Looking for an education on Drift and Slope

Dino Sarti

 I have been spending a bit of time on the Yahoo "Wattage" group and the notion of "drift" keeps coming up.  People complain about dirft moving around and ideas around zero drift.  Then concepts about the meter or rings settling in.  I am a bit confused, but curious.

Is this the number the Garmin 500 shows you when hit the "calibrate" fuction on the Ant+ Power screen?  If so, how aften do you monitor and measure it?  I do it at the start of ride and usually it is a number around 580.  How does it impact the watts number?

Also, what is the deal with changing the slope?  I know this needs to be adjusted if you change the chain rings, but what's the theory here?

 

Thanks in advance for the education.

benvanmarcke

Changing the slope when replacing chainrings may be something that applies to crank based powermeters but I don't see how it would impact the data from a powertap? I mean, that would imply shifting from a big chainring to a small chainring would also require you to change a setting on your garmin... makes no sense.

GMAN 19030

Dino, are you using a Quarq?  If so, the 580 "calibration" number is outside their acceptable limits (mine is usually around -10).

Anyhoo...

http://www.quarq.com/faq (read the Calibration Info and Technical Info sections)

http://www.quarq.com/forum/quarq-discussion/ (read around for some info about how the calibration # effects the watts number)

Dino Sarti

@Bob - Yes in the 580 example above I was referring to my Quarq/S975/Garmin combo. But, I have an SRM on my Camppy/Colnago bike and a Powertap/ Zipp 404 on my TT BIke. I only put that out there to explain that I have a lot to cross compare with. The Quarq and the SRM seem really close in terms of numbers and RPE. The Powertap is generally lower in a very consistent way and I think that is the fact that I make less power on that geometry combined with it measuring in the hub after any power train loss versus the crank.

The 580 number is very consistent before, during and after the ride within 5-10 "values" ever since I installed it in March. I was concerned by your posted numbers above, but based on this "cut and paste" form the Quarq link above, I think I am in the clear:

"My 'Manual Calibration' is approaching 500, should I be concerned?
The 'Manual Calibration' performed through a Garmin returns the 'Zero Offset', or tare value of the CinQo. Though most CinQo's settle well below 500, approaching or exceeding 500 is not a need for concern. What is more important is intra-ride calibration values. If your pre- and post- ride values are regularly more than 50 units apart or are quite erratic varying greatly by 100's or several 100's, then it is recommended you contact us.

Adjusting or the torque on chainring bolts may return the initial offset toward 0, but in time will most likely return to values previously seen."

Does yours move around Bob? Did it ever settle in? Just curious....

@Ben - I think you are right. All the slope conversations I have seen have been around chainring changes and have to do with people using the Qalvin iPhone app to adjust. I have the app, but have never used it. No need to, but I am curious in a sort of "tear it apart to tinker and learn" kinda.

Thanks, guys!

Dino Sarti

Oh - Final thought. Looks like "+" or "-" is relevant. Mine is generally -580 or so.

GMAN 19030

Mine used to drift a lot more before I sent it in late last year after changing chain rings. With the old rings it started at or near zero and ended up settling around -300 or so. The new rings have consistently held steady between -20 and 0.

Dino Sarti

Such an interesting piece of gear. I have been nosing around the Quarq forum and it is funny to see how these things get glitchy and drive guys totally nuts!

GMAN 19030

Some of the guys bitching endlessly on the Quarq forums crack me up because they seem to think that a difference of say 100 in the offset number is some huge deal when in reality it's only a couple of watts. There was even some guy going crazy because his offset number drifted a couple of points. I guess 99.9% is not good enough for some.

MattAaronson

I had my drivetrain cleaned in June after the OS anfbefore my A race andmy calibration value went from the -10 to -30 range all the way to the -400-ish range. I assume this was because the shop torqued the chainring bolts. This really, really irritated me for a while but not it seems okay and the value are consistent with RPE, etc, etc. I will probably email Quarq to see if there is some value to sending the unit in for recalibration before the OS starts, but realistically I'll probably get lazy and not bother.