Minimising aerodynamic resistance through rider position is one of the most effective ways to improve performance among well-trained athletes. Recent studies utilising modern aerodynamic bicycle geometries suggest that the rider contributes \(\approx\)80% to the total aerodynamic resistance acting on the bicycle–rider system [14]. As the rider contributes the largest proportion to the aerodynamic forces, optimising the aerodynamics of the body will likely see the largest gains in cycling performance. The greatest influence one can have on the aerodynamics of the rider is through the adjustment of cycling position. This was identified in an early wind tunnel study conducted by Kyle and Burke [3] which led them to propose a three-tier hierarchy for reducing cycling resistance: (1) the position of the rider, (2) the geometry of the bicycle (or more generally cycling equipment), and (3) the methods for minimising the rolling resistance and drive-train friction losses. Although the biomechanics and physiological efficiency of cycling are outside the scope of this review, when optimising cycling performance, the power output and fatigue characteristics of cyclists must also be weighed up against any apparent gains in the aerodynamic performance through adjustment to position [47,48,49]. Any changes to rider posture must also be considered along with current UCI rulings on legal rider positions.

Wind tunnel testing of rider position

Fig. 8 Reprinted from Gibertini and Grassi [8], p 32–33, with permission from Springer The traditional positions and the time-trial position. Full size image

The importance of position has prompted many wind tunnel investigations into the main positions used by elite cyclists, which are depicted in Fig. 8. Table 4 shows the reported drag area and drag coefficients from the wind tunnel testing of cyclists in various positions. Overall wind tunnel investigations are largely consistent in the relative ranking of these postures in terms of aerodynamic performance. The time-trial position has the lowest aerodynamic drag followed by the drops position and the upright break hoods and stem positions exhibiting the highest aerodynamic drag. Average wind tunnel data suggest that the reduction in drag between an upright sitting position with straight arms (such as the stem and hoods positions) and a drops position can be as much as 15–20\(\%\), and for the time-trial position as much as 30–35\(\%\). However, these are only average results and drag area and coefficient measurements for the time-trial position widely used today vary by as much as 40\(\%\) between separate wind tunnel studies, and as much as 60\(\%\) between wind tunnel studies and other indirect methods of determining drag [50].

There are a number of reasons why reported aerodynamic forces and coefficients vary significantly for each of the main positions between separate wind tunnel investigations. Differences in atmospheric conditions, drag measurement devices, wind tunnel type, blockage effects, Re effects, and freestream flow quality are all specific characteristics of wind tunnels and all affect aerodynamic force measurements [32]. Another source of variation between wind tunnel investigations into rider position is differences in test methodologies and whether tests have been conducted with static or pedalling riders. The time-averaged drag force is not necessarily well represented by a static cyclist and significant variations in aerodynamic drag between static and pedalling cyclists have been reported [51]. Current research suggests that the drag coefficient of a pedalling cyclist is \(\approx\)6% higher than that of a static cyclist holding a horizontal crank position [12]. Although it is difficult to make direct comparisons between different wind tunnel studies, which may not state the specifics of the testing environment and equipment used, the greatest contribution to the dissimilarities in the research is most likely due to rider aspects such as variation in rider position and anthropometric characteristics (rider size/shape).

Table 4 Reported drag coefficient and drag area measurements from wind tunnel testing of cyclist position Full size table

Despite many wind tunnel investigations into the aerodynamics of cyclists, these have not been able to explain the large variation in aerodynamic drag that is observed between different rider geometries and subtle changes to position. As the drag force is sensitive to rider shape and position, it is difficult to identify specific rider attributes that contribute significantly to the large variations in aerodynamic drag that have been observed among cyclists for a given position. A study by Zdravkovich et al. [53] looked at the drag coefficient for two different athletes of similar height and mass, and a 1:2.5 scale model of a bicycle and rider in the brake hoods position, drops position, crouched drops position, and the time-trial position. Wind tunnel measurements showed that the brake hoods position had the highest drag coefficient followed by the drops and crouched drops position, with the time-trial position recording the lowest drag coefficient. However, there were large variations in the drag coefficient between each of the two athletes and the model for similar positions. This was most noticeable between the two athletes with the drag coefficient varying as much as 30\(\%\) between them for a similar position. This led Zdravkovich to conclude that a single value of drag coefficient cannot be specified for any one position or cyclist, a result of the strong dependence of the drag coefficient on the size and shape of the rider.

