NP values differences between the smart trainer vs power meter

[ahpingko]

Smart Trainer: Magene T300
Powermeter: Favero Assioma Uno

Workout used for testing: Cadence Build

I performed spindown calibration earlier today with 10 min warm up before the spindown.
Also calibrated Assioma prior to workout.

Devices used:

1. Zwift on laptop via ANT+ paired to Assioma pedal, connected for Power&Cadence&HR only
2. Sufferfest on iPad via Bluetooth paired to Magene T300, connected for Power&Controllable with ERG On.

End Result:

1. NP 164w
   

   

2. NP 147w
   

   

Is this difference of 10.93% expected?

[Ross]

Hi there,

That is a huge difference for sure. The difference between my Kickr and my Power2Max NGeco power meter (spider based, not pedal) is approx 1%, usually less.

Clearly one of your devices is inaccurate. I am not sure how accurate or good quality either of your devices are I’m afraid.

The graphs look like they are the same, but one has had the power smoothed, that is usually not a problem.

My advice in this instance, because SUF doesn’t (unfortunately) offer power match would be to just use the power data from the smart trainer on SUF, all of your 4DP values on the SUF app from the trainer, but to record all of your data on your Garmin or Wahoo using data from the power pedals. When you next do a 4DP test you should hit “lap” at the 5min/20min/1min tests and get your 4DP profile that way (aside sprint, but all NM efforts shouldn’t be done in ERG anyway so not too important).

Whilst that doesn’t help your original question it might help you moving forwards.

Thanks

[Martin]

All in can offer is previous experience of Favero (dual sided) pedals vs an Elite direct drive trainer and while I can’t remember the actually specifics - but at times I’m sure they were more than 10% - the difference was a) unpredictable and b) too much of a pain, so I elected to just never use those pedals (early versions of the first model) as even outside I got fed up with calibration and from my own testing, relatively inaccurate results.

All of which meant I then used the trainer only for power indoors after that (back then in another platform) and same when I switched to SUF.

All these devices keep getting better so pretty sure a lot has moved on since then, but one constant for me (I have power cranks now instead of pedals and they’re pretty close (2-3% which about matches drive train loss potentially)) is I still won’t use them to measure indoors. I just want to be measuring from one device inside and given other factors affect outdoors as well, I’ve found that to be my only consistent method.

It’s a tough one.

Lots of other suggestions in here as well re taking some separate measurement to enable comparisons

[ahpingko]

Possibility that I had in mind:

• The difference of power between my left/right legs - resulting in the difference of the NP because I am using single sided power… can the left/right differs by 10%?
• Smart Trainer defect - how likely is this?

[SirAlexanderLee]

You’ve also got to consider the power meter margin of error. If both are +/-3% and are at opposite ends of the spectrum, then your difference is only 4%. I found this to be the case with a Stages PM and a Kickr Snap, although the spread was 4.5% - Snap +/-3% & Stages Gen 3 +/-1.5%.

Also, the remaining difference is where these measures are taking place - literally at the crank or at the wheel/cassette.

Just as in a car dyno, you loose power when measuring at the wheels vs the crank.

I can’t remember where, but I read somewhere, even with a clean drivetrain, the average loss of power on a bike drivetrain can be 1-3w.

So there you go. Nothing wrong with either of them, really.

At the end of the day, this is 17w. You’d have to bike for hours and hours like this to get a real TSS difference.

[ahpingko]

I kinda agree with what you said. Maybe I should just ignore it and just workout. Normal day I will just use my power meter as power source for training with Sufferfest (level mode)…

[SirAlexanderLee]

All PMs are slightly different - even if you have the same brand but different PMs on each bike.

What you need to consider is your consistency. Pick one PM to use indoors and let that be it. You’ll be chasing ghosts otherwise. Same for outdoors. In time, you’ll be able to “feel” your power numbers.

[ahpingko]

Yea, I am planning to use my Favero Assioma outdoor, has been training with it and my dumb trainer all the while. Even did the Full Frontal with it. This smart trainer is new addition to my paincave and after that I tried to use ERG mode paired with my Assioma, and hence I found the issue whereby:

• ERG mode doesn’t work as expected with Favero Assioma due to no Power Match feature
• If use the power source from smart trainer, the power value differs from what I had set in my 4DP power (using Favero Assioma previously)

[SirAlexanderLee]

Yeah, and that’s because of the way your smart trainer measures power in a different way than your DFPM. Direct force PM. Strain gauges don’t work the same way that algorithmically driven electro-magnetic motors do.

Even if you had power match, the numbers would still be off and you’d still have a weird lag with the signal from the DFPM to the Smart trainer then to the App and then back. All that lag eventually creates the smart black hole of death and you get sucked in.

I know. I spent 2 months trying to work out this same issue about two years ago with Stages, Wahoo, and the SUF - before Wahoo bought them.

[ahpingko]

I see, I guess no other option already, I will just use level mode with my DFPM to ensure consistency since I am using it all the way.

Thanks all for the explanation! I truly appreciate that! Ride On.

[Alvaro_Santos_Neto]

I use a Wahoo Kickr Core and a Favero Assioma Duo.
Last Sunday I did FF registering Wahoo Core to Suf via bluetooth and Assioma to a Garmin by ANT+. I did not send the power from Assioma as feedback to the Core trainer.
Difference was 1% or less for the 4DP numbers.
I even skipped zeroing the pedals and doing a spindown with the trainer.

