What you can (and can't) do with Aero in Velogic Studio
"All models are wrong, but some are useful". Please do not rely on the analysis provided by the Aero module without verifying via other means, e.g a wind tunnel or real-world measurement.
This page outlines the limitations of Velogic Studio's Aero module. Please read the Aero Basics page if you're not already familiar with aerodynamics.
Cd, A, and CdA - what is Velogic Studio actually measuring?
In general, it's not possible to directly measure Cd, (Coefficient of Drag) directly. The closest we can come is to measure Cd x A in a wind tunnel. Going back to our equation for Aerodynamic Drag Force:
In a wind tunnel, the density of the air can be measured. Then, velocity of the wind can be varied, and the drag force measured. Those values can be plugged into this equation to give Cd x A, which is referred to as CdA. The goal of a wind tunnel session is often to minimise CdA, in the hope that this will lead to better performance in the real world.
By contrast, Velogic Studio uses a time-of-flight “depth sensing” camera to measure A (frontal area). It's your job as a fitter to estimate the Cd; we provide some guidance but ultimately you need to enter a Cd number for your rider. Velogic Studio makes the assumption that the Cd doesn’t change much for small changes in frontal area, then we can estimate small changes in CdA just from the small changes in A. You can vary the Cd for each position in the fit if you have reason to believe it would have changed, e.g. the clothing changes, the position changes drastically, or an aero helmet is added.
Valid uses of Aero in Velogic Studio
You can use the frontal analysis module of Velogic Studio to:
- Establish a baseline Frontal Area number for a rider
- Make small adjustments to the rider’s position, and see the effect on Frontal Area
- Use basic calculations to illustrate the effect of a change in position:
- At the start of the fit session, you can specify a baseline course outcome (distance and time)
- For the initial baseline position, you can supply a Cd number, either from your own experience, or estimated from lookup tables, or measured on a test course or wind tunnel
- Velogic Studio will then calculate, for each different position (with a different measured Frontal Area:
- The change in power required to achieve the same course outcome at different positions.
- Or, the change in course outcome which could be achieved if the same power is maintained with each different position.
- Measure the rider’s Frontal Area at regular intervals over a longer test period (e.g. every 5 minutes over a 1 hour period) to determine how well the rider can hold a desired position over time.
- Measure changes in a rider’s Frontal Area across larger spans of time, e.g. a season.
Limitations and invalid uses of Aero in Velogic Studio
- The camera that’s used by Velogic Studio can’t actually see all of the surface of the rider that’s making contact with the air. For example, if the rider’s head is tucked down, Velogic Studio may not be able to see the top of the rider’s back, even though this area should be part of the frontal area. This means that although it can detect changes in frontal area, it isn’t accurate as an absolute measure.
- The frontal camera obviously can't see behind the rider's head; and yet we know that teardrop helmets make a difference to CdA. This is a good example of how CdA can be affected by Cd or A; Velogic Studio can only measure A.
- The course outcome calculation makes very many assumptions, so should only be used for illustrative purposes. It doesn’t account for course terrain, variations in speed, variations in air density, cross winds, changes in rolling resistance and other factors.
- It’s possible that a position change which results in a higher A could actually result in a lower CdA, e.g. if the new position has a significantly better Cd. For this reason, we only recommend the use of Velogic Studio to investigate relatively small changes in A, which are less likely to affect the Cd.
Where does Velogic Studio fit into the landscape of aero assessment tools?
We think of Velogic Studio as a relatively low-cost way to begin an Aero analysis. Different positions can be quickly tried out in the studio, and promising positions selected for further analysis in the wind tunnel or on the track.
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