Aero lab
Where do your watts actually go on a flat road — and what would the usual aero upgrades really buy you, at your speed? Build your setup in the list below: every row shows what it's worth at your speed, and the box at the bottom turns your ticks into a new speed — or the same speed for fewer watts. Add your average watts if you know them and we'll calibrate the model to you.
Know your power on a flat, steady stretch at that speed? Put it in and we work out your own CdA instead of a typical one. A whole-ride average will not do — that includes climbs, descents and coasting, and the sum below assumes none of those.
Tick only what you would be adding. Anything you already ride with is inside your own number already — leave it unticked and the tool works from where you actually are.
Blue = air · amber = rolling resistance · dark = drivetrain loss. Flat road, no wind, riding solo.
Position — pick one
Kit — tick what you would add
Tyres — what would you switch to?
And the body
Compared with the plain baseline: hands on the hoods, regular kit, butyl tubes, today's weight — at your speed from the top.
Where it bends: flat-road physics with published wind-tunnel mid-range numbers (position CdA ≈ 0.40 / 0.36 / 0.31 scaled to your size; helmet −0.006, loose-to-race-fit jersey −0.012, socks −0.004 CdA; wheels ≈ −0.006 CdA at 50 mm, maxing out at −0.009 around 63 mm; tyres: butyl Crr 0.0050, TPU/latex 0.0043, tubeless 0.0040; weight loss shrinks only the body's share of drag; 8 kg bike, sea-level air, 2.5% drivetrain). Real bodies aren't mannequins: your gains can be half or double these. Some of these marketing claims are on our own list to take apart properly — until then we've kept the numbers deliberately conservative. Average-ride watts include hills, stops and wheelsucking, so the calibration is honest but rough — and it politely refuses numbers that imply an implausible CdA.
Under the hood
On the flat, nearly everything you do goes into moving air out of the way — and it grows with the cube of your speed, which is why the same aero upgrade is worth little at 25 km/h and a lot at 40. The tool splits your power into the two things absorbing it, then works out what each change to your kit does to the air half.
CdA is the number the whole tool turns on: your frontal area multiplied by how cleanly the air flows around it. Every kit item below is a small change to it.
If you give it your watts
Then it stops guessing. The sum above has one unknown left once you supply speed, weight and power, so we solve it for your CdA and work from that instead of a typical figure — which is the only part of this tool that is about you rather than about a representative rider.
Two things follow from that. The figure it works out belongs to the position you selected, so switching position afterwards compares against your own number rather than adding the same gain twice. And it starts from standard tyres, because the tyre choice below is a switch you are considering rather than a description of what you ride — the same rule as the kit.
One thing it cannot know: whether you were already wearing some of the kit listed below when you measured. If you were, that gain is inside your CdA already, and ticking the same item would count it twice. So the rule is simple, and it is written above the boxes: tick only what you would be adding. Anything you already ride with needs no tick — it is in your own number, which is exactly where you want it.
It only works with power from a flat, steady stretch. A whole-ride average includes climbing, descending and coasting, and this sum assumes none of them; feed it one and the CdA it reports will be wrong in ways it cannot detect. When the result falls outside what a road position can plausibly be, the tool says so and keeps the typical figure.
The numbers that go in
| Number | What it is | Where it comes from |
|---|---|---|
| CdA 0.40 / 0.36 / 0.31 | tops · hoods · drops | Our assumption, and the biggest lever here. Moving to the drops does more than every kit item on this page combined. We looked for a published measurement for these three positions and did not find one we could reach. |
| −0.0004 CdA | a race-fit jersey instead of a loose one | Wind-tunnel testing: essentially nothing — about two seconds over an hour. If you were told otherwise, this is the number to hold that claim against. |
| −0.0124 CdA | a short-sleeve race suit instead of a loose jersey | Same test. This is the change that is actually worth watts — and it is a different garment from an aero jersey. |
| −0.006 CdA | an aero helmet | Wind-tunnel testing measured 0.007–0.010; we use the low end. |
| −0.004 CdA | aero socks | Wind-tunnel testing measured 0.0041. |
| the rim-depth curve | deeper wheels | One measured point — box section to 50 mm, averaged across yaw angles — with a straight line drawn through it by us. The testers add that the deep wheel was slower at low yaw and that they could not conclude with confidence that it was faster overall. |
| Crr per tyre type | butyl · TPU · tubeless | Converted by us. The testing we rely on publishes watts on a drum at a fixed speed and load, not a rolling-resistance coefficient, so the conversion — and the step from drum to road — is our arithmetic, not theirs. The ranking is theirs and is solid; the absolute values are ours. |
| 0.3 | how CdA shrinks if you lose weight | Heil (2001) found frontal area scales with body mass to roughly this power. Note it describes differences between riders, which we apply to one rider losing weight. |
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How we check a performance claim before we print a number — read the shoe check.