lgusto

Anyone have thoughts on how the measurement of G forces has changed over the years? Or the differences between Porsche, BMW and others? Have the measurement devices become more accurate?

The question stems from a deep dive on AiM data. I'm seeing significantly lower Gsum values in an older BMW that has similar lap times of newer Porsches. For example, with a 2:17 lap time at The Glen the BMW has a max Gsum of 1.7 versus a newish GT4 with a max Gsum of 2.3.

I realize speed, tires, driver, etc. all enter the equation but could the difference be attributable to instrumentation?

Thanks.

ProCoach

The BMW measure seems right, but I don’t believe that for the Porsche. Perhaps hitting a curb and a spike, but no way can that be sustained. There has been manipulation in the calculation of gSum as the filtering and measures have changed over software and hardware changes over the the years.

Matt Romanowski

I'd want to look at the data and see where the cars made their time.

The calculation hasn't changed or been manipulated. The biggest difference is in the means of acquisition - GPS or accelerometer- and the increase in GPS logging rates from 10 to 25 Hz (in AiM). The slower rates will
look smoothed and can miss the high points. Accelerometers often have noisy signals that onclude other vibrations that make the actual numbers look larger.

lgusto

Thank you both for the answers. It is now clear that I should be looking for the highest sustained Gsum over a certain period of time rather than just the maximum value in a segment. And then add a cautionary note when comparing different makes of cars or acquisition devices.

It is very cool to be able to compare all sorts of performance metrics for multiple drivers/cars/tires from the same sessions at an event using the AiM data.

lowside67

Not only do you definitely want to look at sustained, but if you are going to compare cars across different tracks, then you need to try to isolate corners that don't have a lot of geometry to them - banking/camber, curbs, etc.

In general, my opinion is that people significantly overestimate their car's G capabilities based on a few cherry picked peak G values, mostly forming from bragging rights. I suspect very few sedans can even sustain 1.7G and I'd venture that none short of an FIA GT3 car or better will get to 2. It will take not only significant functional aero and slick tires, but also good suspension and CoG characteristics.

My sports racer sees in the range of 2.5-2.6G sustained - we definitely can find some peak values that touch 2.8+ for a split second, but I am sure my car will not reliably deliver that on a theoretical perfectly smooth circuit with 0 banking.

Cheers,
Mark

ProCoach

Agree 100%

FIFY, but all very good points.

Unfortunately, vertical acceleration is often added into the end result. Which, to your point, distorts the ultimate measure. That's where a lot of the "cherry-picking" comes from!

I've been amazed at some of the negative Long G measures that I've seen in a lot of quicker GT cars and even top of the line street GT cars.

lgusto

I've only started with cars where I've attended the same event, so it's same track, same weather. Fortunately Sebring, Watkins Glen and Club Motorsports have given me plenty to work on. Too bad I couldn't have found better tracks, right?

Matt Romanowski

I've never heard or seen anyone using vertical acceleration in G Sum. No book or resource suggest that either. Where have you seen someone using all three axis?

lgusto

Based on feedback do these figures look more reasonable? I'm using the maximum Gsum values that were sustained for a minimum of one second. The best duration for measurement might be longer or shorter but I started with one second. Also, this looks at all laps over the course of a single DE event.

Thanks