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Lateral and Longitudinal G Confusion

Data Analysis Fundamentals

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I'm using an AEM Vehicle Dynamics Module with an AEM CD Dash. I'm new to data analysis and discovered something odd in my logs. The longitudinal and lateral G's are set by AEM in the VDM and can't be changed, but it seems that they are switched, as in longitudinal is reading as lateral and vise versa. I verified that the module was mounted correctly in the car and then contacted AEM customer support. I sent them a log file to review, and they said it was reading correctly, but it just doesn't seem correct to me. I attached an image to show what I'm describing.

At the end of the main straight, I'm off the throttle, braking hard in a straight line, and it shows my lateral G's as 0.95 and my longitudinal G's as 0.07. AEM said it was normal to see lateral G's while braking, but braking in a straight line, would they be that high and the longitudinal G's be almost nothing in a braking zone?

Attached Files

Doesn't make sense to me, either.

I could see a 'lateral' acc'n if the vehicle had a brake and/or chassis imbalance that caused a 'skew', in a straight line, but I'd expect it to be negligible.

I've only logged 2 tracks, but both show the same thing, high lateral G's, low longitudinal G's in the braking zones, then higher longitudinal G's through the turns. It makes me think the dbc file may have them mislabeled, but AEM says they are correct. Does anyone use AEM Data? I could send a log file to get a second opinion.

Hi Dennis, it looks to me like you're right, they seem switched to me. Where is the G sensor physically fitted? Is it in the dash or an external unit? If you're sure you have it oriented correctly then it seems like there's a mistake somewhere. Either in how the logger has been setup and configured, the pinouts on the accelerometer, or in the software somewhere.

The longitudinal accel should drop sharply as the speed trace does before the lateral accel builds up as the driver turns in. I've pasted an example below of a car approaching a low corner from a fast straight.

Also remember that the normal convention is negative long accel = braking and positive = drive

Let us know what you find!


Hi Tim, thanks for the reply. The Vehicle Dynamics Module is a separate unit logged by the dash. It's mounted to the tunnel floor in the center of the vehicle. Unfortunately, it is a plug and play unit that isn't configurable. It connects to the dash through AEM's CAN and they provide the dbc file, so it would have been configured that way by them.

I think I can correct it through the dash software by creating new channels using their channels as the primary inputs and labeling them correctly that way.

Is the normal convention for right turns a positive or negative force? In the manual for the module they configure it with right hand turns and braking being positive forces. I can correct the braking by applying math to the new channel?

Thanks for the help!

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No prob, Dennis. It's what we're here for 🙌

If it's pre-configured and you've mounted it with the correct orientation, then making use of math channels to make some corrected channels is the way to go as you say.

The sign conventions you use are up to you, but the most common in my experience for G force are: -ve for braking and turning right, +ve for accelerating and turning left. I've also attached a sign conventions cheat sheet with is actually part of our Professional Motorsport Data Analysis course, but I think it might be useful for you in this case.

Attached Files

Personally, as I would know what the current orientation is, and what I want it to do, I'd probably just reposition it to suite my requirements, as there is a high probability this will be an option.

Also a good option, Gord!

Thanks, Gordon. I will keep that as an option, but I'm more confused than ever now. I just got off the phone with one of AEM's engineers. I explained that I didn't understand how my speed could drop from 112.5 to 54.7 and not register any longitudinal G's. He said the movement of longitudinal was front to back, but the force is side to side?

That was one of the stupidist things I've read on these forums, or anywhere outside the FE or geo-centric earth forums...

Either you ask for someone there who has at least THE most basic grasp of physics - Newton's Three Laws of Motion are usually the introduction - or disregard anthing they say about it - he/she is an idiot!

Copy that. Thanks, Gordon.

If that's what they told you Dennis, sounds like a bit of re-training might be in order at that particular company 😳

So bizarre because they are an excellent company. I really appreciate you guys. I'm learning so much from your courses on all three platforms. My only regret is not pursuing my motorsports interest earlier in life. #oldman, thanks again!

Our pleasure, glad you're getting a lot out of it Dennis!

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