# Is it possible to accurately measure airspeed on a bike, and is it useful?

So riding into a 15mph headwind at 15mph, your airspeed would presumably show as 30mph. In no wind, airspeed of 15mph.

What would the airspeed be if cycling at 15mph with a 15mph crosswind?

And would viewing airspeed be at all useful to a training cyclist, apart from relatively determining what the wind speed is? The only reason I'm even thinking about it is because so don't have a power meter. :)

• This used to be a "thing" when I was a kid -- folks would rig up an anemometer atop their front wheel and somehow (pre-microelectronics) read out the speed. But I think it mostly stopped when it became practical (for some reason). Jan 8, 2020 at 22:33

## 4 Answers

is it possible?

Yes it is possible.
Here is a link to an instructable on how to build one.
Here is a link to Velosense - a company that builds a sensor.
Here is a link to a Cycling Weekly article that discusses a device by Notio Konect

It is possible to adapt an anemometer for bicycle use

is it useful?

Usefulness is a subjective value

A summary of the Cycling Weekly article:

By measuring wind speed, humidity, air pressure, air density, temperature and cross referencing this with power, GPS and speed data, the Notio Konect can calculate your drag coefficient (CdA) in real time.

Later in the article

This is particularly exciting as it will allow riders to experiment with things like arm and head positions in order to establish which is faster. It is important to point out that the applications of this extend well beyond time trial, but also into standard road bikes too.

So, if you had an airspeed sensor with humidity, air pressure, air density, and temperature, and referenced the data with power, GPS and speed data with you had a headwind of 15mph you could adjust your riding position and find the most aerodynamic riding position.
You could also modify your bicycle to be more aerodynamic and gather data to determine if there is an actual improvement.

You'd have to decide for yourself it doing all that work is "useful".
Most riders change riding position in a headwind by feel.

What would the airspeed be if cycling at 15mph with a 15mph crosswind?

It depends.
You would have a (some degree) vector of force with a crosswind of 15mph. If the crosswind was - for example - 10 degrees off of dead ahead it would contribute to airspeed. At 90 degrees off of dead ahead it would not contribute to airspeed. Here is a discussion on the effect of crosswind on an plane which is not exactly like a bicycle but principles apply.

The airspeed and the wind speed are both vector quantities having a magnitude and a direction. The chief effect of the cross wind is to deflect the flight path in the direction of the wind.

• Thanks for this. Yes maybe 'is it useful' could be changed to 'is it worth it'. :) A plane is moved sideways by a 90° crosswind whereas a bike isn't, so does the crosswind still not contribute to airspeed, either positively or negatively? Jan 7, 2020 at 17:41
• This type of sensor with a rotor won't be too accurate. A pitot tube is better for airplanes, they are faster and always sort of moving forward. A hot-wire would be better. Especially when you want to take into account such details as humidity. Seriously, it is completely enough to just take a humidity value from the weather forecast or just ignore it altogether, the dependence for the density is pretty small and the effect will be MUCH smaller than the wind speed measurement error. (a meteorologist here) Jan 7, 2020 at 17:48
• You are correct @VladimirF - the anemometer taped to a handlebar won't be as accurate as a pitot tube. "Accurate enough" depends on the use case. Jan 7, 2020 at 18:15
• @Wilskt A bicycle can be blown off a straight line by a powerful cross wind. Depending on the angle of the crosswind to forward motion it might contribute either positively or negatively to airspeed. Jan 7, 2020 at 18:18
• @VladimirF One of the early versions of what later became the Garmin Aerostick used a hot wire sensor (there were other prototypes based on other sensor types, too). I don't recall exactly why the final decision was made but hot wire was absolutely considered for this application and then ultimately rejected. Jan 7, 2020 at 23:36

There are devices designed for bicycles that measure airspeed. And older example is the iBike, but there are others. The iBike is intended to be used as a stand-alone power meter, and makes assumptions about the drag area and rolling resistance of the bike/rider, then combines those assumptions with speed and gradient information to estimate power.

The other airspeed devices (for example, the Notio Konect) are used in conjunction with an on-bike power meter to determine aerodynamic drag; the on-bike power meter measures power, an on-bike speed sensor measures speed and acceleration, and (sometimes) other sensors will measure gradient and air density. From these measurements, aerodynamic and rolling resistance drag can be estimated. Of the airspeed measurement devices that are currently available as of January 2020, none measure yaw -- they only measure airspeed from directly ahead. There are a few devices that are in development that do measure yaw.

This SE.bikes question and answer describes some of the calculations involved in the estimation. A brief description of how the iBike estimates power is discussed in this SE.bikes topic here.

• For some more insight, DC Rainmaker did a review of the "PowerPod" in 2016, which is (was?) one such product: dcrainmaker.com/2016/03/powerpod-depth-review.html Jan 7, 2020 at 16:32
• Thanks for the other links, feel I'm going down a rabbit hole here! Jan 7, 2020 at 17:43
• @AndrewHenle Yeah, the PowerPod is produced by the same company as the iBike. As an aside, I went to a wind tunnel with Ray Maker (DC Rainmaker) a while back to test the PowerPod, the Konect, and the Garmin Aerostick. Jan 7, 2020 at 18:29

What would the airspeed be if cycling at 15mph with a 15mph crosswind?

