ilovebiking

Hello all: I use the Vector 3S pedals on my road bike while outdoors. I display the power on a Wahoo Elemnt Bolt bike computer. (power, Ave workout power and 3-sec power). Indoors I use a Wahoo Kickr smart trainer, which has a built-in power meter. I recently tried both together while on the trainer, running Zwift, to compare the power reported. The power displayed on Zwift, from the Kickr, was at least 25% lower than the power displayed on my bike computer, coming from the Vector 3S pedals (ex., 150W from the Kickr, 190W from the Vector 3S pedals). I have calibrated each unit several times. Can anyone help me understand why this is, how I might fix it, and which one is correct? This appears to be a common question judging from my Google searches, but this discrepancy seems pretty large. Possibly I have set something up incorrectly, but not sure what. Crank length is correct.

Thanks in advance for your help.

WhyFi

You're not going to definitively know which is (more) correct, but if you have the 3S, with single-sided power reading, a part of that discrepancy could be a leg imbalance, meaning that the leg with the power meter pedal is contributing more than the one with the dummy pedal.

colnago62

Do you know someone else with a power meter on their bike to compare?

ingo

You can "calibrate" your power metrics by climbing a steep hill over 5 minutes long that has a know elevation change. Plug the numbers into a online power calculator and verify which measurement is correct. I am guessing the Kickr will be correct. Most people will have imbalance from right and left power, it will also vary with level of effort and fatigue. I have two sided Vector power meter, 4iii single sided power meter, and a Kickr. My R/L imbalance is 2% at low effort and 0% at high effort. Once calibrated they will vary less than 2% overall but the Kickr lags on high power spikes.

MikeEckhaus

I have a 4iiii single side meter on the crank and a kicker Snap. They read almost exactly 20 watts different as most normal power ranges. I suppose some of that is loss through the drivetrain and some is just accuracy. For me the difference isn't big enough to worry about, I use the kicker power meter on the trainer and obviously the other one outside.

guachi

I've read some reviews by websites that have tested both smart trainer power meters and bike-based power meters to ensure accuracy and I've never seen them that far apart except in short bursts. One I recall the reviewer was testing the new Kickr against some pedal power meters and the Kickr would overshoot during acceleration but was otherwise very close on the average (1-3% off). The reviewer said that Wahoo would fix the problem in the release unit.

So my takeaway is that there shouldn't be a large difference between your Kickr and Vector 3s Pedals.

I've checked between my 4iiis single-sided meter and my Kickr Core and they are quite close.

Iride01 

With power meters becoming more and more common and users now having more than one, seems that we are getting back to the same type this device said one thing and my other device said this. My my.

sfrider 

My Favero Assioma Duos agree very closely with my Kickr Core. Maybe around 1% or so. If two PMs differ by 25% then something is seriously wrong.

sfrider 

Make sure your computer connects to the correct pedal, for my Favero Assiomas it's the left one.

guachi

Assiomas were the pedals the Youtuber (Shane Miller) compared with the Kickr.

Barry2 

You are comparing power at the crankshaft vs Power at the wheels.

There are losses in a bicycle transmission, but not the difference you are reporting.
That is, not until you take into account "you".

A single sided PM will measure the pressure of the left foot as you press down on the pedal.
But it won't take into account the weight of the right leg if you fail to take the pressure off the pedal as that foot raises.
A single sided also just doubles the watts to allow for the other leg.

With dual sided PM's you get these variations taken into account.

All the best

Barry

MinnMan

These differences seem too large (unless you have a really pronounced and unusual L/R imbalance.

My Favero Assiomas (two-sided) have been spot on my trainer power every time I've checked. - within 5 watts - and my trainer power is just the Kurt Kinetic InRide, which is merely a calibrated power estimate from the flywheel spin-down time. Exceptions are as people have noted - during brief accelerations, etc. The Kickr should be far better.

one or the other of your devices is giving faulty numbers.

Now I use the Favero Assiomas almost exclusively with Zwift and don't bother with the inRide, because the pedal calibration is much less fuss than the inRide calibration, which took a little care.

Sy Reene

Are you only talking about differences at any given moment in time on your readout (laptop vs. garmin)? Or is this post-ride and you're finding this difference for the whole ride (eg. avg power)?

If the former, could be something in how you have preferences set in displaying power (eg. are both platforms Zwift vs Bolt using 1-second or 3-second -- IOW the same preference)?

PoorInRichfield

One issue that has kept me from buying a power meter at all is this: How does one know if any power meter is "accurate"? The OP is asking why his Vector 3S pedals aren't reading the same as his Wahoo Kickr (which I also have). Most of us will conclude that one of the power meters is right and the other is wrong. However, what if they're both wrong!??!?! How would you know? Does it really matter if your goal is to simply measure your own performance against yourself over time?

