Humsquirto wrote:
There is a GPS option available for many data acquisition systems most
drivers find that you get more repeatable results from a lateral
accelerometer and a wheel speed indicator non driven wheel. There is a
fair amount of error in the gps system, my thoughts also when you get
height changes z axis the calculations are not that accurate vs on a
road where you just measuring x y axis changes


Downhill and alun,

thanks for the info, that is more of what I was hoping for with this
discussion. I like the mention of the z-axis change - makes sense. I
still however profess that 70 mph is absolutely attainible. Downhill
racers typically max at upper 80's to 90 mph, I don't think reaching
75% of this speed in ideal conditions (long wide straight run with a
consistent yet steep slope, iced over groomed run, no bumps, tuck the
whole way, not worrying about ruts, turns, gates, etc.) is that hard
to believe. Even with a 10% error, that is still mid 60's, which is
splitting hairs. I posed the question to the folks at the USA speed
skiing association, who said that under the right conditions that was
certainly doable (difficult, yes, but impossible, no).

What if the GPS was actually close to accurate? Errors are not
automatic. So maybe it was closer to 60 mph, but even that means that
70 mph is still possible. Next time I will take my Trimble unit.

Ask yourself, how fast do you think you could go? 60+ mph is not that
unreasonable.


Not necessarily about your particular questions, but:
A friend of mine has the GPS app for his I-Phone. He
also has a dedicated GPS (he's a toy freak). He finds
that both the I-phone and the dedicated GPS show speed
quite accurately when compared to his speedometer.

Having said that, he finds this is so over a distance
and when he accelerates or decelerates there is a lag
before the GPS receivers w adjust to the new speed, so
what I would suggest is that your GPS may give you
a accurate max speed reading but that at any given
instant, the reading could be wildly off. For example,
it is several seconds before my friends GPS will
register the fact that the car has stopped, still
recording 25 or 30 mph while the car is stopped.

So... I would tend to believe you that in the
conditions that you indicate that you really are
going 70 mph (or so with a fairly small error)

As for me, I believe that I top out at somewhere
around 50. I have another friend who likes to
"speed ski". He does so early in the morning
(before crowds and bumps form) and only when he
is alone or almost so on the hill. I suspect he hits
downhill racer speeds. Big guy, big skis, lots of
skill. Has been stopped by patrol, who listened to
his story and said that under the conditions he
described, going fast is OK. Neither one of us has
had our speed measured.
..