Other studies by Gibertini and Grassi [8] have also looked at the effect that position can have on how streamlined a rider is. In contrast to findings by Zdravkovich et al. [53], wind tunnel tests of an experienced rider in the stem, brakes hoods, drops, and time-trial positions revealed that the most streamlined position for this rider (indicated by the drag coefficient) was not that of the time-trial position (0.792) but of the brakes hood position (0.760). This was despite the projected frontal surface area being 37\(\%\) higher for the brakes hood position. Drag area measurements for the brakes hood position however were 30\(\%\) higher than those for the time-trial position, indicating that it was more important to reduce the frontal area for this particular rider.

Although minimising frontal area is clearly important, as demonstrated by the widespread use of the time-trial position, frontal area is not always the dominant factor when comparing the aerodynamic drag of different riders in similar positions. It is a common misconception that the most aerodynamic riders and positions are the ones that also exhibit the smallest possible frontal area. As the drag coefficient will vary with frontal area (due to change in rider position), minimising one will not necessarily result in a minimum in the drag area. The degree to which the drag coefficient can affect the performance of a cyclist is highlighted in two separate studies reported on by Bassett et al. [59]. Both investigations involved measurements of aerodynamic drag and frontal surface area of cyclists in a wind tunnel at 13.3 m/s. The findings demonstrated a weak correlation between measured aerodynamic drag and frontal area, of which the frontal area only accounted for \(\approx\)50\(\%\) of the variation in drag between the different athletes and their positions.

There have been many ‘rules of thumb’ developed regarding optimal positioning of a cyclist’s arms, legs, torso, and head [57, 60,61,62]. Even relatively minor alterations to one’s time-trial position can have a large effect on aerodynamic drag. Broker [61] and Kyle [62] note that rider positions that result in a flat back, a low tucked head and forearms positioned parallel to the bicycle frame generally have low aerodynamic drag. Wind tunnel investigations into a wide range of modifications to standard road cycling positions by Barry et al. [55] showed that that lowering the head and torso and bringing the arms inside the silhouette of the hips reduced the aerodynamic drag. Positions that resulted in reductions in aerodynamic drag were also related to a lower velocity deficit and turbulence levels in the wake. Studies by García-López et al. [51] and Underwood et al. [49] have shown that reducing the torso angle generally results in a reduction in aerodynamic drag. However, these studies also showed that minimising torso angle did not always lead to the lowest aerodynamic drag readings.

The effectiveness of rider equipment, such as bicycles and helmets, is also dependent on the position and type of rider [51, 63,64,65]. For these reasons, the most effective method to optimise a cyclist’s aerodynamic performance to date has largely been through a trial-and-error approach to force measurements in a wind tunnel. The position of the cyclist, usually defined by the set-up of the bicycle (handle bar and seat positions), and cycling equipment are continually refined until rider position and equipment configurations are identified which result in a lower drag compared to baseline force measurements. Current studies into cycling position have primarily focused on the variation in aerodynamic drag with posture as this directly relates to cycling performance. The direct link between the measured variations in the aerodynamic drag force and the flow field around different cyclist geometries is currently not well understood.

