My Christmas morning, however, was spent doing the now legendary morning sea swim with the TriDubai crew, followed by a little 10km run after refuelling on mince pies and cookies. Not the usual fuel of choice but that famous line "it's Christmas" came out again.
With a start to the day like that it made me not feel so bad going back for a third plate of desert at the brunch. I sit here writing this after spending the best part of four hours battling the unforgiving winds in the desert on a training ride. While tapping away on the pedals trying to keep my power steady and position low, it reminded me of a text from a friend I received recently saying he hates riding into the wind.
He finds it demoralising, it becomes a struggle for him and he gets no enjoyment from it whatsoever. The wind is an element every rider has to deal with, especially here in the Middle East with it being so flat. There are no hills to break up the onslaught of the winds when they decide to pick up.
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Yes it may feel like you are suddenly riding with the brake pads stuck on and I am sure everyone who has spent some time on Al Qudra knows that feeling as you swing on to the home straight, as I like to call it. You come off the last descent, take a right and head towards the mosque — it feels like you have just hit a brick wall, but you can look at it two ways. You use it to your advantage to make you stronger and a better rider, or you quit. The second option isn't an option, right? If you are a quitter then I hear chess is a good sport. So stick with the first option and use it to your advantage.
With the lack of hills where we train for most of our rides the wind can help us. Yes, it may be annoying when your speed decreases and you are working hard to go forward, especially when it can last for long periods of time, but think positive. Get into the right mindset and focus on the goal at hand. The more you moan, the more you sit up and struggle, the more energy and the longer it's going to take you to get back to that cup of coffee and cake waiting for you at the end.
Why not incorporate the hard sections into the wind into your training? Use it to do some increased efforts. The wind acts as great resistance for this or big gear work to help strengthen the legs. I hope those few tips help you next time you hit that wind. One thing to mention when racing or even training is that if you are finding it hard into the wind I am sure 99 per cent of the others are too. Yes some people cope better in the wind that but could be to do with a few things — their aerodynamics because of a good bike fit, their mindset, experience of riding in tougher conditions and some people are just hardened riders who love the extra pain and suffering it can cause.
If you try and keep the same speed as you have been averaging throughout the race as you hit a headwind I can guarantee it will be detrimental to your race. It will slow you down — it's a fact. Use the tips and get through it without blowing up. You can manage them any time by clicking on the notification icon. Tuesday, October 8, All Sections. Riding in the wind at Al Qudra. The TriDubai team on Christmas day. Back to the training: I sit here writing this after spending the best part of four hours battling the unforgiving winds in the desert on a training ride.
With a little pre-planning before the ride this can easily be done. Top five tips for riding in the wind: Stay positive and don't let it get the better of you.
The more you moan or sulk it doesn't get any easier. Suck it up and let there be only one winner. Drop into an easier gear to keep the cadence higher. This probably won't make you any quicker but it should keep the effort a little more consistent. If you are in a big gear grinding it out then you are probably going to fade much quicker.
Share the work load. Sitting behind someone called drafting can save up to 40 per cent effort. These findings have also been supported by recent studies that include the dynamic motion of the legs for realistic racing cadences [ 13 , 16 , 17 ]. These studies show that the large-scale wake structures identified in quasi-steady experiments are still the dominant flow features at elite-level time-trial speeds and cadences. The limit at which a quasi-steady assumption will no longer be representative of dynamic pedalling scenarios, which one would expect to occur at a higher pedalling speed-to-forward riding speed ratio than currently studied, remains unknown.
Most wind tunnels are designed for low-freestream turbulence levels and a uniform velocity distribution in the test section. As a result, wind tunnels offer a simplification of real cycling environments. On-road and track flows are dictated by factors such as turbulent atmospheric boundary layers, air currents driven by temperature gradients, the wind direction, and the turbulent wakes of other bodies [ 19 , 20 ].
Fluid mechanical processes such as flow separation and the transition to turbulence can be influenced by freestream turbulence levels, flow uniformity, flow angularity, and pressure gradients in the test section. Currently, there are no standard wind tunnel test conditions for the aerodynamic evaluation of cyclists. One of the main differences between the two designs is how the flow in the test section is altered compared to free-air conditions. In fully closed test sections, the wind tunnel walls restrict the displacement of the streamlines around the cyclists. One effect of this is that the local air velocity around the cyclist is increased and can have a significant effect on aerodynamic force and pressure coefficients.
