aircraft, automobiles, and yachts, enabling the development of faster and more efficient means of travel.
Thus, research on drag extends beyond bicycles, influencing various mechanical devices humans utilize. For instance, minimizing drag is crucial in the design of aircraft wings and automobile bodies. This not only allows traveling farther distances with less fuel but also has a positive environmental impact. Ultimately, research and technological advancement concerning drag are occurring across all aspects of human life. Someday, we will overcome drag and enjoy perpetual speed.
