IRS problems

[MidnightGT]

I have a 01 GT w/ Roush body and exhaust, and put in an 01 IRS. Also new 03 Cobra springs, Bilstien shocks, ABS sensors, new mufflers. On the initial test drive, I thought I was getting a little oversteer. After a few more miles, I noticed the IRS was shifting front to rear (slight play). I came back, put it back up on the rack. Could not find any play by hand. So I went to retighten ALL bolts and found none that were loose. All bushings were tight. Any thoughts? The IRS has only 5K miles on it.

[Lumpydogs]

How do you know the IRS is shifting front and back? The IRS sub-frame is most likely not flexing much. It is a very robust and rigid design.

Your IRS bushings could worn out. The stock rubber bushings allow a noticeable amount of movement.

Did you use the correct size bolt on the front IRS mounts? I believe Ford had an issue with an undersized bolt on some IRS models.

Could it be that your differential has worn bushings and is moving around?

Maybe the Control Arm bushings are shot?

Just a few ideas.

[MidnightGT]

Thanks for the reply Lumpydogs,

All bushings have been replaced with urathane ones. When I am in a 35MPH turn, interstate off-ramp, and turn harder into the turn, the rear of the car shifts like an oversteer, but with a diffinative jerk. Going in a straight line and say change lanes, it drifts back & forth. I used the stock bolts on the front mounts. They are tight and don't show any signs of movement.

[Lumpydogs]

That kinda sounds like a shock and spring issue. Your first post said Bilsteins so they should be good. I don't know how well the 03 cobra springs work on a GT with an 01 IRS. Seems like the front springs would be a little stiff, but the backs should be OK.

Sorry, but I don't know what to tell you.

[JROC]

IDK, thats strange. Are the 03's control arms the same as the 01's. I would assume so.

I'm no expert but here are some of my thoughts.

Maybe your springs rates and shock/strut valving is off for your cars weight. I have heard people with Terminators that went with an aluminum block say that the 80-100 lbs off the frontend messed up their handling and they had to get struts and spring to match their car weight and weight distribution. With that kind of logic you would think that adding a turbo kit or heavier blower would mess up the handling to though.

Bumpsteer kit - Do you have one? Aftermarket bushings can/most likely will cause some bumpsteer. http://www.maximummotorsports.com/

Better bushings - Even though your car shouldn't do that with any bushings that aren't worn, maybe stiffer ones will help. Heres what I have.
http://www.svtperformance.com/forums/showthread.php?t=260665

03/04 Cobra crossmember brace - Their better than the 99-01's. You may be able to get this cheaper from Ford as its a oem piece.
http://www.maximummotorsports.com/store/index.php?main_page=product_info&cPath=21_88_196&products_id=490

IRS brackets - I dout they would really help, but IDK.
http://www.billetflow.com/IRS_Brackets.htm

You probably shouldn't go spending to much on mods to try and fix this. They are just some of the things I thought up. I would defanently get a bumpsteer kit if you don't have one though.

[MidnightGT]

JROC,

Thanks for the info. I put it up on the rack today and realized that I had a bad left wheel bearing. I had checked everything with the installation and bushings, not even looking at the wheels. I shouldn't have taken it for granted that the bearings were in good condition with only 5K miles on the IRS.
I would like to thank everybody who replied to this. I will be seeing some of you guys at VIR. It was fun last year...

[brkntrxn]

Glad to hear you got it figured out. Don't forget to get a 4 wheel alignment.

[forestgreen95gt]

Thanks for the reply Lumpydogs,

All bushings have been replaced with urathane ones. When I am in a 35MPH turn, interstate off-ramp, and turn harder into the turn, the rear of the car shifts like an oversteer, but with a diffinative jerk. Going in a straight line and say change lanes, it drifts back & forth. I used the stock bolts on the front mounts. They are tight and don't show any signs of movement.I had the same issue myself with just about the same exact mods to my IRS as you have. One day I was fooling around with some polyurathane bushings I never used and noticed how easily they deform in a bench vice with little effort applied. So I eventually sourced some aluminum differential and subframe bushings. They are from Maenalon Autosport. I have their email from my order: maenalon@yahoo.com. Guess they are new since they told me that they were just getting a store on ebay setup. Anyways I just put these in and the aluminum obviously does not give like polyurathane so the rear of the car feels more connected to the road now. Plus I don't have to worry about breaking the diff housing or axles since the diff housing is rigidly mounted to the subframe. Oh and there instructions tell you how to correct the bolt size issue that you hear about with cobras. Now with these bushings and a torsen t2r the car hooks up really well out of tight low speed corners.

[MidnightGT]

Thanks for the info