Goblin Center of Gravity

Desert Sasqwatch

Desert Sasqwatch

Goblin Guru
Lonny,

Is there a measured or calculated center of gravity for the Goblin - height above the bottom frame rail and fore and aft, left to right?

Based on some very unscientific positioning of the frame with the engine and transmission installed, and guesstimate for the rest of the parts installed (on jack stands) - looks like it might be at or near the top of the gas tank, midway back and slightly toward the passenger side? I'm trying to determine the chassis dynamics for setup and knowing the position of the center of gravity is an essential part.

Thanks! :)
 
Desert Sasqwatch

Desert Sasqwatch

Goblin Guru
The CG should be within a couple inches or so of a common point no matter which model you build. The standard frame will be about an inch further aft than the extended frame and adding the hood and other front end accessories will move it forward about an inch. The NA may be slightly lower and more toward the driver's side with no SC or TC weight on the passenger's side. Overall, getting a CG that is close is the intent. :D

And my speedholes are pretty evenly distributed front to rear and left to right! :p
 
Dale E

Dale E

Well-Known Member
For you mathematicians in the group -- maybe this will help??

I'm wild guessing about 13 inches off the ground and somewhere between the gas tank and the seat back. No guess as to side to side from center of tunnel.

After you figure the CG, then you get to figure the roll center (this point moves around during movement of car, you can set it statically) and pitch angles.

Have great fun!!
 
Last edited:
Desert Sasqwatch

Desert Sasqwatch

Goblin Guru
Holy cow, good information, way to fast talking and somewhat hard to understand - sorry just me. There are appears to be several assumptions made to get some of the measurements being called out to perform these calculations, especially for the height of CG. Jacking up the rear of the car until it wants to move forward is not safe - or did I mishear what was being said? I hope Lonny can reply to this with a helpful answer, because I'm not doing the jacking measurement. o_O
 
JBINTX

JBINTX

Goblin Guru
Tim you can interpolate with these numbers for two of the dimensions.
Just another data point for you.
I have the extended frame that is now standard.
of course, your’s will be way less. All my speed holes are still filled….. ;)

dfkitcar.com

Jason in Texas - ext. track - 09 SS/TC

Would love to hear that explanation to his insurance company, "I swear, it was Mario Andretti driving a race car on the street and he had Christie Brinkley with him in the passenger seat..." :p:cool:
dfkitcar.com dfkitcar.com
 
Desert Sasqwatch

Desert Sasqwatch

Goblin Guru
Jason, the fore and aft, left to right location is the easy measurement/calculation. The height is the tough one, since the weights of the engine, transmission, and all other bolt-ons and accessories not only have vertical weight, but a 'lateral' specific weight per inch of its height. Example: the engine sitting on a scale weighs 405 lbs, but the CG of the engine is typically several inches above the center line of the crankshaft. Bolt on the transmission and the CG for the whole drivetrain shifts with the added transmission weight.

Doing the design CG for an aircraft requires weighing each component, determining its location in reference to a 3 dimensional datum (wing main spar and thrust centerline nominally), and performing a calculation of how the CG shifts from the design datum. I'm not certain this is totally possible to do with the Goblin, but was hoping that the design software Lonny used to pull the Goblin together had this feature???
 
C

Chubbs

Well-Known Member
Here are my weight measurements with and without my 216 lbs sitting in the car. That's with nearly full fuel. Lsj donor, track frame, easy entry doors.

I should be able to get the vertical CG by tilting the car and remeasuring. As long as I can get an accurate measurement of the tilt. I'll experiment a bit, and see how accurate I can be.
 

Attachments

W

WarmPepsi

Active Member
Chubbs said:
Here are my weight measurements with and without my 216 lbs sitting in the car. That's with nearly full fuel. Lsj donor, track frame, easy entry doors.

I should be able to get the vertical CG by tilting the car and remeasuring. As long as I can get an accurate measurement of the tilt. I'll experiment a bit, and see how accurate I can be.
Click to expand...
Been meaning to do this with my scales as well.... Just encouraged me to do it.
 
C

Chubbs

Well-Known Member
You may have already found this

Center of Gravity Height

Finding the center of gravity height can be done in several ways, none of which are accomplished very easily and without some work. Presented here is the e
www.longacreracing.com www.longacreracing.com

They already did the math for us. if we can just get some measurements and weights, it should be easy to get a vertical CG. It's just been hard for me to find the time.
 
Desert Sasqwatch

Desert Sasqwatch

Goblin Guru
Chubbs, WarmPepsi, did either of you get that chance to do the weights and measurements for the CG height calculation on your Goblin? Don't want this thread to fall off the table. Thanks! :D
 
W

WarmPepsi

Active Member
Desert Sasqwatch said:
Chubbs, WarmPepsi, did either of you get that chance to do the weights and measurements for the CG height calculation on your Goblin? Don't want this thread to fall off the table. Thanks! :D
Click to expand...
:goes and hides in corner: No. Guilty as charged. I put in roughly 30' of french drain in my swamp of my soon to be backyard on Saturday, and did nothing on Sunday to recover. I'll bump it up my priority list.
 
W

WarmPepsi

Active Member
Desert Sasqwatch said:
Doing septic system or running water line?
Click to expand...
Running an “anti-water” line. Have springs in my area and one is a doozy. 75-80’ long trench and it’s flowing 2-3 gallon a minute; nonstop. Trying to slow it down/divert so I can finish main drain install in what will be a back yard. Have a man made 10 acre lake below me that takes the running water
 

Attachments

W

WarmPepsi

Active Member
I’ve actually got a decent idea on how to do this and I think I can make it slightly easier. I’m gonna level my trailer (tilt deck). Put the car on it flat (confirm level), get weights. Tilt car, get height difference, get weights.
 
C

Chubbs

Well-Known Member
I took an initial swing at this, but It unfortunately didn't work (I ended up with a CG of 75" above ground).

Issue #1, is getting the car 10" elevated. That's higher than I thought it was, and it's quite precarious having the thing teetering on a jack that high. I'll need to use the lift, but that requires shuffling cars around, and was more effort than I had in me tonight.

Issue #2 (the main source of my inaccuracy) is the car must be supported by the tire contact patch once elevated. I foolishly thought I could just jack it up from the rear subframe, but that changes the weight distribution of the car, and I've effectively changed the wheelbase.

I'll try again soon. Hopefully with better results.
 
You must log in or register to reply here.
Top