Yes, you can. The only thing you have to watch is if you're using 3 magnets at top speed, you might saturate the reed switches on the speed sensor.


It's not necessary to mark the start/end of a lap. It becomes very clear where your lap boundaries are.

Surveying a course: there's plenty already written in this thread near the beginning of the thread.

Hey, thanks for using Aerolab and be safe!

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AndyF
bike geek

Thanks

I consistently max out around 28mph on my short little loop so I'll give it a try :)

Thanks

It just occurred to me that I updated my edge 1000 recently and that the speed preferences might be botched and it's using GPS for speed.

Remember that the larger your speed range are, the better your results will be.

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AndyF
bike geek

Did you try this in the end?

Does it make any notable improvements in the VE data or profile?

The furthest I've gotten is gluing magnets to my disc wheel haha. Hopefully this weekend I'll get to it

Cool, well keep us posted, it sounds like a very neat idea if it can boost the quality of the data.

So this weekend, I had a try at using multiple hill runs to decouple CdA and Crr as detailed on pg 43 of Robert Chung's Estimating CdA with a Power Meter presentation. The conditions weren't exactly perfect but the data that I retrieved from it look pretty decent for a first attempt, (at least to my untrained eye!). I read through this entire thread beforehand (as well as numerous other resources) and didn't see this particular protocol discussed that much, so I thought I'd share my experiences and see what you experts think!

Test conditions:

• Test course was 1.70 km with 87.6 m elevation gain; average gradient 5.2%.
• Rider: 5'7" (170cm) and 148 lbs (65 kg); weighed accurately with bike beforehand.
• Collecting data with a Quarq DZero power meter, Garmin 820 and Garmin GSC-10 with GPS off.
• Riding road bike on the hoods (i.e. typical climbing position), held the same position for all runs.
• Tight fitting winter clothing.
• Winds conditions were extremely favourable; essentially windless and well sheltered.
• Road surface conditions were typically British, slightly rough chip-seal and wet surface.
• Running 25mm GP4000s IIs with latex tubes @80 PSI.
• The first two runs each had a couple of instances of car-passing but because of the hill, the speed differential was low and they passed relatively wide (~1.5m).

As a quick summary, I did four consecutive runs, alternating high-speed/low-speed/high-speed/low-speed; the high-speed runs were @285W (~21 kph) and the low-speed runs were @200W (~15 kph). I calculated the Virtual Elevation from these data in Excel and when I put in what I thought would be reasonable CdA and Crr values there was a clear separation of the high-speed vs low-speed runs, but each equivalent run aligned pretty well. I adjusted the CdA until the four runs aligned as best I could and then I adjusted the Crr to match the known elevation gain of the climb. The resultant VE profile for the four runs looked like this:


Before tweaking CdA and Crr values (CdA = 0.350, Crr = 0.0060):
Click to enlarge.




After tweaking CdA and Crr values (CdA = 0.218, Crr = 0.0084):
Click to enlarge.


Apologies for the picture quality of the graphs; I could make the lines thicker and more visible, but then they all just obscure each other.


The striking thing is that the CdA seems way too low and the Crr seems somewhat high; I guess I have a few questions that I was wondering if you guys could share some experiences on:

1. First and foremost, was the speed differential in these runs (15 vs 21 kph) sufficient for this kind of testing? I could've hammered the fast runs harder but wanted to focus on maintaining a consistent and stable position.
2. Were the instances of car passing likely to have ruined the data from the first two runs? I tried just analysing the second two runs (which were cleaner in this regard) and whilst the profiles aligned much more closely, the resultant CdA and Crr values were very similar to above.
3. How much rolling resistance could you expect a wet road surface to add compared to a dry one?
4. Does tweaking the "known" elevation gain of the hill only affect the calculated Crr? The CdA is determined by the alignment of the runs at differing speeds, right?
5. I haven't accounted for drivetrain power losses downstream of the Quarq at all; how significant might this be? What might be a good way to account for this, a fixed value of power loss (e.g. subtract 10 Watts), a fixed-percentage reduction in power values (e.g. multiply by 95%) or something else?
6. What other factors might have produced an artificially low CdA and high Crr?

I'm sure I can improve on this given experience and better conditions, but I thought I'd see if you guys can help me work out where my biggest sources of error are coming from.

I really appreciate all the hard work that you guys (Andy Froncioni, Robert Chung and Alex Simmons in particular!) have put in to developing these methods and especially for documenting them in such an easy to digest and replicate fashion. Thanks!

First, congratulations on your first aero test! It takes a bit of work and, as you found out, quite a lot of reading. And thanks for being so clear about your protocol.

