Aerodynamic philosophy of the Kú TF1
BY RICHARD MCAINSH
Aerodynamics have preoccupied bicycle designers since the later part of last century. The most advanced bicycles today are deployed in the bike leg of a triathlon event.
As the bicycle and its rider move along the road, the air exerts a force that increases sharply with speed. The force is due to friction between the air and the exposed surfaces of the rider and bicycle. At high speed, this drag force can be the most important source of resistance, and with a cross wind blowing, it can also lead to significant side forces.
Forces acting on a bike
The aerodynamic forces on any vehicle come primarily from differences in pressure and viscous shearing stresses. Both types of forces, “pressure drag and “friction drag”, arise because all real fluids have viscosity. Viscosity, or very simply the ‘stickiness’, is a basic property of fluids. The viscosity of honey is high, the viscosity of air is very small, but it is not negligible.
Some simple aerodynamic fundamentals
There are two kinds of bodies, streamlined bodies and bluff bodies. A streamlined body looks like a fish, or an airfoil, and the air flows smoothly around the body.
A bluff body looks like a brick, or a cylinder, or a rider and the airflow breaks away whenever a sharp change in direction occurs. For streamlined bodies the frictional drag, simply the air rubbing against the surface, is the dominant source of air resistance.
Effect of shapes on streamlined flow
Frictional drag comes from friction between the air flow and the surfaces exposed to the air flow. This friction arises because air has viscosity. Viscosity is the ability of a fluid to flow freely. For example, honey has about 100 times the viscosity of water, and it’s obvious that the two fluids flow differently, and that it takes much greater force to stir honey than water.
The frictional drag also depends on the Reynolds number, because the flow is laminar (smooth and attached to the surface) at low Reynolds numbers, and turbulent at high Reynolds numbers.
The Reynolds number is the ratio of inertial forces to viscous forces within a fluid which is subjected to relative internal movement due to different fluid velocities.
It is always true that, for a given frontal area and speed, a streamlined body will have a lower air resistance than a bluff body.
For a bluff body, the dominant source of drag is the pressure drag. In bluff bodies, unfavourable pressure gradients, imagine microscopic tornadoes at the surface, cause the flow to separate creating large turbulent wakes which dissipate a great deal of energy (imagine the power of a tornado!) increasing the drag.
Pressure drag comes from the eddying motions that are set up in air by the passage of the rider. This part of the flow is called the wake, and it is similar to the flow left behind a passing boat. It is generally not a strong function of Reynolds number.
The biggest difference between streamlined and bluff bodies is that in streamlined flow, the regions where losses occur are inside boundary layers and wakes that remain reasonably thin, whereas in bluff bodies, adverse pressure gradients cause the boundary layers to separate, which creates a large wake filled with energetic eddies which dissipate a great deal of mechanical energy and thereby increase the drag.
More on the Drag of Streamlined and Bluff Bodies
A bike and rider moving through the air experiences a drag force, which is usually divided into two components: frictional drag, and pressure drag. Frictional drag comes from friction between the fluid and the surfaces over which it is flowing. This friction is associated with the friction between the air and the surface over which it is flowing.
Pressure drag comes from the eddying motions that are set up in the air by the bike and riser passing through it. This drag is associated with the formation of a wake, this is much easier to see in the wake left behind in the water by a passing boat. Frictional drag is important for attached flows, where the air remains ‘stuck’ to the surface and is related to the surface area exposed to the flow.
Pressure drag is important for separated flows where the air has broken away from the surface and formed turbulent eddies, and is related to the cross sectional area of the body.
triathlon bikes - low form-draG designs
The aerodynamic bicycles we see today are the culmination of a twenty year design trend that largely ignores the bluff body of the rider to focus entirely on the aerodynamic problem of how to streamline and reduce the surface area the frame.
Following such a single, dominating, parameter leads to very developed designs that converge towards a singular optimum solution. The latest Tri bikes are the pinnacle of low form drag design: they offer minimum frontal area with narrow forks, low head tubes, thin aero sections, mono-post armrest supports and most notably, despite the lack of enthusiasm from riders, internal bladder systems for hydration.
This is where we are now. Where it seems only logos and paint distinguish appearance. As Slowtwitch concluded in their article, ‘Triathlon Bikes in the Age of Peak Aero’, these designs have reached a nadir of aero development.
In reality we know that regardless of how well the designer has done to achieve a low form-drag frame design there will always be the bluff body of the rider attached to it.
In this reality parts of the rider and bicycle combination act like a streamlined body, whereas other parts behave like a bluff body.
There is an important connection between velocity and pressure, expressed in Bernoulli’s equation (simply put as velocity goes up pressure goes down) and between velocity and cross-sectional area, expressed in the continuity equation (again in the most simplistic of terms if cross section goes down flow velocity goes up). Without going into complex mathematics it is these relationships in flow behaviour that lets a wing produce lift, and lets us manipulate flow around the rider.
By applying this knowledge of friction and pressure drag with these basic aerodynamic principles, it’s possible to reduce the overall drag of the rider and bike system by altering and exploiting the shapes and positions of the components of the bike.
When an additional object is placed on the vehicle, its topology (in simple terms, shape) is modified. This modification will alter the flow around the body. The basic aerodynamic ‘bluff body shape’ of well fitted riders, regardless of size, tend to be very similar.
Starting with a well fitted rider, (thank you Kú authorised fitters!) placed in the airstream we can begin to add, position, and shape all the components in such a way that they act to ma nipulate the air flow and streamline the body. The obvious and extreme example would be to enclose the whole system in an aerodynamic shell.
It’s clear how the topology of the road car (see left picture) has been modified to the race car (see right picture) not just from its position relative to the ground but by the addition of additional surfaces around the front and rear of the car.
These objects can be categorized into smaller categories, such as winglets, wings, flaps, spoilers, and splitter plates. These modifications can generate longitudinal vortices or delay flow separation. A ‘Vortex Generator’ or VG, is a small surface that alters a flow along or around a larger surface with the specific design intent of reducing drag.
VG’s are common on any aerodynamic surface where the control of the boundary layer and post separation wakes are important to the performance of the vehicle.
Suggested media watching
The Kú TF1 brings the concept of topology modification to the problem of aerodynamic bike design. Can we turn fast bikes into true race bikes!
Once we accept the presence of the rider and the fact that they are a natural bluff body is a significant contribution to overall drag then:
Our ‘F1’ philosophy is any component can (and should!) be shaped and positioned so every piece however small can play its part in dictating how the air will pass around the vehicle, streamlining and reducing the drag of the complete rider and bike system.
Instead of looking to make the bike invisible to the air by streamlining it in isolation we seek to use the frame and its components to manipulate the flow around the rider. The obvious manifestation of the philosophy adopted by Kú Cycle with the Kú TF1 is the FAST fork, and the positioning of the steerer pivot box high between the riders arms.
The FAST fork allows us to develop flow through the large duct created above the wheel and tire onto and around the riders legs.
The deep section, set back, down tube traces the path of the riders knee and largely separates the turbulent air across the bike allowing us to regain some control of the wake from each leg.
The riders arms present the first significant contribution to pressure drag as the air sheds wakes from the trailing edge of the riders elbow and upper arm. The ‘F-Duct’ formed between the carefully shaped and positioned extension supports and the side wall of the steerer pivot box help to regain control of this turbulent air.
In low form-drag bikes the turbulence created by the riders arms is completely uncontrolled. Even more so in mono-post style arm rest cup supports.
The arms are only a part of the riders bluff body characteristics contributing to pressure drag. Most significant is the torso, despite a good aero position getting the riders back low and flat, dirty air re-entering behind the arms or coming through directly from the front creates a high pressure region at the riders torso, and large turbulent wake behind the hips.
HOW MUCH POWER OUTPUT IS REQUIRED TO RIDE FASTER?
Current wisdom when fitting a rider to a low form-drag bike is to force the rider into a narrow elbows and high hand position in order to minimise this effect. However this can be a difficult position to maintain for the duration of a long course triathlon leg. It’s important not to underestimate the value of producing good power from the aero position and within reason it is always worth sacrificing aero points for watts.
Hydration and storage:
Hydration and storage:
Examples of how components can be used to modify the aerodynamic configuration can occasionally be seen in bikes over the last few years. Most notably in how storage and nutrition have been used as part of the total aerodynamic configuration. In order to compete in a triathlon a competitor needs to refuel strategically during the race, taking on fluids and carbohydrates to provide enough energy that the body can keep going. This fuel needs to be stored on the bike.
