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Smart Trainer Under-Reports Power in Big Ring
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The evidence is all in this table:



Test details:
  • 1 minute intervals comparing the power from crankset Quarq power meter vs 1 minute at Zwift ERG-mode interval set wattage.


Setup details:
  • Trainer = Tacx Flux S
  • Date of Purchase = January 2021 (trainer is ~2 weeks old as of posting)
  • Primary training platform = Zwift
  • Cassette = brand new Shimano 105 10 speed 11-28 (cassette on my wheel is Ultegra 10s 11-28)
  • Chain has ~500 miles on it.
  • Chain ring = 53/39
  • It took me a while to properly index the gearing on the new cassette. But now it feels pretty well indexed when in both big and small chain ring (maybe slightly better in the small than the large). I would say that the trainer is louder when riding in the big chain ring. It feels like there might be some amount of chain rubbing. When I spin the chain backwards in the big ring it seems like the chain slips a bit more than if I do the same motion in the small ring.

Background:
  • New to Zwift and direct drive trainers. But in the first couple races I did I noticed that I had a super tough time putting out wattage in the big ring. I thought maybe I just wasn't used to the trainer. But once I did several of the workouts in ERG mode I realized that hitting wattage targets in the small ring was way easier than in the big ring. For high wattage intervals it felt like I was putting 30+ extra watts when in the big ring. So I decided to collect some actual data. And as the table above shows, there is some evidence that at high wattage 300+ (and perhaps exponentially increasing) the trainer under-reports wattage.
  • I will note that for the data I collected I would roughly estimate I averaged ~65 rpm for the big ring intervals and ~75 rpm for the small ring intervals.

Questions and Concerns:
  • First, what is happening and why?
  • Second, is this a trainer issue or a bike issue?
  • Third, my biggest concern is how this data translates to racing my bike outside. Honestly I don't care if Zwift under-reports my data but I do care if I was somehow sacrificing 20+ watts in the big ring outside and I just never knew. Could this be the case?
  • I know that power meters at the rear wheel will read lower than those under foot or at the crank. But given that the power readings match when in the small ring I suppose this is not a transfer of power issue? Or maybe it is? Could the big ring cause the chain to move at an angle that somehow rubs the cassette more and loses 20+ watts?
  • Does cadence have anything to do with this?
  • Could I have installed the cassette on the trainer incorrectly? Is the indexing or other derailleur adjustments incorrect?
  • Is an older (stretched but not overly so) chain on a new cassette somehow responsible?
  • Why do the loses seem exaggerated at 300+ watts but more negligible at <200?
  • My Quarq power meter is drive-side on my 'stronger' leg. I don't actually know if there is any imbalance. But could imbalance be relevant?
Last edited by: nightrider47: Jan 26, 21 20:16
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Re: Smart Trainer Under-Reports Power in Big Ring [nightrider47] [ In reply to ]
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That's interesting. I'm a low cadence type, and I've had some unquantified suspicions about my Tacx Neo II on high Wattage efforts. I might try a similar protocol to yours, and let you know if I see anything.
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Re: Smart Trainer Under-Reports Power in Big Ring [nightrider47] [ In reply to ]
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Thats because youre on ERG mode.

ERG works best when the front to back ratio is closest to 1. Typically you use erg in the lowest gear. This is because the trainer is attempting to estimate resistance to produce a set wattage. If you have a larger ratio, erg has more difficulty estimating the watts since 1 revolution of the pedals = many more to the rear gear

Strava
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Re: Smart Trainer Under-Reports Power in Big Ring [nightrider47] [ In reply to ]
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In short - What's causing your issues here is the trainer flywheel speed. ERG really doesn't work that well when the flywheel is SCREAMING along at a few 1000rpm (or so...). The FLUX units are notorious for this. Related to the brake strength and how the units measure power. Your on-bike meters will be closer to accurate than the FLUX. I've added a basic flywheel/ERG speed test to the tests I do... almost every trainer fails to hold 225W ERG in the 53/11... or if it does, the power reporting isn't as accurate as it is with a slower flywheel speed.

