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      03-22-2016, 12:21 PM   #70
NISFAN
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Drives: BMW M2
Join Date: Aug 2012
Location: Bedford UK

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Quote:
Originally Posted by Laertes
For what it's worth, I have a full data acquisition system in my track car (Racekeeper) w/ 10hz GPS. Before I had this, I used the Harry's Lap Timer app on my iPhone, using the built in GPS. Just for the heck of it, I tried them both at the same time last year, and it was pretty amazing how accurate the iPhone based lap timer was. It was within 0.1 sec (and sometimes less) on ALL laps. I have to assume from this that the app, though only sampling position at 1 hz, extrapolates from that what point in time you crossed the start/finish line. It doesn't just wait until the first position fix after you cross the line and call it good, it figures out how far past the start finish you are at that fix and then uses your velocity to figure out what the time was when you went past the line. Clever.

I'm less skeptical about the lap time based on the timing device than I am about the fact that there was a passenger. At a short track like Laguna Seca the effect is less than at someplace like VIR, but I'd still bet it's a second/lap difference or so. Laguna Seca is a HP/Braking track, and both are very affected by weight in the car.


Quote:
Originally Posted by Robin_NL View Post
Then 1:38.88 sounds fine with me.



About the laptimer: Why should BMW and Porsche use that stuff then if it's far off? Please explain...


Cheers
Robin
Agreed, I've done track days using the tracks own lap timer modules together with using the M Laptimer app and the results are way less than a tenth out. For example my fastest lap on my local circuit was a 1:30.999 according to the circuits timer and 1:30.97 according to MLaptimer. In fact my AIM data logger using a higher GPS sampling rate was less accurate against the two above. I wouldn't doubt M lap figures.
Appreciate 1