Quote:
Originally Posted by X25
GPS speed also goes a bit wrong when you're taking a corner (since you're no longer on a straight line)
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GPS data of course can be slightly off when not going at constant speed in a straight line, but...
Looking at your video, it's obvious that you simply have the GPS data not aligned with the video correctly. Look at the gear shift at 0:18 in the video. The GPS speed keeps going up for a third of a second or so, which is of course incorrect. The OBD speed channel though is well aligned with the video and the speed briefly stops going up as you shift.
The GPS-vs-OBD offset will become even more apparent if you plot those two speeds at the same time.
Good news is that RaceChrono provides an ability to easily offset any data source, you can find it in the devices/channels list in the "Analysis" mode. I suspect you need to delay your OBDLink MX+ data by ~350 ms.
Once you align the OBD data to the GPS data, try re-making that video and make sure to re-align the OBD+GPS data to the video.
I'm pretty sure the scale of this timing problem is orders of magnitude larger than the scale of the theoretical "GPS speed is incorrect when cornering" you mentioned.
Quote:
Originally Posted by X25
so you should use this data for speedometer anyway.
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There are many real-life examples where this statement is incorrect.
Try doing a burnout and you'll see 40 mph on your speedometer while the car is clearly standing still.
Drop the brake to start accelerating, and gently go back to moderate throttle — you'll see the speedometer data doing DOWN while the car is still accelerating.
If you're driving fast enough for tires to start micro-slipping, the speedometer will read low as you decelerate, and high while accelerating.
If you want to compare
how fast you're going between two laps, using the speedometer data can result in wrong conclusions.
GPS speed has none of those effects. It has other bad effects, e.g. when driving under a bridge. But as long as the bridge is not in the middle of the corner, and you don't have other GPS issues (e.g. bad mounting location), comparing GPS speeds between two laps should be apples-to-apples.
That being said, you should really log both, not just say that one is bad and ignore it. When one of them looks suspicious, look at the other one for sanity checking.