Racerender data overlay different from Trackaddict

[Gary T]

Anyone notices that the speed data overlay in Racerender is different from what you see in the Trackaddict app? I have transferred my Trackaddict video and data to Racerender for processing but for whatever reason Racerender show speeds that are much higher than what I see in Trackaddict for the same lap. Any idea why?

[HoboBob]

I've sometimes noticed a difference between my speed calculated by GPS and the speed retrieved from my OBDII reader (i.e. how fast my car thinks it's going). Can you post your data file here (or in a dropbox/Google Drive link) for us to look at?

[aUser]

There can be many things at play here:

- the OBD-supplied data is likely obtained via a request/response protocol (unless it's a passive CAN-bus read like is possible with a RaceCapture unit) so that means there's a time-delay and thus a timing difference between getting the data and where the car is ... at speed, tens of milliseconds are meaningful and can skew the data being presented

- what data you're getting via the OBD port can vary greatly -- if it's directly from the wheel-speed sensors (depending upon how it's calculated/averaged/etc.) it can be close to the actual (and normally shown by a high-speed GPS) speed, but if it's from the data that's fed to the speedo display, it'll almost always be greater than the actual speed by 2-10%, often depending upon the speed (higher percentage is added at lower speeds)

- the GPS-derived speed can also be skewed by timing, depending upon how the data is obtained -- if it's obtained from an external GPS unit (e.g., XGPS160) then there's a protocol/acquisition timing/skew introduced which may or may not be significant

- to accurately compute speed under heavy acceleration/deceleration and especially when rapidly changing from one to the other (e.g., aggressive braking at end of high-speed straight) the higher the GPS update rate, the more accurate the reading ... and this assumes that you have a high quality GPS signal

- finally, there's the actual calculation of the speed from the GPS data -- how this is calculated/filtered/smoothed/etc. also affects the data and, in the case of smoothing, can also introduce some timing skew

(and there are likely to be other factors not listed above ... as the saying goes "it's complicated")

IMO, one of the great features of RaceRender is that it has good tools to allow you to synchronize different streams of data to help account for timing-based data skew.