center of gravity and instant center

[72opelguy]

i need to know what the cg height is on a gt
i need it to determine the suspensions instant center and how i need to change it to improve traction
i was thinking of using a gen 3 camaro rearend so i can use stock suspension but i also want to mak it work the best it can

[GTJIM]

Put "Instant Center" into your world wide web search engine - lots of info comes up. Then add "torque tube" and see if anything like the GT rear suspension turns up. Try "Instant Centre" to as that is the other spelling of "center" sometimes used in "other" parts of the world!

[72opelguy]

i know how to find the instant center and where it is but i need to know where the center of gravity is on the car so i can find the antisquat line i can explain all this info if anyone is interested

[Travis]

Getting the corner weights of your car weighed will tell you where the CG is located front to rear but measuring it's height is going to a bit more involved

Also, as you move the trailing arms around you need to keep an eye on the roll steer characteristics and watch out for introduction of rear wheel hop under braking. You'll likely need to experiment to find a happy medium between the three. My suggestion would be to make the suspension adjustable and tune based on real world performance....

-Travis

[72opelguy]

i plan on making trailing arms with hiem joints and at least three different positions on the axle to adjust everything
and i need to find this info because i am setting it up for drag racing mostly
and i have to set it up properly so i have ligt under acceleration to get traction
and i am going to figur that the cg height on the car is about the height of the top of the trans tunnel it close enough

[GTJIM]

Some where in the trans tunnel area about halfway along the bell housing and may be just inside the bell housing looks about right to me. Equal amount of weight fore and aft plus up and down.

BTW in the old days of drag racing I remember 'Dandy' Dick Landy and the Chrysler 'ramcharger' boys theorised that for drag racing the car should evenly rise up on the suspension so that the "equal and opposite" forces pushed down on the tyres as the body rose thus providing more weight on the rear wheels and more traction.
So if a car "squats" as it launches it is actually pulling its wheels up off the tarmac. I remember seeing some awsome footage of his cars launching and the whole body rising as they did.

[baz]

jims raising body is the way to go and easy to do
1 use air blocks in the rear coil springs to stiffen them up (i think the passenger side needs to be stiffest for torque effect on axle will look it up) and adjustable shocks set full hard
2 use adjustable front shocks and set to full soft when on track so the front will lift as quick as poss
this will give good hook up and the slight ofset on rear will help keep line straght

[72opelguy]

your talking about a whole nother side of gettign traction weight distribution thats why you use 90/10 shocks up front so the front jumps up when you launch to get traction
GTJIM
your right about what the ramchargers did to get traction by raising up in the rear but thats where the instant center for the rear suspension comes in the anitsquatline is a line that runs from the bottom center of the rear tires to the intersection of the front spindle and the cg height
the instant center for the suspension will lay above or below this line
if it is below like most stock suspensions the rear will squat on a launch if it is above it will lift the car
and if its dead inline then i believe thats best for autocross

[GTJIM]

I have searched my reference stuff on GTs' - even the book in German! Plenty of stuff about aerodynamics but not a word on centre of gravity.

My suggestion is weigh the front and rear ends of your car and the individual wheel weights ( a bored cop checking truck overloading can be useful here :-)) remember to sit in the car yourself as you are part of the total weight.

Now do the math and plot it out on a schematic of the GT wheels (ie a rectangle on paper) this will give you the position of the CG in the horizontal plane.

Just use the top of the bell housing at the position above for your vertical position.

Now you have a "working" position for the CG

Do the instant centre thingy with this rough position and see how it looks and use your judgement to do an initial setup.

See what happens in the real world and adjust from there.

Tell us all the results - then we won't have to do it too

[72opelguy]

well the next thign i was going to find was the suspesion angles and then i saw a picture of a gt that Travis drew a rollcage in and from what i see the stock suspesion really really sucks
the control arms are going in completely wrong direction
they down down rear to front rather than up like a camaro so i guess i am going to just rebuild the whoel rear suspension to make it adjustable in the correct direction
but i still ned the other info so i could find the antisquat line to make the most of the suspension

[Sporqster]

I was planning on modeling the suspension for my GT in AutoCAD, for lack or more advanced software - have you got anything already put together?

[oldopelguy]

One thing to consider here is that the Opel uses a torque arm, effectivly making it's launch characteristics more like those of a ladder bar set-up. The instant center is effectivly the mounting point for the forward end of the torque tube. (That spot is also the forward end of the rear axle's roll axis, with the rear determined by the angle on the panhard rod.)

This means the percentage rise for the car is going to be pretty high, and you're probably looking at an antisquat in the 140% range. Lowering the car will lower the torque tube mounting point (IC) and the center of gravity as well, but the effect will be greater on the IC to effectivly reduce the antisquat of the car. This will negativly affect the rear trailing arms and panhard rod, however, and they will need to be lower off the axle to compensate and keep them more level.

When all is said and done, there really isn't a lot available for tuning the Opel rear suspension without some serious fabrication. Good luck.

[davegt27]

my performance analyzer book says measure from the ground to the center of the crank bolt then add 5" (i think that's what it said its been awhile)

anyway i could dig the book back up if you like

TQ arm TQ tube are the same thing or i should say work the same


my notes on my GT for my calculations i used 18" for cg ht but i have a note saying its closer to 12"

never made the actual measurement (the Automath book tell how)

my corner weights before the roll cage
LF = 598 RF = 519
LR = 485 RR = 449

so if my math is right Cg 43.58 behind front wheels and 1.40" from center

my trailing arms have been moved to below the stock location

with 5.28 gears my short time was 2.2 SEC and never a problem with wheel hoop but the car had no power anyway

HTH
davegt27

[72opelguy]

well this is just like a torque arm setup in a camaro and the way you find the instant cent on those is draw line along the trailing arms and the torque arm and where the lines meet thats your point but on the gt its more liek a ladder bar because the trailing arms go up front to rear which is opposite of a camaro so that suspension really rules that out so if i werent going to just rebuild it so it was like a camaro the best thing i could do to make it raise up would be to shorten the torque tube and move the mount back so the angle is greater and makes it lift more

[jackcartwright]

Does anyone have the corner weights of a stock GT? I would like to compare to my turbo GT. Thanks.

[RallyBob]

Does anyone have the corner weights of a stock GT? I would like to compare to my turbo GT. Thanks.

A few years ago when I went to VIR to crew for a race team I met with a forum member ('Soybean') and he had his stock GT scaled at the tech shed. You might PM him and ask him the numbers. IIRC the car was decently balanced diagonally and side to side. I can't recall the front/rear bias however.

Or someone could stop by my shop with a stock GT and I could throw it on my scales, just for the record.

Bob

[baz]

dont think soybean is active in the forum any more , been a couple of years since he posted