These instructions pertain to a spreadsheet that will allow you to
determine aerodynamic drag area (CDA) from six field trials. Data must be recorded by an SRM or other
accurate power measuring device.
Please refer to our recent paper for additional details (Martin et al., 2006). Note that in the paper we used a two-step process to first
determine friction (m) and
then to determine CDA. This process worked
quite well with a large number of samples (several data points from many
riders) but m tends to be unstable when evaluated
with a few points. In the current
spreadsheet we have given some typical values for m that will facilitate more robust calculation of CDA. While this approach will introduce some
error, it will likely be a small error in a small term, so the overall impact
should be negligible.
Start by going to the Air Density Calculator tab. Enter a value for temperature in degrees Celsius
in cell A2. Enter a value for relative
humidity in percent in cell B2. Enter a value for barometric pressure in cell
C2. Barometric pressure must be in
units of millibars and the values must be absolute. This may be difficult to determine because
weather stations often report barometric pressure values corrected to sea
level. The most direct way to
determine barometric pressure would be for you to have a barometer on site
during your testing. These can be
purchased for as little as US$30. Alternatively,
if you are in the USA you can go to the website and find a station near your testing site. You will need to set the output to metric
Both the road and track calculations must be based on six
trials. Do not leave cells blank. If
you cannot do six trials, you could enter data for some of the trials twice,
but that is not ideal. Each trial
should be done a constant speed throughout the test section. The range of speeds you use should be as wide
as possible, from say 10 km/h to as high a speed as the rider can hold steady
for the test section. By setting the
speedometer to km/h you will get maximum resolution. You will need to convert the speed to
meters per second for the spreadsheet calculations.
Instructions for a Straight Flat Road
We have not specified a length for the “test section” but we
recommend as great a length as you can find that is straight and flat. We have used 470 m with very good
Start by going to the CDA for a straight road tab. You will need to enter the following values
for each trial:
Column A: Bike and rider mass
Column B: The road grade in
percent. Note that this will depend on
the direction. Positive grade denotes
uphill and negative grade denotes downhill. These values can be obtained from
local government agencies. Ask for “as
built plans” or “as built surveys”.
Column C: Wind velocity in
meters per second must be entered for each trial. To enter the proper value you will need to
know the wind velocity and direction as well as the direction of the road. From those data, you must determine the
component of the wind that is parallel with the road. We will assume that you can make those
calculations and will not attempt to explain them. Negative values denote wind in the same
direction as the bicycle is traveling and positive values denote wind that
opposes the rider.
Column D and E: Velocity and
the beginning (initial) and end (final) of the measurement interval in meters
per second. These can be read off of the SRM data but you must know the time
associated with the start and end.
Column F: Average velocity
over the measured section in meters per second.
Column H cell H2 only, enter
a value for rolling friction. Typical
values are given for a track, a rough road, and a smooth road.
Column I: Enter the average power
recorded during the measurement section.
Column J: Enter the time required to cover the
measurement section for each trial.
When you have entered all
these values the bike and rider CDA will appear in cell B12.
10. The goodness of fit of your data can be seen in cell Q9. That value should be around 0.98 or better.
Instructions for a Velodrome
The spreadsheet does not account for wind in a velodrome. Wind will tend to cancel for a complete
lap, however these effects are non-linear and you should do testing in the
calmest conditions you can or on an indoor velodrome. The spreadsheet assumes a complete lap and
your data might be improved by doing 2 or more laps. This will be difficult at high speed so you
may want to use more laps for lower speeds and less laps at the higher
Start by going to the CDA on a velodrome tab. You will need to enter the following values
one time only:
11. Cell B2: The length of the track in meters
12. Cell C2: The height of the saddle from the ground in meters.
13. Cell H2: Enter a value for rolling friction. Typical values are for a track are 0.0025.
will need to enter the following values for each trial:
14. Column A: Bike and rider mass in kg.
15. Column D and E: Velocity and the beginning (initial) and end
(final) of the measurement interval (e.g., one lap) in meters per second.
These can be read off of the SRM data but you must know the time associated
with the start and end.
16. Column F: Average velocity over the measured section in meters per
second. This could be obtained from
the SRM or calculated from the track length and the lap time.
17. Column I: Enter the average power recorded during the measurement
18. Column J: Enter the time
required to cover the measurement section for each trial.
19. When you have entered all these values the bike and rider CDA will appear in
20. The goodness of fit of your data can be seen in cell Q9. That value should be around 0.98 or better.
Martin JC, Gardner
AS, Barras M, Martin DT (2006). Modeling sprint cycling using field-derived
parameters and forward integration. Medicine and Science in Sports and Exercise
Published Dec 2006