You clearly can define a single airspeed (and effective wind direction) from the maths - it's 15*sqrt(2) mph at 45° in your example, but it's not very useful. Consider a sailing boat. Sailing perpendicular to the wind is close to the fastest direction, because the wind is deflected backwards pushing you forwards.

If you had a sail on your bike, apart from making balance tricky, it could be angled to propel you forwards, or to act as a brake. In strong winds at low speeds it's actually possible to feel this effect just by angling your body as you ride perpendicular to the wind - sitting up straight, if you push your upwind shoulder forwards you'll be faster for the same pedalling effort as pushing your downwind shoulder forwards. I've only had good chances to try this uphill, but on the flat an interesting experiment would be to see how far you could freewheel in each of these two body positions.

• I played with a sail on my bike when I was maybe 14. IIRC it was a piece of broom handle with a sheet attached, somehow moored to my bike (I forget the details). Jan 8, 2020 at 22:35
• @DanielRHicks on a tandem, operated by the stoker, it might be interesting, but solo you'd want something you could set and forget. I might get the chance to play with body angle this weekend - a long ride planned with side winds Jan 8, 2020 at 22:39
• " because the wind is deflected backwards pushing you forwards" - The wind is a vector that hits the sail, and there is a combination of the direct force of the vector of the wind that is pushing 'forward' (in line with the keel), and there is a component of 'lift' from the air traveling around the curve of the sail, like the wing of an airplane. there is no 'deflected backwards pushing you forward' forces involved in a sailboat Jan 9, 2020 at 15:35
• @Cinderhaze You're right - I'm simplifying deliberately, but I was thinking in terms of mass flow/conservation of momentum on a beam reach (where the wind vector doesn't have a forwards component) or a close reach where it has a backwards (negative forwards) component; that's appropriate when you're generating a headwind by pedalling. I also ignored anything to do with the curve of the sail as you can get propulsion from a flat board, and while a torso isn't a flat board, it's not an aerofoil either Jan 9, 2020 at 16:04

Here's 'the math':

So, "No, your airspeed would not show as 30."

The magnitude of your velocity, and thus your airspeed, would be 21.2 mph. Velocity is not a `scalar` value, but a `vector` value because your "air velocity" in this situation has two components:

• magnitude
• direction

In other words, the `magnitude` is your airspeed, and its value is 21.2 mph. Its `direction` is shown in the diagram with a crosswind that's perpendicular to the direction of travel. Of course in the 'real world', the crosswind could be at any angle. I have assumed the crosswind to be perpendicular (90 deg) to your direction of travel to make the math easier for me :) I hope it's now clear that the direction of the crosswind relative to your forward velocity will affect your airspeed. This is because your airspeed is just the `magnitude` of the resultant (R) velocity, and the Resultant velocity is the vector sum of the forward velocity of your cycle and the crosswind.

Put another way, the airspeed of 21.2 mph is what would be measured by a pitot tube, and by most airspeed sensors. A windsock or vane would measure direction.

IMHO, knowing the airspeed on a cycle might be useful to a training cyclist, but not so much for training and evaluation of progress toward a goal. I say this because relating airspeed to power requires more data. Consider the following:

If you have measured your airspeed and you know your `drag` coefficient you can determine the force required to move the cycle. You apply a force to the pedals, and as the cycle moves forward you are said to have done `work`. If you do `work` over a period of time, you have generated `power`. (Some references for your reading pleasure).

In other words, you will run into some complications: Force on the pedals will change depending not ONLY on airspeed, but also on gear selection and incline (among other things). If you cycled the same stretch of road (same incline), with the same gear selection, and the same cadence, then airspeed would be a reasonable indicator of how much power you expended relative to a reference.

But, that's probably neither practical nor meaningful in terms of training, and measuring progress toward a goal. And so my conclusion is not an `answer`, but rather my opinion: If you're serious about training, and measuring your progress toward some objective, an airspeed sensor is a poor substitute for a cycling power meter.

• Years ago, when power meters were very expensive, Ciclosport sold an altimeter-equipped head unit with special firmware as an inexpensive power meter. I analyzed the data and concluded that a power meter was a better altimeter than an altimeter worked as a power meter. I suspect something similar here: a power meter could do a better job as an anemometer than an anemometer could do as a power meter. Jan 8, 2020 at 23:02
• For those riding without a powermeter, Strava calculates power following a proprietary algorithm depending on rider weight, incline and speed. Jan 13, 2020 at 21:10
• @Carel: That seems (mostly) reasonable... but is it `air speed`, or `speed over ground` they're using? I say "mostly reasonable" because it almost sounds that Strava would like to make math & physics `proprietary` to themselves. Can you provide a link to Strava's claim? Jan 13, 2020 at 22:27