dnovotny

I've seen some posts comparing the C5 vs. C4 IRS. How does the C4 IRS compare to the IRS in the CTS, 3 or 5-series BMW, Mercedes or the new 350Z. Does the C4 IRS show its age with a 20-year old design, or is it still cutting edge with respect to other new rear drive cars?

onedef92

Well, the IRS on my 92's certainly a little more advanced than the one on my 1987 325 Bimmer. Still, there's no way I'd go back to a traditional panhard rod or solid axle like my old Acura used to have.

Vette9d1

I think it is still considered a very good suspension. First off there were no huge changes from 84-96 right? That right there tells you something- they pretty much got it right the first time. Also, how many times do you see in a magazine or a car show how to swap a C4 suspension into an old muscle car or antique corvette etc...quite a bit. I don't know much about the specifics of the suspension all i know is that it was designed pretty good the first time and people like to swap in into older high performance cars. Sure there are some better setups out there but there always will be something better in the future.

Darkness

It might be old but its been updated throughout the times, at least its not like the one in the cobras that brake the shafts like something fierce. It has wheel hoop like nothing else and weighs over 1000# for nothing.

jmr302

It's been said that the original Shelby Cobra's performed well despite their chassis. Interestingly enough the Shelby Series 1 cars that came out just a couple of years ago used C4 suspension.

I think most of the racers shuffle the spring rates around, especially on Z51 cars, to improve the front to rear balance of the car. They talk about this stuff all the time in the Roadracing forum.



[Modified by jmr302, 5:23 PM 8/13/2003]

tkrussell

The C4 rear IRS, especially when modified with the DRM parts (Revised Trailing Arm Brackets, Camber Brackets, and Steering Knuckle) and poly bushings will compare very favorably with most any other IRS.

The knock on the C4 is overall chassis rigidity. The C5 is said to be 300% or so stiffer. One wonders if the RD Camber Brace and X Brace, and a Roll Bar together would offer stiffness comparable to the C5. I've never seen a C4 with those mods tested for stiffness. John Greenwood opines that the C4 is weakest at the A Pillar (windshield) area.

dnovotny

It's been said that the original Shelby Cobra's performed well despite their chassis.

This can also be said for my 3rd/4th gen F-bodies (Trans Ams) which I drive. I've posted before that I plan on putting in a C4 IRS into my '01 T/A. However, saving for a house and additional chassis reinforcement have become more of a priority as of late. Nevertheless, C4 IRS is next on my list after I complete the chassis reinforcement of my '01.

The knock on the C4 is overall chassis rigidity. The C5 is said to be 300% or so stiffer.

Agreed. The 4th gen is somewhere in between a C4 and C5 when it comes to chassis rigidity. I installed some SFCs (subframe connectors) when my '01 was brand new, but I chose poorly and am removing and replacing with a beefier set. Also, I'm also going to run 1.2"x3.2"x6.5 foot SS square tubing running underneath my doors (and connected to the SFCs), similar to the hydroformed rail of a C5.

John Greenwood opines that the C4 is weakest at the A Pillar (windshield) area.

The 4th gen had its front suspension/chassis redesigned reasonably well, leaving the weakness along the floorpan and rear subframes torsionally. SFCs address the floorpan weakness (mostly up and down flex), unfortunately rear cross-braces are needed to fix the lack of torsional rigidity in the rear. This will also create a frame for the C4 IRS, so I'm installing several 2"x2"x5 feet cross-members in the rear (which eliminates the rear seat).

I'm getting really excited because I'm measuring and ordering the SS tubing as we speak, and I'm arranging a time to drop off the car for all this work to be done. So the first step towards installing a C4 IRS is finally happening. :)


One wonders if the RD Camber Brace and X Brace, and a Roll Bar together would offer stiffness comparable to the C5.

Try it out yourself to find out. :cool: I'm fortunate that another 4th gen owner tried out the cross-bracing I've mentioned above, which has totally changed the character of the car and made the chassis stiffness world-class. This has emboldened me to go even further.

jburnett

I'll have to beg to differ on one point above... I do not in any way believe that a 4th gen F-body is "between a C4 and C5 in stiffness." I've got more friends with F-bodies than any other car and they exhibit more chassis flex and cowl shake than ANY C4 I've ever been in. Maybe with subframe connectors they approach the stiffness of a C4 but the simple fact is an F-body is a monocoque chassis where the C4 is a full frame chassis and IS stiffer. The C5's are stiffer still... Wanna make your C4 as rigid as a C5? Two words...Roll cage. Mine is stiff as hell with the 6 pt cage!
-Jeb

dnovotny

I do not in any way believe that a 4th gen F-body is "between a C4 and C5 in stiffness."