Cycling equipment—design for aerodynamic performance

Fig. 9 a CFD simulations by Fintelman et al. [43] comparing isosurfaces of the pressure coefficient coloured by velocity for 0 and 60\(^\circ\) flow yaw angles. It is evident that cross-wind conditions will induce asymmetries in the location at which flow stagnation and separation will occur leading to asymmetric pressure and flow field distributions around the bicycle and rider. Reprinted from Fintelman et al. [43], p 37, ©2015, with permission from Elsevier. b Generalised depiction of the influence freestream turbulence can have on transition and mixing from Bearman and Morel [66]. Reprinted from Bearman and Morel [66], p 103, ©1984, with permission from Elsevier Full size image

Despite tight UCI regulations on streamlining equipment, aerodynamics is a major design criterion of elite-level cycling equipment. The footprints of aerodynamic styling are embedded all over the designs of bicycle frames, wheels, helmets, and skin suits. In addition to reducing weight, improving power transmission, and bicycle stability and bicycle control, enhanced aerodynamics offers equipment manufacturers a direct link to increasing rider speed and improving cycling performance. Savings in aerodynamic drag due to superior equipment that does not involve altering rider position are often referred to as ‘free energy’ as performance gains do not require lengthy training programmes or changes to cycling technique. Although aerodynamic styling targeting drag reduction is often the most visual and recognised aspect of streamlined equipment design, aerodynamics is also critical to other equipment design criteria. These include maintaining stability and control during windy on-road conditions and improving athlete cooling and heat transfer, which is important for endurance events.

To effectively improve aerodynamic performance, cycling equipment must be designed for the local flow field in which it is operating. The true measure of the aerodynamic performance of equipment is not how well it performs in isolation, but how well it is integrated with the complete bicycle–rider flow field. Much of the early work on improving the aerodynamic performance of cycling equipment was done separately from the rider. There are many examples where measured aerodynamic savings resulting from new equipment designs have been significantly reduced or are non-existent when the rider is added to the system [61]. Clearly, the dominant impact of the rider on the global flow field and flow interactions occurring between equipment and rider must be considered to effectively optimise equipment and rider aerodynamics. Performance parameters resulting from studies and equipment designed in isolation of a complete bicycle/rider system should be treated with caution.

The other main consideration when optimising the aerodynamic performance of equipment is the environmental conditions that will likely be encountered on the road or track. Road cyclists compete within a turbulent atmospheric boundary layer that exhibits gusty wind profiles that are rarely aligned with the direction of travel. Cross-winds result in flow asymmetries being generated around the bicycle and rider, as demonstrated in Fig. 9a, which not only affects the magnitude of the aerodynamic drag force but also generates additional side forces, rolling, and yaw moments. These forces and moments can result in a cyclist being unable to maintain control of their bicycle. Typically, aerodynamic styling to minimise drag is at odds with reducing aerodynamic side loads, rolling, and yaw moments and is why aerodynamic design to minimise these forces and moments is particularly important at the elite level. Gusty cross-wind conditions have resulted in a number of elite cyclists losing control during windy road racing events [67, 68]. Although not as severe as on the road, cyclists in a velodrome also experience asymmetric flow conditions when in close proximity to another athlete or while negotiating corners of the track. Recently, this has led to the development of bicycle frames and wheels by equipment manufacturers specifically for asymmetric flow conditions experienced while circling the velodrome [69].

Atmospheric and freestream turbulence characteristics are another critical aspect of environmental flow field conditions that can have a significant impact on aerodynamics performance. Effective design for turbulent ‘on-road and on-track’ conditions is an area that is not well understood for complex three-dimensional geometries, even in much more advanced fields of bluff body aerodynamics such as road vehicles. In the relatively controlled environment of the velodrome, cyclists are still embedded in a turbulent flow field resulting from wind currents generated by natural or forced convection and also the decaying remnants of turbulent eddies left in the wakes of team members and other competitors. The exact mechanisms by which freestream turbulence influences flows around bluff body aerodynamics are complex and often difficult to predict. For simple geometries, the effects of freestream turbulence are known to induce transition to turbulent boundary layers sooner (effectively reducing the critical Reynolds number) and increase mixing and spreading rate characteristics of turbulent wakes, both of which can have significant implications on the magnitude of aerodynamic forces. A simplified schematic of these processes from Bearman and Morel [66] is depicted in Fig. 9b. Given that current standard practice is to set rider position and optimise equipment designs in low-speed, low-turbulence wind tunnels, that in many scenarios will not be representative of track conditions, techniques and methods for tailoring equipment aerodynamic performance for turbulent flow fields are currently not well developed.