1. Yes, that was a very small range of speeds. You could do better with a factor of 2 speed range.

2. Any passing cars will upset your CdA estimate. Anything up to 15m in front of you or 6m to the side will have a big impact. It then depends on how long they stayed near you.

3. Most slightly wet surfaces exhibit a lower Crr than dry ones. However my experience with British chip seal in wet weather is limited. Still, I would expect no more than a Crr of 0.006.

4. I don't know what you mean by tweaking the known elevation gain. Could you elaborate, please? May I suggest turning your 1-way course into an out-and-back by turning around (safely!!) at the top of the hill? This would allow you to elevate-mate the up and down segments.

5. When you don't know, use 0.975. :-)

6. A high Crr estimate will artificially produce a low CdA. So I would start by trying to make the curves match by using a guessed Crr of 0.005. See how good a CdA estimate you can get from the data you have.

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AndyF
bike geek

Good work! I'd be very chuffed if my first attempts had been half as good as that. You're doing the right thing in learning what works test wise now.

Just to add to what Andy wrote, and with experience of British chip seal (unfortunately). Crr can be up around 0.006 in the dry. I've not looked at the effects of damp though.

Definitely go for out and back. A headwind or tailwind (even a barely noticible one) will affect you on single direction runs.

Where the heck in the UK were drivers giving you 1.5m clear space?! ;) you should see lumps and bumps from cars passing on the VE plot that would step it away from the true elevation plot.

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Developing aero, fit and other fun stuff at Red is Faster

Thanks for the replies guys, that's all very helpful info!

So, I just had a quick play with reducing the power values by 2.5% to account for drivetrain efficiency and this immediately brings the Crr down to something slightly more realistic (0.0069). What kind of values are typical for drivetrain efficiency? This test was on my dirty winter road bike, so I'd guess that it'd be at the higher-end of typical losses; would a value of 3, 4 or 5% still be realistic? It certainly makes me think I should give it a good clean and service!

Unfortunately on this course it's not really possible to make this loop an out-and-back, I could turn at the top easily enough, but descending without braking would be a bit sketchy on that long 8-10% stretch in the middle, given the layout of the road. I'm definitely going to give the halfpipe protocol a try next time round; I tried to do some runs on a looped-section after these runs, but quickly found out that my chosen loop had a corner on a short downhill section and it was very tough to take it without using the brakes or changing position. I'll find a more suitable testing ground and give it another shot as soon as I can.

To be honest, I'm surprised I picked up a differential at all between the slow and fast runs because, as you say, 15 vs 21 kph isn't a huge range and even at the top end, that's not the kind of speed where I would be particularly concerned about staying aero although perhaps I should be!

That all sounds right, awenborn.

Whatever you do, please be safe! We appreciate your excellent documentation skills and would miss you if you got hurt.

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AndyF
bike geek

First off, that was a fantastic first attempt!

It's impressive that you were able to pry apart the (CdA, Crr) pairs with that small differential in speed. I don't usually do one-way hill climbs to separate CdA and Crr but when I came up with this protocol I think I did something like a 50% spread in speed and got pretty good matching.

Your Crr estimate will definitely depend on getting the exact total elevation change nailed. Crr scales exactly like slope, so being off by 1 m in total elevation on a 1km hill will mean an error in Crr of .001. Fortunately in my case, I had a USGS topo map (the equivalent of your ordnance maps) with verified benchmarks that had the exact altitude at two points, so I knew the true gain. This is a situation where GPS altitude definitely isn't good enough.

I came up with this version of the protocol to deal with situations where one couldn't find an appropriate (=safe) out-and-back. One nice thing about "twice up the same hill at different powers" is that you can sometimes find a wind-sheltered hill more easily than a wind-sheltered flat or half-pipe venue. As always, you want to find a place where you can do this safely, without interference from cars, so this variant expands the number of places you can test.

I would think 2.5% drive train loss for a dirty chain is on the low end.

Glad you tried this! Some people think that one can't use VE to pry apart the (CdA, Crr) parameters, but it's just math and experimental technique. If you can nail down the exact elevation change (and your speed and power sensors are accurate and precise) then things will work.

BTW, when I do this, I use the approach I mention near the bottom of page 70. In your case, since you made four runs you'd end up with four equations and two unknowns (CdA and Crr), which you could solve in one of the usual ways. A nice thing about this is that you can then do sensitivity analysis to see how much a difference in the drive train loss of 1% (from 2.5% to 3.5%) makes in the estimates, or a small difference in the total climb (from 87.6m to 87 or 88m), or a 1 kg difference in mass. I usually found that small differences in these variables affected Crr more (in percentage terms) than CdA so I really had to know the change in elevation pretty well, and weigh myself and the bike carefully. I use a Power Tap so I don't have to worry about drive train losses.

