Finished car CG reference point for non-extended frame

JBINTX

Goblin Guru
I have extended. But, you should be able to get to the number you want.
1000 lbs rear, 600 front (for me).
97" wheel base
60.625" from front axle C/L should be CG for mine.
Subtract 2" for short frame.
Should get you close!
 

Brian74

Goblin Guru
I have extended. But, you should be able to get to the number you want.
1000 lbs rear, 600 front (for me).
97" wheel base
60.625" from front axle C/L should be CG for mine.
Subtract 2" for short frame.
Should get you close!
I’m trying to brainstorm any last minute design changes and figure out about where I’m at now, lol.
 

Classy

Well-Known Member
CG is hard to calculate off of pure front and rear weights. Assuming your goblin is fairly complete, you could put it on it’s wheels and put jack stands on both sides where you think center is, and see whether the front or rear wants to stay up and go from there. Whenever I would do this while adjusting for heavy lifts, I would get it where I would think it would be, start to pick up on the part, once it starts to lift push down on the side that lifts and see how easily it tips. That will help you find center, getting it perfect will be next to impossible, but you will get a good idea of where it is. Being a mid engine vehicle, 50/50 weight distribution is actually not desirable. Is that what you are after?
 

Brian74

Goblin Guru
CG is hard to calculate off of pure front and rear weights. Assuming your goblin is fairly complete, you could put it on it’s wheels and put jack stands on both sides where you think center is, and see whether the front or rear wants to stay up and go from there. Whenever I would do this while adjusting for heavy lifts, I would get it where I would think it would be, start to pick up on the part, once it starts to lift push down on the side that lifts and see how easily it tips. That will help you find center, getting it perfect will be next to impossible, but you will get a good idea of where it is. Being a mid engine vehicle, 50/50 weight distribution is actually not desirable. Is that what you are after?
Not really. I’m nowhere near complete. Just trying to make a few last minute design decisions.
 

Desert Sasqwatch

Goblin Guru
Sunday morning rantings:
The CG is a 3 dimensional point in space of the front-to-back, side-to-side and top-to-bottom balance of these weights, somewhere around the rear firewall between the seats above the center tunnel. The length and width balance points can be readily measured (4 corner weight), but the height of CG would be somewhat more difficult to get. You could use the aviation build method and weigh each individual component, then do the math to determine how it moves the CG from a datum reference point. (Too complicated to think about this early in the morning) Fuel tank level and driver/passenger occupancy/mass will shift the CG from its 'empty' position and will affect things like suspension calculations.

Keep in mind the relative CG point on the roll axis is different over the front suspension versus the rear suspension. The imaginary line that goes through the CG front-to-back in angled upward toward the rear of the car, since the drivetrain mass is greater and 'taller' in the chassis than most of the rest of the car. So the front suspension CG is shorter coupled to the roll center than the rear suspension, which feels the effects of the higher CG with a longer 'lever arm' acting to produce body roll. Keeping the rear body roll under control is important to setting up the suspension for the Goblin to handle effectively, but it is harder to achieve with the McPherson struts. Roll center changes are more difficult to make that can have an adverse affect on other suspension factors.

Suspension calculations are dependent upon the position of the CG for roll center, weight transfer, dive/squat and several others. With the Goblin being light weight and short wheelbase, it is easy to shift the CG with relatively minor changes in weight and location. If someone can calculate the 3d CG for an empty Goblin it could benefit those who race their car or are tweaking the suspension. Ok, I'll stop now, think I need a nap. o_O
 
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