On 2/16/2020 2:55 AM, wrote:
On Sunday, February 16, 2020 at 12:57:47 AM UTC+1, jbeattie wrote:
On Saturday, February 15, 2020 at 2:30:14 PM UTC-8, Frank Krygowski wrote:

Mine was a real world experience. Conditions were constant except for
the sudden engagement of the dyno. The main weakness is that I know
the before and after speeds only approximately. But I know that there
was very little reduction in speed, which was all I cared about.

That is what you confirmed from your own experience. Yet you argue
with me! How odd.

Not odd at all. I don't doubt the drag scrubbed off some speed, but even your story suggests confounding factors like tracking down a noise, looking around for dragging brakes, etc. It's hard to do all those things while maintaining speed.

-- Jay Beattie.

The big problem with this 'test' is maintaining the same power. The natural tendency is to increase power when the resistance increases. Everyone who has ridden with a power meter know that the power varies significant even when trying to keep it constant. Plus minus 10 Watt is about the accuracy when you try to keep it constant if the restistance varies. Like I said the speed difference is crappy data.

Speed difference is what the dyno complainers worry about. I described
the speed difference I experienced in the real world under what
certainly seemed like perfectly constant conditions. The speed decrease
was minor.

Yes, it wasn't a laboratory test. Maybe an undetected tailwind popped up
at precisely the same time. Maybe my worries pumped adrenaline into my
system. Maybe the barometric pressure dropped. Maybe I hit a pocket of
warm air. And maybe all those factors went precisely away when I
realized what happened and turned the dyno back off.

But it occurs to me, this practical speed reduction shouldn't be too
hard to test, at least in an approximate way. Everyone here who does use
dynos of any type could pick an appropriate downhill and coast down it a
few times, with dyno turned on or turned off. Report your results.

My main point remains: Contrary to what Tom said, the decrease in speed
was, for me, minor. If I were going to race, I would keep the dyno
switched off. YMMV.

--
- Frank Krygowski