Re: Schlumpf 29" (guni) calculations


It looks like you are mixing apples and oranges (basically what Greg
said). If you are using the GPS to measure speed, that is ground
speed. The actual distance travelled to achieve that ground speed is
what a traditional cyclometer would measure. Your cadence would have
to be higher than that calculated from the GPS to actually achieve the
GPS speed because of higher-frequency changes in path direction (wobble
of various types).

Another factor is how the GPS actually arrived at the top speed you
mentioned. Something probably not knowable except by the engineers
that designed that particular GPS. This includes number of averages
and/or curve fitting of the speed data, noisy data rejection, ghost
handling, predictive elements to the algorithm, and the like.

Finally, what's the GPS accuracy anyway? I think it's about 1m these
days in absolutely clear conditions with 5 satellites and no tree cover
or surrounding buildings, with a stationary measurement over time. Or
at least something like that. Can someone confirm or shoot that down
(it's been a few years)? Anyhow, under typical recreational conditions,
there's going to be plenty of measurement error to make this kind of
calculation, which is dependent on one single measurement, essentially
meaningless. For recreational civilian use with no controls on
environmental or experimental conditions, treating a max speed as
anything near truth is probably a misuse of the instrument. An average
speed, though, taken over a reasonable time (minutes?) should be good
under many conditions, though not all.

I've seen people try to map single-track under less than dense leaf
cover, and have a heck of a time trying to get a GPS to work properly.

My measurement of the BA diameter has been just about 30", though I did
not do a rollout on it. This was the fatter BA at about 60 psi,
unloaded.

Whatever the calculations, given what cadences track racers reach, and
world-class cyclists train at, and your much shorter cranks, 178 cps is
not an overwhelmingly high cadence to reach for a top speed.


--
U-Turn

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-- Dave Stockton
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