AIRFLOW Thermoregulated Cooling for Indoor Cycling

The AIRFLOW is a smart device that accurately generates the ideal airflow velocity for an optimally thermoregulated human body when heat production balances heat loss. Every minute the AIRFLOW smart device determines the very appropriate airflow velocity to reconcile the Heat Balance Equation.

For indoor cycling on a static trainer a stable Heat Balance is paramount. Thermal heat stress may seriously affect the performance, overall productivity, safety and health of an individual. Discomfort at least, even heat illness and at worst heat stroke are three phases of the reaction of the human body when exposed to an unstable Heat Balance (hot, humid environment and inadequate airflow).

In our specific case we are, for practical reasons, interested in loosing heat during indoor bike training on a stationary trainer. Usually, one or more fans on the ground face the indoors setup, to cool down the (over)heated body during the workout or during intense virtual races like with Zwift. In that situation you want to minimize the rate of internal heat change. Front facing airflow around the body, is the best way to minimize sweating, as we all know from cycling outdoors. The faster you go outdoors the better you cool the body. When confronted with your indoor trainer setup, the question remains how hard should the fans have to blow to result in a maximal effect without causing shivering due to overcooling when the exercise is not that intense anymore?

Maintaining a relatively constant body temperature requires a fine balance between heat production and heat losses.

If body temperature is to remain unchanged, increases or decreases in heat production must be balanced by increases or decreases in heat loss, resulting in negligible heat storage within the body. If the body is at constant mass, the whole-body heat-balance equation expresses this concept.

Metabolism (M) is the consumption of energy from the cellular oxidation of carbohydrates, fats, and proteins. For a cyclist, the useful work on the environment (W) might be the energy spent in riding a Time Trial or a Granfondo. However, because of a long list of inefficiencies - the inherent inefficiency of metabolic transformations as well as frictional losses (e.g., blood flowing through vessels, air flowing through airways, tissues sliding passed one another) - most metabolic energy consumption ends up as heat production (H = M − W).

The AIRFLOW smart device continuously tunes accordingly the requested airflow velocity of the fan(s) for a stable Heat Balance during all phases of an indoor cycling workout, warm-up, intensity intervals, intermittent recovery and at cooldown. The cyclist has no on-the-way interference and can fully concentrate on the demands of the stationary trainer workout, always facing the ideal airstream that will cool him/her appropriately.

In a separate document the Heat Balance Equation and the applied algorithms are explained and elaborated see:

• Heart Rate Monitor (Dual ANT+ and) Bluetooth LE transmitter

The AIRFLOW device needs continuous measurement of the heart rate to estimate the critical internal body temperature which is proportional to the netto heat that is stored in the body. Most cyclists are used to wear a heart rate band during a workout and when that band is transmitting data over BLE it will be suitable for the AIRFLOW device. Notice that many of the (older) devices allow only one BLE connection at the time, which means that you cannot concurrently connect your cyling computer and Zwift computer and/or AIRFLOW device over BLE with the heart rate band... If possible: Use ANT+ for regular connections and only BLE for the AIRFLOW device!

• A Power meter with (Dual ANT+ and) Bluetooth LE transmitter

The AIRFLOW device needs continuous measurement of the critical variable: cycling power, produced during a workout, to determine how much energy the body is generating. The Power meter of your smart indoor trainer can supply the power measurements that you push during a workout. Or a dedicated power meter mounted on the bike at the crank, pedals or rear hub as long as it is capable of transmitting power data over Bluetooth Low Energy (BLE). Notice that many of the (older) devices allow only one BLE connection at the time, which means that you cannot concurrently connect your cycling computer and Zwift computer and/or AIRFLOW device over BLE with the power meter... If possible: Use ANT+ for regular connections and only BLE for the AIRFLOW device!

• Temperature & Humidity Sensor

Ambient air temperature and relative humidity are critical variables during any serious workout. These are measured with a Adafruit sensor continuously and is part of the electronic circuitry that processes all measurements!

The mechanical construction to hold 2 fans in front of your trainer setup is not the most complex and one can follow their own design. In principle one stand-alone fan of large diameter and capacity could suffice. However, 2 fans vertically aligned, create a more elongated air flow sphere and wrap your upright frontal area completely from head to toe! Furthermore the software allows to balance this setup to your preference with different operation capacities for the lower and the upper fan!

• 2 Table Fans 340 mm fan house diameter, 220/110 Volt AC 50/60 hz

For this project I have remodeled 2 low cost table fans that were bought years ago for domestic use and that I later always arranged on the floor around my indoor trainer setup for cooling. The fans had 3 buttons to select the preferred capacity and one button (#0) to switch it off.

Dismantle the fans and mark the (220 V) hot wire that is connected to button # 3, selecting full blowing capacity. Isolate carefully the other 2 wires, of buttons # 1 and #2, in such a way that they do no longer pose a danger. You should end up with 2 Hot wires and the Ground wire that produce full blowing capacity when connected correctly to a wall socket. Be aware of the danger of working with 220 Volt connections and appliances!

• 1 sheet of plywood (1220*610*15 mm)
• 2 Threaded rods M 5.0 and 400 mm length
• 2 Heating pipe, 15 mm outer diameter and 360 mm length
• 4 Acorn nuts
• Hand full of wood screws

Use an electric jigsaw to saw out carefully the curved shapes from a plywood board as indicated in the attached worksheet. Mount the plywood parts as shown, using threaded rods of the right length. Notice that the distance between left and right side is dependent on the diameter of the fans that you apply. In my case it was 360 mm that gave a stable construction and enough space for the lower fan housing.

• 2 Threaded rods M 5.0 and 55 mm length
• 4 Wing Nuts

The dismantled fans have been mounted with a clasping construction using a short piece of threaded rod (M 5.0) and wing nuts (M 5.0). In this way their position can be slightly rotated (up-down) to adjust the preferred airflow direction.

