Aerolab

What does it do?

The Aerolab analysis allows determining the Coefficient of Rolling Resistance (Crr) and Coefficient of Drag Area (CdA) for a bike + rider + bike position combination and allows determining the efficiency, from an aero perspective, different bike setups and rider positions.

This implementation is based on the presentation estimating CdA with a power meter by R. Chung, and there are many online resources on how to perform this kind of analysis, but this document discusses some specifics or the ActivityLog2 Aerolab implementation.

How does it work?

This section provides an overview of how Aerolab Crr and CdA estimation works, but some things are simplified, and does not go into mathematical details. For a full explanation see the estimating CdA with a power meter presentation

The power generated by the while pedaling a bike is used to:

• Wrr -- overcome rolling resistance between tires and the road -- this is the force resisting the motion when the bike tire rolls on a surface, overcomming deformation forces on the tire.

• Wpe -- overcome changes in elevation, when climbing or descending -- this component will generate power if going downhill.

• Wke -- overcome changes in speed, such as accelerating or decelerating. this component can generate power if slowing down (decelerating).

• Waero -- overcome the air resistance

For example, when cycling on a flat road at a constant speed, Wpe and Wke will be zero, and the power generated by the cyclist will be used to overcome the rolling resistance between tires and road, Wrr, as well as resistance of the air, Waero. When going downhill, the Wpe component will generate power assisting the cyclist of either maintaining the same speed for less power output from pushing the pedals or even accelerating by transferring the power to the Wke component.

Each of the power components depends on one or more of the following factors:

• speed of the bike -- both rolling resistance and air resistance increase at higher speeds

• slope of the ground -- as the slope goes steeper more power is required to maintain the same speed -- going uphill is harder

• acceleration or deceleration -- accelerating, or increasing the speed, requires more power than just maintaining the same speed, decelerating or decreasing the speed requires less power.

• air density -- this is a measure of how dense the air is, and affects the air resistance of the bike. It is calculated from weather observations, such as barometric pressure, temperature, dew point and humidity.

• air speed -- this is the wind contribution to the Waero component -- helping out if the wind if from the back, or hindering the rider if it is from the front.

• mass of rider + bike -- heavier riders require more power to maintain the same speed or climb hills.

• Crr and CdA -- rolling resistance and air resistance are also modelled by some coefficients: Coefficient of Rolling Resistance and Coefficient of Drag Area.

All but the last two parameters depend on the road / course being ridden as well as the weather, while Crr depends on the type of tire and its inflation pressure as well as the road surface, while CdA depends on type of bike and rider position (among other things).

The power, speed of the bike and slope of the ground can be recorded in an activity, the air density and air speed can be determined from weather data at the time the activity was recorded, while the mass of the rider + bike can be known by weighing the rider and the bike.

This data, together with some guessed values for Crr and CdA can be used to calculate a "virtual elevation" profile for the ride -- that is, the slope of the ground can be calculated based on the power output and power used by the different components, and this "virtual elevation" can be compared against the actual elevation of the route. The Crr and CdA parameters can than be adjusted so that the "virtual elevation" matches the actual elevation as close as possible. This process can be done either manually, by adjusting all values and looking at the data on the graphs, or automatically by the program.

Planning the testing

There are two considerations for planning an Aerolab test: selecting a route and looking at the weather.

Selecting A route: you'll need to find a route which is a loop (that is, it ends where it starts), which you can complete without having to stop -- this includes no stop signs and no, or very little, other traffic. For best results you should be able to traverse the route by always pedaling and generating power, with NO BRAKING and NO COASTING. The route should be about 1 - 2km long as you'll need to complete 3-4 laps of it for each test scenario.

Looking at the Weather: perhaps the most important thing is to select a day with light or no wind -- while some amount of wind can be accounted for, unfortunately, stronger winds have a variable effect around a circular route, and will lower the precision of the estimates.

You'll also want to select a time when other weather data is relatively constant over the duration of the test (e.g. avoid testing when a weather front is coming in, and barometric pressure changes every 10 minutes, or early morning/late afternoon, when temperature changes every 10 minutes).

Collecting the data

• You will need a power meter and a bike computer, and, of course, a bike.

• Weigh yourself and the bike before the ride -- the model needs the total weight of rider + bike.

• Write down weather information at the time of the ride. In particular, you'll need to record temperature, humidity, dew point (if available) and the barometric pressure -- this needs to be the pressure at the location, most weather applications report a "sea level equivalent pressure" which is not what we need (unless you are at sea level, of course).

• Complete several laps (at least 3 - 4) of the course in each aero configuration you want to test. Each aero position should correspond to a separate activity, since Crr and CdA parameter estimation is done for the entire activity.

This is not a power/strength test: you would use a power level such that you are always pedaling and generating power, while maintaining full control of the bike and you can safely slow down, so you can turn. Remember that for best results: NO BRAKING and NO COASTING should be done.

Analyzing the data

Once you have recorded the activities, import them into ActivtiyLog2, than for each of them, you will need to open the activity and enable the "Aerolab" analysis tab (this can be done from the "Activity/Show Aerolab Analisys" menu option.

On the Aerolab tab in the session inspector, you will need to fill in the values in the boxes on the right form the data you noted down (mass of rider and bike and weather conditions). The virtual elevation graph on the right will update as changes are made. You can use the sliders to try to make the virtual elevation profile match the actual elevation profile, however a better option is to use "Estimate Parameters" which will find a better fit for Crr, Cda and wind speed and direction.