Originally Posted by lees02WS6

Things to consider with the C4 rear suspension:

The C4 uses four virtual swing arms created by drawing an imaginary line through each of the arms to their intersection points, starting from the bottom center of the wheel for the strut rods and half shafts.


(Courtesy of 1MeanZ)

As an engineer from Art Morrison pointed out, the operating range to keep the geometry on the C4 optimal is pretty tight. How you arrange those arms affects a lot, including anti-dive, anti-squat, camber, and toe just to name a few.

To extend his point, the C4 has a number of draw backs. Because it’s a trailing arm suspension those trailing arms force the wheels to travel in an arc from front to rear, as opposed to side to side with a solid axle located with a Panhard bar. The wheel base length changes slightly as the wheel travels through its motions. This is why trailing arm angle, length, and placement are so crucial! At the edges of this arc bind occurs with the strut rods which control camber. Add camber, and/or toe, and you can really exacerbate this issue, hence the reason poly really is a poor choice in this application and rod ends are optimal.

CATIA V5 DMU Kinematic IRS (C4 Corvette-5 Link) - YouTube

(Courtesy of billharbin)

Again, because of these things, the window to keep the camber curve optimal is smaller than say a double wishbone setup, like the C5 or C6. Many points on the C4 are rubber mounted, the diff carrier (bat wing), and all the control arms. Extreme cornering will distort the center points on each of these bushings.

In addition, because of its placement not in line with the half shafts, the toe control causes some toe change in compression and rebound.
So why use the C4 IRS at all? The handling isn’t terrible, it’s ok, and from a fitment standpoint it’s easy to customize for a variety of chassis’s. It’s reasonably inexpensive ($1,500 for a Dana 44, even cheaper for the Dana 36, $500 or less), and can be narrowed if necessary, and lots of stuff is available for it.

Really too, there isn’t much suspension travel with an F-body to begin with. At a common height for lowered F-bodies, there will be somewhere between 2.5” and 3”, more if the car is at stock height, of compression travel before the wheel touches the inner fender well. Limiting the suspension travel really mitigates the C4’s short comings. Also with proper geometry and a little sacrifice in ride quality, the handling can be quite impressive and far exceed the solid axle setups available for the F-body.

How do you get it on the car?

Securing the pinion: Torque arm, C-beam, custom pinion mount. There is somewhat of a space issue, and this presents a challenge to mounting a traditional torque arm. The C4 rear has the mount on the passenger side unlike the 10 bolt and replacement axles.



(Courtesy of Jay Cutshaw/Zeus Performance)





The standard C4 geometry may not be optimal for the f-body, and depending on your requirements may not be desirable anyway. The C4 is lighter, has a 53/47 ( vs the f-bodies 56/44) weight distribution ratio, and therefore different center of gravity than the Corvette. The vette’s wheel base is also 4.9” shorter than the f-body.




Stock Geometry




(Courtesy of 1Meanz)

Modified Geometry



(Courtesy of Jay Cutshaw/Zeus Performance)

Corvette also doesn’t have a back seat. Keeping the backseat and the C4 geometry is possible, but comes with some sacrifice.




(Courtesy of FlyDoc)

Do you want a sway bar?

If you keep the leaf spring, it will provide some anti-roll. Currently 99_orange_SS is using a VBandP race monospring with no sway bar which allows for great riding characteristics while still digging hard in the corners. Regardless, mounting the swaybar can be accomplished a couple of different ways. Stock corvette style, in front of the rear assembly, or a custom sway bar. Mounting the sway bar from the rear using the C4’s stock mounts will require removing more material from the frame and more fabrication.



(Courtesy of Jay Cutshaw/Zeus Performance)







Braking, and Traction Control:

The rear brake discs are a little thinner than the f-body’s, but that won’t make a noticeable difference. Some road racers say the rears are a little too helpful on the f-body to begin with, so some reduction in rear braking maybe helpful.

You could run the c5 rears on a c4 , but you will need the C5 calipers also. Doing this will eliminate the stock parking brake. You can, however, buy an aftermarket parking brake system, from Precision Brakes, it’s called a “Mr. Parker”. According to 99_orange_SS, he has the CTS-V 4 piston Brembo's up front with the stock rear C4 setup and he says it performs flawlessly; brake bias is perfect, the rears don't lock prematurely, and there is little nose dive.

