Module 004
Aero test: measure your CdA with your iPhone
Set your iPhone in front of your indoor trainer, get on and pedal: the camera measures your frontal area and the app derives your CdA. Repeat on the hoods, in the drops or on aerobars, and read the difference in km/h and minutes on your ride. No wind tunnel, no power meter.
Included in the annual subscription
The test, in practice
How an aero test works
-
Set your iPhone facing the bike
On your indoor trainer, the iPhone about 2.5 m (8 ft) away, at hip height, facing you. You need to be fully in frame, head to feet. An iPad works too.
-
Pick your position, get on, pedal
The measurement starts by itself as soon as you pedal, and a voice guides you: no need to come back and touch the screen. Hold the position for about thirty seconds, torso and head still.
-
The camera measures your frontal area
The app cuts out your silhouette frame by frame and converts it into square metres. The scale comes from the LiDAR sensor on iPhone Pro models, otherwise from your shoulder width or your height. Your CdA follows, based on the position held. Everything is computed on your iPhone: no image leaves it.
-
Measure another position, compare
Hoods, drops, aerobars: each position gets its own measurement. The app gives you the difference in km/h and minutes, and keeps the history so you can track your progress from one session to the next.
To turn your CdA into speed, the app relies on the formulas of Martin et al., 1998, the reference power model in cycling: aerodynamic drag, rolling resistance, gradient and wind. No power meter is needed for the measurement: the power you enter is only used for that conversion.
The methods
Testing your aero: four methods
There are four ways to find your CdA. They do not measure the same thing and do not need the same equipment.
| Method | What it measures | What you need |
|---|---|---|
| Wind tunnel | The actual drag of rider and bike, in controlled wind | A wind tunnel session, booked in advance, expensive |
| Road or track field test | CdA derived from your power, speed and deceleration over several runs | A power meter, a flat course, little wind, rigour |
| Computer simulation (CFD) | Airflow computed on a 3D model of you | Photos and your body measurements, no riding |
| Camera measurement | Your actual frontal area while pedalling, from which CdA follows | An iPhone or a webcam, an indoor trainer |
FramIQ uses camera measurement. It does not replace a wind tunnel for an absolute number, but it is the simplest way to compare your positions at home, as often as you like, and see what every centimetre of lower torso is worth.
Definition
CdA, in plain terms
CdA reads as drag coefficient multiplied by frontal area. The first term describes how air flows around your body and your bike, the second the surface you present to the wind. They are not separated, because it is their product that slows you down. The result is expressed in square metres, and it drops when you make yourself small and clean in the air.
The power needed to beat the air rises with the cube of speed. At 20 km/h, air already accounts for more than half of your effort; the rest goes into rolling resistance and friction. At 35 km/h, it exceeds 80%. That is why aero is not discussed the same way at every pace: at low speed, rolling resistance still takes a sizeable share of your effort; above 30 km/h, air takes three quarters or more.
And this is where position outweighs equipment. A pair of deep-section wheels saves in the order of 0.01 m² of CdA. Moving from the hoods to the drops saves about 0.10 m², ten times more, for zero euros. On an amateur cyclist's budget, your position is the only lever that weighs this much without costing anything.
Reference points
Four positions, four CdA values
These four positions cover almost everything a road cyclist actually holds on a ride. The gap between the highest and the lowest reaches a factor of three on CdA: it is the biggest lever available on a bike, before any equipment consideration.
How to read the values: they come from FramIQ's engine for a reference cyclist of 1.75 m and 75 kg (5'9", 165 lb) on a standard road bike. The bike alone accounts for 0.035 m² in each of them; the rest comes from you. A taller or broader cyclist presents a larger frontal area and reads higher CdA values in every position. What never changes is the order.
- Upright, hands on the bar tops: CdA 0.64 m²
- Hands on the hoods: CdA 0.40 m²
- Hands in the drops: CdA 0.30 m²
- Aerobars, triathlon position: CdA 0.21 m²
None of these values is a measurement made on you: they are reference points. In the app, the camera measures your own frontal area in each position, and your CdA also depends on your handlebar, your clothing and your helmet: two cyclists in the same position do not read the same number.
Translation
What it changes in speed
A CdA in square metres speaks to no one. What speaks is the speed held at constant power, and the time over a ride. Here is the same series of positions, converted at 200 W on flat ground.
| Position | CdA | Speed at 200 W | 40 km |
|---|---|---|---|
| Upright | 0.64 m² | 27.2 km/h | 1:28 |
| Hoods | 0.40 m² | 31.5 km/h | 1:16 |
| Drops | 0.30 m² | 34.5 km/h | 1:10 |
| Aerobars | 0.21 m² | 38.5 km/h | 1:02 |
The reasoning, not just the result
The power spent against the air equals half the air density, multiplied by your CdA, multiplied by the cube of your speed. At constant power, cutting your CdA by a quarter therefore does not gain you a quarter of speed: the gain follows a cube root, and is much more modest than the CdA difference suggests.
That explains the hoods-versus-drops line. On paper, the CdA gap is 25%. On the road, it is 2.9 km/h, a little over 6 minutes on 40 km. A real gain, obtained without a single extra watt, but not the transformation the percentage promised.
Order of magnitude to keep in mind: reckon on 3 to 4.5 km/h per position step, at 200 W on flat ground. The faster you ride, the more each step pays. The slower you ride, the less it counts.
Compare two positions
+2.9 km/h
Estimate on flat ground, no wind, at 200 W, for a combined rider-and-bike weight of 82 kg, on good tarmac, at sea level. Change a single one of these parameters and the numbers move.
Honesty
The limits, stated plainly
- This is not a wind tunnel. Your frontal area is measured, but each position's drag coefficient comes from the model: your CdA remains an estimate. It is most reliable for comparing your positions with each other, on the same bike and in the same kit.
- The app does not measure your power. It does not need it to measure your CdA, but the speed shown depends on the power you enter: without a power meter, that is an assumption.
- The rest of the system counts too. Wheels, tyre pressure, wind, road surface: the measurement is done on an indoor trainer and isolates your position. Helmet and clothing, on the other hand, are part of your silhouette.
- A position you cannot hold is worth nothing. This is the most important limit of the four.
The useful rule: your best aero position is the lowest one you can hold without losing power, without pain and without losing your line.
Trade-off
Aero and position: what it costs the body
Gaining aero, concretely, means lowering the torso and closing the hip angle. The body absorbs that closure somewhere, and not always where you expect.
When the hip angle closes beyond what your mobility allows, the pelvis can no longer tilt forward: it tilts backward, the lower back rounds and the load goes into the lumbar spine, in flexion, on every pedal stroke. It is the most common complaint I see after an overly aggressive bar drop. Higher up, the closure is also paid for at the shoulder and the neck, which has to hold the head up for longer.
The knee is not spared either. In a low position, the pelvis slides forward on the saddle, the foot enters top dead centre at a more closed angle, and pressure on the kneecap increases. Anterior knee pain that appears after an aero position change is almost never a knee problem: it is a saddle setback or height problem.
So the order matters: first set a healthy position, check that it holds over time, and only then chase the hundredths of CdA. Doing it the other way round is buying 2 km/h at the cost of lower back pain.
-
Setting your saddle height
The foundation of any position. Before lowering the torso, the saddle must be right, otherwise the hip closure is paid for twice.
-
Knee pain from cycling
Where the pain sits, what it says about your position, and which setting to correct first.
-
Cleat setup
The anchor point the whole chain starts from. A misplaced cleat cancels the benefit of a good position.
If you want the full trade-off, position and aero handled together, that is the work of a full in-person bike fit.
FAQ
Aero test: your questions
Can you do an aero test without a wind tunnel?
Yes. With FramIQ, your iPhone camera measures your frontal area on an indoor trainer and the app derives your CdA from it. It is not wind tunnel accuracy, but it is enough to compare your positions and see which one saves you time.
Do you need a power meter to measure your CdA?
Not with camera measurement: it relies on your silhouette, not on your watts. Power is only used to convert your CdA into speed, and you can enter it yourself. A road test, on the other hand, requires a power meter.
What equipment do you need for the test?
A compatible iPhone (iOS 18.2 or later), your bike on an indoor trainer and about 3 m (10 ft) of space in front of you. On iPhone Pro models, the LiDAR sensor measures the distance and makes the scale more reliable. On other models, the app calibrates on your shoulder width or your height.
How accurate is it?
Your frontal area is measured on you. The position's drag coefficient comes from the model, so the absolute number remains an estimate. The most reliable figures are the differences between two positions measured in the same conditions, and those are the ones to look at.
How much does the aero module cost?
It is included in the annual subscription, €89 per year, along with the frame comparator and the video analyses. Details are on the Pricing page.
What about testing your aero on the road?
FramIQ measures on an indoor trainer, out of the wind. On the road, you use a field test with a power meter: several runs on a flat course, in calm weather. The two complement each other: the indoor trainer to choose your position, the road to confirm it.
Test your positions before your next ride
Estimate your CdA, compare two positions and read the gain in km/h and in minutes. The rest is training.
Aero module included in the annual subscription · €89/year