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Purpose

Field testing is time consuming. This app aims to accelerate the task by computing on the bike the Coefficient of Drag Area (CdA) or Coefficient of Rolling Resistance (Crr) on several laps carefully done at different speed, using the virtual elevation method (see here for the theory). The app is free but some features are locked. You will need a key to unlock the premium version.

Support
Post here on Slowtwitch forum in the dedicated thread if you need help or have question.

Requirements 

• The app should be installed on an Android phone
• The phone should be set on the handlebar. We highly recommend this one (universal, solid, cheap).
• The bike should have ANT+/BLE power and speed sensors (except if you have a PowerTap hub) 
• A network connection is required to get weather station data and upload the CSV files for post analysis

Protocol

The virtual elevation method is described in practice in this thread. It is recommended to read it if you are not familiar with field testing. The app can be used to do CdA field testing (laps at high speed), or Crr field testing (laps at slow speed). Each lap with an average speed between 16 km/h (10 mph)  and 32 km/h (20 mph) will be rejected. These values may be changed into the settings thoiugh.

App use

When the app is launched, the BLE devices are scanned immediately and if found, you can click to connect or discard the device (see also these video for the first connection):  

 

Once the power and speed sensors are connected, the red button switches, according to the selected mode to "START RUN" or "AUTO START RIDE": 

In the middle part of the screen, environmental data from the nearest weather station and possible device sensors are also shown. At least, the values used in the model are indicated.

See here how to change the mode or add different data on the screen.

Settings

Before your first run, check all the settings in the menu. You would change at least the mass and your location (in order for the app to detect the nearest weather station). If no weather data is retrieved (in a zone with no network for instance), it is interesting to provide local altitude, temperature and barometric pressure to let the app estimate the air density. You can also hide some environmental data (to have a bigger green part on small screens for instance) by deselecting "Show all data". The other settings you could change are:

-wheel circumference (default 2096 mm)

-chain loss (default 2%). This figure will only be used for crank or pedal power-meters

-an estimation of Crr (default 0.003) for the CdA runs

-an estimation of CdA (default 0.4) for the Crr runs

-...

Crr or CdA measure

Now you are ready to start your run. A good advice is to first try to measure the Crr of your tires (and refine the base estimate of the Crr in the settings) during this run. Ride slowly on a loop (the length should be at least 100 meters) several times. 

For that, once you cross the mark you defined to begin the run, click "START RUN" and complete the whole loop, before clicking again on the red button, each time you finish a lap. After a lap is completed, the Crr value of the previous lap is printed into the green part at the bottom of the screen.

During the lap, the current distance is written on the red button.

Clicking each lap is defined as the "lap classic mode". There are some other modes, like the "defined lap length mode" for velodrome testing or the "training mode" to monitor CdA on the fly during a ride. See here for more details.

Once you decide to finish the run, just stop the bike and after 3 seconds, the red button will return to "START RUN" and some statistics (mean, root mean square, CV) for the run will be printed. With good experience, you would obtain CV of 5% (resp 2%) for Crr (resp CdA) field testing.

At the end of the run, it is possible, at this moment to record via a voice message, some information of the run (tested configuration, if a car drafted you during a lap, etc...).

During a run, it is not possible to access the settings or to record a message.

Once the session is finished, you can get the CSV files (by mail via the menu) to a post analysis, for example with the Aerolab tool of Golden Cheetah software to double check the CdaCrr results.

Some advices for field testing

Keep the same position on each lap of the same run

For Crr field testing, lap length should be at least 200 meters

For CdA field testing, lap length should be at least 600 meters 

Flat loops are better

The lower the speed average on Crr runs, the better

The higher the average speed on CdA runs, the better

Keep coasting when crossing the line (PowerTap only)

Keep pedaling during all the lap (Crank based powermeter only)

Try to keep the same sprocket during runs

Register a voice message after the run to remember the cause of the problem (ex: A car passed me during the second lap)

Try to find a velodrome and read this before

And, during a field testing session, always remember the three rules: no wind, no car, no brake.

If you are drafted by cars on your testing location or have a bit of wind (less than 10 km/h), you will take advantage of connecting an anemometer on your bike. See here for more information.

Ride safe, and enjoy.