Bicycle frames

Surprisingly, little has been published in peer-reviewed articles that focus specifically on the aerodynamics of bicycle frames. The most notable exceptions are that of Zdravkovich [70] and Parker et al. [65] who performed early investigations into methods to improve bicycle frame aerodynamics. In an attempt to streamline a traditional round tube frame, Zdravkovich [70] looked at the effectiveness of adding splitter plates. Aerodynamic savings were limited using splitter plates and it was concluded that a much more practical method of reducing drag on the frame was streamlining the tubing (using tear-dropped or airfoil cross-sections). Parker et al. [65] showed that the aerodynamics of the frame could be improved by adding a faring to close the main triangle of an open frame, which is now illegal under current UCI regulations. Parker et al. [65] also highlighted the importance of rider position on frame aerodynamics. They showed the potential to improve rider aerodynamics through decreasing the width of the bottom bracket and reducing the gap between the legs and both an open and a closed frame geometry. Apart from these and other minor studies, the vast majority of bicycle frame development has occurred within industry and the exact design details and flow physics of their frames are not easily assessable. Despite this, we can see the impact that aerodynamics has had on the design of modern bicycles.

Fig. 10 Bicycles used by Olympic gold medallist competing in the individual pursuit compared with a traditional round tube frame and double diamond frame geometry common until the early 80s in elite cycling Full size image

The main driving forces behind bicycle design for elite athletes over the past 50 years have been primarily a result of a greater understanding of the importance of aerodynamics on cycling performance, advances in materials, and composite layup techniques and regulations on bicycle design set by the UCI. These influencing forces on bicycle design are evident in Fig. 10 which compares bicycles used by Olympic gold medallists in the individual time-trial (now part of the Omnium) over the past 35 years to a traditional round tube frame that was typical prior to the 1980s (in this case the bicycle used by Eddy Merckx in his successful 1972 world hour record attempt).

One of the first bicycles designed with aerodynamics in mind was a result of the ‘Elite Athlete Project’ started by the US Olympic committee. To improve its chances at cycling success at the 1984 Olympics, the US, who had not won a medal in cycling in over 70 years, developed track cycles for the US Olympic track cycling team using a low-speed wind tunnel test programme with a focus on minimising aerodynamic wind resistance. The bicycles, known as ‘funny bikes’, employed a number of features to reduce aerodynamics resistance. These included streamlined aluminium alloy tubing to construct the frames, cow horn handlebars, and frame geometry to improve rider position, and disc and flat spoke wheels. The bikes were also designed with the use of smaller than standard wheels at the time. Smaller wheels were said to improve the drafting effect in team events, as riders could sit closer together in a pace-line. For individual events, a smaller front wheel in combination with a standard size rear wheel (now illegal under current UCI rules) was said to improve the aerodynamics of rider position.

Towards the end of the 1980s, advances in the use of composites to construct light-weight frames led to the development of several exotic bikes used in competition that departed substantially from the traditional double diamond frame. Several companies, Zipp and Lotus being two notable examples, developed what they considered “super bikes” which consisted of monocoque frames. These bikes capitalised on the moldability of carbon fibre layups to create stiff structures that served not only as structural members but also as aerodynamic fairings, and often did away with extraneous tubing such as the top or down tube, and occasionally one or two of the stays in the rear triangle of the frame. When tested in isolation of a rider, these bikes proved to produce substantially less drag than their more conventional counterparts. In the early 2000s, the UCI mandated a return to more conventional geometries for competition, effectively ending much of the work that was being done on the monocoque super bikes. The UCI added a further restriction in 2009, known as the “3:1 rule”, restricting the cross-sections of the tubes that make up the frame to a length-to-width ratio of 3:1 [71, 72].

