IRS / Solid Axle Ride Quality Comparison with Poly Bushings

[Stock Motor]

Hey all, let me preface by saying I have a fox body I would like to install an 03-04 Cobra IRS into for the purposes of improving the car's compliance over broken pavement. The car is a 100% street car, so I'm not concerned with lap times, or 1/4 mile. The car makes 600whp and is backed by an auto trans and I roll around on 275/40/17 ET Street R's. Right now I run stock fox upper control arms, MM lowers with poly bushings on the chassis side and the articulating bushing on the rear end side, Bilstein HD's, and H&R sport springs. When I do the IRS I will go fully poly (to cure wheel hop) and I'll run stock 03-04 Cobra spring rates on Bilstein HD coilovers.

Does anyone have any before/ after feedback regarding the IRS? Obviously a bone stock 99-04 Cobra will ride waaaay better than a lowered SRA fox. But how does a stock spring/ poly equipped IRS car ride? Even if the poly harshens things up a bit, my thinking is there is NO way it could be harsher than a solid axle car.

 

[01yellercobra]

My closest comparison is my wife's old 06 to my Cobra. Her car had Koni shocks, Eibach springs, and BMR LCA's. My car has the full FTBR bushing kit and I felt my car rode way better. Both with the H&R springs and the current coilovers. There's a small groove in the off ramp by our house and her car would skip while mine will just soak it up.

 

[Blkkbgt]

Hey all, let me preface by saying I have a fox body I would like to install an 03-04 Cobra IRS into for the purposes of improving the car's compliance over broken pavement. The car is a 100% street car, so I'm not concerned with lap times, or 1/4 mile. The car makes 600whp and is backed by an auto trans and I roll around on 275/40/17 ET Street R's. Right now I run stock fox upper control arms, MM lowers with poly bushings on the chassis side and the articulating bushing on the rear end side, Bilstein HD's, and H&R sport springs. When I do the IRS I will go fully poly (to cure wheel hop) and I'll run stock 03-04 Cobra spring rates on Bilstein HD coilovers.

Does anyone have any before/ after feedback regarding the IRS? Obviously a bone stock 99-04 Cobra will ride waaaay better than a lowered SRA fox. But how does a stock spring/ poly equipped IRS car ride? Even if the poly harshens things up a bit, my thinking is there is NO way it could be harsher than a solid axle car.

I am one of those that swapped the stick axle in my gt for an 03-04 IRS. My stick axle had stock upper and lower control arms an MM PHB, Tokico illumina shocks and matching springs. In this configuration I hated the ride and the PHB caused a lot of additional NVH over the smallest of bumps. The handling was great on smooth pavement but god forbid the road needed work. I always felt like the rear end was back their doing its own thing as it caused most of my ride harshness.

I ran that setup for about 2 years and decided to swap in an IRS and you'd have to put a gun to my head to get me to swap back. Now I will admit that I never ran the IRS on the stock bushings as I added a complete FTBR bushing kit before the swap. I did this after talking to others who had installed the kit and came to the conclusion that the NVH would probably be less then what I had been dealing with on the old setup. I also decided against poly after my experiences with it on the front end of the car. No matter how much grease I added to the bushings they would eventually start creaking when it was cold and it drove me insane.

Even with the FTBR kit, H&R Red Race springs and Bilstein HD's the car is far smoother and enjoyable to drive. Cruising down the highway at 80 mph is actually enjoyable. I feel like the rear end works with the front and rough pavement is not really an issue. The NVH is without a doubt lower then the old setup and like I said I'll never go back. If something happened to my car and I decided to build another new edge I would not do it without and IRS in the car. The only issue I had was around 60 mph I would get a slight hum from the rear end, this is due to the diff being solidly mounted with the FTBR kit. I pretty much cured that with a little dyna mat over that area. The most noise I get from the rear of the car is actually my flowmasters droning at certain RPM's

If you decide to go through with the swap I think you'd be making a massive mistake by going with poly bushings. They will not allow the control arms to articulate properly and whether or not they cure wheel hop is a toss up. Some claim they do and others claim they don't. I side with Bruce and feel a lot of the wheel hop issues are caused by stiff sidewalls. Lastly The FTBR kit comes with grease fittings so you don't have to disassemble the rear end to grease everything, can't do that with poly once it begins to make noise.