How much time does an aero change actually save?
Less than the watts suggest, and more for slow riders than fast ones — both of which follow from doing the sum honestly. An aero change doesn't hand you watts; it lowers the power a given speed costs, and what you do with that is ride faster at the same effort. So the right calculation solves speed for both drag areas at your power and takes the difference over your distance. For an 83 kg rider-plus-bike holding 200 W over 40 km, going from the hoods (0.32) to the drops (0.28):
| On the hoods (CdA 0.32) | In the drops (CdA 0.28) | |
|---|---|---|
| Speed at 200 W | 33.7 km/h (20.9 mph) | 35.1 km/h (21.8 mph) |
| Time for 40 km | 71 min 12 s | 68 min 18 s |
| Saved | 2 min 54 s — or 20.6 W if you held the old speed instead | |
Flat road, still air, sea level, road tires (Crr 0.0045), 97.6% drivetrain — the Martin et al. power model this site's speed, climb and CdA tools all run on.
Watts saved vs time saved — why they're different numbers
At a steady speed on the flat, nearly all your power fights air, and air power rises with the cube of speed. Run that backwards: cut the power a speed costs by 10.3% (the 20.6 W above) and the speed you get for your original power rises by only about a third of that — 4.2% here. That's why "saves 20 watts" never turns into "20 watts faster": the same change is worth 20.6 W at a fixed speed or 2 min 54 s over 40 km at a fixed power, and the marketing prefers the first number because it sounds bigger. The calculator shows both, and calls them what they are.
Who saves the most from aero?
The slower rider — which sounds wrong until you notice they're out there longer. The same hoods-to-drops change, at five power levels over 40 km:
| Power | Speed before → after | Time on course | Time saved | Watts saved at that speed |
|---|---|---|---|---|
| 125 W | 28.1 → 29.3 km/h | 85 min 21 s | 3 min 22 s | 12 W |
| 150 W | 30.2 → 31.5 km/h | 79 min 29 s | 3 min 11 s | 14.8 W |
| 200 W | 33.7 → 35.1 km/h | 71 min 12 s | 2 min 54 s | 20.6 W |
| 250 W | 36.6 → 38.2 km/h | 65 min 30 s | 2 min 41 s | 26.5 W |
| 300 W | 39.2 → 40.9 km/h | 61 min 14 s | 2 min 32 s | 32.4 W |
83 kg system, CdA 0.32 → 0.28, flat, still air. Watts saved are at each rider's own speed — the fast rider's change is worth more watts and fewer seconds.
Same bodies, same change. The fast rider gets 32.4 W of help but is on the road for 61 min 14 s; the slow rider gets only 12 W but has 85 min 21 s for the gain to accumulate, and a percentage of a longer time is more seconds. Aero is usually sold to racers. By the clock, it's the century rider and the first-time triathlete who have the most to gain from getting low.
What is each aero change worth?
Every change the calculator offers, priced for the 200 W rider over 40 km. The drag-area deltas are typical values from wind-tunnel reports and manufacturer claims; the seconds are what the physics makes of them.
| Change | ΔCdA | Watts at 34 km/h | Saved over 40 km | Typical cost | $ per second |
|---|---|---|---|---|---|
| Zip the jacket — or lose it — for a fitted jersey | -0.030 | 15.5 W | 2 min 09 s | Free | — |
| Hoods → drops | -0.040 | 20.6 W | 2 min 54 s | Free | — |
| Tuck on the hoods (forearms level, elbows in) | -0.020 | 10.3 W | 1 min 25 s | Free | — |
| Shaved legs | -0.005 | 2.6 W | 21 s | $5 | $0.24 |
| Aero socks / shoe covers | -0.003 | 1.5 W | 13 s | $40 | $3.08 |
| Clip-on aero bars (drops → aero) | -0.050 | 25.8 W | 3 min 40 s | $150 | $0.68 |
| Aero road helmet vs vented | -0.006 | 3.1 W | 25 s | $250 | $10 |
| Skinsuit / aero jersey vs standard kit | -0.015 | 7.7 W | 1 min 03 s | $300 | $4.76 |
| Deep-section wheels (50–60 mm) vs shallow alloy | -0.008 | 4.1 W | 34 s | $2,000 | $59 |
| Aero road frame vs round-tube frame | -0.008 | 4.1 W | 34 s | $4,000 | $118 |
Starting CdA 0.32; each row applied alone. Costs are rough US retail for the dollars-per-second column only — your prices will differ; your deltas will too (±50%). Changes don't simply add: a skinsuit over a shaved leg saves less than the two rows summed.
Why is every aero claim quoted at 45 km/h?
Because the cube law makes the number biggest there. A product "saves 10 watts" at 45 km/h — pro time-trial speed — and the same product at the speed most riders actually hold saves a fraction of it. Here's a 0.01 m² improvement, the size of a good helmet or a set of deep wheels, rescaled:
| Speed | Watts saved by 0.01 m² | Share of the 45 km/h claim |
|---|---|---|
| 25 km/h (15.5 mph) | 2.1 W | 17% |
| 30 km/h (18.6 mph) | 3.6 W | 29% |
| 35 km/h (21.7 mph) | 5.8 W | 47% |
| 40 km/h (24.9 mph) | 8.6 W | 70% |
| 45 km/h (28 mph) | 12.3 W | 100% |
Flat, still air, 83 kg, from CdA 0.32. The calculator's "same change at 45 km/h" line shows you the marketing number beside your own, so you can see the 71% that evaporates on the way down to 30 km/h.
Position first: the free watts
Read the change table from the top and the pattern is hard to miss: the two biggest savings cost nothing. Getting into the drops properly — elbows bent, back flat, head down rather than just hands lower — is worth about 20.6 W at 34 km/h, 2 min 54 s over 40 km. Swapping a flapping jacket for a zipped, fitted jersey is worth about 15.5 W. An aero helmet, by comparison, saves 3.1 W for $250, and it only works if you keep your head where the wind tunnel had it. Equipment is real; it's just the third thing to buy after two free ones, and the narrower bars that shrink the shoulders are the cheapest hardware change of all.
Deep wheels save about 4.1 W at this rider's speed, 34 s over 40 km, for around $2,000 — $59 a second. The drops save 2 min 54 s for nothing, and the zipped jersey 2 min 09 s. None of that makes the wheels a bad buy — they're also stiffer, they look fast, and in a race where you've already done the free things, seconds are seconds. It just puts the receipt next to the result.
When does aero stop mattering?
When you slow down. Aero power falls with the cube of speed while gravity's share climbs linearly with grade, so the same hoods-to-drops change over 10 km is worth less on every steeper road:
| Grade | Speed at 200 W | Time for 10 km | Saved by CdA 0.32 → 0.28 |
|---|---|---|---|
| 0% | 33.7 km/h | 17 min 48 s | 43 s |
| 2% | 24.3 km/h | 24 min 40 s | 37 s |
| 4% | 17.3 km/h | 34 min 45 s | 24 s |
| 6% | 12.8 km/h | 46 min 52 s | 15 s |
| 8% | 10.0 km/h | 59 min 50 s | 9 s |
83 kg system, still air. On an 8% climb the position change is worth 9 s over 10 km; there, weight is the lever — which is what the wheel-weight page prices, and why nobody has ever won a mountain stage in the drops.
The other place aero matters less is in a group. Sitting in a wheel cuts your aero drag by roughly a quarter to two fifths depending on the gap and the size of the rider in front, so in a road race or a fast club ride the equipment gains shrink with it — until the moment you're alone off the front, which is exactly when they count. Time trials, triathlons, solo centuries and the last 200 meters are where every row of the change table applies in full.
Measure it, then decide
Every preset above is somebody else's number. Yours comes from a steady lap with a power meter: the CdA calculator back-calculates your drag area from power, speed and weight, with the day's air density and an honest error bar, and saves the result to your bike in the Garage — after which this page starts from your real drag area instead of a preset. Test the position change before the purchase; if the drops save you what the table says, the wheels can wait.