How do I determine best instant center / anti squat value for my car?

[BuickMike]

So I did some measurements and was playing around with the IC calculator on Baseline Suspension's website.

Instant Center Calculator for drag car suspensions

I looked at what I have currently and it is bad:

Then I liiked at what would happen if I dropped the LCA's 2"

Then I looked at what would happen if I raise the UCA's 2"

Raising the UCA's brings the IC way back. Both ways get my anti-squat value close. So...how do these different IC values affect the handling and launch of a car that is more on the street than the track? Which would be better for me?

Thanks!

 

[Phatman]

sub.......

 

[Slow91z]

What car is this? Stock suspension?
How do you get the rear wt to be 523# more than the ft??

Think your backwards chuck

 

[BuickMike]

The front is heavier. It is a '87 GN with stock suspension as far as the rear control arms / geometry goes. I have SPC springs and Bilstein HD shocks all around. I have custom UB Machine front upper A arms and 1/2" longer ball joints in the upper and lower arms. I built the car with handling to be more of a priority than hooking, but I would like to get my IC rearward a bit if possible without causing major problems when cornering. Also, this car is only mid 11's capable and I will be foot braking my lanches so I don't think I am in danger of making it wheelie with my setup. Thouhgts?

 

[Slow91z]

No wheelie danger period...you get it back to where you are talking and it would be more likely to wheelie than where it is now...if you ever make that much power that is.

 

[BuickMike]

No wheelie danger period...you get it back to where you are talking and it would be more likely to wheelie than where it is now...if you ever make that much power that is.

I was talking about wheelie danger after I move the IC back. I'm thinking that even if I get the IC back to 49" in front of the rear axle I won't be able to get the wheels off the ground foot braking it at 8-10psi on my build. That being said, my concern is that I choose the method that least affects the handling of the car. I don't want to cause massive oversteer or have the front get light in turns. With the usupension the way it is I feel the rear end lift up from under the car when I stab the throttle. Since the car is lowered with stiff springs / shocks the front won't come up and I just skate around. I threw on some DR's and airbags as a band aid and it has made a huge difference, but I'd like to get that rear end to push down instead of pull up.

 

[charlief1]

Idealy you want the IC to be centered below and slightly behind the CG. You also don't raise the UCA point on the axle, you lower it on the frame. Your LCA's need to be parallel to the ground as well. It looks like the lowers are sitting at an angle that's pointing down from rear to front and the mount on the axle needs to be lowered by .4" to level them out.

 

[BuickMike]

Idealy you want the IC to be centered below and slightly behind the CG. You also don't raise the UCA point on the axle, you lower it on the frame. Your LCA's need to be parallel to the ground as well. It looks like the lowers are sitting at an angle that's pointing down from rear to front and the mount on the axle needs to be lowered by .4" to level them out.

Thanks Charlie. Lowering the front points of the UCA's works better. I think I am starting to figure this out better. After playing around, I was able to achieve:
AntiSquat value of 92.92%
Instant Center Lenth: 51.20"
Instant Center Height: 9.25"

This can be obtained by leveling the LCA's and dropping the front mount points of the UCA's 1 3/8". I think this is the best I can do without dropping the rear mount points of the LCA's below level. This should be a night and day difference from what I am at now.

 

[charlief1]

Post up another diagram so everyone can get a better idea of what you're talking about Mike. This can get confusing unless you do a lot of reading and understand how the IC and CG work in the car. If you want to see something interesting try locating the CG down about 1" and back .5". I think it will suprise you what happens then.:biggrin:

 

[BuickMike]

Here is the plot. Let me know your thoughts:

 

[BuickMike]

Now, on to figure out how to get this done. I like the Baseline Suspensions kit, but I'm really limited on funds right now and have other things I need to buy for the car. I will have to see if I can just make my own relocation brackets and continue to use my stock, boxed control arms. Relocating the lowers will be easy. I will just oval the mounting hole, set them where I want, and weld some washers in place. I don't think I have enough play on the front mounts to relocate that far, but maybe I could do it with offset bushings and some creativity.

 

[charlief1]

If you drop the front UCA down to 16" the anti squat goes to 116% so you're in the neighborhood. What you might want to do is a little reading on DonWG's suspension thread. He's been working wth them for a while and has some interesting data he's collected on using the relocation brackets. You can get them by themselves but then you have to figure out which uppers you can use with them. If you can figure the best location for them and get the IC centered right on the red line I think you'll be more than happy with the way it works.

 

[Alky V6]

It looks like you're on the right track. Move the rear of the lowers down to get the lowers level, then move the upper front mounting down to get the desired IC length and AS%. That's pretty much what you've already determined needs to be done.
I strongly suggest getting the buckets for the upper frame mounts from Kevin. They are very well made, easy to install, and you can easily remove them to play with the mounting hole locations to dial in your IC. You will need shorter upper arms, though. Kevin's are well made and have been working fine for me.
I know this sounds like a commercial for Kevin, but his system is simply the easiest solution for changing the upper frame mount location, and also allowing for easy changes to the IC, if found needed at sometime in the future.

Good luck with the IC hunting. I like your target.

 

[Alky V6]

There is some interesting dynamics with the IC that occurs that can be used to control some suspension aspects such as body roll at launch, but for your application I wouldn't go that extra hurdle. Set the static IC locations as symetrical, left to right, as you can get it.

 

[BuickMike]

Thanks for the info Don. I wish I had the extra money to spend on Kevin's setup. I played with the calculator using shorter UCA's (I am guessing his are around 1.5-1.75" shorder) and dropped the front 1.75". That got me almost identical to my last results. Maybe for now I will just put this project on the back burner and live with my airbags. Once I get the car looking pretty I can revisit this and just get his setup.

 

[Louie L.]

How did you measure the control arm length? Just a reminder they are to be measured at a 90 deg angle from the axle housing. Is this a Drag radial car ?