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Old 04-12-11, 10:57 PM   #1
brianappleby
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Replaced my powertap, gained 20W!

I just replaced my 6 year old powertap with a relatively new one and my LT jumped up by 20W. If I want to use both, I'm going to have to find a way to make them consistent. Is there a way to calibrate them without sending them to Saris?

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Old 04-12-11, 11:34 PM   #2
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What did I tell ya!

I think the wattage list is your best bet for info about powermeter calibration.
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Old 04-13-11, 08:02 AM   #3
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There's no way to make them consistent. Stomp test both of them and adjust your numbers accordingly. If you send them back to PT for calibration, they'll both come back being off in different directions. Maybe they'll be closer, maybe they won't.

I know mine reads 2.8% low. I know my "display" threshold and my real threshold. I train zones based on what the PT reports. Interestingly, my training partner's PT reads 7% high. Between my -2.8% and his +7%, we end up targeting the same numbers for intervals (due to rider weight differences). Makes the power talk easier.
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Old 04-13-11, 08:32 AM   #4
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ALL instruments have error. Thankfully we don't base race and fitness off of the absolute numbers that come from devices - rather from actual riding and performance. With that in mind I usually recuse myself from discussions like these because I see them as really pointless....

...as long as you acknowledge and know that power is a relative number when using these devices then focus on what they are there for - to train. Therefore base your training off of the numbers you have and it will be effective for you. Having to swap between devices frequently can make it a bit of a hassle, but honestly I swap between at least 5 different power devices and 3 different head unit (it's good to be the wheel guy) and they are all really close to each other. It makes me crawl the wall when I run into someone who is freaking about their calibration (like someone freaking out because the computrainer is reading 5w higher than hat their head unit is reading from the PT they are riding). I do usually find that a calibration will bring most high quality power devices into a few % points of each other.
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Old 04-13-11, 09:07 AM   #5
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I'm not familiar with a "stomp test". I'm guessing that you hold the brakes and put your weight on a horizontal pedal, then look at the torque reading? Is there more to it than that?

Psimet, I'm fully aware of instrument error. I choose to believe the error is systematic, and that 300W on one powertap today is the same as 300W on the same powertap tomorrow. I'm just in the lucky position that I have 2 of these things now, and I don't want to have to remember which one I'm using when I'm choking for air at the end of an effort.

The good news, I suppose, is that I can sell one now. I'll charge more for the one that reads high...
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Old 04-13-11, 10:56 AM   #6
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I dunno. An ~6% variation between powermeters is a pretty large error. I would call Saris and ask their opinion on the subject.
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Old 04-13-11, 11:04 AM   #7
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I'm not familiar with a "stomp test". I'm guessing that you hold the brakes and put your weight on a horizontal pedal, then look at the torque reading? Is there more to it than that?
I shift to my small ring, biggest cog, and hang a known weight from a pedal. I weighed this weight (just under 40 lbs) on a postal scale. I point the crank about 20 degrees down, forward. I change PT to torque mode, then zero it. I lock up the rear brake, and pull the bike back into a wheelie until the crank is level. I watch the torque as I slowly swing the bike up and down through the level crank zone, noting the max torque.

Then I lower the rig, spin the wheel ~1/4 turn, and repeat. I do this for four total readings around the wheel and average them. Then use the stomp test equation to determine the predicted torque.
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Old 04-13-11, 11:41 AM   #8
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Then I lower the rig, spin the wheel ~1/4 turn, and repeat. I do this for four total readings around the wheel and average them. Then use the stomp test equation to determine the predicted torque.
Any idea why the readings are different based on wheel rotation? It seems like it should always be the same torque on the hub regardless of wheel postion (assuming same crank position and weight).
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Old 04-13-11, 12:58 PM   #9
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Could be an analog data transmission to my wired sensor. Maybe distance from the transmitter as it revolves around the axle? Dunno. I like samples and data though.
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Old 04-13-11, 01:04 PM   #10
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Isn't there also more than one strain guage? Torque is angle dependent, so doesn't the angle change the compensation between the gauges?
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Old 04-13-11, 02:34 PM   #11
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Yeah, that's a good point. It will be bowing toward the crank from chain tension. Strain gauges will be on differing parts of that bow.
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Old 04-13-11, 02:46 PM   #12
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I shift to my small ring, biggest cog, and hang a known weight from a pedal. I weighed this weight (just under 40 lbs) on a postal scale. I point the crank about 20 degrees down, forward. I change PT to torque mode, then zero it. I lock up the rear brake, and pull the bike back into a wheelie until the crank is level. I watch the torque as I slowly swing the bike up and down through the level crank zone, noting the max torque.

Then I lower the rig, spin the wheel ~1/4 turn, and repeat. I do this for four total readings around the wheel and average them. Then use the stomp test equation to determine the predicted torque.
if you'd stop doing all this witchcraft you'd probably have like 16 hrs/week to train
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Old 04-13-11, 02:50 PM   #13
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if you'd stop doing all this witchcraft you'd probably have like 16 hrs/week to train
.....he's got a point there...
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Old 04-13-11, 02:59 PM   #14
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It's only twice/year or so, and takes 10 minutes. (if you don't count shredding the bat wings to make the powder, but I use that for other recipes anyway)
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