Mustang Cobra IRS

[CraigS]

I am posting this here as this seems to be the best forum to talk about this subject based on several people who have upgraded the FFR design.
Did any of you look at the Mustang Cobra IRS? Maybe too much work but it has wide based UCA and LCA and an easily adjustable toe link. Pics here.
https://www.google.com/search?q=2003+mustang+cobra+rear+suspension&rls=com.microsoft:en-US:IE-Address&tbm=isch&imgil=BteO3yLxtkQdvM%253A%253Bhttps%253A%252F%252F encrypted-tbn2.gstatic.com%252Fimages%253Fq%253Dtbn%253AANd9 GcSzKvc8HT8jBX28gS-4WNyxFxousDWYMao0YrXWdfmBOIk5o8wO%253B1536%253B102 4%253BAFXFIE5sX7fVrM%253Bhttp%25253A%25252F%25252F www.kennybrown.com%25252Fpress%25252Fkenny-brown-advanced-geometry-independent-rear-suspension-1999-2004-svt-cobra-and-2000-cobra-r&source=iu&usg=__07dfpF7Mzha3e2uvkGnn_mWSmgU%3D&sa=X&ei=9osTU4qRCMXp0AG5moFI&ved=0CEsQ9QEwBA&biw=1536&bih=719#facrc=_&imgdii=_&imgrc=AdV9t_mki0YPgM%253A%3BWUrZEWg-BFHtNM%3Bhttp%253A%252F%252Fwww.svtperformance.com %252Farticles%252Firs%252FIRS_final_6x9.jpg%3Bhttp %253A%252F%252Fwww.svtperformance.com%252Fforums%2 52Ffront-page-news-143%252F610897-mystery-cobra-independent-rear-suspension.html%3B600%3B400
I was thinking that using this spindle might be a way to get toe adjustment and then fairly easy to design and build UCA and LCAs. I ask because I keep dreaming of doing an IRS upgrade to my car but the FFR design has enough drawbacks to keep me from investing the time and $. Of course the $ may kill the idea either way but I would appreciate your thoughts. The track is listed as 60.2 inches but flange to flange width may be greater than that. OTOH, I guess that could be worked around as the Thunderbird has been.

[loeffler1]

As we all are aware anything can be done if a person has the skills. I'm sure there would be a lot of welding involved. However, on the other hand the expense might be formidable even if you were lucky enough to find a boneyard Mustang Cobra. I dont know if you could buy the complete assy from the factory but I'll bet you'ld pay a pretty penny for it. I wonder why would you go to all this trouble when the FFR IRS seems to be pretty decent. I don't have one but a friend does and he doesn't seem to have any problems with it. It seems as you might be trying to reinvent the wheel here as this would seem to be a pretty complicated task. What drawbacks are you aware of with the FFR IRS? I was thinking in the future I might upgrade to IRS, and even with the FFR IRS it would be a monumental task with an allready built coupe. Has anyone done this upgrade with a finished coupe?

[Garry Bopp]

Craig,

You need to decide what you wish to do with the coupe. Will it be an "all out racecar" or just street usage with occasional track day weekends. If you want to build a racecar, yes, the FFR IRS has drawbacks which require modifications. The race coupes that are being built right now tend to use the 3 link live axle setup. My coupe is primarily used on the street with occasional track day weekends and the IRS works quite well. I do have "wheel hop" if I try a dragrace type start but since I don't drag race the car, it is not an issue. I did have the Forte adjusters built into my upper and lower control arms, which make alignment very easy.

Garry

[Jacob McCrea]

Craig,

If I recall correctly Hank and I considered grafting the Cobra IRS upper control arm, or something like it, into the system, but ultimately settled on the design(s) shown on my long-dormant build thread as the latter was easier to add into the existing system. I don't think we ever considered making the existing system work with the Cobra IRS knuckle, or installing the whole Cobra IRS system.

As for installing the complete Cobra IRS unit, what stands out to me is that there is a great deal of componentry (some of the lower control arm, some of the "cage" structure, an anti-sway bar?) that sits in front of the drive shaft flange. So, making new mounts for the front of the Cobra IRS cage looks like it would take massive modifications to the Roadster's 4" tubes, the cross member that the differential bolts to, probably the 2" x 3" vertical tubing, and probably the seat area as well. And once those issues are resolved, one would have to turn his or her attention to making sure that the rear IRS cage mounts married up to the frame and didn't interfere with the gas tank.

I have no idea whether the Cobra IRS knuckle, in and of itself, could be cost-effectively grafted into the system (along with custom control arms), although at first glance it looks to be very difficult.

I have never had my car on the road, much less raced it, so take this for what it's worth: I think you could resolve the weaknesses with the existing FFR IRS system many times over for what it would take, in both time and money, to graft in either the Cobra IRS system as a whole unit, or its spindles. I think that the Driveshaft Shop sells axles that are rated to 1,000 horsepower, if axles are the concern. If heat in the differential is a concern, a Tilton oil pump and cooler should fix that. Also, I think I read that the Cobra IRS still has problems with the axle nuts loosening. Safety wire should solve that. The add-on to the upper control arm that Hank and I did is far from simple, and I found locating the new pivot point to be especially difficult. Still, it is simpler than adding in the whole Cobra IRS system, or making all-new upper and lower control arms.

[Hankl]

What Jake said!

Hank :cool:

[CraigS]

Jacob and Hankl, thanks for responding. I was looking for you guys who have done the IRS upgrades to give your thoughts. Looks like the Cobra IRS wouldn't be worth the trouble.

[tirod]

There is also what the factory intended in the design. They were working with an supersized McMustang that needed to have very driver friendly - ie liability reducing - handling. The average buyer was not track licensed and as youtube has conveniently documented, can get in over their head in a heartbeat.

It was designed with the intent to keep a 40% loaded rear end with 300 to 400hp from swapping ends. Transplanting it directly into a car 1,000 pounds lighter with 50/50 distribution is no guarantee it would be remotely appropriate. It's been a long term discussion donor brakes are less than optimal in that same swap.

From my first cursory readings on the subject, it seems the better thing to do is get heim joints into the system and eliminate as many rubber bushings as possible. That way the spindles/hub carriers don't start taking alternate settings under stress. That's not cheap as it is, and looking at the Cobra IRS, I see a lot of rubber. Detroit likes using it because they deform and will introduce more understeer as the car goes faster. Power oversteer is also a concern (the 427's were notoriously uncontrollable.)

Likely much more could be done, cheaper, just sorting out the existing F5 suspension. Spring rates, shock valving, adding a set of anti roll bars, etc are pretty much unexplored territory for the average kit. It's what Ford would have spent months on getting right, down to moving a suspension pickup point 1/2" to optimize safe handling from their lawyers viewpoint.

Those prototypes also tend to be used in collision testing. Keeps them from having to recall 93 loaners to send to the crushers. : (