Your belief that a C4 is more rigid is only psychological, I'm afraid. C4s measure approximately 5 Hz. 4th gen F-bodies are 14 Hz (t-tops). C5s are around 24 Hz for hardtop, 22 Hz for coupe. The facts beg to differ.

However, I will agree that if the car is abused and not re-inforced with SFCs, the chassis will degrade over time. But installing SFCs when the car is new (as I've done), doesn't let any chassis degradation creep in, because the SFCs create a full frame, attaching the front/rear subframes together.

jburnett

Where is this information...I wanna see it... Not trying to debunk it but in real world feel even my wife's 155k mile '85 exhibits much less deflection than my best friend's 1995 T/A (which is highly modded for auto-x) with 80k miles... Compared to my '90 (even before the cage) both are considerably less stiff with much more cowl shake and chassis flex evidenced...
-Jeb
PS- Is that "5 Hz" with the targa removed? If so I would probably concur...

dnovotny

The C4/C5 rigidity have been published in quite a few articles. The best source would '97/'98 reviews where they compare rigidity. I don't know if the targa was removed or not.

F4 rigidity: I looked long and hard for this info, I bought several books that compiled magazine articles which discuss '93-'02 F-bodies, and found one article about convertibles (can't remember the cars year, but it was an LS1). There they pointed out T-top cars are used as the basis of a convertible conversion, and the rigidity of those T-top cars is 14 Hz, to point out that a convertible's rigidity is south of that. You probably won't find any info on the web, I didn't.

As for real-world feel, I test drove some earlier 4th gens when they were new, the '01s I drove seemed more rigid. Just like with 3rd gens, the welds/assembly continually improved over the years. Also, I owned one '91 3rd gen since brand new, but also bought a '91 GTA used several years back. Even though the GTA was a hardtop, it still flexed more along the cowl and rear hatch than my T-top. With two SFCs on the car, the GTA is more rigid in the floorpan, but the cowl and rear hatch still flex more. How a person treats a unibody car and what type of roads its been driven on seem to have a significant effect.

Nathan Plemons

Another thing that has to be brought up here other than just rigidity is the handling aspect. Although an F-body might be "more rigid" that doesn't mean that it will handle the same as the C4 will. The C4 simply has a better suspension setup than the F-body, and thus handles better. This can be "felt" as a tighter car.

Although the car might actually flex more that doesn't mean it can't pull more G's on the skidpad. I have no doubt in my mind that an F-body is stiffer than my C4, however I also have no doubt that I can out handle them all day long.

Disclaimer: This assumes we are talking stock - stock. If you wanna say that an F-body with $xxxx worth of suspension upgrades will outhandle a stock vette, that's all well and good, but spend the same money on a vette and you still have a better handling car.

Just my $.02

I can't wait to see your results BTW, a 4th gen with a C4 IRS should be a very intresting setup indeed. :cheers:

Darkness

Lets not also forget the overall weight, and weight distribution. I know for a fact that the C4 has a F/R% 51/49, I highly doubt that the F-body can compare. Moreover, the F-body is a T-tops, and the C4 is targa that makes a difference when tops are off.

Later :cheers:

dnovotny

I know for a fact that the C4 has a F/R% 51/49, I highly doubt that the F-body can compare.

Weight distribution is 57/43. No doubt, I won't have as good weight distribution as a C4/C5. But it'll be interesting to see how the handling compares once I'm done.

dnovotny

Disclaimer: This assumes we are talking stock - stock. If you wanna say that an F-body with $xxxx worth of suspension upgrades will outhandle a stock vette, that's all well and good, but spend the same money on a vette and you still have a better handling car.


True, at least for the rear suspension. The problem with solid axle setups is they will never outperform an IRS on rough surfaces, but additionally you have to sacrifice comfort to really push the performance envelope of a solid axle. This is my main motivation for going to an IRS setup. As for the front, I really don't know enough to compare C4/C5 setups to F-bodies, but the 4th gen has an SLA setup which is at least modern unlike the ancient McPherson Strut setup on 3rd gens.