CdA is the drag coefficient (Cd) times frontal area (A). The aerodynamic drag force is simply CdA times dynamic pressure. Dynamic pressure is a function of air speed, specifically ½ x density x speed2. CdA should remain constant for a given bicycle configuration across the speed range, so is a useful measure of how aerodynamic you are.
It is not possible to directly measure aerodynamic drag of the bike and rider combination. Instead, we measure the power input from the rider, subtract measured kinetic power (power required to accelerate), gravitational power (power required to go uphill) and make an assumption for frictional losses (which are all relatively small), which leaves aero power. From this we can easily calculate CdA.
Dynamic pressure is typically measured using a pitot-static tube, which is accurate over a wind yaw angle range of around +/-10°.
The 3-hole probe is a type of pitot tube which also measures yaw angle. However, this has a very complicated calibration due to separation of the flow from the probe head. As a results we found it to give poor results across the large speed and yaw angle range that bicycles operate at.
Our patented probe design maintains clean attached flow through the probe central channel, giving a very linear and easily-calibrated response across a large yaw angle range.
CdA is a function of yaw angle. Up to a certain angle, typically 8-12° the drag falls away as the wheels create a “sail” effect. Beyond that the drag can rise very steeply. At the relatively low speeds the bicycles travel at it doesn’t take a lot of wind to generate large yaw angles. For example the UK mean wind speed is around 15kph. At 40kph bike speed a crosswind of 15kph equates to a wind yaw angle of over 20°.
The image here shows a typical aero wheel drag behaviour with wind yaw angle.
The aerobody device sits on the stem, and measures relative position of the rider’s head, chest, and waist using a non-contact optical sensor. This gives you a measure of rider position which is accurate to around 1mm, allowing the cyclist to check that they are maintaining their optimal position throughout their ride, be it on the road, in the velodrome or even in a wind tunnel. The device can also be used on static trainers, allowing the rider to train in their best aero position.
In velodromes the track shape has an influence on the drag measurement due to the lean angle of the bike. Knowing where the bike is on the track allows us to account for this and give a highly repeatable measure of drag from one lap to the next.
Out on the road, one of the biggest sources of error is the altitude measurement, which we calculate through measurement of atmospheric pressure. Although Aerosensor has industry leading accuracy in this respect, it is still not perfect. Using two lap triggers we can get a much better altitude measurement between your start and end points, improving the CdA measurement.
aerodrome sends the lap start/end signal direct to Aerosensor with an accuracy better than 1ms, allowing the internal drag calculation to be synchronised with track position to a very high degree of accuracy.
Our Indiegogo is live as of 9am 1st August 2022! Be one of the first in the world to own our groundbreaking aerodynamic cycling system, so you can optimise your performance on the road. The adventure starts now and we hope that you can join us on this exciting journey.