ABS and Traction Control (ASR/TCS): Is it 3 channel or 4 channel? If it is a 4 channel it’s a lot easier to retain. The 95/96 corvettes came with the Bosch 5.0 system, and the 98+ f-body’s came with the Bosch 5.3 system.

The sensors from the 95/96 vettes will plug in to the existing wiring harness on the 98+ fourth gen four channel. The sensors will need to be salvaged from a junk yard. Most parts companies, for whatever reason, only keep the ’96 right-hand sensor in stock, if they stock them at all. The sensor pictured below is the correct type.


The prior versions of the sensors (BOSCH 2/2S) yellow connectors, those won't work.



Then there’s the exciter ring. The 98+ ABS/ASR(TCS) cars came with 47 tooth exciter rings, good news here is so did the 92+ Corvettes. If the car has a 53 tooth four channel ABS unit (97 and earlier) then new rings will need to be made. Costs will vary, but expect to pay around $300 to do so.

Things are murkier for the 3 channel cars. The best approach maybe to mount a smaller ring on the pinion if possible. This might require a different yoke depending the year of the C4 IRS that is being used.

Then there’s the option of just flat out deleting ABS altogether. SJM makes a kit for doing just that.

Track width, Wheels, and Tires:

The fourth gen rear is 64.72” flange to flange. The 90 to 96 vette rears are 63.25”, the earlier rears are 62.5” wide.

The weights:

10 bolt (without brakes and with gear oil): 146.4 lbs
Dana 44 C4 IRS (without brakes and without gear oil): 165.4 lbs

Overall weight increases by roughly 20 lbs, but unsprung weight falls by more than half.





Wheels:

A spacer or adapter may work if it doesn’t push wheels out past the fender. If that is not an option then the backspacing on the wheels could be changed. This option is trickier and more costly. If the wheels can’t be rehooped the only option is to order a new set.

If wide wheels (315 or wider) are being used special attention must be given to how much static negative camber is set, how much will be gained during compression, and the amount of suspension travel allowed. There isn’t a lot of clearance with the stock 10 bolt running 17x11’s, and with a boxed steel frames now occupying the spring pockets, for the C4 IRS, options for clearance are further limited.

The drive shaft needs to be dealt with as well, as it is too long. The stock shaft can be shortened, or a new one made. The rear U-joint will have to be replaced as well to allow it to bolt into the Dana 36 yoke.


Shocks and Springs:

It must be decided how the weight of the car will be sprung and what kind of shock will control those springs. Coilovers, or reuse the leaf? Coilovers will require additional bracing of the knuckle and the frame mounts. Neither was designed to carry the weight of the car; however there are kits out there to do this (Vansteel, Qa1, AFCO, DRM, Guldstrand). The lighter unsprung weight of the IRS will also change the required dampening. Depending on the new rates and setup, new antiroll rates will have to be established and a custom bar will need to be fabricated; unless you setup your system around the factory bar which will create its own set of issues.

Originally Posted by Heat Seeker WS6

I'm going with the IRS from Heidts. It's made specifically for the 4th gen and is a 9 inch setup.
Don't have it yet but but I took these pics during their open house a few months ago. Their Camaro is running 315/30/18s.

Originally Posted by smitty2919

Is the track width of the GTO/ATS correct for a 4th gen?

Originally Posted by sjsingle1

some people want what they want.............no matter how expensive ............or bad it is..........and you can get a irs in a camaro............its called a 5th gen !!!!!!

Originally Posted by Chrisingermany

I like that they did that video, I just hate the way they did it. Didnt really learn crap about the car in the little canyon drive. No shots of the suspension sucked as well. Was excited at first but was let down real quick. Thank you for sharing it though. Just wish they would have went into more details.

Originally Posted by dojob

As long as this IRS setup has been around, it has never been properly tested or reivewed on a track. That's what it needs. Either delcared snake oil or a worthwhile item.

Originally Posted by Chrisingermany

I agree. All done and said my MWC Fab 9" with a Watts link and driveshaft is very close to the price of the IRS setup (if the $5k +/- stated in the video is accurate). I road race and auto-x my car almost exclusively and if this IRS setup is everything that they claim it is, I would be really sad that I went the 9" route. Not that I regret the 9" but for what I do that may have been a better option.

EDIT: I just went online and pricing with the 3rd member and depending on what shafts you pick, it was well into the $8k price range (and that was without brakes). That was nowhere near what I was with the Fab 9" with a much better 3rd member....I'll stick with my Fab 9".