Shane Miller - GPLama
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Re: Smart Trainer Under-Reports Power in Big Ring [nightrider47] [ In reply to ]
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If you want a more reliable data set, repeat the test but don't use ERG mode and record during steady state, not including significant accelerations as you would have during a 1 minute ERG controlled interval. You don't need to test at an exact figure. Just pedal consistently at around 100W, 200W and 300W or whatever range you want to cover, and record power data from both the trainer and power meter for comparison. Remember that the trainer and power meter both have declared accuracy tolerances, and that the two devices are measuring different things. The trainer is measuring power at the rear hub and a crank based PM is measuring almost your entire output, missing only any small losses at the pedal bearing.
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Re: Smart Trainer Under-Reports Power in Big Ring [Ai_1] [ In reply to ]
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both what GP Lama and Ai_1 say seem to make sense. on the latter point if you assume each has an error of +/-3% then it could be that one is on the high side the other the low side and the resultant effect is a deviation of 6%, so if in you example one was reading low 3% say the trainer, and the other was high 3% then what you see is the error of 6% difference. At 300 watts that is 18 watts difference between the two. I realize this may be unlikely but that can add to the complication. I suspect instead of erg mode try slope mode and hold a consistent wattage and then compare, also vary which gearing so that you can see how fly wheel speed and pedalling cadence comes into play.

For me it is just easier to use the data from my bike power meter because it should still be more similar to power I see outside vs inside on the trainer. The trainer just changes the effort.
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Re: Smart Trainer Under-Reports Power in Big Ring [s5100e] [ In reply to ]
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s5100e wrote:
both what GP Lama and Ai_1 say seem to make sense. on the latter point if you assume each has an error of +/-3% then it could be that one is on the high side the other the low side and the resultant effect is a deviation of 6%, so if in you example one was reading low 3% say the trainer, and the other was high 3% then what you see is the error of 6% difference. At 300 watts that is 18 watts difference between the two. I realize this may be unlikely but that can add to the complication. I suspect instead of erg mode try slope mode and hold a consistent wattage and then compare, also vary which gearing so that you can see how fly wheel speed and pedalling cadence comes into play.

For me it is just easier to use the data from my bike power meter because it should still be more similar to power I see outside vs inside on the trainer. The trainer just changes the effort.

It doesn't even need to be half that total error to explain his data in my opinion.
Let's assume those 100W, 200W and 300W figures are exact according to the trainer's measurment? There could easily be say 10W lost in the power train at the 300W range. That only leaves a 8.7W differential for his biggest gap. 8.7/300=2.9% total error. I think the Flux and Quarg have declared accuracies of 2.5% and 1.5% repectively. That's 4% when comparing data between the two. It only takes 2.9% to explain the OPs results without anything being out of spec, assuming 10W lost in the drive train.
The difference between small and large ring may be a separate issue, likely due to a flawed test method (ERG), and that can easily be checked.
Last edited by: Ai_1: Jan 27, 21 7:03
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Re: Smart Trainer Under-Reports Power in Big Ring [nightrider47] [ In reply to ]
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I don't have a quarq so this might be taken care of, but don't they have to be calibrated for each chainring size?
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Re: Smart Trainer Under-Reports Power in Big Ring [nightrider47] [ In reply to ]
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1) I love that it's always presented as the power is under reported in big ring, not over reported in little ring. ;-)
2) This is a known issue of the Neo, which is essentially the same basic system as the Flux S. It is down to issues of the power at different axle speeds. There was another post on here about a year back with this exact issue. (edit: 2 years ago almost exactly)
3) There is a difference from the quarq (I have 3x quarqs, they all do this) between inner and outer rings - demonstrated by calibrating back to back from the inner and outer and you'll get slightly different calibration. Only slight though, later units better than the old ones.

There is an old thread somewhere on the internet that also discussed this in detail from 2016/17 - some informal neo support forum from memory but didn't pop up in first page of my first two googles then I lost interest in helping you.... sorry.
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