Yeah, it's sort of like separating Cd and A. For most of us who ride bikes, we don't really have to get separate estimates of Cd and A. All we need is their product, CdA. Likewise, Crr comes into the equation as Crr*mass*g, but g isn't actually constant across the surface of the earth. The earth is actually kinda squashed in shape and there are mountains so on the surface of the earth in the Andes near the equator g is about 0.7% lower than at sea level at the North Pole. But most velodromes aren't quite that big, so we just treat g as if it were a global constant.

I am new to VE testing and did a set today. It is a saddle with the out being a bit more uphill and then on the turn coming back to the start it was more downhill than I thought so I had to soft pedal it. I did sit up at each turn but it was consistent each time.

The first set I did was my current setup but I pushed stopped at a bit different spot than when I started to the beginning and end aren't the same.

The second set I did I lowered my bars 15mm. I just had a bike fit and the fitter thought based on my position that I could ride a bit higher and not suffer an aero penalty. I was hoping I could have some VE gurus take a look here.

I used a speed sensor and had GPS turned off on my head unit. I did 5-6 laps on the saddle for each setup.

Try to find a way to share the fit files.

Also post your weight, power meter type, tire type, location date and time of day (or even better yet weather station data you collected).

Weird I attached the files but they didn't show up. Hmmmm?

Setup weight 181 lbs. (82.1kg)
Tire: GP Force 700x25
Location: Lander, WY 8:30 AM
Weather Station: Used Weather Underground facility that measures was .75 miles away

Power meter type ?

Powertap P1

That wasn't to shame you if you used a single side powermeter :-)
It was to know to use a 0.97 ETA for a PM at the crank or 1.0 for a PM at the hub.

Ya, I got that, lol. It is dual sided. I did my best to research the eta portion of it and I figured that is what you wanted.

If you all PM me I can email you my spreadsheet for VE.

You may need to do some edits or pastes or small updates to make it work for your file, but it's enough to get the idea. You can paste in the rider info (weight, CRR, etc...).

I did this a while back for folks. I accept no liability of errors. Just be good enough at Excel to catch errors when you're working with it.

Here are some screenshots
I did them quickly in GC, you can fiddle with the data but these are the fields you enter
I attached the weather data and RHO calculator
This is not normally how I do it personally.

Marc, lonniecdams,

The rho calculator in GC uses "station" barometric pressure. Unfortunately for us, official weather stations like those at airports standardize to sea level. Lander WY is at an altitude of 1600m, so if you're using an official barometric reading rather than "station" pressure, you need to adjust for that.

What this means is that air density in Lander was close to 1.00 rather than 1.219. Since the density was lower, it unfortunately means that lonniecdams CdA was higher. rho multiplies the CdA, so if rho is lower by 1%, then CdA is higher by 1%. In this case, rho appears to be about 17% lower, so CdA will be 17% higher.

(As an aside, a quick rule of thumb is that air density decreases by about 1% for every 100m in elevation gain, so since Lander is at 1600m ASL, the rule of thumb is that rho would be about 16% lower. The rule of thumb isn't bad).

In any event, the difference in the CdA between the two tests is about .008 m^2. That much difference in drag area is roughly half way between the drag of a standard business card and a 3x5 index card; or, using Coggan's rule of thumb, at racing speed that's close to 30 seconds in a 40km TT.

Thanks

I don't normally use weather station data.
a) I use air pressure in the air component of the VE
b) I do measure barometric pressure

So the weather station is giving PA at sea level ?

Official ones do, and then they tell you what altitude they're at. Alternatively, if you have a portable handheld one (like I do) then I set it at my home (which happens to be at sea level) and then when I get to the top of the ridge behind my house I can just read off the proper rho.

Here's what I got... I used darksky's historical weather data and adjusted for altitude. I wasn't so generous with crr :) I don't think you'll get better than 0.005 on average asphalt with those tires, but I could be wrong.

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What's your CdA?

which one do you use ?

What do you plan to use when darksky goes away ?

do you adjust for ETA ?

I have the Kestrel 5100. I don't actually use it very much for the anemometer function any more. I got it mostly so I could learn and "calibrate" my sense of windspeed, and figure out how much difference there is in wind at bicycle height and up a little higher. The coolest use was to verify windspeed inside the velodrome.

So the Kestrel gives you barometric pressure at the point you are standing or you have to do some kind of calibration like sitting in your living room to 0m :-)

Hah, I had no idea it was going away. Looks like the API will still be online until the end of 2021. I guess I'll be going back to using OpenWeatherMap. Sucks because darsky's API was so much better.