In industrial applications where instruments are used for precise measurements, there are companies that specialize in calibrating equipment to make sure the equipment is accurate. I've heard of such a service for cycling power meters, but there is no such thing local to me and even if such a service existed, it'd probably be too expensive for most cyclists. As such, I'm of the opinion that if you can figure-out how to calibrate both power meters to at least read close to each other, that's as good as you're going to get. At least with that configuration, you can compare your own power output to previous measurements to determine if you're doing better or worse. </end confusing monologue >

WhyFi

Power meters are most often in close agreement, so this kind of disparity is unusual. If there's doubt, one could probably do a hanging weight test - I'm sure that Garmin has something in their manual someplace.

RChung

There are ways that you can check accuracy (and precision) by yourself but, depending on what your needs are, they range from relatively simple to quite complex. These methods are well-understood if not well-known. Most power meter companies don't actually want you to know how to check accuracy or precision.

At the easiest end, you can do a "static" calibration check. This involves getting some known weights. You don't really need gram accuracy but I work on a university campus and I had a friend who worked in a lab so he weighed some of my barbell weights on a scale and we marked their mass on the plates with a sharpie. You could do the same with the Post Office or UPS or FedEx. They have to get their scales checked and calibrated regularly so you know they're accurate.

Then you put your bike in a sturdy stand or on a trainer, hang your weights from the pedal or crank arm and, depending on whether your power meter is a pedal, crank, crank spider, or rear hub type, do a little calculation for the torque. Then read that off your head unit. Try different combos of mass to check that it reads accurately across the entire range of weights you have.

A dynamic check is a little (a lot) more involved but it's the same principle: you set up a situation with known drag force and then check to see if the power meter is reading properly. Because it's dynamic there are more things you have to keep track of, but you can still do it at home if you're motivated enough. I've done it but it's enough of a hassle that I normally just do static checks (unless there's a good enough reason to go through the hassle).

It's really not a mystery. People who say it's impossible (and thus conclude that all power meters are equally accurate/inaccurate) just haven't thought about it.

billridesbikes

If this is a wheel on trainer?

Next, before you buy weights and calculate work done through a pedal rotation, try to determine if this is a constant offset or not. If you ride at 300W on the pedals is the difference only 40W or is it 25%? Same for very low watts, say 50w on the pedals? Try to compare the instant power instead of the average for comparison purposes.

I have a tacx neo and powertap pedals and difference is <10W at 300W on the pedals, slightly lower on the Neo which I just chalk up to a few percent from some drive train loss.

sfrider 

It's not that simple. With the Favero Assiomas each leg gets its own full 360-degree dynamic power data. So how you lift each leg, the peak power phase, even off-center pressure (inside/outside) is all captured as cycling dynamics. Unlike a spider based meters, which can only determine net force; pedals and crank based PMs can tell if you lift one leg while pushing the other. Needless to say the Assioma Duos are awesome, except well, their use of Look Keo cleats, which is less than awesome. (That said, the Xpedo cleats work well with the Assioma pedals; the original Keo cleats not as well. I used to have Keo pedals back in the dark ages, and found some old spare cleats. Very tight to clip out.)

MinnMan

DC Rainmaker has a fairly extensive write-up on issues related to Power Meter accuracy.
https://www.dcrainmaker.com/2018/12/...cy-issues.html

Barry2 

If you own a watch, you’ll always know the time.
if you own two watches you’ll never be sure!

Barry

PoorInRichfield

Bwahaha... my point exactly!

ilovebiking

Thanks to all for your input, I appreciate the time you all took to respond.

I get that it really should not matter if you are just measuring against yourself. However, while I am no great racer, I do some age-group road racing and some time trials. I'm not a good judge of "perceived effort," so I like to know how many watts I can ride at in a TT race, or maintain in a short breakaway, and remain just under my puke level. My plan was to work the offseason to get some good power numbers, then use that during the warmer season. So it would be useful going into the summer to know what those numbers are.

But from all the feedback I have received, I don't think I can plan on using Kickr numbers when I get back on the road. But I think I can do some more trial and error when that happens, and go from there.

Thanks to all again.

OP

Dancing Skeleton

I've had small differences with my Favero Assioma Duo's & my Kickr18. The difference is up to ~3-4%, but seems to lessen on longer Zwift rides.
Last winter, I did a static weight test, made some adjustments, did several spin-downs & pedal calibrations & got average power within one watt.
This year it's back to ~3% after the pedals were on another bike for the season.
I'll go through the same process of adjustments soon to get everything to agree.

I also have a Favero Assioma Uno, and the difference with that varied greatly, so I don't use it anymore.

A difference of a few watts is normal (+/- accuracy, drivetrain loss) but 25% is too far off.

jadocs

Was the 25% to the positive or negative in terms of power? In other words were your Uno's telling you, you were putting out more power than you actually were or was it the other way around?

RChung

None of us are, at first. Just as it's a good idea to calibrate our power meters against a known standard, it's a good idea to "calibrate" our perceived exertion against a good power meter.