PeteHski

You can estimate both parameters reasonably well using BBS (Best Bike Splits) pacing software using a few loops you have ridden several times.

GhostRider62

Chung field testing is very easy to do reliably as long as you have a good speed sensor, power meter, and take the time to get and record the atmospheric conditions. No real fancy equipment needed, even an idiot like me can use it to minimize one's CdA.

burnthesheep

Plugging their stuff into aeroweenie necessitated a cda of .380 unless I made the weight pretty high with a crr of .004.

tomato coupe

Yep.

mschwett 

i checked out the aeroweenie calc with data from four laps around the oval, each separately, and then 4 miles back and forth on a dead flat road. there's a pretty strong crosswind on the back and forth, but the results seem pretty consistent, suggesting a CdA (with this bike) of around .36. this does not seem unreasonable for a 6'2" person with broad shoulders riding a regular road bike on the hoods, hairy legs, no lycra, etc.



for previous runs - different bike, different power meter, same oval, over 7 laps, an average speed of 34.1 kph showed an average power of 254, which yields a required CdA of .396. given how much of the CdA is the rider, not the bike, i'd guess that power meter is reading 5-10% high or something, and the CdA is really more like .37. unfortunately i have no way to know which of them is "right," or what the actual CdA is, but i've ridden up enough hills and on enough flats to know that for me, at least, the same recorded power output at the same heart rate feels WAY harder and less sustainable when going uphill. what i'd love to know is if there's any kind of training that could work to neutralize that!

PeteHski

It's just physics. Whatever parameters you plug into "their stuff" work the same as in any other "stuff". As long as you use fairly calm days to calibrate I find it plenty accurate enough. I presume you were using the paid version with real time weather data etc.

GhostRider62

Crr of 0.004 is actually extremely conservative WRT estimating CdA. Believable on good surfaces with the best tires, otherwise, Crr is higher and CdA is lower.

0.004 is the lowest I have ever measured. That was with Vittoria Speed tubeless on one year old decent pavement. IIRC, I got 0.0042 with GP5000 and latex on the same road. I typically get 0.005 with the GP5000 and latex tubes on more typical roads where I live.

burnthesheep

With CdA testing, overstating a high CRR is one of the number one reasons people publish overly low CdA's. This is a pretty accepted conclusion in multiple sensor/aero/tunnel topics across both Slowtwitch and Timetriallinguk forums. A conclusion I didn't make, but the people who are in the industry made. I had a video chat with the guy over winter that was paid to consult the development of the Notio and that came up in that chat.

Starting out, I typically threw in something a lot higher for CRR. But have gravitated closer to the BRR website values, despite their testing method not matching the tarmac.

So when you hear of trackies going for the hour with a "0.160" CdA, that probably ain't entirely true. There's a blend of CRR and CdA there and often they're guilty of not measuring or updating the really low track CRR's.

The reason any of this "my whole ride" calculated stuff isn't that great is that there are factors to calculate CdA that can really be thrown off in a longer uncontrolled ride. Acceleration and deceleration from stops. Accel and decel from improperly detected elevation change. Not having an aero sensor to detect airspeed. Anyone that's setup the laps for doing the testing knows how bad acceleration can affect the test. Acceleration as in a change in speed over time. Not jumping to a sprint per se, but any change in speed over time.

Which is why the "my cda couldn't be that high" from guessing from a calculator over an out/back or giant 20mi loop may as well be throwing crap at the wall.

GhostRider62

I had confirmed my "overstating a high CRR" with Chung and Coggan. I was surprised at my results. A little research on the interwebs shows that real world numbers are much higher than the steel drum and even Tom A. had given his real world numbers, on ST IIRC.

You might have really good roads where you live. Like buttery concrete with perfect expansion joints. In most of the real world, Crr under 0.004 is completely unrealistic and 0.005 isn't out of the question.....for good tires. I've spent a lot of money and time testing tires. One mistake out there is the assumption that Crr is a constant, it is not. Some tires Crr are much better at speed than others.

GhostRider62

I am starting to setup my upright (Cervelo S) for time trialing and will be doing some Chung testing, it will be interesting to see what this tall fat buff (sic) body gets.

When I went thru a similar 18 month process on my recumbent, I started with 0.210 m^2 and got down below 0.150.

I hope to achieve 0.230 m^2 or lower. I have a set of Trig Aero bars to install. Bike will have 1x aero crank, covered rear, M5 2-spoke front with 22 mm tire, aerocoach LS skinsuit, and Bell Javelin as a baseline. Pedals and shoes are a problem but first things first. I guess it will be 0.250 out of the box.

burnthesheep

FWIW, doing some double-brick duathlon style training at lunch I did a 10mi out/back on the road bike as part of the workout. Mostly so-so country road pavement with some bumpy junk. 24.3mph, 250w, no autopause on the u-turn waiting for a truck to go past. I was moving and adding up time. Road helmet, road kit, road bike, 60mm wheels, gp5000.

About 250ft elevation for the 10mi, so not much.

That's so-so crr on a country road like this and pretty good road bike CdA. Perhaps the meter reads a bit low, but before uploading to Trainingpeaks I do boost the power 3 to 4% because of it being a left-only Stages and knowing that my left/right ratio is normally from a meter I own that has left/right.

GhostRider62

Sea Level? Do you know what the Rho was?

That is pretty impressive if at sea level (and no aero clip ons)!

I can to 24.3 mph on less than 170 watts but I am 220 lbs and cheat (recumbent)

burnthesheep

340 feet, per the Google lookup on the area. Yeah, no clip ons.

The last thing though is I really want to get a new meter for that bike as sometimes it just defies logic the "aero versus power" and elevation of some rides. IMO. That just seems low even though I'm in the camp on this topic of feeling it doesn't take a ton of power.

I need to put the TT bike with the Quarq onto the Kickr and do a Zwift ramped warmup then repeat it with the left-only Stages on the other bike and see what the gps file for each says for power during each step. I'm betting the Quarq reads either "just right" or maybe 5w high, and that the left-Stages reads really low.

ZHVelo

Interesting, for me it is exactly the other way round, the same watts feels easier on climbs than on a flat.

ZHVelo

I was going to already reply saying I agree with you after checking my first data for a flat (which is hard to come by here) but then I checked a few more and the results are vastly different. One ride I did 248W but only averaged 34.3km/h another I did 231W but had 35.7km/h.

So why is this?

I think it is mostly wind, because on the same ride as the latter data, I also did 229W for one somewhat flat stretch (+27m but also -27m) but averaged 37.3km/h and that was on the other side of the lake where often wind is from the rear (and when you come around on the way back home it is often headwind). That is a 1.6km/h difference on the same ride at almost identical power output.

mschwett 

i discovered that some of the explanation for the surprisingly low numbers i got from one of my bikes is partially explained by leg asymmetry. my dual sided meter was only transmitting the left side power (doubled) which works out to 88% of the typical total power as measured on a long ride. (44/56 leg imbalance!)


the numbers are still quite a bit lower (10% generally) than what the aeroweenie or bikecalc algorithms return when riding flat loops, but who knows what that's about. i'd say the grade measurements are off but i've verified a few, and it's always 10% low. could be wind, scale being off, PM being off...

burnthesheep

To beat the horse some more...............Today, out and back route. 22.75 mi in 1hr 5 seconds. 22.7mph on the road bike, decent kit and helmet, gp5000's and 750ft of elevation. There were 4 slows to full stops and restarts.

237w. That's all.