Cycling pacing and aerodynamics glossary: CdA, Crr, W′bal, critical power and more
Performance against the clock depends on three things: how much power the rider produces, how much of it is lost beating air, rolling resistance and gradient1,5, and how effort is distributed over the course4. These are the terms that describe it.
Physics: air, rolling and gradient
CdA (drag area)
Drag coefficient multiplied by frontal area, in m². It sums up how aerodynamic the rider and bike are: a lower CdA means more speed for the same power. In time trials, typical values range from 0.20 to 0.30 m². Read more →
Crr (rolling resistance coefficient)
A dimensionless number which, multiplied by weight, gives the force resisting motion due to tyre deformation. On the road it usually ranges from 0.003 to 0.006. Read more →
Air density (ρ)
Mass of air per cubic metre. It falls with altitude and heat, and aerodynamic drag falls by the same proportion. At sea level and 15 °C it is 1.225 kg/m³. Read more →
Virtual elevation (Chung method)
A method to estimate CdA and Crr with a power meter and GPS, by rebuilding the elevation profile that would “explain” the measured power. If the virtual profile matches the real one, the coefficients are right. Read more →
Physiology: how hard you can go
Critical power (CP)
The highest power that can be held in a near steady state for a long time. It separates the sustainable domain from the one that drains a finite reserve. Read more →
W′ (W prime)
A finite amount of work, in joules, that can be done above critical power. When it runs out, power has to drop below CP. Read more →
W′bal (W′ balance)
What remains of W′ at each moment: it falls above CP and recovers below it. It is the basis for distributing power without blowing up before the finish. Read more →
Pacing and planning
Pacing
How effort is distributed over an event. In a time trial the best distribution is not constant power: push harder where you are slow (climbs, headwind) and less where you are fast. Read more →
Normalized power (NP)
An estimate of the constant power that would have carried the same physiological cost as a variable effort. It is always equal to or higher than average power. Read more →
Variability index (VI)
Normalized power divided by average power. Close to 1.00 means a very steady effort; in a well-ridden flat time trial it usually stays below 1.05. Read more →
ETA (estimated time of arrival)
The time the rider will pass each point of the course. Planning with the weather at the ETA, not at the start, changes the plan when wind or temperature change during the event. Read more →
Drafting
The reduction in drag from riding behind another rider. Deep inside a large peloton, drag can fall below 10 % of an isolated rider’s. Read more →
Gear development
The distance the bike travels per pedal revolution. On the track, with a fixed gear, it sets the cadence at each speed. Read more →
Frequently asked questions
What is CdA in cycling?
Drag area: how much the air slows the rider and bike together. It is measured in m², and in time trials it usually ranges from 0.20 to 0.30.
What is the difference between FTP and critical power?
Critical power comes from a mathematical model of several maximal efforts and separates sustainable effort from effort that drains W′. FTP is a practical estimate of one-hour power. They are usually similar, but they are not the same.
References
- Martin JC, Milliken DL, Cobb JE, McFadden KL, Coggan AR. Validation of a mathematical model for road cycling power. J Appl Biomech. 1998;14(3):276-91. doi:10.1123/jab.14.3.276
- Jones AM, Vanhatalo A, Burnley M, Morton RH, Poole DC. Critical power: implications for determination of VO2max and exercise tolerance. Med Sci Sports Exerc. 2010;42(10):1876-90. doi:10.1249/MSS.0b013e3181d9cf7f
- Skiba PF, Chidnok W, Vanhatalo A, Jones AM. Modeling the expenditure and reconstitution of work capacity above critical power. Med Sci Sports Exerc. 2012;44(8):1526-32. doi:10.1249/MSS.0b013e3182517a80
- Abbiss CR, Laursen PB. Describing and understanding pacing strategies during athletic competition. Sports Med. 2008;38(3):239-52. doi:10.2165/00007256-200838030-00004
- Debraux P, Grappe F, Manolova AV, Bertucci W. Aerodynamic drag in cycling: methods of assessment. Sports Biomech. 2011;10(3):197-218. doi:10.1080/14763141.2011.592209