Although reducing wind resistance on the frame is important, it will always be limited as the majority of the wind resistance acts on the rider. Bicycles that have resulted in the largest gains in elite cycling performance have been achieved through designs that target the aerodynamics of rider position. Today, time-trial bars, which act to both reduce frontal area and streamline the rider, are a must have for any serious time-trial competitor. When they first started appearing on the scene in the late 80s however this was not the case. In the final stage of the 1989 Tour de France, a 25-km time-trial to Paris, Greg LeMond, who was 50 s behind the race leader Laurent Fignon going into the final stage, rode with time-trial bars and an aero-helmet, whereas Fignon rode with a wide dropped position and no helmet. Lemond, who was thought to have little to no chance of claiming victory, ended up winning the 1989 Tour by just 8 s over Fignon who conceded 58 s to LeMond on the final stage. To this day, this is the smallest winning margin in the history of the Tour de France. It is widely accepted that the superior position and aerodynamics of LeMond had the most significant impact on his 1989 victory. Other classic innovations in bicycle design, with a focus on improving rider position, can be seen in bicycles developed by Graeme Obree for the world hour record (see Sect. 1.1).

Compared to bicycle frame development of the early 90s, restrictions imposed by the UCI after 1996 have meant that aerodynamic improvements today are achieved through relatively minor modifications to a standard frame with aerodynamic tubing. Modern frames adhering to the “3:1 rule” are designed using both wind tunnel and CFD techniques with a focus on improving the aerodynamic interactions between the frame, front and rear wheels, and the rider. Currently, the major area for development in bicycle technology has occurred in triathlon. Relaxed rules on frame geometry, rider position, and the addition of food storage, hydration, and electric gear shifting systems gives bicycle designers much more room to move to improve bicycle aerodynamics. Today, these low-profile bikes incorporate internal cabling, concealed brakes, frame cutouts to hold moulded hydration systems, and electric battery packs integrated into the frame design all in an attempt to minimise wind resistance and set them apart from their competitors.

Wheels

Fig. 11 Reprinted from Tew and Sayers [73], p 213, ©1999, with permission from Elsevier Various commercially available wheel designs tested for aerodynamic properties by Tew and Sayers [73], including a traditional 36-spoke, b 16-spoke, c 12-spoke, d quad-blade-spoke, e tri-blade-spoke, and f disc wheel designs. Full size image

Wheels make up a major component of the bicycle and have been the subject of a substantial amount of analysis into cycling aerodynamic performance. The magnitude of the aerodynamic forces and moments acting on wheels is highly variable, particularly when we consider the large range of shapes and designs (spoke wheels to deep rim wheels to disc wheels) and the environmental conditions in which they operate. The aerodynamic properties of spoked wheels received a substantial amount of study beginning in the early 20th century on aircraft with fixed landing gear, and later for their application on motorcycles [70]. In contrast to those earlier studies, however, the form factor of the bicycle wheel, as classified by the ratio of the wheel diameter to the tyre diameter, is much higher, owing to their small size and relatively high inflation pressures.

Over the last 15 years, a number of studies have looked at cycling wheels under yawed flow conditions. These studies have looked at spoked wheels with various rim profiles, as well as unconventional spoked wheels and disc wheels. A substantial body of work on the specifics of wheels, however, remains either proprietary or has been published as unreviewed white papers or articles. Nonetheless, there have been a number of studies conducted both in wind tunnels and, more recently, using CFD.

Tew and Sayers [73] performed a wind tunnel study, examining six different wheels: a conventional spoked wheel, a low-spoke count wheel, a bladed spoke wheel, two wheels with a small number (three or four) of structural bladed carbon spokes, and a disc wheel, which are depicted in Fig. 11. Drag and side force coefficients were measured for yaw angles up to 30\(^\circ\). With the exception of the conventional spoked wheel, all of the remaining spoked wheels featured deep rim profiles, nominally intended to reduce the wake behind the rim and, thus, the drag of the wheel. For non-yawed conditions, the disc showed a 70% reduction in the drag coefficient over the conventional wheel, while spoked, deep section wheels were well clustered about 60% below the conventional wheel. A critical characteristic of the deep section wheels that the authors observed was a nearly flat drag coefficient across the yaw angles and wind speeds. The disc, however, showed a sudden increase in the drag coefficient at intermediate yaw angles, particularly at low speeds. The critical angle increases with speed, and the sudden nature of this rise suggests a boundary layer separation effect.