In open-jet facilities, the blockage effects tend to be less; however, the correction methods are more complex. For example, the curvature of the jet boundary is increased or over-expanded, resulting in a lower velocity profile around the cyclist. Blockage corrections have been developed for both closed and open-jet test sections that take into account the solid blockage and wake blockage effects [ 21 , 22 , 23 , 24 ].
One of the key parameters to the corrections is the blockage ratio given as the ratio of the frontal area to the area of the wind tunnel cross-section. Particularly, when testing high blockage, long models, or large cycling arrangements drafting cyclists in a pace-line formation for example , blockage effects and the type of wind tunnel facility must be considered and taken into account when interpreting findings.
One of the major problems with athlete wind tunnel measurements is the repeatability of rider position. The validity of assigning forces and any aerodynamic quantity associated to a particular rider shape and position depends upon the ability of the rider to maintain their position throughout the testing period. Further, many testing scenarios require the rider to dismount from the bicycle in the wind tunnel and re-mount the bicycle in the same position.
Small variances in rider position also result in a change in the physical geometry of the rider, making it difficult to isolate exactly what variables are influencing aerodynamic force measurements of rider position. In an effort to control for rider positioning, some wind tunnels have implemented camera and motion tracking systems to monitor and record the position of athletes throughout testing.
By performing wind tunnel experiments on a mannequin, rider positions can be accurately repeated and maintained for extended periods of time and tested on demand. Testing with mannequins also allows for the geometry and position to be decoupled. Cycling mannequins are increasingly being used both in fundamental research and industry for detailed wind tunnel investigations.
Testing with cycling mannequins opens up additional wind tunnel testing methods that are impractical or simply cannot be performed with athletes.
Velocity field, surface pressure, and flow visualisation measurement techniques provide additional information on the nature of the aerodynamic forces acting on cyclists. Detailed knowledge of the link between the flow field, surface pressure distributions, and the aerodynamic forces is critical to our understanding of these flows, the further development and design of cycling equipment, and the validation of numerical codes. With improvements in meshing methods, increases in computing power, and advances in turbulence modelling and prediction of flow separation, computational fluid dynamics CFD is now capable of being practically utilised as another tool for investigating viscous flows around complex three-dimensional geometries such as a cyclist.
As outlined in a review of the impact of CFD in sport by Hanna [ 33 ], CFD is being increasingly used to solve aerodynamics problems in sports ranging from car racing such as formula one, yacht racing, swimming, soccer, cricket, and cycling. In recent years, numerical codes have been used to simulate flows around bicycle components such as wheels [ 34 , 35 ] and investigate the aerodynamics of different rider positions [ 27 , 36 ].
Relative contribution of various parts of the body and the bicycle to the total aerodynamic resistance. For a more detailed breakdown of the magnitude of the aerodynamic forces acting on the body, the reader is referred to these articles. For practical applications involving high Reynolds numbers and complex geometries, such as cyclists who operate at Re numbers orders of magnitude larger than what can currently be solved using DNS, various averaging and turbulence modelling techniques must be utilised. Instead of solving the NSE equations directly they are averaged and the time-averaged flow field is resolved.
This averaging process requires the use of turbulence models to close the equations so that they can be solved. Various other modelling techniques also exist which blend solving the flow directly and using models such as Large Eddy Simulations LES and Detached Eddy Simulations DES where only the large turbulent motions are resolved in space and time and the influence of the small scales is modelled.
As the outputs from CFD are sensitive to the initial input conditions, mesh size, time step size, turbulence models, and whether steady or transient simulations have been computed, the best outcomes, both in research and industry, arise when CFD is used in combination with an experimental test programme. A number of turbulence models and numerical modelling techniques have been applied to model flows around bicycles and cyclists holding a static leg position. This model has also been used by others to model the flow around bicycle components [ 44 ]. Although steady-state simulations modelled asymmetrical leg positions reasonably well, they did not accurately model the flow field when the cranks were close to a horizontal position.
In these phases of the crank cycle, time-averaged transient simulation results provided the best comparison with time-averaged velocity fields obtained from wind tunnel experiments. The improved accuracy came at a significant cost, but with the computing time required for the transient simulation compared the steady state increasing by up to a factor of While wind tunnel testing and, more recently, CFD have taken over as the primary analysis techniques for studying cycling aerodynamics due largely to their accuracy, reproducibility, and the insight that they can provide into the basic mechanisms that create drag over a cyclist, field testing methods have played and will continue to play a major role in the development of this field of research.