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What's your CdA?

So, it gives you "station pressure" if you know your altitude -- and then automatically calculates rho. Alternatively, if you're trying to use it as an altimeter to measure change in elevation, you need to enter your starting baseline baro pressure. But I hardly ever use it this way. Mostly, I'm at a particular location and I have a pretty good idea what its altitude is. Then it measures the pressure, temperature, humidity, and tells me rho.

I guess I was looking more for was my course a good course, but thanks for all the other information.

The Rule 28 data that Dan posted shows the same Cd-Re characteristics, not surprisingly perhaps, as the characteristics of a cylinder or sphere with roughness that is often illustrated in fluid mechanics text books. For example:



http://brennen.caltech.edu/fluidbook/externalflows/drag/dragonasphere.pdf[/url]

This shows, if it wasn’t already obvious, that these aerostripe fabrics are working by tripping the boundary layer, and that you’d want to choose a fabric with protuberances that are just large enough to cause transition for the Reynolds number of interest, but not larger than is necessary (otherwise that’ll just overly thicken the turbulent boundary layer).

I have a couple of thoughts and questions, having just read the posts on this thread from the last few months about speed dependence on CdA (and possibly Crr):

1) Many of the things that people would want to test with the Chung method will affect in some way the points of flow separation on the bike or rider (e.g. socks, skinsuits, torso angles). Should the protocol recommended in this thread suggest that the person targets the speed range that is relevant to their race/event/discipline? That might affect the location that somebody chooses for testing. For example (a real example for me), if I want to check the benefit of trip socks, and my usual event is a flat-ish TT done at 40-45 kph, doing a VE test where half of the lap is uphill at 25kph may introduce uncertainties about whether the trips are working at the low speed sections. A velodrome would be a better choice. Of course, our choice of test location is usually a compromise, with several considerations.

2) For the Tom Compton Challenge, I’ve always wondered whether a sphere is a good object to use, in view of it’s Reynolds sensitivity. Personally, I’ve never found the time to do the test. However, the polystyrene ball, that has been patiently sitting in my garage for a few years, does not have a smooth surface. I’m not sure which side of the drag crisis it would sit. I wonder if it would be better to instead use an object that is less Reynolds-sensitive, such as a flat disc perpendicular to the flow.

Nick:

Nice to see you back.

Yes, in general you should try to test at the speeds and under the conditions you'll be racing -- though that's not always easy for practical reasons. In addition, if you *are* concerned about speed dependence and you want to identify that, you're certainly going to have to test at varying speeds.

For the Compton Challenge, I tried to use a wooden cube on a stick. It was actually a wood block my kid played with when he was a year old but once he graduated from school I figured he wasn't going to play with it anymore. To get to clean air and out of the bow wave, you can either go out to the side, or way way in front, or up high. I mounted a dowel vertically to the back of my saddle, and figured out how to mount the block to the top of it. Then to "tare" the dowel, on a calm day I went on a test run with just the bare dowel, varying my speed. But when the test run was done I forgot the dowel was back there so when I dismounted I snapped it off and stabbed myself in the butt. Don't do that.


If what you're trying to do is to determine how small of an effect you can reliably detect, I think a simpler test is the delta mass test. I've mentioned "delta mass" a couple of times in this thread before but I don't think I explained why I prefer it. Now you know. All you're really trying to do is to see how small of a drag force you can measure reliably, and it doesn't matter whether that drag force is aero drag or rolling drag. So do test runs with an empty water bottle and then the bottle filled with something heavy. I've used sand and small nails but an alternative is to use pennies. You can stuff a kilo of mass into a water bottle without risking your bottle cages.

Thanks Robert. I’ve seen your suggestion of the delta mass technique in previous posts, but I haven’t tried that yet. I suspect most people haven't gone for that variant of the Compton Challenge because the most obvious way to check a method for determining CdA is to change the CdA and see what can be detected (I‘m assuming the majority of people that are using your method are doing it to get a CdA estimate).

Thinking about it though, the delta mass test is probably easier to perform than rigging up support to hold a dragging object in undisturbed flow (as well as avoid any potential injuries!). I have a feeling my test technique and choice of low-wind days is not good enough to detect a kilogram of extra rolling resistance though, so I’d probably have to go for two bottles or heavier contents to be successful. I also think a lot can be learnt about the method precision from the observed differences in ABAB repeats.

The idea to tare the support is also a really neat idea. When I’ve read other people’s accounts of their Compton Challenge attempts, I think they calculated the incremental CdA of the support instead, which must be tricky because part of the support will always be in disturbed air.