In other FF before I used to register power directly from the pedals via Bluetooth to The SUF and ANT+ to Garmin, numbers used to be almost identical, differing just a bit more at the 5 sec max effort.

During regular workouts I have also tested the function where Wahoo Core reads the power from pedals via ANT+ and adjusts the resistance based on that. However, I noticed the trainer is used to do a good job alone and therefore I avoid the extra step of connecting the pedals and the trainer each other every time I will ride.

[Peteski]

You would expect the true power measured at the pedals to be in the order of 3-4% higher than true power measured at the rear hub/trainer due to drivetrain losses (bicycle chain drive efficiency is 96-97%), but the difference you are seeing of 11% is well beyond that and implies that at least one of your devices is reading inaccurately. Unfortunately it is not really possible to say where the discrepancy lies without adding a third power meter.

The most important thing here is consistency. I would use your trainer power for everything indoors and monitor the % difference to your pedal PM. If the difference consistently remains at 10-11% then at least you can work with that and set your outdoor power targets accordingly. I do this with my Elite trainer and crank PM and consistently see a difference of 4-5% higher at the crank.

[digininja]

Can I jump in with the same problem. I’ve got 4iiii single crank power meter and Wahoo Kickr trainer. I’ve always noticed that there is a difference between what the two report but never really looked into it before. This is the second block of FTP progression, about 6 mins of data.

This is 4iiii

This is Kickr

My FTP is 171 and the block should be just over and so 173 from the trainer is the right value. The average from the 4iiii is 152 which is 13% different.

I do regular spindowns and after I finally got the 4iiii to pair with my phone I zero’d it about a week before the ride when conditions were similar.

I’ve told SUF to take the data from the crank and so the graph on screen while I’m riding is the non-normalised spiky mess and as I said, it always reads under what the workout says I should be hitting.

I’m tempted to go back and check my FF data as with this much difference there is no way I’ll be hitting my FTP when out on the road and looking and crank data rather than trainer data.

[Peteski]

That’s a big difference and especially so when you add in another 4% for drivetrain losses, making the effective difference 17%.

Again I would work with your trainer power indoors and log your crank power in the background to get an equivalent outdoor FTP and power profile, which in this case will appear to be 13% lower if the difference you are seeing is consistent.

Of course you will never be sure which of these values is closest to your true FTP unless you can reference a third source of power measurement. But this shouldn’t affect your training providing the power readings on both sensors are consistent.

[digininja]

I’ll try to work out an average difference and see if it stays consistent, I’ve got quite a bit of data to work with.

If you had to guess, what could be causing it? If one is faulty I’d like to get it fixed or replaced but without getting hold of a third device that wouldn’t be easy to work out which.

[digininja]

Here are a pair of graphs from Endurance +. A 10 minute constant effort block, 4iiii has me at average 76 bouncing all over the place, Kickr has 100 solid. That is a 30% difference, something is definitely broken!

These are the graphs for the full 35 minutes:

Is there any way to calibrate the devices in some way? Dangle a fixed weight from a pedal and record a reaction or something like that?

[Peteski]

Firstly don’t be fooled by the apparent smoothness of your trainer power. That’s just a software smoothing filter. It’s also completely normal to get a more spiky power reading at the crank as that is where you are directly applying force. Power measured downstream at the wheel hub will always be smoother.

As for accuracy, they should be both reasonably good (within 2%) and power measured at the crank should be around 4% higher than at the rear hub. But obviously you are not seeing this at all!

I would suggest a simple test where you ride for a few mins at a range of steady power levels within your comfort zone eg 75, 100, 125, 150 etc and a couple of max effort sprints. Log both power sources during the test and compare the results. This will show whether the difference you are seeing is a zero offset or a difference in calibration curve, or both. Also make sure you warm up on the trainer for at least 10 mins and then calibrate both sensors before the test.

My own experience of using a single-sided crank PM (Specialized 4iiii) has been very consistent with my trainer (Elite Direto X). It consistently reads 3-4% higher at my average power.

[digininja]

I understand the spikiness,I think it is just the range on what are supposed to be constant power sessions that surprised me.

Would you suggest using level mode to do it or using one of the open workouts and adjusting the power on that?

[Peteski]

Even with the smoothest of pedalling technique, we are still applying torque to the pedals in distinct pulses and that just gets reflected in the spiky power data measured from the crank. This is why most riders display power on their head units as a rolling 3s average to get a more steady reading.

Yes I would use level mode for the power test and try to keep your cadence very consistent. But it would work fine in ERG mode too and require a little less concentration. What you are looking for are nice steady intervals at several stepped points along your personal power curve. So if you have an FTP of around 150W as measured by the trainer, then 75, 100, 125, 150, 175, 200 and 250W should give you a really good idea how both power measurements really compare across your power range. You might find, for example, that you see a much bigger percentage difference in readings at lower power levels. The results might give you some clue where the discrepancy lies and whether or not you can work with the differences.

Edit: Just make sure your trainer is well warmed up before the spin-down and before you start the test. Also make sure you zero the crank PM too. My Elite trainer definitely reads low on power when it’s cold and takes a good 10 mins at around 150W to warm up.

[digininja]

I’ll try to give it a go, sounds like an interesting challenge