Authors: Paul Mannion 1, Magdalena Hajdukiewicz 2, Bert Blocken 3, Eoghan Clifford 4 Affiliations: 1 PhD researcher, 2 postdoctoral researcher, 4 lecturer - all civil engineering, College of Engineering and Informatics, NUI Galway & Informatics Research Unit for Sustainable Engineering, NUI Galway; 3 professor, Department of the Built Environment, Eindhoven University of Technology (Netherlands) & Department of Civil Engineering, Leuven University (Belgium)
Background
Cycling has become one of the most popular recreational and sporting activities in Ireland. Indeed, in 2014, Ireland and Northern Ireland hosted three stages of the Giro d’Italia (one of the world’s most prestigious races). The Tour de France is probably the one cycling race that evokes images within the general public; indeed it is the largest annual sporting event in the world and is widely considered the hardest (and has been one of the most controversial) professional sporting events in the world. Furthermore the role of cycling in transport policy is becoming internationally recognised.
At the highest level, engineers are often the key to increasing the gains sought by cyclists. Engineers strive to develop components and frames that are lightweight without compromising robustness and stiffness. The push for improved aerodynamics to decrease the power required to move at speed, and performance measurement requirements are driving the development of new technologies.
This article discusses four key areas where engineers play a major role in competitive cycling namely (i) cycling aerodynamics, (ii) structural design, (iii) development of disk brakes and (iv) performance measurement.
Aerodynamics
Aerodynamics holds the greatest potential for improvements in cycling speed (Blocken, Defraeye, Koninckx, Carmeliet, & Hespel, 2013). Aerodynamic drag is the resistance experienced by the cyclists from the surrounding air as he/she is in motion. It can be responsible for 90 per cent of the total resistance experienced by a cyclist at speeds greater than 50km/h (Defraeye, Blocken, Koninckx, Hespel, & Carmeliet, 2010).
On flat roads, descents and slight inclines, air resistance is the single biggest force a cyclist must overcome. Thus, even an increase in speed from 35km/h to 40km/h requires a huge increase in power. The faster one goes, the harder it is to go faster. A doubling of speed requires eight times the power to overcome aerodynamic drag.
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Figure 1: Overview of key forces, power measurement and equipment design impacting racing cyclists(click to enlarge)[/caption]
Aerodynamic drag originates from two sources; viscous and form drag. Viscous drag is a resistive force experienced by the cyclist caused by the friction effects in the boundary layer of the bike’s and cyclist’s surfaces. It can be reduced by wearing a smooth suit or by shaving the skin. Form drag is the drag on a body moving through a fluid as a result of the shape of the body and thus relates to the anthropometric characteristics of a cyclist.
It is the most important contributor to drag (up to 90 per cent) (Defraeye et al., 2010). Form drag can be reduced by streamlining the bluff body shape of a cyclist, by reducing the cyclist’s frontal area or by moving the so-called flow separation points on the body of the cyclist and bicycle further downstream. To this extent, recent innovations in skin suits include dimples located on the sides of the cyclist’s biceps and thighs for improved aerodynamic performance. Similarly, innovations in bicycle aerodynamics include placing roughness strips on parts of the frame.
Elite cyclists optimise their cycling position by means of wind tunnel testing and field tests. Previous research conducted using wind tunnel experiments and field testing shows that even minor adjustments to a cyclist’s position can result in decreased aerodynamic drag (Defraeye et al., 2013). It is important to facilitate optimal respirator functions and metabolic performance when attempting to reduce aerodynamic drag.
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Table 1 (click to enlarge)[/caption]
The optimal position therefore, is always a compromise between aerodynamics and power output. While the position of a cyclist on a bicycle is key to reducing form drag, the cyclist’s equipment can also have a significant impact (e.g. an aerodynamic helmet). Table 1 illustrates costs of aerodynamic improvements in cycling along with the associated time gain of each cost.
Studying the flow field around a cyclist is a difficult task. Performing smoke tests in a wind tunnel can shed some light on the complex flow interactions. However the use of computational fluid dynamics (CFD) tools is an engineer’s greatest asset when studying the wake flow field in detail. CFD provides the ability to analyse the wake flows of athletes; thus, identifying the causes of drag. Figure 2 illustrates how CFD can be used to explore the flow features around a cyclist.
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Figure 2: Typical visualisation of the air flow for a team time trial using CFD (Blocken and Toparlar, 2015)[/caption]
The benefits of CFD are now being widely recognised within the cycling industry. While the use of CFD is well established within motorsport, other elite sports have only recently embraced its full potential. This is visible in the Olympics, with CFD being used to analyse swimming, skiing, bobsleighs, running and many more. Olympic gold medals can be one by tenths of a second, (Hart, 2006) and CFD can be used to acquire this additional time.
