Primarily, the sub-frame that the whole IRS is attached to.

On a live axle car, the solid axle (which as you point out weighs about the same as the IRS) serves as it's own rigid structure.

On IRS (save the one like on the GTO, which depends on relatively heavier and larger arms to support the suspension than those on a live axle) you are going to need a "cage" to hold the IRS system, and bolt the whole unit (IRS & the structure holding it) to the chassis.

As for the chassis, the area where it's going to be attached is going to have to be reinforced, since istaead of keeping the live axle located, it's going to have to firmly secure a heavier sub-structure.

Mustang Cobra & GT are perfect examples of what to expect since it's a unibody car. Any IRS system in a Uni is going to be modular, an assembled mini structure containing the IRS. This is what Sigma, LX, Mercedes, and most other unibody cars use. The GTO and the former version 3 series used the lighter system from paragraph before last.

Corvette is not a good example of a lightweight IRS. First, it's a full frame structure, second GM went to extremes to trim weight from the car, and third GM has a higher profitable pricepoint on the car.

The argument would then move to "Well, you'll sell more Camaros than Corvettes to make up for the pricepoint difference." What fails here is that it costs more to make the IRS than a live axle. More labor intensive, more parts, more bushings. Any doubts, sit a live axle assembly from a Mustang GT next to one from a Cobra. In short, though engineering costs will be spread around better, it's going to cost more per car to make than a live axle due to labor and parts. This is offset by more use of steel.... which equals weight.


From there, the next example of a lightweight inexpensive IRS that's going to be brought up is the Kappa chassis. However, this isn't a good example either. The Kappa is as long as a Miata, yet weighs 3000 pounds. It would have been alot heavier if GM hadn't eliminated all the power options, added in the V8 and all the components that could withstand the horsepower & torque the engine would make. Also remember, a 4 cylinder Pontiac Solstice is only about 150 pounds lighter than the 2 feet longer, far more electric motor and electronics laden, V8 powered, heavy duty transmissioned, heavier duty drivetrain, Corvette.


No one has yet come up with an explanation (plauseable or otherwise) as to WHY an automaker with armies of engineers, computers that would put the ones at the pentagon to shame, CAFE and more stringent safety standards brething down their backs, would purposely create a heavy IRS system if it could make a light weight one without costing itself any additional money.


Summary;
1. If you want IRS, it's going to cost weight or money.
2. GM, Ford, and Chrysler aren't BMW or Mercedes.
3. GM, Ford, and Chrysler all have smarter people working in engineering than any of us here.
4. These cars need to be engineered to sell at prices people expect.
5. You simply are not going to get a $50,000 BMW 5 series suspension on a $20,000 Chevrolet.
6. Finally, that we are getting more difficult and sophisticated IRS on US made RWD cars, but then to think that engineers at Chrysler AND Ford AND General Motors are too dumb or are going to ignore some low cost light weight way of manufacturing them is really, really, really wrong.

Chrysler LX weigh 4100 lbs. Cadillac's STS weigh 4300 lbs. The VE is going to weigh about 4000 pounds. All are about the same size.

GTO and Mustang Cobra both weighed about 3750 pounds. Both were the same size.

If we get a 6.0L V8 Camaro the same size as the Mustang, with IRS, priced in the mid $20Ks range, and weighing even a pound under 3800#, we should line up to buy the engineers steak dinners because they would have done a HELL of a job.

I suppose this is about as a in depth an answer possible without drawing pictures or a field trip to a local junkyard.

 

You keep using the SN95 Mustang Cobra/Mustang GT to show the affects of "live axle vs. IRS", but like Charlie pointed out the SN95 had to be retrofitted with IRS...it certainly stands to reason that any pony car-type structure with IRS baked in from the start would have inherent advantages in terms of weight and cost over something that has to be rigged up to an older chassis....then again I ain't no injuneer.

