There is a thread on IRS vs Live axle on Camaroz28.com asking why the IRS Ccobras suck.
I thought they did well in AI but don't really remember.

Does anyone have info on the IRS Cobras vs solid axle?

http://web.camaross.com/forums/showthread....583#post4517583

Evan

I have some 3rd hand data to add to this.

At my previous employer I worked with a guy that wrote a book on chassis engineering and I asked him this exact question. He didn't do the testing first hand, but someone that he knew in the Trans Am series did what seemed to be a pretty fare test and found that the beam axle was consistantly 1-2 seconds faster on a road course. I think he said that the beam axle car was 100 lbs heavier (is a beam axle package heavier? If not it was the other way around), and I can't remember if they added weight to the IRS car to make it equal or not.

Of course I said "why?" He thought it was because of reduced wheel deflections (camber and toe) and the increased antisquat that you can get with a beam axle suspension allowed the car to put the power down sooner coming out of the corners. I didn't know this, but evidently you can't get as much antisquat with an independent.

There was a thread on CC a while back that someone was dropping hints that with some tweaking and testing the IRS was as fast as a solid axle in an AI car on a smooth track while the IRS was faster on a bumpy course.

You'll have to go digging for more info because I don't remember, but my guess would be that the Cobra IRS is too sloppy (rubber bushings/geometry) out of the box because it was designed for a street car to have good street manners.

Oh, and the IRS is heavier overall then the solid axle but it has lower unsprung weight, FWIW.

Hey Evan - good to see you here, too.

Sorry if my tone is a little off on cz28 - I tend to get, um, pissed at members if I browse too much there. So don't take anything personal by it.

Last I heard (and, admittedly, it was a few years ago), the IRS Mustangs were doing very poorly on track, and most were swapping out the IRS for a solid rear.

Yes, an IRS can have many advantages. It's not just packaging reasons that cause people to use them. But a properly sorted IRS is much, much harder to accomplish than a properly sorted solid rear, so for most people its not worth the effort. A C4 IRS (going by your icon both here and there) has severe limitations in toe change during travel, etc - issues that a solid rear will not have. Camber control is better in a solid axle (modulo the fact that getting some camber requires bending tubes) - the tire does not go through a camber curve through suspension compression, but stays put. There are movements involved from the panhard rod, but with the length of rods we have in these cars, that's a minimal effect.

Yes, the LS1 Camaro brake hops like crazy - but that's not so much a function of the stock torque arm as it is a cycling issue with the ABS. Yes, a decoupled torque arm works better - but here again, we get a feature that an IRS can not have - insane levels of anti-dive and anti-squat at the same time!

Now I'm not crazy. I'd much rather have an IRS myself, and work around all the inherent limitations it has to get the benefits that only it can give. But considering that major manufacturers have a hard time designing a suspension that works better than a simple solid rear (C4, Mustang, BMW E30, etc)...

According to Herb Adam's book Chassis Engineering a well set-up live axle configuration and a well set-up IRS setup will yield very similar results on the track.

I have been told (by an SVT engineer) that the Mustang IRS wasn't done properly and requires tweaking to improve it's performance.

I like my live axle setup.

Not to mention that quite a few of IRS's advantages get negated on a nice smooth racetrack, whereas all of its deficiencies are still there and possibly exaggerated by an ideal surface.

Ok, thanks, it was the other way then. I think they added weight to the beam axle car for the test.

I also forgot to mention that this test is probably 20 years old (IMG:http://www.frrax.com/rrforum/style_emoticons/default/rolleyes.gif)

I think the reputation for bad road course manners had to do with stock-bushing IRS assemblies allowing the wheels to toe out under braking, making the cars oversteer on corner entry? There was also wheel hop under acceleration (which Vettes and the CTS-V also have) and a reputation for breaking half shafts at the dragstrip. The earlier cars were more prone to that then the 03/04s, which have beefier axles. The broken axle thing is why a lot of drag racers did the swap back to an IRS. (There's lots of info on this on corner-carvers if you dig around.) There are ways to fix most of the handling issues, now, thanks to Maximum Motorsports.

Here's my experience: I have been autocrossing with a bunch of IRS Cobras for several years, now. I think, with the exception of live axle Cobra guy I know that can just flat-out drive, the IRS cars have been consistently faster than the stick-axle cars. I haven't seen any torque arm/panhard bar-equipped Cobras show up to compete against the IRS guys, however. So I'm generally comparing 4-link live-axle cars to IRS cars.

Having driven and or ridden in several IRS Cobras (with varying degrees of suspension mods) many times, now, I think the IRS is a big (no HUGE) improvement over the stick axle, especially the stock 4-link (quadra-bind) setup. The IRS is also a HUGE improvement over the stick axle cars in road manners.

I was really impressed with how an 03 I drove, recently, rode and handled. But I still think my Camaro is more comfortable to drive. I don't know why Ford put the shifter so far forward in the mustang, for example.

So how does an IRS Cobra stack up to an live axle F-body at the autocross? My experience has been that, when well-driven, our cars are more than capable of handling them... (IMG:http://www.frrax.com/rrforum/style_emoticons/default/smile.gif)

-John
(four mustangs before buying a Z28)

This post has been edited by Spooner: Apr 4 2007, 12:57 PM