# Thread: CoG of human body

1. ## CoG of human body

Hello FSAE world

My name is Stefanos Stamoulis, i am part of TEIWM Racing Team from Western Macedonia, Greece. I would like to ask, if anyone in FS has calculated the center of gravity of the human body/driver and if it is necessary to do it.
The guys from the University of Western Macedonia which participated in the MotoStudent told us that is absolutely necessary

2. Hi StefStam!

I am sure that some FS team has measured their driver's CoG.
Easy enough with a fulcrum and a beam for the driver to lay on.

But what do you think?
You should have some idea for the driver's COG.
But how accurate do you think you need to know their CoG?
+/- 0.5mm? +/- 50mmm?
Do you think you could model it in CAD to make an estimation?

-William

3. Originally Posted by StefStam
Hello FSAE world

My name is Stefanos Stamoulis, i am part of TEIWM Racing Team from Western Macedonia, Greece. I would like to ask, if anyone in FS has calculated the center of gravity of the human body/driver and if it is necessary to do it.
The guys from the University of Western Macedonia which participated in the MotoStudent told us that is absolutely necessary

Find the three heaviest components on your car. These will dominate your mass distribution in X,Y,Z.

Usually in this order:
1) Driver
2) Engine
3) Chassis tubing/panels

When you consider the "95% Average Human Male" weighs about 100 kg and an average SAE car weighs about 185kg without driver -- yes, it is absolutely necessary.
Even the 50% Average Human Male weighs 78kg...

Reference this document for some guidance.

I would also suggest to find the CG of your drivers in their seated position either in the car or by seating them on scales in X,Y and Z. You can tilt test your drivers too.
We found that our CG in X varied up to about 2.5% of F/R depending on who was seated in the car, our lay down style seating contributes to this significantly.

4. Absolutely necessary and fairly straightforward. You'll need to put the driver in a seated position resting across two scales to get the x location, then rotate the whole assembly around y while the driver holds still to change the axis and allow you to get the z location. At A&M we tried to test y-location and found values that were clearly smaller than measurement uncertainty, concluding that our mass is fairly well centered laterally.

If you'll accept my sounding like Claude here, 100 kg is clearly excessive for the mass of the driver and he or she would have to be spectacularly good behind the wheel to make up for it. Even Travis Renegar, who is larger than Percy, got down to 83kg to race. Try Ultimate Frisbee or running and eat more sweet potatoes.

5. What I learned from working with sport doctors and engineers working on racing crash test simulation is that in the usual driver position ("bath tub"), the human body CG is not far away from the spine 2nd lumbar.

6. I ended up buying a very cheap student license for Catia v5 to make use of their "Human Builder" for solving this problem. The basic driver model we were using in Solidworks worked fairly well for packaging and light ergonomics, but the mass properties were a shot in the dark. I seem to remember just manipulating the density of the driver model until it matched the driver mass we anticipated... a pretty terrible way to represent the variable density of a real human.

I ended up exporting the posed solidworks driver into catia, and then "built" a human until the Catia Manikin was aligned with the old driver. I then copied the CoG and Moments of Inertia from the Catia human (which does account for variable density inside a human) back into the solidworks model and locked the properties. Moving the Manikin into the right position using Catia's simulation of biological joints was very interesting. Driver ergonomics seem much less abstract after that exercise.

Anyways, this revision ended up moving the predicted cg so much that I had to translate the sprung mass something like 30 mm to maintain the target longitudinal weight distribution. That was quite a horrifying "adjustment" to be making in the later stages of design.

This screencap I took during the process might give you some idea of where the CoM is for a 95th percentile driver (without a helmet on) in a FSAE cockpit orientation. This was my first experience with Catia though, so the chance that I've done something wrong is high.

7. No need to obsess about it.
Make a choice of the driver body X,Y,Z coordinates that seems logical for your simulation input.
Then when the car is finished measure the car CG with and without driver.
Most of the time what you measure is at the end more reliable than what you simulate.

8. There's a joke in there somewhere about how much cojones your driver has.

9. And light brain.....