We consider it an upside to be sharing testing protocols. :-) Thank you for doing it!

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AndyF
bike geek

i have read through all 11 pages but still have a few questions. I have a couple of potential courses picked out, 2 of which are loops and 2 are out and backs. Is there any way to do this with wind? Living in Oklahoma there is only about one day a month without 10-15 mph winds. I get the part about doing multiple loops for one session. The part I am not clear on is varying the speed. Do you vary the speed by each loop or within the loop? I am guessing within each loop so speed/power will be varying through the entire run of say 5 loops for 1 session. My next question with knowing the answer to the previous is, after an equipment change for example, you do not have to match the speed/power variations with the previous run correct? You just do another series of 5 loops for the one session but the course needs to match the exact same as the previous run? Sorry if these seem like common sense answers. I am trying to learn about this as I go and want to make sure I am interpreting the test measures correctly.

Wind is tricky, so I will let the more knowledgeable people answer that.

The Chung method uses "virtual" elevation, so what you are doing is calculating what the elevation change has to be. For example, if you are doing 250 watts on the flats at 26 mph, but you hold 250 watts and slow down to 20 mph, the physics of the system says you must have been going up a hill. The equations take all the known inputs to the system (weight, power, rolling resistance, air density,..) and determines the elevation change for any CdA value. On each lap your net elevation gain is 0. Repeat this 5 or so times and you just choose the slider values that says you started and ended at the same elevation.

You don't have to vary your power output at the same place each lap I do easy laps and hard laps. My one suggestion would be to not go too easy or too hard. Going super hard might cause you to change position you can have an uneven cadence if you are coasting down hills. You are looking to hold a consistent position and make the route you ride as consistent as possible.

Hi LSchmitt,

Wow, you read through everything? You are very diligent -- that'll come in handy.

The best way to vary speed is by choosing a course with a little elevation. A speed range of 3-5 m/s is usually enough.

What to do with wind, huh? Well, if you have an out-and-back you can assume the wind is the same going out as coming back (ie. that headwind reverses sign on the way back). And then you can add a "headwind" column in the Editor tab and insert whatever you think the wind might've been. When the elevation is right, you have the right headwind.

I'd also try to keep as much the same as possible. Same course until you can reproduce a CdA that makes sense 4 or 5 times on different occasions.

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AndyF
bike geek

Reading all this is quite an effort, but the right thing to do to get useful results. Andy beat me to it with the 'headwind' column in GC, as he should as he wrote it.

If you can find a course that's out and back or a long racetrack shape where the wind is predominantly along the long bit that'll help too. Just so you know, it's not really headwind, it's air so you don't have to factor in the bike's ground speed.

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Developing aero, fit and other fun stuff at Red is Faster

Not replying to anyone in particular, but just wanted to ask a bit more about discussion on page 5 regarding sources of weather data. Has anyone ever attempted to obtain data from on-sight, perhaps from a DIY mobile weather station (raspberry pi or otherwise)?

When I'm analyzing my races I sometimes have difficulty finding atmospheric pressure and dew point from a source that is reasonably nearby. Further, one course in particular is often very much in the wind but I'm unsure how accurate the wind data I get from those same sources that aren't on-site. I've considered building my own weather station that I'd set up at a race or during my testing protocol but wasn't sure if anyone has attempted this before and if it worked well. Or should I continue using the data I'm not too confident in?

The easiest thing would be to buy a weather station. They're pretty good for Rho, and ok for wind speed. Not that published weather stations also normalise wind speed to a set height so its lower than reported at 'bike height'. (Wind speed gradient). Some of the stations out there will display Rho for you. E.g. Kestrel 5100.

Some of us may have tried building weather stations for this purpose. I would suggest that it's a lot easier to buy one.

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Developing aero, fit and other fun stuff at Red is Faster

Thanks for the tip. I had considered a bought weather station but wasn't sure how great they would be for this purpose. I'll give them some more consideration. The DIY method was more of an idea for a summer project in what little free time I have these days.

i thought that reported winds are at 5m and are typically higher than what is experienced at bike height. am i mistaken?

I think they're at 10m.

I have a Kestrel 5100, which reports rho. The wind meter doesn't read quite as low as I'd like (sometimes it says there's 0 wind when I can feel a tiny breeze) but it's still pretty handy.

Correct, 10 meters.