+1 on all this.

It would be great to have it at one of indoor velodromes - Colorado Springs or Carson !

Have you seen or measured this ?

I've seen data from the Notio and Velocomp devices, and have not been impressed. Also the mounting location alone is a dead giveaway that they aren't intended for serious testing, since the influence of the rider+bike on airflow is too great.

If you are computing CdA on the fly and not using laps, then you have to determine elevation to high precision. How do you do this? GPS isn't good enough, and neither is barometric pressure. Velocomp uses a calibrated accelerometer, but it's influenced by the tilt of the device relative to the road, which is going to vary depending on weight shifts, bumps, and vibration. Ordinarily they calibrate the tilt based on 5 min of barometric data. For instance if the accelerometer says your elevation change over the last 5 min is zero but the barometric sensor says otherwise, the tilt will be recalibrated to match the barometric data. Obviously this isn't ideal because the barometric pressure changes over time, plus airspeed tends to affect it on any of the devices I've used. On the Aeropod they stopped doing calibrations when in CdA mode, but that just reverts to the old way of doing things (iBike days) where you run with the initial (imprecise) tilt calibration and call it good.

Another one is the airspeed measurement. Let's say you've calibrated it really well, and airspeed will be accurate regardless of the conditions to +-1% (which would be extremely good IMO). That still translates to a +-2% error range for CdA, which is more than is acceptable for testing, and we haven't even looked at all the other variables we have to contend with.

Both of these issues can be dealt with using an out-back lap protocol. Elevation zeros out by crossing the same points, and your runs are used to perform the airspeed calibration.

As much as I'd like to mount a simple unobtrusive device on my bike and get CdA values that are good enough for position and equipment evaluation and optimization... by just riding wherever I want... it isn't happening. Getting good actionable values outdoors in variable environmental conditions is tough even if you are using the most precise and accurate methods possible.

Aren't you placing an anemometer in the same location ?



There are was to get very high precision elevation change using sensor fusion algorithms. Using 5min of barometric data to calibrate an accelerometer is pretty primitive. Gyroscopes, accelerometers, magnetometers, barometric pressure and position can be used to correct each other. Drones and quad copters have been doing it for a while. How well Velocomp and Notio do it, I don't know.



Yes, an out and back helps a lots with overall/average CDA It also helps to calibrate your airspeed. I would further argue ii's not enough to calibrate your airspeed. There are other techniques to achieve equal or better results BTW, once calibrated I think you are overstating the impact of minor changes like hand position. Yes, go from 0 to 20 degree extension angle and you will need to calibrate, but not minor changes.



There ate still limitations of how to ride for calibration But you are not restricted to riding closed loops all the time.
Riding loops will always be better. But riding in a variety of configurations brings out data you don't always get in closed loop testing

Early on I used to use a 3 mile long route with a VABM (vertical angle bench mark) at each end. That meant that I didn't need to do closed loops since I had an anchor for the elevation change over the entire run.

Several years ago I noticed the city was surveying a street near my house, so I called up the Public Works Dept and asked if I could get the data. The guy got a lot more cooperative when I explained that I was a professor at Cal and wanted the info to test bike drag. He was both a recent graduate and a bike rider, so I got the data. Unfortunately, he left that job after a couple of years and the person who replaced him wasn't nearly so nice.

I know Notio paid land surveyors to survey their most popular test site.

A bike racer whose day job was a land surveyor had laid out a housing subdivision with roads and hills, so knew the contours. He tried to use VE on one of the hills he'd laid out but couldn't get the estimates to match up: for any reasonable set of CdA and Crr, the hill should've been a few feet higher. So he pulled out his surveying instrument, went back to the hilll, and discovered that the contractor hadn't graded the hill to his specification. When he put in the correct hill height the CdA and Crr estimates made sense.

"When a power meter is a better altimeter than a land surveyor's specification"? ;-)

Makes me wonder how often/little grading surveys are checked...

---

http://bikeblather.blogspot.com/

"The contractor built the road with 3.51' more vertical drop than it was supposed to have according to my design (I guess he wanted to save some cash on fill dirt)."

http://jasperga.blogspot.com/...thod-is-no-joke.html

My anemometer is mounted ~18cm up and out from the basebar handle, but I'm not sure that is far enough away.

I don't know what Notio uses for elevation, but if you have one it should be pretty easy to demonstrate how good it is at recreating a lap profile. I just had a look at their webpage and there is nothing about its accuracy or exactly how it works. Calibration looks like the Aeropod... ride out and back once and assume wind cancels. I can guarantee that you need to do many passes for that to be true if there is any wind! This thing has been kicking around for a couple years now and still no demonstrations of how good it is, from the company or users.

What "better way to calibrate airspeed" are you referring to?

 

Sounds similar to my pitot tube. I would have to measure but it's closer to the tip of my extensions, One reason some manufacturers place it closer to the basebar is to reduce vibration that comes from the extensions. This "helps" with alittude, but there are better ways to get altitude without compromising airspeed measurement


I agree the need to provide more data on testing, accuracy....I hope the do. If not someone else will :-) I was hoping Robert and Tom would with DCR. This will force manufacturers to deliver.



Agreed. I know my calibration factor better than my anniversary date. But I got it with a lot of testing. By using other data than just cancelling wind you can greatly accelerate the calibration

How do you do it? Or is that secret info?

The most stressful part of that day was realizing we were missing a mounting clamp for one of the devices we were trying to test, and getting the specs emailed so we could 3-D print a new one.

Guess who was at the receiving end of that email.

I hope you guys continue thpse tests in the future

Not secret info but probably drags in conversation that I'm not sure this thread is the best forum for.

Cool !