offset

[Ridewt]

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?

[JimB]

Yes, this is all about right. In the winter, my calibration will
change about 5 points when the bike goes from inside, to below
freezing outside. It takes mine at least 20 minutes to stabilize.
Ignore any data while you warm-up. Stop and recal after about 20min,
then data is good. Same rules apply when going from air conditioned
inside to sweltering heat outside. Obviously the cal numbers move in
opposite directions for each scenario. When you got the windows open
in the house, you should be able to calibrate and ride right away...
no temp change there.

The amount of watts per cal point increases with crank RPM, but most
of your time is spent in a norrow RPM band so the amount is pretty
constant. You mention 5W was about right for you. On my units (at my
cadence), it's about 6 or 7W.

All and all, your theory is sound... or at least it checks out with
everything I've seen!!