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Old 08-07-2008, 08:39 AM   #1
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Default How do you find the car's center of gravity in the Z axis?

I know how to find it in the X (length) and Y (width) axis,
but how do you find the car's center of gravity in the Z (height) axis?
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Old 08-07-2008, 08:51 AM   #2
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This might help you out. http://www.teamassociated.com/racerh..._handling.html
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Old 08-07-2008, 09:02 AM   #3
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I'm going to guess that it would be similar technique as you would in the X-Y direction. This is what I'm guessing and how I would do it (please correct me if I'm wrong):

1) Stick a car in a box (this is so the faces can be drawn on)

2) Attach two sticks along two of the edges that will go from the nose of the car inside to the tail of the vehicle (front to rear of the vehicle). If the car is placed in a box, you should have four edges to choose from, but you only need two

3) Hold the assembly at the tips of one stick, minimizing friction if possible... Draw a straight line down from that corner to the floor/ground. Repeat with the second stick

4) Proceed to draw your intersections at the two faces of the box. The two faces would be if the where the ends of the sticks are coming towards you, or looking at the car head-on or tail-on...

Your intersection would reveal the X,Z coordinate of the center of gravity, just strip away the "X" and use the Z-component and you should get your COG in Z-axis... I'm assuming your X-axis is the axis from the left side of the car to right side looking at the car bird's-eye-view, with the nose of the vehicle pointing upwards...
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Old 08-07-2008, 09:12 AM   #4
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Here's a crude drawing of what I mean

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Old 08-07-2008, 05:40 PM   #5
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I'm going to try this method described here with the suspension removed:
http://www.oople.com/forums/showthread.php?t=12996
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Old 08-07-2008, 06:36 PM   #6
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I actually have the book that the associated site suggests at home.. (I R nerd and I bought it while I was still in undergraduate engineering studies) I'll go home and see if that gives any valuable information.
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Old 08-07-2008, 11:08 PM   #7
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Here ya go, this is for full scale cars but you could adapt it with some mathematics for your 1/10 scale car. I am pretty sure you will need to remove the shocks though and replace it with something solid.

http://www.longacreracing.com/articles/art.asp?ARTID=22
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Old 08-08-2008, 12:57 AM   #8
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mattnin,
That's awesome! I can't believe it's that easy. Dang.
Thanks!
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Old 08-08-2008, 08:00 AM   #9
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Well now I feel like a real dumba** for reading this post.
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Old 08-08-2008, 12:13 PM   #10
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The site that squeaks links is the roll center. They draw a line from the top sus member (your turnbuckle) and another from the bottom sus member (your a-arm) in the direction inside of the wheel and see where it intersects. This is called the Instant Center. The roll center is then measured by drawing a line from the center of the contact patch of each tire. Where both of the lines intersect (the lines from the contact patch of each tire) determine roll center. Looking at my RC10B4, the roll center will be very close to the ground.

Now for Vertical location of the CG. The method that Miliken and Miliken describes is to lift the rear of the car with the front wheels prevented from rotating.

You need to know the following:

-(theta) the angle the rear wheels are lifted from horizontal.

-(Wtotal) total weight of the vehicle

-(Wf) Weight as measured at the front wheels when the vehicle is lifted off the ground. (just place the front wheels on a scale and lift the rear of the car off the ground with your hands)

- (b) horizontal distance from the rear wheels to the vehicle CG. (the pitch axis)

- (l) wheelbase length

Now, the height from the center of the wheels to the CG (h1) would be:

h1=(Wf*l-W*b)/(W*tan(theta))

If the wheel centers are the same distance off the ground from front to rear (which you can safely assume for any buggy or truck unless you have a drag racer or something)

HCG= Radius of wheel + h1

With this method, i think something simple can be rigged. We aren't going for pinpoint accuracy here, but just something so that we know where the general vertical location of the CG is.

It was interesting reading the book and finding this info. Makes me feel like a grad student again.
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Old 08-08-2008, 03:27 PM   #11
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Yeah, feels like university again...but notice, it's all high school trig!
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Old 08-08-2008, 05:15 PM   #12
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If you have postal scales, that would make this job easier too. If you are going to get serious about doing this, you could make your own solid links pretty easy too using some turnbuckles and threaded rod from a hardware store. It would be pretty easy to do with a hacksaw. Just make it like the rock crawlers do when they make their own custom links.
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