In recent years, a significant amount of work has been done using CFD. Godo et al., in particular, have produced some of the most extensive CFD analyses of wheels [34, 35] and the interaction between the front wheel, the fork, and the down tube [74]. Going beyond the capabilities of wind tunnel analysis, Godo et al. [34, 35] were able to resolve the contributions of the various components of a wheel—hub, spokes, and rim—to the overall drag of the system. These studies simulated the flow around the wheel in isolation of the bicycle–rider system. Steady-state simulations were run from 0\(^\circ\) to 20\(^\circ\) yaw. Transient simulations were also performed that simulated the rotation of the wheels at an equivalent ground speed of 20 and 30 mph.

Both of these studies by Godo et al. compared their simulation data to various published wind tunnel results for the various wheels, taking data from both peer-reviewed sources and equipment manufacturers’ white papers. The authors noted the similar discrepancies to those that have been noted above, with the drag coefficient at zero yaw (theoretically the cleanest and simplest case) varying by a factor of two across many of the different experimental studies. This highlights the magnitude of uncertainty associated with aerodynamic forces and moments acting on wheels as a result of variability in test fixtures, measurement apparatuses, and wind tunnel conditions. As such, while the results by Godo et al. followed qualitatively similar trends as much of the experimental data and generally fell within the quantitative range of the data, a direct comparison is not really possible.

For the deep profile spoked wheels (the Zipp 404, 808, and 1080), the CFD data showed very good agreement in the trends, and the CFD analysis showed that all three wheels had a minimum drag coefficient occurring at 10\(^\circ\) for all three wheels, whereas the drag coefficient of the conventional spoked wheel remained flat up through 14\(^\circ\) before beginning a slow rise. Curiously, these results show that the three deep profile wheels only perform substantially better than the conventional wheel over a small range of yaw angles centred around 10\(^\circ\), although as the rim depth increases, that range increases. For the disc wheel, the drag dropped over the entire range of yaw angles; however, the study was unable to replicate a proprietary result by Zipp, which showed that the drag coefficient dropped below zero over a small range, supposedly producing a net propulsive force. By resolving the pressure and viscous contributions to drag separately, however, they did show that the pressure force on the disc was negative (propulsive) at \(8^\circ\) and above 14\(^\circ\), but was overwhelmed by the viscous (friction) component of the drag. This suggests that the “sail” effect is real, but that the total drag on the wheel is sensitive to the boundary layer properties (and consequently the freestream turbulence).

A time-resolved analysis of the wheels showed the formation of several recirculation zones at the upper and lower sections of the wheel. These recirculation zones were seen to be the largest on the disc and trispoke compared to the conventionally spoked wheels. Mechanistically, it seems clear that the formation of these flow structures and their periodic disruption by the spokes play a critical role in the production of drag; however, the analyses have not yet gone into sufficient depth to understand their role. The studies did explore other aerodynamic forces and moments experienced by the wheels, including side force, vertical force, and turning moments, were also examined; however, those are omitted here, as their role in performance is less clear.

Helmets

The location of a rider’s head relative to the flow and its size relative to the rest of the system mean that the choice of helmet can have a significant effect on the net drag force that the rider must overcome. As the effects of aerodynamic drag on performance have become more widely acknowledged, helmets initially designed to meet the safety standards set forth in various jurisdictions while providing substantial ventilation for thermal comfort have given rise to specially designed time-trial helmets. Modern time-trial helmets are designed for speed over comfort and, more recently, has led to the development of hybrid helmets that attempt to reduce drag without compromising ventilation and mobility. This focus on helmets arises from the relative magnitude that a rider’s head and helmet have on the overall drag, noting that some studies have shown that the difference between well-performing helmets and poorly performing helmets can be greater than the difference between fast and slow wheels [75].