It is very important that expected aerodynamic improvements within a wind tunnel are compared to the measured performance in the field, as ultimately performance in the field is the true metric of success for any aerodynamic optimisation. Finally, field testing methods, while prone to larger uncertainties, may be more readily performed if wind tunnels or high-end workstations for CFD analysis are not available. In preparation for the Olympics, the United States pursuit team underwent on-track pacing sessions, in order to investigate rider positions and equipment [ 45 ].
Furthermore, while this procedure did ascertain the total resistive force on the cyclist which was most relevant for the purpose of the US Olympic Team , it did not distinguish between aerodynamic drag and other sources of resistance such as wheel rolling and bearing resistance. Minimising aerodynamic resistance through rider position is one of the most effective ways to improve performance among well-trained athletes.
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. Reprinted from Gibertini and Grassi [ 8 ], p 32—33, with permission from Springer.
Reported drag coefficient and drag area measurements from wind tunnel testing of cyclist position. 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.
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 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. 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.
Both investigations involved measurements of aerodynamic drag and frontal surface area of cyclists in a wind tunnel at 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.
Positions that resulted in reductions in aerodynamic drag were also related to a lower velocity deficit and turbulence levels in the wake. 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 ].
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.
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. 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. 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. 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.
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.
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. 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.
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. To improve its chances at cycling success at the 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.
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 s, 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.
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 s, 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.
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 Tour de France, a 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 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 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.
Compared to bicycle frame development of the early 90s, restrictions imposed by the UCI after have meant that aerodynamic improvements today are achieved through relatively minor modifications to a standard frame with aerodynamic tubing. 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.
Various commercially available wheel designs tested for aerodynamic properties by Tew and Sayers [ 73 ], including a traditional spoke, b spoke, c spoke, d quad-blade-spoke, e tri-blade-spoke, and f disc wheel designs. 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.
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. 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. These studies simulated the flow around the wheel in isolation of the bicycle—rider system. 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. 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.
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. A time-resolved analysis of the wheels showed the formation of several recirculation zones at the upper and lower sections of the wheel.
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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.
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. 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. Extremely high inclination angles resulted in high drag across the board; however, no mechanistic correlation between helmet design and performance was identified. 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. 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.
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. Careful placement of vents allows for some measure of cooling in time-trial helmets without significantly compromising their performance.
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. The arms and legs exhibit transitional type behaviour for Re relevant to cycling.
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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.
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. 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. 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.
The ability of the riders to shelter themselves in the wake of others known as drafting , and thereby reduce their own drag, is one of the defining aspects of most bicycle racing with the exception of individual timed events such as time-trials and individual pursuits. The addition of other riders, however, has received little prior attention due to the complexity of the problem, sensitivity of the results, and difficulty in carrying out experiments and computations. In other fields of bluff body aerodynamics ranging from simplified 2D cylinders, surface-mounted cubes, and more complex bluff body geometries such as racing cars, interaction effects between flows around multiple bodies are known to influence the aerodynamic force on both trailing and upwind bodies [ 86 , 87 , 88 , 89 , 90 , 91 , 92 ].
Over the past decade, advances in computing power and experimental techniques have opened up a line of enquiry into drafting effects in cycling, and in particular the team pursuit has provided the motivation to study multi-rider aerodynamics. In the team pursuit, two teams of four riders compete by attempting to cover 4 km on the track in the fastest possible time. Team time-trials on the road are run in a similar configuration, often with up to nine riders on a team competing to complete a course in the fastest time.
As both of these events are cooperative, the riders seek to both minimise their own drag and provide shelter to the other riders on the team.
This is demonstrated in Fig. Studies focusing on these cooperative race schemes have sought to primarily answer four questions: how the aerodynamic drag force varies as a function of spacing from the lead rider, how much drag reduction do the multiple trailing riders experience, does the lead rider also experience a drag reduction, and what are the sensitivities of these results?
Studies addressing the influence of drafting distance on the aerodynamics drag of a single trailing rider are summarised in Fig. This type of behaviour has previously been observed with other drafting bluff bodies such as racing cars [ 90 ]. This is despite significant differences in upstream flow conditions that exist for isolated and trailing riders and highlights the robustness of the formation of the large-scale wake vortices.
Kyle [ 94 ] was one of the first to establish a relationship between aerodynamic drag and in-line drafting distance using the coast down method. Tests were performed with a number of athletes over a m coast down track which resulted in the relationship reproduced in Fig. Although no quantitative analysis of the variability of the coast down test was provided, it was noted that large variation in the data was present.
This was likely due to the inability of the drafting riders to maintain a constant separation distance and axial alignment with the lead rider, an inherent issue with this sort of test technique. As drafting riders experience lower resistive forces than the lead rider, they will tend to decelerate at a lower rate. Despite the uncertainties associated with the coast down method to investigate drafting effects, the findings of Kyle [ 94 ] agree reasonably well with much more recent studies conducted in the controlled environment of a wind tunnel [ 95 ].
Edwards and Byrnes [ 96 ] attempted to address how individual rider characteristics influence the drafting effect. The study found that not only is the drag area of the leader of critical importance, with a greater drag area for the lead cyclist corresponding to a greater drafting effect, but there appeared to be some interaction between the particular lead and trailing cyclists. The authors were unable to strongly correlate this interaction with anthropometric measurements of either the lead or drafting rider and postulated that, beyond drag area, drafting skill was the most probably one of the dominant factors determining the magnitude of the drafting effect.
Both studies note the importance of the drafting effect on the shape, size, and position of the riders. It is not reported whether these findings considered the close proximity of the wind tunnel walls to the test subjects. In addition to characterising drag savings of in-line riders, both of these investigations also studied lateral offset positions of the trailing rider. This type of behaviour has also been observed in the aerodynamics of racing car manoeuvres [ 90 ] and also simpler bluff body geometries such as 2D cylinders [ 86 ].
The CFD simulations were performed for a lead and trailing rider in upright, drops, and aero-bar positions without the bicycle. Simulations were primarily validated by comparison to a single-rider wind tunnel set-up with limited comparison with experiments performed using a two-rider set-up.
A drafting rider was found to experience both a reduction in the stagnation pressures acting on frontal surfaces and also an increase in the base pressure acting on the back. Both of these effects contributed to reductions in aerodynamic drag of a drafting rider. The magnitude of the drafting effect was dependent on rider position and varied between Compared to an isolated rider position, it was found that the relative size of the drag reduction for a trailing rider reduced for the more streamlined lower drag positions.
The reported drag savings of a drafting rider are significantly lower than those found by experimental studies. It would be expected that the exclusion of the bicycle would reduce the drafting effect. Personal correspondence with the lead author of this study and unpublished results show that the exclusion of the bicycle from the simulations is likely the cause of the discrepancy between other studies that include the influence of the bicycle on the drafting effect. The authors were also able to show that the lead rider experiences a reduction in aerodynamic drag as the spacing between the lead and trailing rider is reduced to a minimum.
In contrast to the trailing rider, when both riders were simulated in more aerodynamic positions, the magnitude of the drag reduction on the lead rider increased. The mechanism that was clearly identified by the authors was an interaction between the pressure field of the trailing rider and the base pressure of the leading cyclist. The high-pressure region generated in front of the trailing rider was found to increase the pressure in the wake of the lead cyclist.
The drafting cyclist had negligible influence on the pressure filed immediately upstream of the lead rider. This resulted in a reduction in the pressure differential between the front and back of the lead cyclist resulting in lower pressure drag which is consistent with research into other bluff bodies [ 86 ]. Additionally, Blocken and Toparlar [ 40 ] investigated the effect that a following car has on the drag of a lead cyclist—a situation one might find in a professional time-trial or in a single-rider breakaway.
Similar to a following car, the aerodynamic effects of a close trailing motorcycle, even for relatively short following durations of a typical length road time-trial, was also found to be significant enough to dictate the outcome of the race. As a result of these findings, the authors made recommendations to the UCI to not only increase the current 10 m minimum separation distance between cars and motorcycles but also to implement measures that strictly enforce the minimum separation distance.
As the number of riders in close formation increases, the number of riding configurations and flow interactions between group members also grows in complexity. The most widely studied group formation is that of an in-line team pursuit team. Despite differences in methods used to characterise the savings in each position, and the spacing between each rider, trends developed are relatively consistent among the various studies investigating drafting effects within a four-rider inline pace-line. The tests were conducted in team pursuit configurations of three to four riders at an outdoor velodrome at speeds between Power measurements for each rider were normalised based on their mean power in a given position compared to their mean power while on the front.
On average, for an optimised spacing between team members riders needed to produce The study did note substantial variability, however, due to rider position, size and mass, the order of the riders, and the drafting technique of the riders.