The general design approach to hydration and storage has largely been to follow the same low form-drag philosophy applied to the frames with bottles for liquids and ‘hard storage’ for food, energy gels and bars etc becoming extensions of the frame:
These bespoke solutions have race management implications as they cannot use the standard round ‘bidon’ style bottle handed out at aid stations during the event. They are difficult to clean and maintain, often with flexible bladder style liners for liquids which are prone to puncturing and leakage.
Despite our polling of triathletes in the early stages of defining the Kú TF1 suggesting a general dislike for bladder and internal systems several 2021 model year bikes have launched with this style of hydration system hidden in openings in the frame. Apart from the obvious question of how the frame stiffness is compromised by these large openings, (the steerer pivot box of the Kú TF1 was specifically designed not to have any holes for maximum structural integrity). The practicality of sucking a viscous energy gel through a length of tube passing through the frame when trying to manage breathing for oxygen uptake does not seem adequately addressed.
The ‘bottle between the arms’, or BTA solution has long been used in triathlon, it was found that there was little aero penalty for a round bottle in this position. Intuitively it closes a gap between the arms that could be a source of turbulence.
The Kú TF1 design takes this concept to close the gap between the arms to the extreme by raising the steerer pivot box into this zone. The aero intent of the Kú TF1 design is to close the large void, (see picture above and here on the left) between the riders upper arms, torso and top tube. We did this by developing a bottle shaped to take down stream flow from the riders arms, coming though the F-Ducts and guide it outwards around and over the riders thighs (picture right). Although even a standard round bottle on the Kú TF1 can help in this regard.
As we saw above; air re-entering behind the arms or coming through directly from the front creates a large high-pressure region at the riders torso, and large turbulent wake behind their hips. By closing the gap to the riders chest we can use our storage and hydration solution to add a leading edge to the bluff body of the torso. Instead of hiding it we use it! Topology modification in action.
Practicality also has to drive our thinking and while the aero bottle tested well within expectations it shared the race management issues with the low form-drag hidden versions on competitor’s bikes. This particular design also severely compromised top tube stand-over clearance. While not entirely off our development radar as an extreme race solution aero cannot always win out.
For some riders the round bottle in this position could hit their knees.
The problem becomes how can we follow our topology modification design thinking with careful positioning of a standard bottle and storage unit that does not overly compromise top tube clearance, all while still defining aerodynamics.
By going back to Jane, our CFD rider mannequin, we can optimise the position for a standard bottle mount in the BTA zone. This ends up being slightly higher and further back than a conventional BTA solution. The hands, arms and bottle then work together to form a nose cone, or tip of the ‘rocket’, in front of the torso. The bottle being set further back allows for angled extensions and raised hand positions. The bottle will steer with the front of the bike.
As we saw with the aero bottle it is not practical to extend the semi rigid storage unit back along the top tube to where it compromises stand over clearance. The revised ‘dorsal fin’ style unit extends the back of the bottle and partially closes the void in front of the torso helping to prevent turbulent air from the arms re-entering that area, but is short enough to afford top tube stand over clearance.
As we have lost the extended and carefully shaped sides of the aero bottle two small VG’s are added to the trailing edge of the dorsal fin unit to compensate.
The vortices generated by these VG’s will help draw high pressure air in the small void that remains in front of the torso outwards and around the riders hips and thighs.
This style of storage, closing the void to the chest may not suit everybody, and in addition to the dorsal fin unit we are collaborating with a specialist plastics design company to produce a gel rack similar in concept to the ‘gelrila’ that was available some years ago but specifically for mounting up to 6 gel sachets on the steerer pivot box.
Our choice of the Ritchey saddle clamp means there are numerous options for aftermarket behind the saddle storage options. This is an interesting position for mounting bottles as it is right in the heart of the huge turbulent wake left by the riders hips and back.
Not every one’s race goes as planned and punctures are inevitable at some stage. Kú TF1’s ship as standard with state-of-the-art tubeless tires from the top brands in the market. These are highly resistant to flats but just incase the easy to access nose storage pocket just under the bottle houses a ‘Dynaplug pill’ repair kit including CO2 inflator to get you back in the race.
Further storage under the BB is available for pocket tools or a secure place for keys and small personal items that won’t interfere with your race set up.