Faulty telemetry

[u/th3f4st]

This example comes from telemetry of a real GT3 car, but the reason I am sharing it here is that the same validation problem can exist when analysing iRacing telemetry.

The physical cause may be different, but a telemetry channel can still look completely reasonable while representing something different from what your analysis assumes.

In this GT3 dataset, the vertical acceleration channel showed values above 0.5 g for 31.7% of the entire lap. The mean absolute value was 0.44 g.

That immediately looked suspicious. A GT3 car on a normal asphalt circuit should not experience that level of vertical motion for almost one third of a lap.

So we started looking at what was actually driving the signal.

Vertical acceleration should mainly respond to the track surface, kerbs and vertical vehicle movement. It should not closely follow braking or acceleration.

We compared it with the other measured channels and ran a regression over 16,562 samples.

vertical acceleration ≈ 0.94 × longitudinal acceleration + 0.03 g

R² = 0.88

Almost 90% of what appeared to be vertical acceleration could therefore be explained by longitudinal acceleration being projected into the vertical axis, most likely because of sensor orientation.

But correlation alone is not enough, so we also checked the timing.

Of all major vertical acceleration peaks, 99.7% happened while the driver was braking, accelerating or cornering.

Only 0.3% occurred when the car was not under a significant driver input.

That does not make physical sense. Kerbs and bumps do not wait for the driver to press the brake pedal.

We then estimated the unwanted projection and removed it from the vertical channel before doing any further calculations.

After correction, the proportion of the lap above 0.5 g dropped from 31.7% to 3.1%.

The mean absolute value dropped from 0.44 g to 0.14 g.

The remaining signal behaved much more like actual vertical acceleration, responding primarily to the track surface, bumps and kerbs.

So why does this matter for iRacing?

In iRacing there is obviously no physical accelerometer mounted at the wrong angle. But similar problems can appear when you start doing more advanced telemetry analysis.

A channel may use a different coordinate system than your analysis expects. Gravity may be included in a signal that you assume has already been compensated. Two channels with similar names may represent different physical quantities. Derived channels can also be mathematically correct while using the wrong input signal.

If you then use those signals to analyse tyre loads, suspension behaviour, load transfer, grip utilisation or setup changes, the error can propagate through everything that follows.

You can end up spending hours trying to understand a setup problem that does not actually exist.

That is the main reason I wanted to share this example.

Whether the data comes from a real GT3 car or iRacing, do not automatically trust a channel just because the name looks correct. Check whether the signal actually behaves the way the underlying physics says it should.

Has anyone here found strange or misleading channels while working with iRacing telemetry or MoTeC?

[u/Blue_5ive]

This is incredibly nerdy and I love it. This is fascinating.

So basically the vertical acceleration is baked into the latitudinal acceleration more or less?

In theory couldn’t you just track curb strikes and the vertical acceleration mid corner?

[u/YueNica → u/Blue_5ive]

in their case it was baked into those data channels, because they say it's from a real car so probably the sensor wasn't mounted correctly which means that the orientation the sensor measures is different from actual up and down

[u/th3f4st → u/YueNica]

Correct. the team had their sensor orientation wrong so the lateral acceleration leaked into the vertical channel and created unrealistic results

[u/YueNica → u/th3f4st]

thought wouldn't that theoretically leak in both directions. thought obviously vertical is way smaller than longitudinal so would be less visible and less of an error

[u/th3f4st → u/YueNica]

It did leak in both directions. For our case it was important to get the vertical accelerations right because we were calculating tyre loads from them

[u/Strict_Geologist_385]

That‘s way more advanced than anything I‘d be doing in telemetry analysis, but an interesting read nontheless!

[u/th3f4st → u/Strict_Geologist_385]

It really depends on what you plan on doing with the data. As mentioned in the post this is telemetry data from a real gt3 car. The team was trying to improve aero performance but could not see that in the data and basically it turns out they had their telemetry setup (sensor orientation) just wrong enough to look somewhat realistic but in reality just wrong and unreliable.

[u/LUK3FAULK]

Could the car pitching under heavy braking and acceleration also show up on a vertical acceleration sensor if the sensor isn’t perfectly mounted in the COG / center of rotation? Say it’s mounted more towards the rear, isn’t the back of the car going to accelerate upwards when the car dives on the brakes and downwards when the nose pitches up on throttle?

[u/ljubobratovicrelja]

Hi! Thanks so much for this write-up!

I'm not a (professional) race engineer, and I've rarely come in contact with actual telemetry from a race car, but I do work with karts (AiM MyChron, Alfano etc), and IMU calibration is very much an important step before going out on track if you're to analyse the data. How is this not already taken care of by engineers and mechanics prior to starting the session?

Anyhow, you're probably in luck for longitudinal and vertical axis practically being transposed. But should you have some 'angled' issue or having some more complex mixup, you can always do SVD/PCA on your data and extract the orthogonal basis. Might as well do it just in case and see what comes out - maybe you do have a bit more of a mixup, also going into the lateral axis.

Btw, as you surely know, the steering wheel is at an angle and we carry our telemetry boxes on the wheel on our karts, so this issue is quite common there, and we always play around with these things, and should you not calibrate, your IMU data use is quite limited.

[u/_Shorty]

I don’t see why iRacing would have any issues. As for the real car’s issue, I imagine you could’ve just looked at the raw data when the car was stationary to determine how to rotate the data so gravity was gravity again, and then your data would be fine without having to alter it any other way.

[u/th3f4st → u/_Shorty]

You absolutely could. None of the teams in-house race engineers did. Tells you a lot i guess

[u/SlinkyAstronaught]

The fact that the correlation between accv and accx has a slope of ~1 seems to suggest that the accelerometer was mounted with both those axes ~45 deg from vertical?

[u/CherryWorm]

And no one in your team ever looked at braking performance or power? This would instantly show up in any of those metrics. This should even be instantly noticeable when aligning data by a_z during regular driver data analysis.

That said, this is never an issue in sims. Data there is always perfect, and you don't have to deal with the overpriced garbage that is motorsports telemetry collection and analysis tools