SuperRecord.

Aerodynamics

How do you measure CdA without a wind tunnel?

CdA matters most in a time trial and is the least well known. A wind tunnel is expensive and far away; a power meter, a loop and the right method are enough to measure it with the rider’s real position and equipment.

The problem

  • The plan’s CdA comes from a table or a guess, and nobody knows how wrong it is.
  • There is no way to know whether the new helmet, the skinsuit or the lower position is really worth it.
  • Two road tests give different numbers and you do not know which to believe.
  • The wind tunnel is expensive and measures a static position, not the rider pedalling in a race.

Why it happens

At time-trial speed the air takes most of the effort. An error in CdA turns almost entirely into an error in the watts asked of the rider.

Measuring it on the road is possible because power, speed and the terrain profile have to fit together: only one CdA makes them fit. Wind and a careless protocol are what spoil the measurement, not the method.

How SuperRecord solves it

  1. 1

    Run the test

    Several laps of a flat loop or out-and-back runs, no stops, in calm air and at race effort, in the position you want to measure.

  2. 2

    Upload the .FIT to the lab

    With the real total mass and the tyre you rode. If you pass the same point several times, the lab uses it to cope better with wind and altimeter drift.

  3. 3

    See how far to trust it

    Next to the CdA there is a quality indicator saying whether the measurement is excellent, good, fair or needs repeating, and a warning if it was measured on an easy ride rather than at race effort.

  4. 4

    Save it on the bike

    It is stored by position (road, TT, triathlon, track) and from then on plans use it. If the equipment changes or the measurement ages, the plan warns you.

  5. 5

    Compare equipment

    To choose between two helmets or two positions, the guide explains the out-and-back protocol and how many runs you need for the difference to be credible.

What you need

  • A reliable, calibrated power meter.
  • A flat loop or an out-and-back stretch with no traffic.
  • A day with little wind.

Try it on a real time trial

The demo is a real time trial: change the power, the CdA or the wind and watch the time move. No sign-up.

See the demoStart your free trial

Frequently asked questions

Can you measure CdA without a wind tunnel?

Yes. With a power meter and a well-chosen loop it can be measured on the road or in a velodrome. On the track it works even better, because there is no wind.

How accurate is a road CdA test?

It depends mostly on the wind and the protocol. That is why the result comes with a quality indicator: a good test in calm air is enough for a race plan.

Can it tell whether a helmet or skinsuit is faster?

Yes, but comparing takes more care than measuring: out-and-back runs, alternating setups and knowing how much your own test varies. The guide explains it step by step.

Do I need an aero sensor?

No. The power meter and the head unit’s .FIT file are enough.

Further reading