mschwett 

took about a 90 minute ride on a new bike today; third ride with it. it has a factory installed 4iiii precision pro dual sided power meter. i believe i’ve only got it connected as single sided, need to dig into that.

on my first two rides, the readings were about 5-10% higher than what i’d expect based on previous bikes, calculators, hill climbs, etc. before the third ride, i did a zero offset while the bike was still inside the basement of a climate controlled high rise. readings looked normal at first, but then i rode into cool, foggy weather (perhaps 15 or 20 degrees cooler than the interior space) and the readings seemed quite low. i did another zero offset an hour into the ride at a stop, and the reading seemed higher.

i haven’t done a careful study of this, but exactly how sensitive is this type of meter to changes in temperature and humidity? the three rides were of similar effort, an hour and a half, similar heart rates, time of day, first two 240 average power, third one 210.

chaadster

Generally, PMs are not very sensitive to climate change, asI understand it, and it takes moving from extremes to affect it, like from an actually cold start.

I don’t think that you’re using sufficiently discreet assessment tools to determine anything about PM accuracy, but you’ve certainly not given us sufficient info to diagnose anything.

So, just guessing, I’d say you’re experiencing normal PM operation, if only because PMs are typically accurate, but also because variability is just part of how they, and you, both work.

Here’s a link to a troubleshooting article which will give a better sense of what you need to do if you want to delve into accuracy issues:

https://www.dcrainmaker.com/2018/12/...ssues.html/amp

Iride01 

Most people are probably surprised their power readings are so low.

Also, after I haven't ridden in a few weeks, my first ride out I have a lot of power. My second ride not so much.

I'm not sure you've given us anything to go on though if there is an actual issue. But I just got my 4iiii left side PM this last X-mas and it seems to work correctly with my Garmin Edge 530. Nothing suspicious except I'm not the power pushing cyclist I wish I could be!


Are your examples the same ride? If so, then are the times for the ride the same and were the wind conditions significantly different?

What about weight you may have carried with you?

mschwett 

thanks all.

same time of day, approx temperature range, average heart rate, etc, but different rides. same load, i never really ride with any heavy gear.

the difference between readings of similar effort was surprisingly large before and after the zero offset, on the order of 15 percent.

the absolute value (compared to other power meter or online calculations) is not something i’m equipped to verify, not super important, but i’d like to think it’s consistent to within a few % if the temperature or humidity changes by a large amount day to day or during a ride.

Seattle Forrest

I've been hesitant to join this thread, because you're asking about a specific device I haven't used. I've been riding with power for a long time now and agree with @chaadster that this is complicated stuff and nothing has been proven at this point. Zeroing your PM appears to have had an effect, but who knows at this point? The reason we spend $$ on power meters in the first place is that relative effort isn't that sensitive. I would keep poking at it because it feels suspicious.

mschwett 

here's the best example i can find from the four rides i've now done with this meter.

similar time of day (which is critical for me b/c my medications will cause heart rate to be off by about 15% compared to "effort"), similar weather (63f, 44%, 8mph WSW for one, 62f, 36%, 8mph W for the other). heart rate within a few BPM. one says 271w, the other 219w! a HUGE difference. power meter was zeroed before the ride on the right, not before the ride on the left.


for grins, here's what bikecalculator says it should take to do it at 17.1mph, eerily close to the 219w average power of that segment. my guess is that the data from the ride on the left is just wrong, and that the zero offset was WAY OFF at that point in time? all references i can find to this subject for this meter say that the differences are very small, like a few percent at most, but this is 24% higher!

Riveting

Still too many variables. The 0.6 mph avg. spd. delta between the two rides accounts for 16 watts. So with that alone you're down to just a 36 watt delta between the two rides. And then the wind direction was different. I can't tell which direction this ride was East-West, or West-East, so one might be much more susceptible to the wind direction difference, than the other, but using that same calculator (which I've used for years), just 1 mph of headwind delta accounts for another 14 watts. So that's only 20 watts total delta for the two variables that you for some reason disregarded. And that 20 watts could be explained away by yet a third unknown variable, such as the duration of your aero tucks, or coefficient of drag (CdA) differences via clothing choice on the two rides.

Maybe try this same experiment with the wind, avg speed, and CdA variables removed, one ride on a trainer in your home (or maybe several at the exact same avg speeds so you can see the standard deviations that may occur with your specific PM), and then X number of rides outside (or maybe in your home garage) where it's warmer or colder, on the same trainer, at the same exact sustained top speed, and ramp up duration.

Iride01 

Maybe it gets back to the logging interval on your device. Particularly if set to what Garmin infamously refers to as smart logging (or whatever your device manufacturer's version of that is). Also the short distance you are selecting out seems like it'd be more prone to larger errors.

If you have this as a .fit file, then there are some tools that can pull the info out of them. Been a long time since I've used them. Don't even remember their names. If you can get is as a .gpx file, then you can run that into excel or something and pull out the entries between the beginning and end of that segment. Then you could see if the number of data points is the same and whether their values going into and finishing that segment might be enough to skew the watt number.

mschwett 

all very possible. wouldn't explain why it felt like the same effort, with the same heart rate, but i suppose the takeaway is that how hard it "feels" is not very reliable? i am almost positive the first one is high, but maybe not as much as i thought.

mschwett 

yeah, i recall reading about that. a bizarre choice on their part to save a few minutes of battery life. i've looked at the fit files that my app produces (i use "cadence" on a iphone) and it seems to be consistent one second intervals. no tall buildings on this bit of the ride, so i think it's pretty clean data. no obvious dropouts when viewed in different viewers.

Seattle Forrest

​​​​​​If it was, or even if HR worked as well as we'd all like, almost nobody would spend $$$ on power meters.

Seattle Forrest

What @Iride01 described, Garmin's smart logging, was invented something like 20 years ago when storage space was too limited to be able to record a data point every second for hours on end. It actually drains the battery faster, because the unit has to constantly calculate whether a new data point is required, so the CPU has less idle time.

In theory it should just log every second because your power is always changing, but nobody outside Garmin really knows. So it's wise to use every second, which it seems like you are. 🙂

chaadster

All of that is simply no way to assess power meter accuracy. A 4min effort? That’s barely enough time for HR to respond to increased wattage output, especially if you consider that one minute of higher effort could significantly affect the 4min average, and HR would not respond to that at all if one someone is reasonably trained.

If you wanna talk about power meter accuracy, OP, you need to get serious.

mschwett 

it was a 4 minute excerpt from a much longer ride, of course, selected to minimize external factors like wind, stop signs, traffic, etc. there was no "increased wattage output" since it's the middle of a more or less steady state effort, but of course there are other factors in any outdoor public road ride on two different days. just not enough other factors to explain a 24% delta.

after a few more rides with this meter, what i conclude is that the zero offset doesn't have a huge effect in the range of environmental conditions that i encounter (50-70f, moderate humidity) but that the initial calibration was somehow WAY off. readings since those first couple rides are consistently 20 percent lower. they're also about 10 percent lower than my other bike, but it has a different type of meter that isn't guaranteed to be apples to apples.