Although manufacturers of bicycle power monitoring devices SRM and Power Tap (PT) claim accuracy to within 2.5%, there are limited scientific data available in support. The purpose of this investigation was to assess the accuracy of SRM and PT under different conditions.
First, 19 SRM were calibrated, raced for 11 months, and retested using a dynamic CALRIG (50-1000 W at 100 rpm).
Second, using the same procedure, five PT were repeat tested on alternate days.
Third, the most accurate SRM and PT were tested for the influence of cadence (60, 80, 100, 120 rpm), temperature (8 and 21 degrees C) and time (1 h at ~300 W) on accuracy.
Finally, the same SRM and PT were downloaded and compared after random cadence and gear surges using the CALRIG and on a training ride.
The mean error scores for SRM and PT factory calibration over a range of 50 - 1000 W were 2.3 +/- 4.9% and -2.5 +/- 0.5%, respectively. A second set of trials provided stable results for 15 calibrated SRM after 11 months (-0.8 +/- 1.7%), and follow-up testing of all PT units confirmed these findings (-2.7 +/- 0.1%).
Accuracy for SRM and PT was not largely influenced by time and cadence; however, power output readings were noticeably influenced by temperature (5.2% for SRM and 8.4% for PT).
During field trials, SRM average and max power were 4.8% and 7.3% lower, respectively, compared with PT.
When operated according to manufacturers instructions, both SRM and PT offer the coach, athlete, and sport scientist the ability to accurately monitor power output in the lab and the field. Calibration procedures matching performance tests (duration, power, cadence, and temperature) are, however, advised as the error associated with each unit may vary.
As you can read/see there is a lot of difference. SRM is super precise but when using a PT on Zwift you will probably get a seriuos BONUS to your power output
I hope Zwiftpower guys are aware of this and are working on a solution …
Next one Kickr vs Neo:
February 21, 2019, 11:37am #4
I’ve got a Kickr and a Tacx Neo along with power meters on all my bikes. My Kickr reads 25 - 30 watts higher than my Quarq power meters. Both the PM and the Kickr are calibrated. I use Power Match, so not a big deal.
My Neo is much closer in power to my Quarqs, usually within 1 - 2 watts.
February 21, 2019, 12:20pm #7 Wheel-on trainers are much less reliable in measuring power than direct drive ones, as the power estimation is calculated from roller speed and resistance level, but the actual load depends strongly on tire pressure and roller position. Spin-down calibration tests attempt to reduce this dependency, but they on the other hand depend themselves on wheel inertia - yet another factor thrown into the mix. So a wheel-on trainer can be consistent, if you keep the same bike with same wheel and same tire at same pressure; but consistent does not mean right.
February 21, 2019, 5:45pm #9
Lately I’ve been using my Kickr again while waiting for a new free hub for my Neo. For fun I sync’d up my Garmin with the Kickr during last night’s workout. It was SSB between 235 - 250 watts (again I was using power match so that was what my Quarq was reading). However my Garmin was reading 265 - 280 (getting data from Kickr).
my watts go up 20-30 watts on my stages gen 1 left only power meter compared to my Tacx neo at home…
max 20 min at home 315 ; max 20 min on the road 342
max 5 min at home 373 ; max 5 min outside 362 [but this was part of a longer effort]
I get almost exactly the same numbers on my set up: Gen 3 Stages Left only PM versus a Tacx Flux smart trainer. My road number are noticeably higher despite calibrating regularly.
Same for me, 4iii left only on road is also 20-30w (~10%) higher than my tacx neo when riding at 200-300w. Tested this by comparing tacx power on zwift vs bike PM on garmin simultaneously. However high end stuff, 700/800w+ were pretty spot on. Figured this was down to being left only reading… think 10% is reasonable, I just make a mental note to lower the intensity on my indoor power based workout. Not sure which is more accurate now that I think about it… if I was zwift racing I guess I should hook up the 4iiii PM instead
Zwift races is the only time it bothers me. I’ve noticed similar differences using Cyclops Hammer vs Quarq Dub
Same. I’m 20-30watts better off on the road. Both on assioma pedals and now dual sided stages gen 3, compared to Tacx Neo.
I ended up setting my KICKR to use my Vector 3 pedals as the source of power to keep everything the same inside and out.
I have the Elite Qubo Digital B+ smart trainer and it’s easily giving me 20% more power than I can actually do, they are flawed in that they don’t have a conventional power metre in them, you have to know your power and set it manually through their app (using the P1, P2 and P3 settings). I don’t have a power metre yet so I can’t calibrate it accurately, but I know I’m not that strong! (mostly through using Wattbike)
the watts on my assioma uno are at least 10% . higher so I calibrated my elite qubo digital smart b+ trainer using the powermeter and now they give me approximately same results.
Have bkool trainer and p2m power meter on my bike, so i ofcourse let the power2max power meter feed data into the trainer to the trainer only controls resistance. But i have tried to compare the 2, trainer was around 30 watts higher.
I don’t see a huge difference using current gen KICKR indoors and SRM, Quarq or Stages dual (depending on which bike outdoors) and my average power for rides of a similar length are nearly the same (within 1-5% of each other) but that is just anecdotal comparison, not trying to collect data from both a KICKR and crank-based power meter and comparing them.
Very interesting but the point is you can’t say much to this experiences. It should be tested by trustworthy people without marketing involved to get the best/stable trainer(s) for using in zwift races. For Training it doesn’t play much of a role everyone has numbers and can train. Of course a HR monitoring system would be much more sensefull/intelligent to use compared to power monitoring system. Power is stable HR is individual! Although using both would be king. But that’s another story …
I’ve just got a Tacx Flux 2 and gave it a whirl this afternoon for the first time. I’ve also got a 4iiii crank power meter and comparing the Flux power reading to my 4iiii via my Garmin there was a consistent 20 – 40 watt under-read of the Tax vs. the 4iiii/Garmin. Is this normal and a function of the fact the 4iiii is measuring power at the crank and the Tacx measuring it at the wheel? a bit like cars on rolling roads where you get a flywheel power reading or an at the wheels power reading? or should I expect the reading to be the same?
I’ve got my Garmin set to sample the 4iiii power over 3 seconds to smooth out the power fluctuations and can’t seem to set the Tacx output in the same way so the tacx fluctuates more, but still getting a sizeable discrepancy with both 4iiii and Tacx freshly calibrated. I’ll look into it.
My Tacx Flux would read about 15-20w lower than my Stages.
For the first few months I just did my FTP tests and TrainerRoad stuff using the Flux. It meant that my FTP wasn’t ‘right’ for outdoor riding, but I wasn’t doing structures intervals outside so it didn’t really matter. Now I use the PowerMatch feature in TR to control the flux from my Stages.
What are you using the trainer for?
I have quite a discrepency on mine – bepro pedal reads something like 8% higher than the kickr core. Seems consistent so it’s not a hindrance for training (as said), but it does give me two different power regimes when I’d prefer to just have one.
@cid did you read the study you referenced or just the abstract? There is a copyright on the paper (so I wont copy and paste) but if you read it you will see that they concluded that the most accurate SRM (out of 19) was more accurate than PT (out of 5) but the least accurate SRM were less accurate than the PT. And by accurate we talk about the 2% range. Also that Paper is from 2004.
All of these arcticles just show 1 thing.
Powermeters differ from model to model
from manufacturer to manufacturer
from person to person
Technic is influenced by the surrounding area or everything the unit is connected to - Organic to anorganic.
In the end if you race on top level 10% difference in FTP is just another level.
5 % will probably give you enough room to make your attack succesfull
2 % is acceptable.
In other words the best you can do is to share every bit of intel about PM to the zwifters so everyone know what is going on and how to solve differences, discrepancies with to high or low Powermeter-Data.