In the above figure the wiring of the different components and circuitry is shown. The Adafruit Feather nRF52840 Express has been chosen as the preferred processor because of its excellent mathematical properties, large working memory, high performance nRF52 timers and furthermore it is capable of concurrent connection to many BLE devices. Notice that the Zero Cross Interrupt lines of both Robotdyn modules have been connected to one interrupt Pin.

• Adafruit Sensirion SHT31-D - Temperature & Humidity Sensor

• Adafruit OLED 128x64 I2C blue display

• Robotdyn AC Light Dimmer Module, 1 Channel, 3.3V/5V logic, AC 50/60hz, 220V/110V

• Adafruit Feather nRF52840 Express

• 2 Table Fans 220/110 Volt AC 50/60 hz

Be aware of the danger of working with 220 Volt connections and appliances!

The AC Dimmer Module by Robotdyn is designed to control Alternating Current/voltage (110/220V). It can control AC levels up to 400V/8А. In most cases, the AC Dimmer Module is used to turn the power ON/OFF or dim lamps or heating elements. It can be used as well with fans, pumps, air cleaners, etcetera. A major benefit of the Robotdyn board is that the 110/220V part is (optically) isolated from the 5V logical control part, to minimize the possibility of high voltage damage of the attached low voltage microcontroller. The logical level of the Dimmer Module is tolerant to 5V and 3.3V, therefore it can be connected to a microcontroller with 5V and 3.3V level logic.

More detailed info can be found at: Github repository reference

The separate modules and components have been tested in different stages of development and construction.

For me it was a step in the dark since I have never worked with these components before!

At first the circuitry was tested in an experimental setup, using light bulbs instead of Fans to see whether the nRF52 timer library that was applied and the software components that regulate the intensity (duty cycle) were working correctly!

Later the setup was connected experimentally to the mounted fans for first software testing of the complete device.

Finally the circuitry was mounted on 2 small and thin plywood sheets that cover the open compartments that are created with the construction of the fan holder (right the 5 Volt components and left the 220 Volt AC dimmers). You can follow the 220 AC wiring (back-view) coming out, attached to the AC dimmers of the upper and lower fans! Yellow-Green is ground, 220 AC Brown is Hot wire and 220 AC Blue is Hot wire.

Be aware of the danger of working with 220 Volt connections and appliances!

The blue OLED display was mounted at the front of the fan holder with a small slot that gives access to the 5 volt circuitry compartment for wiring throughput. The chosen character size of the information displayed on it, allows reading from the indoor trainer setup in front of it!

In the last phase the definitive setup configuration was tested elaborately and fine-tuned for more than 6 months during my intensive indoor winter training program and the 2021 lockdown period.

Only after many weeks of operation I decided to extend the original design with a "table" construction (at the height of my head in cycling position) for placing a separate monitor connected over HDMI with my PC . The monitor presents right in front of me the TACX app and Zwift sessions that I choose for, while the AIRFLOW device is keeping me cool during the workout! Meanwhile I can keep an eye on the critical variables that detail how my body is doing in terms of Heat Balance!

The Feather nRF52840 is programmed to produce a continuous stream of data that the user might interest during a workout. During operation the Blue Oled display is updated very regularly with relevant data sets about how your body responses to the exercise in terms of thermoregulation. The variables shown are the direct outcome of on-the-fly solving the Heat Balance Equation. Most of these data are unique and will not be found on your cycling computer and by monitoring these data you learn about the temperature management of your body during intense exercise.

At start-up (power on) the user is informed about the BLE connection process of the AIRFLOW device with Heart Rate Monitor, Power Meter and Smart Phone. On the top bar, icons are shown that indicate the (BLE or I2C) connection status of: Heart Rate Monitor, Power Meter, Smart Phone, Humidity and Temperature. During operation a sequence of (11) informative screens are shown on the Blue Oled display that detail the measured or calculated critical values that determine the Heat Balance Equation.

On the top bar, the double chevron to the left always indicates whether you netto gain (up) or loose (down) internal heat during the workout! Every 10 seconds the content of a new screen in the sequence (of 11) is shown. 2 Screens in the sequence (#1 and #9) change of data set during display. The user can switch the 11 screens to be shown in the sequence on/off, in accordance with his/her preference and at any time during operation with the help of the Airflow Companion App (Display Settings).

From the start of the project it was decided to develop an AIRFLOW Companion App that would allow changing settings and if all went wrong to allow a strict manual control of the fans. I have little experience with App development and decided to program one (for Android) in the accessible environment of MIT App Inventor 2.

More detailed info can be found at: GITHUB AIRFLOW Repository Reference

Install the Arduino IDE and all the libraries on a PC/Mac. You will find all the Arduino code that controls the AIRFLOW on Github AIRFLOW Repository and also the Arduino test programs (modified for this project) that focus on the components separately. Download all the code from Github AIRFLOW Repository and install in your Arduino IDE.

Download from Github AIRFLOW Repository also the (Android) code that is used for the AIRFLOW Companion App and install it on your smartphone. The Companion app will allow you to change settings during operation.

In addition to an ideal airflow velocity for an optimally thermoregulated human body, the AIRFLOW user can get (as a bonus) insight in the development of his/her Critical Power when training intensity is intense and long enough! The applied Arduino nRF52480 Express CPU is so powerfull that Real Time calculation of CP and W Prime can be accomplished aside all the calculations for determining the Heat Balance terms and setting the fans to the appropriate blowing capacity. Setting on/off this functionality and/or changing your starting CP and W Prime values are also an integral part of the AIRFLOW Companion app! If you want to know in more detail about the science, math and implementation... See Github Repository: RT-Critical-Power