Integrating structural requirements with aerodynamic design
Modern road bikes (figure 3a), though very streamlined, rely on a more rounded frame design for structural and stiffness purposes. However, the time trial (TT) bike relies on a tear drop and very tight frame design (figure 3b). All components are housed within the frame, which is designed to reduce turbulence and drag as much as possible.
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Figure 3a: A standard road bicycle is shown on the left, illustrating a streamlined but stiff design[/caption]
Engineers must also consider the system as a whole and best integrate its components (e.g. the interaction of the brakes/cables with the wheels and frame). CFD modelling can be extremely useful with these tasks alongside wind tunnel testing. Seat-stays and the bicycle frame can be designed to direct air over the brakes and areas of high flow separation can be identified and redesigned/eliminated.
Aerodynamic components are not always the most ideal solution for the conditions in which cyclists have to perform in. Aerodynamic frames can be heavier than their equivalent road bike frame. Tear drop shaped frames can suffer from a lack of stiffness or increased lateral deformation. This results in power losses for the cyclist. Similar issues apply to the wheels where, for example, fewer spokes qhile being more aerodynamic, can mean less rigid wheels.
The expense of aerodynamic components further leads to reduced popularity. UCI (Union Cycliste Internationale) has strict rules on the use of aero components in both road racing and triathlon TT racing. This is to limit the impact of equipment on performance, and to make sure the race victory goes to the best cyclist, instead of the best machine.
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Figure 3b: A triathlon TT bike is shown on the right which takes aerodynamic design to the extreme. Teardrop shapes comprise of almost the entire frame[/caption]
Carbon fibre has become commonplace in bicycles over the past 10 years. It has an extremely high strength to weight ration coupled with high stiffness making it ideal for cycling applications. It is possible to manufacture very light frames and wheels, not only for standard road bikes, but for aerodynamic shapes on triathlon bikes also. Thin and tear drop shaped frames are possible. Furthermore, carbon fibre is anisotropic, thus it can be strengthened as required in particular directions.
Disc brakes
Disc brakes have been legalised for the professional road peloton in 2016, which introduces a dramatic change for all competitive cyclists. There is safety consequences associated with the use of disc brakes which is why they have been banned in road racing since their development, although they are commonly found in mountain cycling events.
However, mountain cyclists are not subject to the same conditions as road cyclists, for example, where heat generated by the brakes during an 80 km/h descent could result in severe skin burns in the event of a fall. Furthermore, a spinning disc can cut through flesh in the event of a crash. Wheel interchangeability may also become a new problem for cyclists with the introduction of hydraulic brakes. Aligning the brake pads so that no rubbing occurs is not a quick job.
Furthermore, the combinations and permutations possible for parts will be a challenge for neutral support at major racing events. While rim brakes transmit power directly to the wheels via the rim, disc brakes transmit power from the rotor to the hub, then to the spokes following on to the rims and tyres. Wheels may need to be evaluated/redesigned to support these additional stresses.
Electronics and performance measurement
Power measurement has seen major advances in recent years with systems becoming lighter, cheaper, more flexible and increasingly accurate. Power systems generally measure power in the rear wheel hub, the cranks, pedals or at the chain wheels (inside the spider of the crank set). These systems, in various ways, use strain gauges to measure the power output by the rider.
Some of the systems allow for other information such as torque, applied force, pedalling efficiency and separate measurement of the left and right legs. All of this information is fed wirelessly back to the rider via an on-board computer and can be very useful when combined with real-time heart rate data.
This data is combined with a multitude of data available from GPS units and other sensors to give the rider a complete picture of their training or racing effort that includes speed, distance, cadence, climbing speed, elevation climbed and air temperature. Such data can now be overlain onto video images from on-board cameras to give the rider or TV audience a real-time view of what the rider sees and their current statistics (e.g. speed, power output etc).
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Figure 4 Development of electronic components in cycling[/caption]
Post racing or training cyclists will upload this information to various online software platforms that enable complete analysis of the training or racing effort and comparison with peers. The systems have become increasingly robust and reliable and are now (with the exception of power which is mostly used by racing cyclists) in widespread use among all cyclists.
The major development in the area of components in recent years, alongside disc brakes, has been electronic gears. Most people will be familiar with mechanical gear shifting systems on bicycles, however component manufacturers have focused heavily on both wired and wireless gear changing systems.
These have required major developments in battery technology and gear design to ensure such systems are suitable for the elite market. The engineer’s challenges include designing robustness against weather, debris from roads and crashes. The development of low maintenance and light components alongside durable battery technology can be seen as a technology revolution in the cycling world.
NUI Galway and Eindhoven University of Technology research project. This article stems from a new research collaboration between the College of Engineering & Informatics at NUI Galway, the Eindhoven University of Technology and is supported Cycling Ireland, the Irish Institute of Sport, and Paralympics Ireland.
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