What is CdA?

CdA is a measure of your aerodynamic "slipperiness". The lower your CdA, the faster you can ride for the same power output.

CdA is the product of two key factors that affect your aerodynamics:

  • Coefficient of Drag (Cd) - which is primarily a function of the shape of an object relative to the fluid (air) moving around it. It can also be affected by the texture of the object's surface.
  • Frontal Area (A) - which is a measure of the size of the object moving through the fluid.

With aero testing we measure your CdA and work to minimise it while not compromising your power output. We can reduce CdA by a combination of:

  • adjusting your bike position to reduce the frontal area your body presents to the wind,
  • changing the shape your body adopts on the bike, and
  • making changes to equipment and clothing.

How much impact does CdA have?

On flatter terrain, aerodynamic drag accounts for up to 90% of the resistance forces a cyclist faces, and so changes in CdA will have a significant impact on speed.

For example, a reduction in CdA from 0.30m2 to 0.25m2 will result in a speed increase of approximately 6% for the same power output, or more than 5 seconds per km quicker.

Try our online calculator to see the impact changing CdA has on your speed.

How long does an aero testing session take?

Testing sessions typically take 3 hours. We have 2½ hours of exclusive use of the track available for test runs and collecting data and we allow ½ an hour for setting up, agreeing on test process and pairing your bike's equipment with our technology.

You'll also need to allow time for travelling to and from the venue and for getting all of your gear inside the velodrome.

You will need to bring:

  • The bike you wish to test on, fitted with a working ANT+ power meter. Check the batteries are in good order and you can attain a stable torque zero
  • Cycling kit to wear, preferably your racing attire
  • Cycling shoes and helmet
  • Any special tools or equipment necessary to make changes to your bike's set up, e.g. custom spacers for aero bars
  • Any extra equipment options you would like to test, e.g. helmets, clothing, wheels
  • Some seasonal clothing to put on in between test runs, espcially if the weather is cool
  • Some drink and perhaps a snack
  • A good sense of humour :)

What do I need to bring?

Do I have to ride at a fixed power or speed?

No. The nice thing about these testing and analysis methods is the rider does not need to think too hard about how hard they are riding.

After we do the initial baseline test runs, we suggest picking a tempo and gear ratio that is solid but manageable for the remainder of the test runs. Often this is close to race pace for your target event so we can assess the impact of position and equipment changes as they would be for you while racing. We monitor your power output during the runs and will advise if we think you need to go a little harder or easier.

I've never ridden on a velodrome before, will that matter?


We've had many clients for whom their aerocoach testing session was their first time on an indoor velodrome. We'll take a little extra time to get you acclimated with riding on the track but it usually doesn't take more than about 5-minutes to get the hang of it and once going you'll be struggling to wipe the grin off your face.

Yes, you will need to have a suitable power meter on your bike. Our technology is compatible with all power meters that transmit data via the ANT+ protocol. This includes all the major brands, and many newer meters.

  • SRM
  • Powertap
  • Quarq
  • Power2Max
  • Garmin Vector

Aerocoach does have a Powertap wheel (10-speed) we can use for those that do not have a power meter.

Power meters that record total power from both legs are recommended. While single sided meters such as Stages are compatible, we cannot be as certain about whether the data reflects changes in aerodynamics and/or changes in power asymmetry.

Do I need a power meter?

In 2015 we attained, on average, an 8% reduction in CdA for our clients. This equates to an average speed gain of 1.1km/h on flat terrain (2.3 seconds per km), or the equivalent of 17W in power.

That's just under a minute for a Calga 25km TT, or nearly 7 minutes for an Ironman bike leg!

Obviously how much each individual can expect to improve does vary, in particular it depends on how aerodynamic you are to begin with. We'll start with any "low hanging fruit" and then move onto seeking out the more marginal gains.

Time and again we receive feedback from riders who have raced and reported speed improvements in line with what we tell them.

What sort of improvement can I expect?

What can I test?



The list is endless! First up we can test you on any bike fitted with a suitable power meter. Road, track, TT or triathlon bikes are all suitable. Broadly there are two key areas of testing:

  • Bike position
  • Equipment and clothing

What bike positions we can test often depends on what adjustments are feasible to perform on your bike. Some bikes are easier to adjust than others, so please keep this in mind when you come for your testing session.

Typically we can modify the height and width of elbow/arm support pads, the height and extension length of the hand placement (and hence forearm angles), and make saddle height and fore/aft placement adjustments. We'll also recommend several positional refinements you can adopt while riding which don't require changes to the bike itself.

Equipment options we often test include:

  • helmets & visors
  • skinsuits / trisuits
  • wheels
  • shoe covers
  • bidons

Aerocoach can detect small differences in aerodynamics, and do so relatively quickly. How fine a difference does depend on a few things, in particular the amount of data we collect for a given set up, the stability of the rider on the bike and the quality of the power meter's data.

We report a standard error for each set up's CdA and it is normally in the range of 0.001m2 - 0.003m2, (around 0.5% to 1.5%). When testing larger and more obvious changes we require less data and time but when testing for very fine differences we take a bit more time to gather data and increase confidence in the results.

If we can't reliably detect a difference then we say so. Indeed knowing that two set ups are not aerodynamically different provides valuable insight as one set up may be more comfortable or powerful, or you can choose from more than one equipment option and know you won't sacrifice speed, and it leads us to focus on other testing options instead.

What is the resolution of the testing?

Do you have helmets/wheels I can test?

Unfortunately not. You will need to bring along any equipment options you wish to test.

While we would love to have an array of equipment options on hand, we have decided on keeping such costs down.

In a typical test session we'll achieve between 8 to 12 test runs per paired test rider and 12 to 20 test runs per solo test rider.

The exact number of test runs depends on how much time is required to perform position or equipment set-up changes. For example, helmet testing and on bike positional changes are quick to do, while making changes to your bike's TT bars often take much longer.

How many things can we test for?

Can I organise for a group testing session?

Of course!

Aerocoach has provided aerodynamics testing for groups and for teams. Just contact us to discuss how we can help your team perform better.

I'm a bike fitter/coach, can we work with you?

For sure - we've worked with many bike fitters and coaches to provide that extra performance edge for their clients. Just contact us and we'll be happy to discuss working with you. You are also very welcome to attend our sessions.

At Aerocoach we place a high value on riders undertaking a quality bike fit before performing aero testing and we recommend our clients first seek out the service and support of a professional fitter. That way when they present to us, we are working with someone that is stable, comfortale and powerful on their bike and we'll have a better understanding of their effective position limits prior to testing aero options.


We've often tested riders and teams at the DISC velodrome in Melbourne, and we are interested in working with riders at the new Anna Meares velodrome in Brisbane.

We can do testing at local outdoor venues, however the benefits of being indoors (benign wind conditions and shelter from the weather) make outdoor testing less reliable.

Just contact us to discuss the options.

Can you test anywhere else other than Sydney?