Why calibrate?

[Deleted User]

This is an addon to a recent thread, but thought it would be better as its own topic. Here's the question: Why calibrate a power meter, and what is going on when you do calibrate?

For the last year and a half I've used a wired powertap and LYC. I would occasionally follow the directions to make sure it was zeroed with no tension on the pedals. It always showed a 0, but sometimes I would calibrate it anyway even though the directions indicated that this was not needed. What happened inside the LYC when I did this? Why do some people here say to do it before every ride?  The powetap instructions do not seem to indicate that is needed.

I'm about to switch to a powertap pro with a Garmin 910xt as a head unit.  Do I need to calibrate that more often than the LYC?  Also, the calibration instructions with the Garmin are a little different than the LYC. The Garmin manual says to refer to the power meter's instructions to calibrate the unit, but then goes on to say that you should keep the power meter active by rotating the pedals until a message appears saying calibration is complete. What's correct - spin the pedals or keep then unweighted during calibration?

Thanks everyone!

GMAN 19030

I just hit calibrate without any pressure on the pedals.  I would assume pressure on the drivetrain would cause an incorrect calibration result.

[Deleted User]

Thanks Bob. That's what I'm thinking is the way to go. Still curious about why do it and what happens when you do...

Daniela

To add on to the questions...

The Joule has an Auto Zero option. Does this mean it never needs manually calibrated?

foolio_mugoolio

Posted By Daniela Williams on 24 Feb 2012 06:47 PM

To add on to the questions...



The Joule has an Auto Zero option. Does this mean it never needs manually calibrated?

I think you don't need to.  I have auto zero turned on and it seems to be fine.

mgraffeo

For me, wired PT with LYC, I check the calibration at the beginning of every ride on the trainer (I ride for about 90 seconds easy to start it to warm up, then stop). About 1 out of every 3-4 rides I find it not at zero, and have to manually zero the torque. If I see it at zero, I do nothing. Zeroing it when it's already at zero doesn't do anything.

Outdoors, I worry about it less, because it auto-zeros every time you coast. Indoors, I seldom coast (and when I do, there's much more resistance than vs. outdoors due to trainer curve), so I prefer to check it.

Paul Hough

I have a PowerBeam Pro that only needs to be calibrated if the rear bike wheel is released. It takes 2 mins to run a calibration test. I've read on SlowTwitch that folks running a PowerTap on their PowerBeam simultaneously are getting identical or near identical results.

Paul Hough

I have a PowerBeam Pro that only needs to be calibrated if the rear bike wheel is released. It takes 2 mins to run a calibration test. I've read on SlowTwitch that folks running a PowerTap on their PowerBeam simultaneously are getting identical or near identical results.

[Deleted User]

Found an interesting discussion about calibration vs zeroing the torque, and why to do it here:
http://forums.roadbikereview.com/racing-training-nutrition-triathlons/garmin-powertap-users-do-you-zero-calibrate-before-every-ride-246316.html

Keith Wick

Well, I guess that having it zeroed really doesn't matter on my PT as it doesn't indicate the torque stuff---interesting, although I've never seen a problem. Usually I turn on the computer and give the wheel spin, which wakes up the hub and lets me see a zero before I flip the lever and engage the wheel on my trainer. I just can't get the zero the torque sequence in my brain, so I'm always looking at the manual when I do this.

Dave Campbell

Keith, what computer/head unit do you have?  My old LYC did not display torque, but my Joule does. With my old wired PT hub/LYC, I just looked for  0 watts when the wheel was turning and no pressure on the drive train. With my new wireless hub and Joule, as completely different process and it does display torque settings on the Joule. No experience with a Garmin head unit.

mgraffeo

All LYC's display torque. it's part of the calibration procedure.

-Use the mode button to put the cursor on the Watts row

-hold down the select button for 3 seconds, until it starts blinking (now you're seeing torque).

-hold down the select button for another 3 seconds to zero out the torque

-tap the select button to return to watts.

You can be seeing zero on the watts, and have a non-zero torque (for example, a negative torque is shown as zero watts).

kkiefer

@Paul, quick remark on your PowerBeam comment, I read the dcrainmaker review yesterday. If do not have PM on the bike, I recomend do your run down test after warm-up at each work-out as tire pressure and tire temprature changes over days. If you have a PM on bike that you are ok.

Peter Greagg

Just a follow up to Mike Graffeo's post, I have a Power Tap with LYC ANT+. I changed the batteries for the first time a month ago. I took it out for a ride and when coasting and not pedalling, the Watts line showed 0 — so I assumed it was zeroed.
I did my 2 x 20 FTP test on my Computrainer with the PT wheel and LYC — the PT gave an FTP of 196 watts and the CT gave 213 watts.
I didn't think too much about it until I saw Mike's post (I used the CT FTP for my inside intervals and the PT FTP for when I rode outside).
So I zeroed the LYC before yesterday's 2 x 20 test — the PT gave 212 watts and the CT gave 217 watts — good enough for government work.
So thanks Mike for the reminder to zero every time.