60ft questions
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Centre of Gravity
I'm trying to learn how to set my stock type suspension up for launch. How does one go about determining the centre of gravitiy on our cars ?
It seems to me that tire size, front/back weight bias, ride height etc. must come into play somehow. Doesn't figuring out the Percentage Rise going to depend on knowing where the CG is ?
Is there a pat formula ??
I'm a real novice at this IC stuff. Any help here would be appreciated.
George
I'm trying to learn how to set my stock type suspension up for launch. How does one go about determining the centre of gravitiy on our cars ?
It seems to me that tire size, front/back weight bias, ride height etc. must come into play somehow. Doesn't figuring out the Percentage Rise going to depend on knowing where the CG is ?
Is there a pat formula ??
I'm a real novice at this IC stuff. Any help here would be appreciated.
George
You will need to measure the weight across the front wheels, rear wheels and overall weight. A truck scale is an easy way to do this if you don't have individual wheel scales.
Lengthwise CG location:
CG location behind front wheels = rear wheel weight divided by overall weight, then muliplied by wheelbase.
Figuring the height of the CG is a little more complicated. Stay at the truck scale and lift one end of the car, if doesn't matter which end, with the other end on the scale. Lift the car 24 inches, if possible. Get the weight on the tires still on the ground. If you want the outcome to be as accurate as possible, air up the tires that will be on the scale to minimize tire deflection when the added weight is put on the tires. Also, the fuel tank will come into play. You should do all the measurements with a full tank and then all again with an empty tank. This will give you 2 different CG results that you can take an average from.
One other thing that will need to be addressed for the most accurate CG height result will be to disable the suspension for the end that will be lifted. Actually, both ends. This can be done by replacing the shocks with solid links with their lengths set with the car resting on the ground.
Lengthwise CG location:
CG location behind front wheels = rear wheel weight divided by overall weight, then muliplied by wheelbase.
Figuring the height of the CG is a little more complicated. Stay at the truck scale and lift one end of the car, if doesn't matter which end, with the other end on the scale. Lift the car 24 inches, if possible. Get the weight on the tires still on the ground. If you want the outcome to be as accurate as possible, air up the tires that will be on the scale to minimize tire deflection when the added weight is put on the tires. Also, the fuel tank will come into play. You should do all the measurements with a full tank and then all again with an empty tank. This will give you 2 different CG results that you can take an average from.
One other thing that will need to be addressed for the most accurate CG height result will be to disable the suspension for the end that will be lifted. Actually, both ends. This can be done by replacing the shocks with solid links with their lengths set with the car resting on the ground.
After you have the weights, you will need 3 measurements.
1)Wheelbase of the car level on the ground.
2)Wheelbase at ground level with the one end raised the amount used when you got the weight measurement.
3)The measurement of the distance the one end of the car was raised.
1)Wheelbase of the car level on the ground.
2)Wheelbase at ground level with the one end raised the amount used when you got the weight measurement.
3)The measurement of the distance the one end of the car was raised.
The formula for finding the CG height:
cg height = level wheelbase multiplied by raised wheelbase multiplied by added weight on scales,... then divided by distance raised multiplied by overall weight.
Easy, right?
If anyone actually ends up doing this, please let me know what you find.
cg height = level wheelbase multiplied by raised wheelbase multiplied by added weight on scales,... then divided by distance raised multiplied by overall weight.
Easy, right?
If anyone actually ends up doing this, please let me know what you find.
Holy crap ... I wonder who does all this .... I know .... The winners and record setters !!
Thanks Don
I found this sight too.
Longacre Racing Online -- Tech Article "Center of Gravity Height"
Anybody know where it would be "approximately" on a 3400 lb. regal. Height and along the wheelbase ? I'd be interested in knowing if I was remotely close.
I have access to a set of Longacre scales so I can play with it in my shop.
George
Thanks Don
I found this sight too.
Longacre Racing Online -- Tech Article "Center of Gravity Height"
Anybody know where it would be "approximately" on a 3400 lb. regal. Height and along the wheelbase ? I'd be interested in knowing if I was remotely close.
