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Computrainer Accuracy - 60-80 watts off - talk me off the ledge
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I'll apologize in advance; I'm sure I can't provide enough information for a firm answer to my question (I have zero experience with computrainer).

I have a newer Shiv with a 10 month old Power2Max Type S. It is mostly used on a Kickr at home.


Today I had a VO2 Max test where they used some variety of computrainer (again, I know nothing about Computrainer).

During the first 1/2 of the 10 minute warm up the computrainer and my PM were pretty close; +/- 5 watts. The person administering the test adjusted something, and then my Power2Max read about 20 watts higher than the computrainer. I asked about it, and was told that there was always a bit of a difference, but the numbers would get closer as the test went on.

There was a 3 minute break between the end of the warm up and the beginning of the test where I sat on the bike but didn't pedal. When we started doing the test my power2max was showing 60 watts higher during the first 3 minute step. By the time we got to the 5th stage where my legs gave out, the Power Meter was reading 80 watts higher.

After I got home I put my bike back on my Kickr, and just controlled the Kickr through the Wahoo app on my iPhone in ERG mode and watched the power from my PM on my Garmin. The power on my garmin showed within 10 watts on the Kickr at 100, 200, 300, and 400 watts (kickr always higher, but the 2 were close).

So, if the person doing the test made a mistake, is it possible to get a 60 to 80 error? I have a hard time believing that my Kickr, my Power2Max, and my FTP (insert sob) are all 60-80 watts off.
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Re: Computrainer Accuracy - 60-80 watts off - talk me off the ledge [captain-tri] [ In reply to ]
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  1. PM ≈ Kickr
  2. PM ≈ Computrainer (for a while)
  3. Human manually adjusts something
  4. PM ≠ Computrainer

Hmmm... looks like PM 2, Manually adjusted Computrainer 0. I vote for the PM.
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Re: Computrainer Accuracy - 60-80 watts off - talk me off the ledge [exxxviii] [ In reply to ]
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That was my line of reasoning, but I wanted to confirm.

Like I said, no experience with computrainer, but I know they have a strong reputation.

Thanks for the reply.
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Re: Computrainer Accuracy - 60-80 watts off - talk me off the ledge [captain-tri] [ In reply to ]
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After 10 minutes a computrainer ( mine anyway) requires a roll down to recalibrate. So after your ten minutes you should have been asked to spin it up to 25km/ hr then let it spin till it stops without pedalling. It will then be calibrated to reflect your power output. You may need to adjust the flywheel tension at this point to get the value close to 2.00 but you would roll it down again to confirm it. If you wait several minutes without spinning after this roll down then I think you will no longer be calibrated properly.

The person performing the test is obviously ( I hope) familiar with the machine and the protocols. If I was having the test done though I would have questioned what was going on based on my experience with computrainer.
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Re: Computrainer Accuracy - 60-80 watts off - talk me off the ledge [rhayden] [ In reply to ]
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After calibrating my CT and P1s read virtually the same thing. I'm going to vote human error.
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Re: Computrainer Accuracy - 60-80 watts off - talk me off the ledge [rhayden] [ In reply to ]
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She did have me do the roll down, and then adjusted something, then another roll down. Then sat for 3 minutes.

She didn't ever ask me what my power was, or try to get the numbers to sync. It was strongly implied afterwards that my power meter was wrong.

If it makes a difference, they also didn't use any of my sensors (power / cadence / speed, etc).
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Re: Computrainer Accuracy - 60-80 watts off - talk me off the ledge [captain-tri] [ In reply to ]
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captain-tri wrote:
She did have me do the roll down, and then adjusted something, then another roll down. Then sat for 3 minutes.

She didn't ever ask me what my power was, or try to get the numbers to sync. It was strongly implied afterwards that my power meter was wrong.

If it makes a difference, they also didn't use any of my sensors (power / cadence / speed, etc).


so it sounds like the computrainer protocols were followed then. so it comes down to whether sitting for 3 minutes messed it up again; or if there is that big of a difference.

i use a stages PM. I dont find it and the computrainer to be in perfect lockstep. im not sure which is more accurate.
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Re: Computrainer Accuracy - 60-80 watts off - talk me off the ledge [rhayden] [ In reply to ]
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Do they track the same, meaning the pm is always 10 watts/10%/whatever high/low?


--Chris
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Re: Computrainer Accuracy - 60-80 watts off - talk me off the ledge [chriselam] [ In reply to ]
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chriselam wrote:
Do they track the same, meaning the pm is always 10 watts/10%/whatever high/low?


they track the same. not always the same per centage difference. I am not consistent though. I dont always get the roll down to exactly 2.00. I dont always have the tire inflated exactly the same. the temperature is not always consistent, the tire is not always perfectly clean etc etc...
in that respect I would say the power meter on the bike may be more accurate as it should be consistent regardless of the conditions.
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Re: Computrainer Accuracy - 60-80 watts off - talk me off the ledge [rhayden] [ In reply to ]
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You're not supposed to do the rolldown to exactly 2. That's the minimum and 5 is the max. I shoot for about 2.75 which is 5 1/2 cranks once the thing touches the tire, if the tire is 120psi.


--Chris
Last edited by: chriselam: May 17, 17 5:56
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Re: Computrainer Accuracy - 60-80 watts off - talk me off the ledge [captain-tri] [ In reply to ]
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My guess is the calibration was done wrong maybe pedaling slightly during the rolldown. I have never heard of a computrainer being that far off the only one I've seen being slightly off was one that was purchased when they used Nintendo for running the software. Also if they do this frequently someone would have mentioned it before you it wouldn't just go completely out of whack like that.

Also like someone else said, with an ftp test I wouldn't have put it at 2. There is definitely a good chance of the tire slipping as the power goes up and the tire heats up when you use 2.
Last edited by: Grant.Reuter: May 17, 17 5:30
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