Understanding Bicycle Aerodynamics with SOLIDWORKS Flow Simulation

by Benjamin Modic

 

Small Improvements Add Up

There are a lot of benefits to having the right gear while cycling. Companies will go out of their way to design and sell you components based on fractional gains. These small improvements will add up and are easily quantifiable when you are at the top of the sport. But for those who cycle for recreation, it has less to do with the total time saved over a 100-mile stage and more to do with bragging rights when you can beat your riding partners to the next stop sign.

My bike frame has a lot of aerodynamic tube shapes, one being the seatpost. It has a very streamlined profile, which is different than the round seatpost used on most bikes of the same style. I thought I would use SOLIDWORKS Flow Simulation to find out how all of these design elements are helping me!

(L) Streamlined Seatpost (R) Round Seatpost

Comparing the Two

For a simple comparison, I am only testing the difference in the seatposts alone. While the air pushed through to the seatpost is fairly clean, other factors affect its performance. Let’s take a look at the study conditions:

  • Analysis type – External
  • Fluid – Air
  • Air Pressure – Sea level
  • Air Density – 1.225 kg/m^3
  • Air Temperature – 20 deg C
  • Velocity – 10 m/s in X
  • Flow type – Laminar

Study Goals:

Drag Force – Force on the profile in X [Used for convergence]
Drag Coefficient –

Cd = (2*{Drag Force N})/(1.225kg/m^3*10m/s^2*0.0114m^2)

Drag Power –

Pd = {Drag Force N}*10m/s = Watts

Mesh plot for both profiles (Level of refining cells at fluid/solid boundary = 3):

Cut plot showing the velocity of airflow for both models:

Both the mesh of the round post and the streamlined post have similar refinements. We can view the simulation results a number of ways. Flow trajectories may be hard to interpret across multiple studies. The cut plot gives us an excellent overview of the results and allows us to view the two studies side by side. Note the larger wake created by the round seatpost.

via GIPHY

What does it all mean?

Check out the goal plots, and you can see an increase of drag force on the round seatpost; this results in a higher drag coefficient and ultimately requires more power to move through the air.

Most professional cyclists, including those competing in the Tour of California, produce about 350 watts on average during a single stage. According to the analysis here, a cyclist with a round seatpost would require an additional 0.719 watts to keep up with a competitor whose bike has an aerodynamic seatpost!

Okay, that isn’t much but as stated before, the little performance gains all add up in the end.

Happy riding!

 

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