Aero module
Aerodynamics: what your position gains you
Above 30 km/h (19 mph), the biggest share of your power no longer moves you forward — it pushes air aside. The aero module estimates your CdA from the position you hold on the bike, then converts the difference between two positions into speed and minutes on your ride. You see what you gain before changing anything on your bike.
Included in the annual subscription
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 accounts for about 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: below 25 km/h your CdA matters little; above 30 km/h it becomes the dominant source of loss.
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.
Estimated CdA
1.75 m · 75 kg
0.21 m²
Values calculated by FramIQ's engine for a reference cyclist, not individual measurements.
None of these values is a measurement made on you. In the app, the calculation starts from your own measurements, 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 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.
In the app
How the module works
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You tell the app which position you ride in
The one you actually hold on a ride, not the one you would like to hold. The module starts from there.
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The app estimates your CdA
It combines the position's drag coefficient and your frontal area, derived from your body measurements. The result is shown in square metres.
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The simulator converts to speed
You choose your power and your distance, the app returns the speed held and the time over the course. The model used is Martin et al., 1998.
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You compare two positions
Side by side, with the difference read in km/h and in minutes. That number tells you whether the change is worth it.
The aero module relies on the formulas of Martin et al., 1998, the reference power model in cycling: aerodynamic drag, rolling resistance, gradient and inertia. These formulas are public and verifiable — there is no black box behind your numbers.
Honesty
The limits, stated plainly
- This is not a wind tunnel. Your CdA is estimated from a position and a build, not measured on you.
- The app does not measure your power. Without a power meter, the value you enter is an assumption, and the result inherits it.
- The rest of the system counts too. Clothing, helmet, wheels, tyre pressure, wind, road surface: the module isolates the effect of position.
- 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 tilts forward and the load moves up into the lower back, 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.
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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.
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Knee pain from cycling
Where the pain sits, what it says about your position, and which setting to correct first.
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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.
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