Rear control arm geometry question?

[pdqwagon]

But if the instant center is too far back, wouldn't the rear of the car try to jump off the ground at launch, throwing everything out of whack and probably resulting in a lot of wheelspin?

That's Antisquat.
I hope I get this right as I'm mostly going from memory, but here goes [-o<

Depending on the car, you can increase traction by increasing or decreasing antisquat. Too much AS hits the tires really hard, but only on lauch. Then as the suspension sets, the ires may spin. Not enough AS makes the rear droop on launch & unloads the suspension, also causing tire spin right off.

When you raise the front of the top you're increasing the IC, but not correctly. You're moving the IC up above the Center of Gravity, too. This increases Percentage of Rise.You have to move all 4 mounts, top & bottom to increase IC without changing the PR. more PR "hits" the tires harder, less PR hits softer.

I'm sure there's stuff to read & diagrams to help you visualize on the internet, but I don't have time to search here at work.

Tommy

This is what I am asking about. The plan is to move the upper and lower arms. As the car sits now the front of the upper arms are pointed down to a point the tires move forward instead of up if you were to hit a bump. The lower arms are actually pointed down in the front. This is why I wanted to raise the front of the upper arms and lower the rear of the lower arms to get the angles back close to where they would be if the car was not so low.

 

[tiedyemike8]

Ok, then it sounds like you should start by making the lower arms parallel to the ground, and measure the change in angle (or should you measure the distance moved?). Then move the uppers the same amount of degrees (or distance?). That should bring all the geometry close to factory, if not better, for the new ride height. I don't know if the lowers are parallel with the ground out of the factory, but it is agreed that parallel is the best place for them.

 

[LS6 Tommy]

Ok, then it sounds like you should start by making the lower arms parallel to the ground, and measure the change in angle (or should you measure the distance moved?). Then move the uppers the same amount of degrees (or distance?). That should bring all the geometry close to factory, if not better, for the new ride height. I don't know if the lowers are parallel with the ground out of the factory, but it is agreed that parallel is the best place for them.

Having the lower links parallel to the ground moves the IC out in front of the CG, which will cause excessive front end lift. I said earlier there has to be good reading on the subject. It was right here on our Sponsors links (Duh!):

http://www.baselinesuspensions.com/info/Launching_A_Drag_Car.htm

Tommy

 

[tiedyemike8]

Ok, then it sounds like you should start by making the lower arms parallel to the ground, and measure the change in angle (or should you measure the distance moved?). Then move the uppers the same amount of degrees (or distance?). That should bring all the geometry close to factory, if not better, for the new ride height. I don't know if the lowers are parallel with the ground out of the factory, but it is agreed that parallel is the best place for them.

Having the lower links parallel to the ground moves the IC out in front of the CG, which will cause excessive front end lift. I said earlier there has to be good reading on the subject. It was right here on our Sponsors links (Duh!):

http://www.baselinesuspensions.com/info/Launching_A_Drag_Car.htm

Tommy

Good read, thanks Tommy.

 

[bbc-olds]

Here is a pic of my Cutlass last year at Pink all out Seattle. Used adjustable arms set at stock length and lowered with coilovers until the lower arms were almost level. Upper arms used the stock mounting points and were at a rather steep angle. Car hooked good with 60ft times in the 1.40's. In my case the pinion angle turned out to be 2 degrees. Unless you need to raise the car for clearance I would leave it as is. Just my 2 cents :lol: