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Discussion Starter · #1 ·
I recently purchase a car that has good hardware and plenty of motor, but the brakes and suspension need sorting badly. The car has FFR tubular A-arms and rear IRS. I didn't do the build so I may not be able to provide details to answer questions or take advantage of advice too quickly. I'd appreciate some ideas.

The car has a front anti-sway bar from Breeze. I saw some notes from the original owner in a binder on suspension setup, tow out etc. He was building to roadrace so at least he had that in mind. My impressions so far are a little more body roll than I'd like and a tendency to snap the rear end around with severe oversteer when I go from mild throttle to neutral or negative throttle in a corner.

I haven't had a chance to play around in a autocross format yet because I need a baseline thats somewhat reliable. Craig gave me some advice so far, but I don't even know if I am asking the right questions yet. Do I need to get a rear IRS antiroll bar on this thing before I even try to get it sorted? I do have many photos of the suspension/underside of the car if there are questions I can't answer verbally.

here are the handwritten notes I found...keep in mind this car needs to stay street driven more than autocrossed. I hope some of this makes sense to somebody!

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-Ground Clearance: Front - 4.5 Rear - 4.75
-Camber: 3/4 degree neg. front
4.5 castor
-Toe: 1/16 in total
-rear:1/2 degree neg camber

note: 1 turn upper control arm equal 1/4 degree camber - rear
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Thanks from a suspension dummy in advance!
 

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Discussion Starter · #2 ·
Anyone......Bueller? Bueller? Bueller?
 

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I would use those alignment settings with the car at the proper ride height (yours sounds a little high). Adjust the ride height to make the lower A-arms parallel with the ground.
I woudl buy a rear bar from Craig and the appropriate springs. Snap oversteer woudl be from a very stiff rear springs - since you have a front bar. I only know a good combo with 3-link. I dont know the motion ratio of FFR IRS. I run 450# ft / 250# rr, craigs bars - full stiff front and 1/2 stiff rear. Mine has very slight oversteer at the extreme limit.
My alignment specs are: 5 degrees castor (PS), 1.5 neg chamber, 1/16 toe in. My car is more track than street.
 

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I'm no expert, but curious.
Is your front sway bar set for fully stiff?
You might need stiffer front springs.
The way I understand it, adding a rear bar will only make your situation worse.
What's the rear gear?
 

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Mike,
I think the spring rates are messed up. I would definately run a rear bar with the IRS low roll center. The builder might have installed very stiff springs in the back to offset roll. Too stiff compared to the front spring rate and sway bar.
 

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Interesting thing in M3Hammerdown's first post: "1 turn on rear upper control arm = 1/4 degree camber [change]"

Thanks! Does anyone know of a similar conversion for rear toe change when adjusting the lower control arms? The "guess and check" method gets really old really fast when you've got to reassemble the suspension and drop the car onto its wheels just to see whether you're close.

I've also just got my car on the road and like M3 I'm more alarmed than thrilled by the handling. I'm not sure what to make of it. It doesn't track well, is difficult to keep in a straight line and seems to "over rotate" on turn in or even making lane changes. The front caster is matched (it doesn't pull to either side), I've got 1/16 toe in front and rear and 1 degree negative camber front and rear. At first I thought my rear toe was off and it needed more, but I've also read a lot of posts refering to "twitchy" cars-- what do the Challege Series guys do with their settings? How much of this can be applied to a street driven IRS car? Should I set the camber to zero?

M3: Trevor sound like he's onto something. There's an old saying about set up: "Soften the end that isn't working." Lift-throttle oversteer shows up on a lot of cars and soft front/ hard rear will make the weight transfer and oversteer more pronounded. I had a pretty bad turn-in understeer problem on my ITB BMW 2002 until I got stiffer rear springs. Maybe going the other way will help here.

Anyway, at least it isn't an early 911...

John Tennyson
 

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I don't have IRS, but my car tracks like an arrow.
Crisp, clean and rock steady.
I haven't had it on a track yet to really test the cornering, but it feels great on the street.
FWIW, I have a MK II w/ fox spindles, 15/1 manual rack, offset rack bushings set high, and Forte's tie rod ends mounted high.
I don't understand why so many have this instability problem. Is it the IRS?
I need to have someone with issues try my car and see how it feels.
 

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Chassis Engineering by Herb Adams is an excellent book and a must have if you have not messed with suspension set-up.

John,
Make sure you settled the suspension properly after making suspension changes and remeasuring (ask how I know). Power steering cars can run more castor, 5-6 degrees. You need to know the castor measurement.

Make sure your rear suspension is aligned true to the centerline of the frame.

I aligned my solid axle car in the garage. It is a 1 hand on the wheel car now (I demonstrated my handy work on Sat to the wife on the interstate). I have a manual 15:1 rack and it will definitely change lanes quickly, but I wouldn't call it twitchy.
 

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I have sway bars front and rear, the car was very twitchy untill I set the wheel alingment to toe out on the front and toe in on the rear. this has totally chaged the car it is now far more predictable to drive
Jeff
 

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When I first put my car on the road it wouldnt stay in a single lane (pre alignment). I figured out some specs and had the car professionally aligned. Now its solid as a rock and perfectly straight. I dont know how to setup a car with IRS. I am sure someone here knows what to do with the toe setting. Changing from 0 toe (front) and 1/16 toe-in made my car feel much more stable and straight. Adding chamber to 5 degrees after power steering made it feel the same as my Audi A4 (normal car).
 

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M3,

Do you know what springs are on the car? My thought would be to lose the sway bar, go back to the stock springs, and align per the book. Then if you want or need to adjust, do so from there.

-Scott
 
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