Av og til henger jeg bort lycra og pappa-flekkete klær og finner fram den hvite labfrakken, og børster håret rett opp.
Gjerne en mørk og dyster kveld så naboen kan se at det lyser i tårnvinduet på Bjölzen Bike Labor. Han er i gang igjen hvisker de til hverandre. Så kommer todenskrallet alle venter på.
Uten ytterligere dramatikk gir jeg dere dialogen mellom undertegnede og Amy som er Knowledge Base Manger hos Stages. Mer drøft og vurder følger under.
Hi!
I am constantly getting readings from my Stages that are 7-8% higher than from my Powertap. I have done a static load calibration test om both meters by hanging a load of about 26 kilo from the pedal and reading the resulting torque from the Powertap . From Stages I have used the values from Hi Speed Recording to read the weight in kilos directly.
My calculations give me accuracy within 1% for Powertap but approximatly 4 kilos or 15% off for Stages.
My question is How should the weight be applied to the crank arm to get the highest accuracy from a static load test on Stages? I have hung the weight on the pedal axle close to the crank arm and taken the measurements with the arm horizontally.
Hi Lars,
The instructions to do a static load test would be to, of course, first zero reset the meter in the 6 o'clock position with no weight on the crank. Then move to meter to the horizontal position as you have, and hang the weight close to the center of the pedal spindle or as close to 5cm away from the crank as possible. 5cm from the crank arm is where the center of almost all road pedals is. We have to hang the weight here in calibration since it accounts for any angular flex on the crank.
Please let me know what your result is (as well as make note of the zero reset values you are getting when you zero the crank), as if it is still coming that far off we may want to set up an RMA for the crank to be returned to our office in Germany, Stages EU, as even with the weight hung close to the pedal spindle that seems like a large margin for it to be reading high.
i Amy!
Thanks for your quick and detailed answer.
The zero reset value I got from the calibration before doing the test was 895. I always get values between 892 an 895.
I have redone the static load test and applied the weight as close to 5 cm from the outside of the crank arm as possible. With Shimano MTB pedals it is not possible to get it to hang at exactly 5 cm, but I did two measurements and got these results
a) 25.12 KG (1312 data points)
b) 24.73 KG (708 data points)
For an average of 24.92 KG. My test load weights 24.97 KG!
The distance from the crank arm (plus a more accurate scale to weight the test load) did the trick as far as the accuracy of the Stages in Static load is concerned.
Hi Lars,
That is good to hear that the second test provided much better results. If they're both looking accurate based on static calibration, it sounds likely that perhaps your left leg is slightly stronger than your right (or there is a fit issue). We do occasionally find customers who have a slight leg length difference or muscle imbalance that results in slightly different readings from the Stages power meter vs another meter.
Aha. Stages er ikke skikkelig unøyaktig. Men avviket å målingene skyldes angular flex eller vridning i krankarmen som ikke var tatt høyde for i mine første målinger av "Static Load Calibration". Så hva er det da som gjør at mine målinger i første post blir såpass off i forhold til Powertap når det viser seg at Stages er ganske så korrekt?
Her er det tre mulige faktorer som jeg kommer på. Tar vi utgangspunkt i posten over med 8-9% forskjell kan jeg sette opp dette regnestykket:
Ovale drev 4-5%
Forskjell i beinstyrke 2%
Drivverkstap 2%
Så har jeg forklart hele differansen.
Når det gjelder de ovale drevene drøfta jeg det i første post og konkluderte med at de ikke ville påvirke side Stages måler snitt over en runde både for vinkelhastighet og effekt. Jeg sjekket det til og med med Stages-Travis
Thanks for checking in. We have found that Stages users have consistently found that they are seeing higher values as compared to many rear-wheel based systems such as PowerTap or Computrainer. This could be for a couple big reasons:
1) We are calculating the torque much closer to the point of force than a rear hub or tire, meaning that there is less to dissipate the force applied.
2) There is a bit of drivetrain loss involved with a rear hub system, meaning that the chain interaction with the chainrings and cassette will cause a dip in power readings as it uses some of the force applied.
The non-round chainrings could definitely cause higher readings (especially on the downstroke), but over the course of a full workout, the average should work out to be about the same. This should also be the case with a rear hub system as well.
I am going to mark this ticket as resolved but please do not hesitate to respond with any additional questions or comments you may have and the ticket will reopen automatically.
Thanks,
Så Stages mente altså at ovale drev ikke skulle ha noe å si med deres algoritme. Men det var den gang. Nå har de endret mening:
Our power measurement through a Stages Power meter is event based, where as an event is one complete pedal revolution. Due to the changes in velocity non-round chain rings produce through the course of an 'event' you will see that your power will be skewed higher than with a round ring, which has a constant velocity throughout each event. Through our own testing, and using a hub-based meter as a control, we conclude that there will be a 4-5% increase on the readings from a Stages Power meter when used with a non-round chain rings. We recommend that our customers take this into account when changing from round to non-round chain rings, as they may need to adjust their functional threshold power accordingly.
Dette samsvarer nå med internettets fasit på det spørsmålet:
https://www.physicsforums.com/threads/bi...inrings.774249/http://bikeblather.blogspot.com.au/2013/01/whats-up-with-those-funky-rings.htmlNår det gjelder drivverkstap gjorde jeg testene med et bakgir og en dropout som hadde gått på en smell. Jeg trodde jeg hadde rettet opp på en grei måte. Men opplevde subjektivt at det ble lettere å snurre pedalene baklengs med nytt gir og dropout. Tallet 2% er subjektiv gjetting.
Så da sitter jeg igjen med en nøyaktig måler som blir lurt av ulik beinstyrke og ovale drev. Det kan jeg ta høyde for i den grad jeg skal kjøre strukturerte intervaller på terrengsykkelen.
Så sliter jeg ut de ovale drevene, men ikke giret og tester i tandem igjen til høsten.
Takk til de som gadd å lese så langt i en tråd med betydelig nerdefaktor.
Nå må jeg ta meg av noen bønder med høygaffel og fakler.
LarsB
