What's the length of the velodrome, what power meter are you using, and what are you using to calculate rho?

400 meter velodrome, Quarq dzero sram red powermeter, and used Golden Cheetah's calculator/estimator for rho. I was a little concerned with the power meter, because I repeated zeroing, and it still seemed to jump around a bit (e.g., 585, 576, 585). For rho, I have a kestrel device I am getting, so next time should be more accurate (I can also more easily check for wind).

OK, so those undulations look to be about 400m apart. Since the velodrome is 400m, I'd expect peaks at 200m unless you're getting something that happens only once per lap -- and the most likely thing is, as you suspected, wind.

The good news is that your eta should be a little less than 1.0 since you're using a crank-spider based power meter, so your total drag is actually a bit lower. I'd use something like 0.97 or so for eta.

The rho calculator in Golden Cheetah assumes you're using "station" barometric pressure, i.e., if you're using "official weather station" baro pressure you have to adjust for altitude. If you're anywhere near sea level you don't have to make any adjustment. If you're at altitude and using official weather station pressure, there are online calculators that'll calculate rho exactly but a rough rule of thumb is that for each 100m gain in altitude above sea level you should reduce rho by 1%.

That makes sense, thanks a lot, Robert.

(And thanks to the others that contributed to this old thread, I've learned a lot.)

Power meter offset issues


(Quarq Dzero Sram Red)


Did some more runs at the velodrome. Up to this point, I have been zeroing my PM after each run (just to make sure) -- getting 3 successive offsets that are equal or very close. Today, I did ABAB (laps 2, 4, 6, 8 below), but zeroed only between 4 and 6. And you can see that the second set of AB have a significantly higher CdA. So I'm thinking it was the PM difference in offset that threw things off.

So, is there something wrong with my PM? In theory, I should be able to zero it more often, and that would be better (judging by earlier posts in this thread)? But actually that makes my results less consistent. I'm usually there for less than an hour, so temperature differences are usually not too significant (and the PM should be able to adjust via calculations anyway). So I can just not zero the PM from here on out (other than once at the beginning), but I'm wondering if there's actually something off with my PM. I haven't done anything like calibrating against a known weight. I should likely update the firmware, although not sure if that would change anything.

I was thinking of just getting a Powertap hub, since it seems like that's what a lot of folks use, and then I wouldn't have to worry about estimating the Eta/drivetrain-loss value. But not sure if it would be susceptible to the same sort of problem anyway.

I don't have any experience with the Dzero.

What were runs 10, 12, 14, 16, and how did they differ from 2, 4, 6, 8? They appear to be pretty consistent.

I adjusted the pad stack a bit before runs >= 10, again alternating ABAB (just a different helmet for A vs B). Yeah, the results look much more consistent after that point. It could be that the offset was off for 4 and 6.

Although, in general my results haven't been very consistent, and I think it was probably because I was getting different offsets for my PM, and zeroing too much. I think I'll try just zeroing once at the beginning of a session, since my sessions don't last that long so I don't expect much change due to temperature. I also updated my firmware, and I may at some point try calibrating the slope with a known weight.

In an ideal world, if your offset is off (but doesn't change) then your Crr will be wrong but your CdA should be OK. If you slope is off (but doesn't change) your CdA will be wrong but your Crr should be okay. If your slope is off (but doesn't change) absolute differences in CdA will be wrong but the relative differences should be okay (so setup A will have the same relationship to setup B).

Since Crr scales like slope, if your offset is off the amplitude of the VE profiles will be off but the peaks and valleys should be in the same places.

At least, I think so.

That seems to make sense. In my case I haven’t been teasing apart cda/crr (eg via Shen method). Instead I have been assuming a crr value based on independently reported rolling resistance for the tire type and multiplying by a small factor for the velodrome surface. So I guess my cda will still be off even if only my pm offset is incorrect (since the error is not “absorbed” by the crr). (Of course, it may already be incorrect since I’m assuming a crr, but it seemed more accurate than what I would estimate via the Shen method — maybe I’m doing it wrong). But I would still get the relative cda’s.

IMO, the offset issues are accentuated on a Powertap hub cause the measured torque is lower on the hub compared to the torque at the chainring because of the bigger gear ratio used at high speed for aero testing. I get more reproducible results since I switched to a Power2max during morning sessions with changing temperature conditions.

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I would do as you suggest yourself and just not zero between runs when the temperature (and pressure) is not changing much anyway. I have seen the same issue with both the older Quarq (Red) and an older Powertap. I strictly use SRM now - one very nice thing about that is that I can check the ZO without actually saving it, so I can go in and check that it still is around the same value without actually introducing any noise by saving the value.

I do not zero while testing but do zero at the end and record it.
I use a Quarq as my reference from tests to test but I also ride with Favero pedals. To date they have always tracked very well.

I also measure asphalt temperature at beginning and end of a session. I find that has a bigger impact on my results than PM drift.

Good to know I'm not the only one, and others have taken a similar approach. Maybe in the future I'll invest in a higher quality PM like the Power2Max Bugno (wow, cool that it is +- 1% accurate), or the SRM.

That's a good point about the surface temperature. Actually, in general I wasn't factoring temperature into the crr. Looking at the following, seems that could affect it quite a bit: https://fitwerx.com/...in-the-cold-weather/ .

What you do with that number?

IME the tire temperature tends to track ~1/3 of the way between the asphalt and air temperatures. This can have pretty huge effect on Crr if it's sunny.

I should clarify. In the past I would take air temperature, asphalt temperature and tire temperature. I still do now and then but much less frequently.
I would then try to correlate each of them to CRR and I could.

But since air temperature is easiest to record, I now use that with pretty good results. I do adjust my CRR to temperature in a way very similar to what Tom A posted here a while back.

What did you find when you measured them all? I'm hoping to eventually get good day to day consistency outdoors, and the effect of road temperature is too big to ignore.

Do you correct CRR with temperature ?

Yes. What I'm wondering is what you got for the relationship between the temperature of air, road, and tire in different conditions.

1%/F is a 20% difference in Crr if the tire is 20F hotter than the air. With my data that's about a 6% difference in CdA... way too much to ignore.

2 deg C, say between 20 and 22 deg (ambient) should give about 2% in CRR difference

So you are using 1%/C. You only saw a 2C difference between tire and air on a sunny day? I get way higher differences.

  
I use a formula that gives % of a reference CRR based on temperature. It's actually closer to 3% if I compare 20 and 22 deg celcius.

Also, I wonder if you may be seeing three things

Imagine

a) your air density is skewed/innaccurate....from day to day
b) your PM is fluctuating with temp
c) your Crr is fluctuating with temp

I am immune to A because I don't need air density in my calculations

I'm measuring temperature and pressure, so air density shouldn't be an issue.


Regardless, it's a fact that on a sunny day the tire temperature will be a lot hotter than the air. Just checked near my house today after I got back from a ride. Air was 70F, road was 103F, tire was 86F... while going ~15mph. So if you are adjusting Crr for temperature, just using air temp doesn't cut it.

For some reason I can't see that image very well so I'm not commenting on that specifically.

Another possibility is the track itself is not flat.

Don't know which track this is but many of the ~400m cycle tracks around country town cricket ovals in Australia have a height differential. There's one in Queanbeyan for instance with a pronounced variance in elevation from one side to the other. Gosford isn't flat, neither is Lidcombe. Goulburn has a slight variance. It's not uncommon for there to be a faster and slower side of an outdoor track unrelated to wind.

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