Curing understeer after IRS swap on fox - Ford Mustang Forums : Mustang Forum
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post #1 of 6 Old 05-02-2019, 12:32 AM Thread Starter
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Curing understeer after IRS swap on fox

My setup consists of a Griggs k member with 350lb coil overs and koni yellows and factory sway bar. Camber is -2 and castor is +7.5 with 0 toe. The rear end is a IRS with redone bushings with Eibach springs and bilstein sports. Factory rear bar. Rear is setup with -2 camber and slight toe in. Lowered by 1.25 inches. The issue I have is strong understeer with some roll after installing the IRS. I was thinking maybe going with a larger rear bar or higher rate springs but curious what others recommend. Thanks

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post #2 of 6 Old 05-02-2019, 01:01 AM
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My car pushed the front bad with upfront being MM everything and unknown coil over spring rates on bistiens. Rear was cobra bilstiens, cut coils, poly control arm bushings, delrin sub bushings, MM toe arms, solid pinion bushings, poly diff bushing, stock bar with adjustable ends. The car had 245 Firestone Firehawk Indy 500 tires. Alignment specs I think were stock 99 cobra specs, and I don't think the alignment shop knew what they were doing. I had never autocrossed before, and when I did the car plowed. Sometimes pretty bad. It was kinda scary. Never experienced that in a car, on dry pavement. The whole cowl and dash would shake. Took a few times to figure out what all the noise was.

The following summer I upgraded to MM bilstiens and coil overs in the back with matched springs in the front, 325lbs for the front and 525 for the rear. 275 Nitto NT05's, and had the car aligned to MM's recommended specs and corner weighted. The car was much more neutral after that with no sense of under-steer.

1993 Mustang LX Coupe H/C/I 306/T5/IRS
1997 Lincoln Mark VIII
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post #3 of 6 Old 05-02-2019, 08:55 AM
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Sounds like you need more rear sway bar to get the car to rotate. You would need an adjustable aftermarket sway so you can go to a stiffer setting. Also, if you had an adjustable front sway i would say loosenen that up a bit at the same time...
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post #4 of 6 Old 05-02-2019, 10:43 AM
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Id try a smaller bar up front, the stock bar is HUGE and is designed for factory soft springs, 1. you went coilovers, 2. your spring rates are much higher than stock.
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post #5 of 6 Old 05-02-2019, 07:29 PM
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The rear wheel rate in the car are absurdly soft compared to the front wheel rate. You need to install rear coilovers with at least 600lbs/in springs.
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post #6 of 6 Old 05-24-2019, 12:37 AM
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Might help to pull the S spring out of the diff also.

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