Long-tailed helmets, which cover the rider’s head in an elongated fairing, have been the subject of several studies that have compared different helmets, as well as the manner in which they are worn as well as their context (geometry of the rider’s head and back). Blair and Sidelko [63] conducted an experimental investigation of 14 time-trial helmets (accounting for helmets that came with a detachable visor) using a mannequin that represented the upper body of a cyclist at several different yaw angles [75]. In addition, the helmets were mounted in three positions, based on the inclination of the leading edge. The results showed a global reduction in drag of up to 10% for well-performing helmets compared to poorly performing helmets. Extremely high inclination angles resulted in high drag across the board; however, no mechanistic correlation between helmet design and performance was identified.

Chabroux et al. [64] further showed that there is a strong interaction between the posture of the rider (comparing a more upright road posture with a low time-trial posture) and the drag force on the helmet. The study further showed that while a visor has a statistically significant effect, reducing the drag of the helmet, forward-facing vents do not tend to result in a drag penalty. Brownlie et al. [76], however, showed several cases in which the visor (or sunglasses) resulted in a slight increase in drag. Furthermore, this study showed that while, in general, a time-trial helmet has superior aerodynamics to a more conventional helmet, a time-trial helmet also produces less drag than a bare mannequin head. (With the absence of rough features and the like, a mannequin’s head can be reasonably assumed to have a lower drag coefficient than an actual human head.)

Chabroux et al. [25] conducted a more detailed wind tunnel investigation using particle image velocimetry to investigate the wake structure behind three long-tailed time-trial helmets. The study experimentally showed the time-averaged velocity deficit behind these three helmets (all of which produced similar total drag); however, in the absence of a comparison to other helmets with substantially different characteristics, the authors were not able to present a mechanistic story of the drag characteristics.

The particular geometry of any particular helmet, as well as the geometry of the riders head and upper back, limits the ability to make generalisations about helmet design. While streamlined shapes are a clear advantage, small geometric effects, as well as visors, can positively or negatively influence the drag force on the helmet, depending on the rider’s posture and shape. Careful placement of vents allows for some measure of cooling in time-trial helmets without significantly compromising their performance.

Skin suits

Textured fabrics have been used to improve the aerodynamic performance of many high-velocity sporting disciplines, most notably skiing, speed skating, and cycling. The fundamental flow mechanism responsible for aerodynamic performance gains using textured skin suits is the delay or movement of the separation point towards the back of the body. This effectively reduces the size of the wake leading to increases in wake pressures and reductions in the pressure drag component of the aerodynamic resistance.

Fig. 12 a The drag coefficient of 2D circular cylinder of varying surface texture as a function of Re, reproduced from Achenbach [77]. The variation in \(C_\mathrm{D}\) is related to changes in the flow regime around cylinders. The main flow regimes are labelled for the smoothest cylinder which is highlighted in red. Schematics demonstrate the relative difference in the wake width between a subcritical regime and the point at which drag crisis is said to have occurred. The actual flow topology of each regime is much richer than what has been depicted here. A summary of the various flow regimes and a more detailed description of the nature of the flow around cylinders for each regime can be found in Tropea et al. [6]. b Results reproduced from Fage and Warsap [78], who conducted some of the earliest studies into the influence of freestream turbulence on the drag coefficient and critical Re of 2D circular cylinders (colour figure online) Full size image

In one of the first detailed investigations into skin suit aerodynamics and design, Brownlie et al. [26, 79] demonstrated the potential to improve cycling performance using a range of textured fabrics to treat specific areas of the body. The relative texture of fabrics is dependent on a number of parameters, such as yarn type and material, stitch pattern and density, thickness, cover factor, porosity, seam positioning, coatings, and fabric tension. All of these variables have been shown to be important when considering the aerodynamic performance of skin suits [80,81,82,83]. Using a range of textured fabrics (> 200), Brownlie et al. [26] performed wind tunnel experiments with cylinders, full-scale leg models, and pedalling athletes that revealed a number of aspects of skin suit design critical to aerodynamic performance. These include the following:

The arms and legs exhibit transitional type behaviour for Re relevant to cycling.

