Re: [ergomo-users] Digest for ergomo-users@googlegroups.com - 1 Message in 1 Topic
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Rodrigo Diaz
May 27, 2011, 1:53:27 PM5/27/11
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I don't think so.
Offset and calibration numbers (k-factor) are different. The k-factor you have to modify manually, the offset is established by "zeroing" the unit by pedaling on the biggest gear.
Offset is deviation from zero (in broad terms, axis-intercept on a chart), k-factor is the linear response to the deflection in the system (slope).
In my experience (3 ergomo units), the offset deviation is not "consistent", I could never really assign a value to it. Like you said, it changed with temperature, or when I cleaned and reinstalled the bottom bracket. It seemed to have little effect on the actual power measured, when compared to a Computrainer. Plus, you can't really control it since it's just basically re-zeroing itself. I rarely see differences more than 2-3 digits on either way.
The k factor DEFINITELY had a big effect. In fact, I fiddled for a couple of weeks trying to match the Computrainer's output for training consistency purposes. It eventually got within 2%, in ranges between 150 and 450 W. More than that, I think the tire started slipping a bit much.
My advice is always calibrate if in doubt. I frequently participate in a weekly TT, and I know more or less if the numbers are in range when pre-riding some small hills on the course. If anything looks out of whack I do an offset test. After all, with hot weather etc. many other things change- physiological response, air density, etc. It is annoying having to "fudge" the power numbers. If you had a good night (comparatively! since the conditions do change for all) and you know that the measuring instrument was wrong - that's it. Imagine if your speed meter went wrong - it might say 25 km/h (and it has happened to me, since the magnet was only reading about once ever two revolutions due to poor placing) but you know you're working hard.
Offset and calibration numbers (k-factor) are different. The k-factor you have to modify manually, the offset is established by "zeroing" the unit by pedaling on the biggest gear.
Offset is deviation from zero (in broad terms, axis-intercept on a chart), k-factor is the linear response to the deflection in the system (slope).
In my experience (3 ergomo units), the offset deviation is not "consistent", I could never really assign a value to it. Like you said, it changed with temperature, or when I cleaned and reinstalled the bottom bracket. It seemed to have little effect on the actual power measured, when compared to a Computrainer. Plus, you can't really control it since it's just basically re-zeroing itself. I rarely see differences more than 2-3 digits on either way.
The k factor DEFINITELY had a big effect. In fact, I fiddled for a couple of weeks trying to match the Computrainer's output for training consistency purposes. It eventually got within 2%, in ranges between 150 and 450 W. More than that, I think the tire started slipping a bit much.
My advice is always calibrate if in doubt. I frequently participate in a weekly TT, and I know more or less if the numbers are in range when pre-riding some small hills on the course. If anything looks out of whack I do an offset test. After all, with hot weather etc. many other things change- physiological response, air density, etc. It is annoying having to "fudge" the power numbers. If you had a good night (comparatively! since the conditions do change for all) and you know that the measuring instrument was wrong - that's it. Imagine if your speed meter went wrong - it might say 25 km/h (and it has happened to me, since the magnet was only reading about once ever two revolutions due to poor placing) but you know you're working hard.
On Thu, May 26, 2011 at 11:45 PM, <ergomo-use...@googlegroups.com> wrote:
Group: http://groups.google.com/group/ergomo-users/topics
- offset [1 Update]
Topic: offsetRidewt <bill.t...@snet.net> May 26 10:44AM -0700 ^
--
Last night I did a local time trial and was trying to ride it at a
specific wattage target. I rode the 10k race but my numbers were low
and I was a bit down about the whole thing until I realized that we
had had a headwind and I had one of the best times. Then, I did an
offset after the race, having forgotten to do it before, and realized
that the offset number dropped by about 7 points. That is a fairly
significant drop for me. I think it was maybe because it was the
first hot day of the year and the weather was a lot different than the
previous time that I did the offset. I've heard that temperature can
affect the offset value.
Anyway, I believe that I've read that 1 point of offset drop is
equivalent to about a gain of 5 watts. So, if I dropped 7 points, I
should add 7x5=35 watts to my average watts. If I did that, my
wattage target would have been just about right on the money.
Is all this correct?
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