...Which is interesting, wouldn't you say? Because the Miata features....brace yourself...a multilink rear suspension. Comparable in size to Kappa...available with similar options, priced similarly and weighs considerably less....Why is that? Please tell me how little ol' Mazda pulled that off? I'm not saying GM didn't do a nice job with Kappa, but I don't understand your point at all. In fact, you've just muddled things.

 

True, an IRS sub-frame adds weight. Of course, you don't have to have one, but it is generally preferred for NVH reasons.

But this sub-frame isn't dead weight - like, for example, that heavy cast iron live axle housing which IRS does NOT have BTW - it also adds structure and rigidity.

The smart platform architect will quantify this in his structural analysis and reduce mass somewhere else because of it.

25-50 lbs. I stand by it.

 

This just reminded me of something. Given the new Camaro's front strut arrangement and the additional IRS weight in the rear, can't we expect something closer to a 50/50 weight distribution? Hmmm, interesting.

 

You wouldn't think they would stop making the stronger 4-spider 9-bolt axle in 89 and then go with the 10-bolt, not for three years, but for the whole next generation.
But yet somehow they came to the conclusion that a 7.5inch 2 spider 10 bolt was a good idea. They could have used the 8.5 inch 10 bolt they had been making for years. And any idiot could tell you 4 spiders are alot stronger then 2, but yet even with all their computing power and engineers we got a 2 spider 7.5inch 10-bolt.

Point. Just because they could do something, just because they should do something, doesn't mean the will do something.

 

Guy ... where are you getting these weights? I stripped a 99 Z28 Hardtop for track use and was stunned by how little weight I was actually able to remove from the car by getting rid of all the "luxury" items, including carpet, headliner, all interior trim rear of the front seats, etc.

I still have most of the parts and a bathroom scale.

 

I agree completely. If it is quick and it handles very well, then to me weight is moot. I dont want a porker Camaro, but with new gov't safety standards and more and more options becoming standard in new vehicles I dont think any of us are gonna be happy with the weight figure we finally see.

 

Their is a reason why the new Z06 does so well in performance, because they rerally focused on making the car lightweight. Hopefully they will take the same concept and apply it towards the new Camaro.

 

GM rates curb weight "wet". Full fluids including gas tank.

 

Some of you guys seem to forget that even though the new Z06 lost a lot of weight (vs. base C6) it also gained a lot of racing gear making it heavier.


Items that add weight pounds
Upgraded brakes 27.6
Dry sump, extra oil, oil cooler 23.1
Three-inch exhaust and valved mufflers 11.7
Larger differential with cooling circuit 11.0
Rear-mounted battery
(cables and bracket) 8.8
Larger wheels and run-flat tires 6.2
Items that subtract weight pounds
Aluminum chassis structure 136.7
Reduced interior content*
*(electric seat bolsters, six-way passenger-seat adjustment, acoustic damping material) 33.0
Magnesium engine cradle 11.9
Removable roof hardware 6.6
Carbon-balsa floor panels 5.3
Carbon-fiber front fender liners 2.2

 

As above, GM curb weight is full fluids, including gas tank.

Shipping weight is a basic way to look at "drag racing" weight. All fluids topped but just a couple of gallons of gas. My 2002 Z06 had a shipping weight of 3040 pounds. A 2006 Z06 2LZ is a 3078 pound shipping weight. Gasoline is heavy.

 

That's what I figured. If it were dry, they'd call it "dry weight", but it's not ... it's the weight at the curb! On the road!!

It does almost seem surprising about the weight of the Solstice though. Granted it's a convertible, but so is the Miata? Who knows.

 

Call me a NVH obsessor but I'd much prefer they left the dampening material in the Z06. That's one thing I am not happy to give up to save weight.

I'll go on a diet before I'd want to make the car louder.

 

You Lexus drivers, you!

I gather the Z06 drivers wouldn't ever turn on their radios because they all want to listen to 7.0L of glorious symphony... hence the reason they omitted the NVH stuff.

 

Some have even pulled the fuse for good, and kept those cutouts open permanently.