The motion of the legs throughout the pedal stroke combined with turbulence generated from upstream components of the bicycle and body reduce the effectiveness of textured fabrics to induce drag crisis on any part of the legs.

In areas of attached flow, smooth fabrics should be used to target reducing skin friction.

In areas of completely separated flow, such as the lower back, surface texture has a negligible effect on aerodynamic drag and any appropriate fabric may be utilised.

Reductions in aerodynamic resistance can be accomplished through tight fitting apparel with few wrinkles and aligning seams with the airflow.

Using these points to guide fabric selection, wind tunnel testing with a pedalling cyclist holding a time-trial position showed that aerodynamic drag could be reduced by \(\approx\)4% using up to five fabrics to construct skin suits, compared to traditional suits not optimised for aerodynamic performance that typically used 1–2 different fabrics.

Fig. 13 Smoke flow visualisation of a team pursuit team being tested in a wind tunnel Full size image

Critical to understanding the aerodynamic performance of skin suits is the process by which turbulence can be induced at lower Reynolds numbers. As the human body has components that resemble cylindrical cross-sections, modern skin suit development has its foundations deeply rooted in early work into the laminar–turbulent transition process of flows around, and the aerodynamic drag acting on, circular cylinders. It is noted in Sect. 2 that a turbulent boundary layer is less susceptible to flow separation over curved surfaces. Figure 12a reproduces results from Achenbach [77] who investigated the influence of the surface texture of circular cylinders in a pure cross flow on aerodynamic drag as a function of Re (where the cylinder diameter is the characteristic length scale). Achenbach’s findings show that not only is the drag coefficient a function of the Re number but also the surface texture. With increasing surface roughness, defined by the roughness parameter k (ratio of the roughness height to the width of the body), the minimum drag coefficient \(C_\mathrm{D,min}\) or the critical point at which drag crisis is said to have occurred is shifted towards lower critical Reynolds numbers \(Re_\mathrm{c}\). One also notes that with increasing surface roughness the \(C_\mathrm{D,min}\) increases and that for \(Re>Re_\mathrm{c}\) the drag coefficient is higher for cylinders treated with a rougher surface finish.

As with cylindrical geometries, the aerodynamic drag acting on body components (particularly the arms and legs) also displays similar dependence on Re and surface texture. When optimising skin suit design, the choice of fabric will depend on the size of the athlete wearing the suit, cycling speed, air properties, and UCI regulations governing allowable fabrics. Modelling the body as a composite of simple geometries in isolation of one another in a pure cross flow has a number of limitations when attempting to minimise aerodynamic drag. This simplification does not take into account flow interactions between limbs and body parts and the influence the motion of the legs has on the flow field around the body.

In addition to this, the relative orientation of limbs, the wind angle, and freestream turbulence levels have all been shown to be the relevant factors when reducing aerodynamic drag [84, 85]. The influence of freestream turbulence intensity on the critical Re on two-dimensional cylinders is shown in Fig. 12b, from the work of Fage and Warsap [78]. One finds that for increasing freestream turbulence intensity the transition process which leads to drag crisis occurs at lower \(Re_\mathrm{c}\). (Note: Intensity is only one characteristic of turbulence that is of importance to bluff body flows. The geometric characteristics and relevant length scales of turbulence are also important to transition and mixing processes.) The defining characteristics of turbulence experienced on the road and track are currently not well understood. As skin suit aerodynamics is sensitive to the wind environment, the size, position, and shape of the rider, there is no one skin suit that will have texture optimised for all cycling conditions, athletes, and cycling positions.