Bike Aerodynamics Simulation — Reducing Cyclist Drag by 30%
Bike aerodynamics is considered to be one of the most complex applications in aerodynamics not only because of the bluff body shape of the rider ― which generates the majority of the drag ― but also due to the transient cycling movement. The latter creates high complexity in the flow pattern and changes the forces acting both on the bike, as well as on the rider dramatically.
Forces Acting on a Bike
If we consider a straight riding without side wind, we can expect the following forces acting on the bike and the rider:
- V the wind direction
- P the propulsion force
- D the aerodynamic drag force
- G the gravity acceleration
- R1, R2 the ground reaction
In the picture above, we can clearly see that the only force preventing the rider from cruising is the aerodynamic drag force (without sloping ground). This is why drag is a very important factor in defining the performance of a bike and why aerodynamicists invest a significant amount of time in optimizing the shape of the bike and the equipment, as well as the rider’s position to achieve maximum performance.
When it comes to bike aerodynamics, the major interest is reducing drag, because this will have an effect on the power required by the cyclist to finish the track cycle.
To illustrate this, let’s consider a cycling race of 250 KM range, a drag reduction of just 2 N and an average speed of 13 m/s. In terms of power, we have:
(2 Newton x 13 m/s) x 250000 m = 65000 KJ
In other words, reducing 2 N of drag force can save 65000 KJ for the rider, and this amount of power is equivalent to 3 bars of chocolate.
The performance of bikes is impacted by many parameters. One of the most important ones is wind direction; the figure below illustrates the effect of side wind on the forces acting on the bike.
In the case of side wind, the forces are shifted with the same angle that the wind makes with the bike:
To calculate the real forces acting on the cyclist, we use the following trigonometry formulas:
Forces in axes
Dr– in the direction of the wind
Str– perpendicular to the wind
Forces in bike axes – calculated
Db – opposing the motion of the bike
Sf – perpendicular to the motion
Related through the yaw angle
D = Dr cos(y) – Str sin(y)
Sf = Dr sin(y) + Str cos(y)
To better understand the effect of the side wind, I performed a couple of simulations using SimScale, the online CAE platform. The following simulations are for the same bike geometry, the same rider position as well as the mesh and simulation setup.
Flow Over a Bike
Clearly, the side wind has a remarkable effect on the flow over the bike. The side wind will shift the pressure force to the side and if we decompose the drag vector force into two components, we remark that this will decrease the drag force but at the same time increase the side force.
This figures above show the effect of 20 degrees side wind on the pressure distribution on the front of the rider.
In the following pictures, we can see the flow over the middle of the bike and rider in a straight riding condition without side wind, and notice that the flow behind the cyclist is a low kinetic energy flow because of the turbulent wake.
This creates the most part of the cyclist’s drag, but in fact this low kinetic energy bubble can be really beneficial for the riders riding behind him, because this will reduce the pressure force acting on the rider from the front and, in consequence, will diminish the pressure difference between the front and the back of the rider. In the end, the drag on the rider will be reduced by 30%.
But when the side wind is considered, the flow pattern over the bike will be totally changed and this can be better visualized in the following animation:
These animations show exactly the effect of the side wind, on the flow pattern over a cyclist by swapping a slice over the bike from the front to the back. If we take a close look, we can see that the flow over the bike without side wind is well aligned and the wake region size is smaller compared to the case with the side wind, where the wake region is larger. This is confirmed with the different drafting positions in the next figures.
These two pictures show the different drafting positions according to the wind direction. The one on the left shows the drafting without side wind, where the cyclists try to stay closely behind each other, because of the aligned small wake region created by the bike in front. The purpose is to take maximum advantage of the low kinetic energy flow behind the rider in front. But with the side wind, this low kinetic energy bubble is shifted to the side, and that’s why cyclists ride beside one another instead of behind each other (drafting riding), to take advantage of the low kinetic energy created from the side of the riders.
In term of forces, we can see from the graphs that the drag is reduced by the side wind as mentioned before, and this shows that there is a gain riding at low side wind below 15 degrees, but after this we can see a significant increase in the drag. This is because of the stalling effect that happened at high angles, where the flow can’t stay attached to the geometry and based on statistics, the riders spent 80% of the riding staged in the range of 2 to 15 degrees.
But for the side force, we can see that we have a linear increase in the side force by the angle of attack.
Bike aerodynamics is one of the most complex aerodynamic applications because of the shape and the cycling movement of the rider, both making this biomechanical system very challenging for aerodynamicists to find the best configuration. It’s also important to mention that for the bike aerodynamics, the rider can also make the difference in terms of performance, regarding his position on the bike as well as in the racing row.
The wind direction can affect the performance of riders dramatically. Finding the right position in the racing row can give the rider significant advantage compared to the other riders. Every gram of drag has a remarkable effect on the performance of the rider on long range race.
To learn more about aerodynamics and different industrial applications of CFD, download this case study showing how the engineers from Carlsson Autotechnik GmbH used SimScale to test the aerodynamic behavior of their sports car.
Front wind riding: