CdA Values by Bike Type: From TT to MTB
CdA is effective drag area — drag coefficient times frontal area. It explains why the same wattage produces completely different times on different bikes.
Reference values for practical use
These are starting points for predictions, not measurements. Height, shoulder width and position shift them considerably.
- Time trial bike, practised position: around 0.22 m²
- Aero road bike in the drops: around 0.27 m²
- Classic road bike: around 0.30 m²
- Gravel bike, no bags: around 0.34 m²
- Mountain bike, upright: around 0.40 m²
What the difference is worth
Going from 0.34 to 0.27 is roughly 20 percent less drag. On a flat 2 km segment at 38 km/h that is easily 8 to 12 seconds — more than most riders gain from several weeks of training.
So on flat segments the bike choice is a strategic decision, not a matter of taste.
Position beats equipment
The rider accounts for roughly 70 percent of total drag. Forearms level, head low, shoulders tucked — that often beats new wheels. The position just has to be sustainable for the length of the segment.
Estimating your own CdA
Ride a flat segment twice in opposite directions in near-still air with a power meter and steady watts. From time, power and a rolling-resistance assumption you can back out CdA. For prediction purposes that is accurate enough.
In short
Use the CdA of your real setup. An over-optimistic value makes every prediction useless.
Frequently asked questions
How much do gravel tyres change things?
Knobbly tyres raise both rolling and aerodynamic drag. On tarmac expect several percent of time compared with fast slicks.
Does jersey choice really matter?
Yes. A flapping jersey can cost several percent of CdA — on a three minute segment that is several seconds.