I have access to a set of Longacre scales so I can play with it in my shop.
George
johnplogii
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I have heard that the shifter handle would be the approximate location of the center of gravity in a stock car.
John
John
Here's some interesting information on the IC for a stock GN.
Instant center length is -110.3". That's 110.3" behind the rear axle.
Instant center height is -4.8". That's 4.8" below ground level.
Anti-squat is 21.4%. Any percentage below 100% is considered to be squatting (rear axle sucking up into the body under load). A percentage over 100% is opposite of squatting, anti-squat (rear axle separating from the body under load). 100% is neither.
Instant center length is -110.3". That's 110.3" behind the rear axle.
Instant center height is -4.8". That's 4.8" below ground level.
Anti-squat is 21.4%. Any percentage below 100% is considered to be squatting (rear axle sucking up into the body under load). A percentage over 100% is opposite of squatting, anti-squat (rear axle separating from the body under load). 100% is neither.
great info
ok..i measured my pinion,tailshaft on the trans, and the driveshaft. i measured the driveshaft before i took it out. the car is level on jackstands with the jack stands on the rearend and on the control arms in the front. then measured the tailshaft and pinion with an angle finder. here are the results..DS 1 1/2 degree up which is positive, pinion is 3 degrees up (positive), and the tailshaft is 3 degrees down (negative) i am guessing i am at 0 degree. if i bring the pinion down 2 degrees i will be at -2 degrees right? i have poly bushings in the control arms...............thanks Bob
ok..i measured my pinion,tailshaft on the trans, and the driveshaft. i measured the driveshaft before i took it out. the car is level on jackstands with the jack stands on the rearend and on the control arms in the front. then measured the tailshaft and pinion with an angle finder. here are the results..DS 1 1/2 degree up which is positive, pinion is 3 degrees up (positive), and the tailshaft is 3 degrees down (negative) i am guessing i am at 0 degree. if i bring the pinion down 2 degrees i will be at -2 degrees right? i have poly bushings in the control arms...............thanks Bob
I'm not sure how much angle difference you can get by raising the tailhousing before it hits the tunnel, but yes, it would also raise the center of gravity and help weight transfer. Again, I don't know by how much it would help.
Pinion angle can be much more easily changed with adjustable uppers.
Someone told me that the aftermarket traction bars on the market for the G body raises the upper/rear control arm mounting 3". If that number is different, please let me know.
Here is the IC numbers for a stock GN with the above mentioned traction bar installed.
Instant center length is 37.7" ahead of the rear axle.
Instant center height is 12.7" above ground level.
Anti-squat is 165.6%.
For a tartget of 11.0 ETs, this new IC is smack dab in chassis expert Dave Morgan's suggestion box for the IC.
Here is the IC numbers for a stock GN with the above mentioned traction bar installed.
Instant center length is 37.7" ahead of the rear axle.
Instant center height is 12.7" above ground level.
Anti-squat is 165.6%.
For a tartget of 11.0 ETs, this new IC is smack dab in chassis expert Dave Morgan's suggestion box for the IC.
As long as the jackstands in the rear are centered on the axle housing tubes, he should be OK.
I agree the front should be supported on the tires.
Don... I've been picking up what you are laying down until this......
Forgive me for questioning......
As I understand it... the instant center is the imaginary line where the upper and lower control arms intersect. This height and forward distance.... in my measurement is out in front of the car..... not behind the rear end......
Please explain.
I'll find my old thread where I measured my car.... and plotted it on Cad....]
I found it....
My Old IC Thread
In my case, I found:
Autocad says intersecting point is 10'-8 15/16" ahead of rear axle centerline which falls about 22" in front of front axle centerline
This point was 14 7/16" off the ground.
When you did the measurements on your car, was the car lowered below the stock riding height? If lowered enough, that would put the IC in front of the car. Way in front. Like around 550".
No... stock height.... I crawled my fat @zz under there and used a magnetic angle finder... and a tape measure.... nothing super accurate... but should suffice OK I would think....
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