Question about Quarq 'zeroing'
Quick question about my quarq…truthfully more for curiosity than anything else. I asked the quarq people about this in an email but the response I got didn't really answer the question.
Before each ride I zero it using the procedure spelled out in the owners manual…I put the drive side crank arm at the 6 o'clock position and then navigate my Garmin to the relevant screen, hit "calibrate" and wait for a number to appear on the screen. Usually the number is between -7 and -1. Once it was as high as 1 and once as low as -13.
What are the factors that drive this number, and what is the meaningfulness? For example, I believe temperature is one factor. So if I zero the quarq and then zero it again with the temperature 10 degrees higher, by what amount will the calibration number change? What about 20 degrees?
And what factors other than temperature impact the calibration value (and in what magnitudes)?
And more importantly, what does a change in calibration number mean for watts? From the quarq website I understand that a change in the offset value by 1 is equal to 1/32nm. But I am unclear what that means for watts. If my body is outputting a certain constant effort but the calibration value is higher by 1, how different will the "watts" be on the display?
I said I want understand this mostly for curiosity and that is true. But the other thing is that my bike is stored in a room that is about 10-15 degrees colder than where I ride it in my basement (~55-60 degrees vs 70). So when I do the "manual zeroing" before each ride, it is at a colder temperature than midway through my intervals. Assuming I don't calibrate later in the ride, is my physical effort likely being overstated or understated by the display? And is the difference material? I have a feeling the answer to the latter is "no" because about half of the time I set my bike up in the warmer room the night before a morning ride, and hitting my intervals on those rides doesn't seem any more or less brutal than if I just pull the bike out of the colder room.
FYI Quarq's response was (a) as long as the numbers are within +/- 500 then don't worry about it, and (b) if you check the calibration number after the ride and it is within 50 of when you started, then the power for your ride should be considered "accurate".
Cheers, Matt
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My Quarq was always around the -200 range but had drifted into the -400 range the last couple of months. I recently took the crank, rings and BB apart for a good cleaning and the new numbers register back around -200. I assume a re-tightening of the chainring bolts helped.
Temperature certainly factors into it. I believe you'd want to re-calibrate if/when the temp changes more than 20 degrees. That's easy as pie to do -- just backpedal four or five times and it will re-zero the Quarq on the fly. I do that about every hour no matter what the temp is.
Btw I followed up with Quarq and they answered half of my question (the other half - what the quantitative impact of temperature is on the calibration number - they'll get back to me on). Their response was:
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The three most common causes for changing calibration numbers are changes in temperature, a shift in the chainring/spider interface (in the chainring bolt locations), and water/moisture intrusion.
A change in calibration in relation to watts is minimal. Let's say you started a ride and your calibration number was -1 and when you finished it was -11. That would be a difference of -10/32 Nm. If you are pedaling at a cadence of 60 at the end of the ride (you last zeroed at -1 and the actual is now -11) your powermeter would be reading 2 watts low. If you were pedaling with a cadence of 120 your powermeter would be reading 4 watts low.
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Btw this is true for all strain gauges and therefore the srm and powertap can shift the same way.
I'm actually surprised the disparity is as much as they stated. Granted, we're only talking a couple of watts but I thought it was actually smaller. It's still basically immaterial unless you had a really big calibration delta.
That's why I'm an FBI Agent and not an engineer.
Mancona is dead on as usual. Do the backpedal zero at the very beginning of the bike and once every hour.
Matt (or anyone), can you explain the manual zero to me... following the steps in the manual for my manual zero (Q/J combo) doesn't work. Others have mentioned backpedaling but nothing happens when I do that as far as I can tell. What buttons do I need to push, when do I push them, when do I start pedaling backwards, what should happen when I do.
sorry - still a newbie to power.
thanks.
Regarding the auto-calibration via back-pedaling, I will do that fore sure once I get outside. I wonder how it works internally to the computer, however, since when backpedaling there is certainly a torque load on the crank arms.
@ Becky - I have an Edge 500 so sorry, no idea how it works with the Joule
Matt, the manual backpedal works because the calibration is
factor and all power related calculations are done by the cinqo not your ant computer. The computer is just showing the value hat is stored on the Garmin for your reference and trouble shooting. You could never do a manual calibration and still have accurate day az long as you were to back pedal at least 4 times prior to the ride.
Fwiw, the the cinqo zeroing works exactly the same if you back pedal or trigger it from an ant computer. If you want to check or change the slope you need weights and quarqD.
Manual zero is not possible with the current released version of the Joule. I think there's a beta that works, but you need to request it directly from Saris.