Anyone swap an IRS rear into a TR?

[MAP]

Greetings folks,

I know this constitutes a radical transformation, but has anyone actually tried adapting/installing an independent rear suspension into these cars? If so, what was the source of the IRS, and how difficult was it to adapt to the G-body chassis? What were the differences in handling? Traction? Weight and car weight distribution?

The stock suspension works well within its design limits, but doesn't lend itself well to high-performance handling, mainly because of a.) approximately 300lb. unsprung mass, and b.) the very high (18") design roll center height, which causes the rear to meander over bumps like "walking with a hip out of joint" (to quote a Motorweek critique from many years ago.)

Any feedback most appreciated!

Best,
MAP

 

[pwrhws]

I have never seen it done but if I were to do it I would go one of two ways:

Rob everything from a c4 corvette, or newer cobra/ mark viii. The cobra/ mark have stronger center sections, but have cv joints as apposed to the u loints on the vette. The vette center is a dana 44, the cobra/ mark is an 8.8. The cobra has a 31 spline axle where as the lincoln has 28, but can be updated to 31.

The vette has alluminum trailing arms to locate the axle, The cobra/mark uses an A arm.

If you do it let me know how it works out.

I wouldn't plan on doing alot of wheelies with either set-up.

 

[MAP]

Thanks powerhouse,

Limited traction - I agree that's the big disadvantage here. IRS's can't be dialed-in for more than about 25% anti-squat, according to Herb Adams. I'm in the data-gathering mode for the project, assuming I even attempt it, given that lots of $$$, time, and basic blood, sweat, and tears lie in store for me.

But if I were to do it, I'd certainly narrow the frame in this area, and use the widest possible tires I could find - say, 345's or so.
That should help to compensate for some of the lost traction inherent with an IRS, plus provide a very big improvement for all-around handling.

It sounds like you're quite familiar with those Cobra rears - have you done any transplants into cars other than TR's, maybe?

Best,
MAP

 

[PCS74]

I am in the planning stages for this as well. I decided to make a clean sweep however and build a new suspension/chassis setup from the ground up.

I am finishing up the chassis drawings, but I am still undecided on what to use for the rear suspension. I originally planned to use a vette IRS, but am leaning more towards a four link setup at present.

I plan to use the car for the texas triathalon, nevada open road races and eventually one-lap, with alot of drag racing in between.

I think the four link(with full adjustability) will offer the best all around performance for my needs.

I plan to run 335's, narrowing the rail 3" per side and mini-tubbing it.


Chris S

 

[zam70]

I've got a c4 vette setup sitting out side right now. It's basically an all aluminum 4 link setup in it's own right. This one is going into a 66 Barracuda of all things...Not familiar with the Cobra setup but one advantage I see in the Vette setup is price....picked this one up (complete) for $350.00. One MAJOR draw back to the vette setup is ring and pinion price. I mean OUTRAGEOS! oc course that would only matter if you want to change the gearing. If you need some pics, let me know. I'd be happy to send them out.

Just a little info.

 

[pwrhws]

I have never swap any of the irs's into anything they didn't belong in, but always kept an eye on possibilities.

If you were to combine the two, use the center section of a ford, and the halfshafts and outer trailing arms of the c4, You would have a relitivly bullit proof set up. I do not think it would be all that difficult, The ford half shafts bolt onto a flat flange.
Gears for an 8.8 are a dime a dozen
Most of the cobra drag race guys are going to straight axle anyways.
You could make a adjustable trailing arm set up for the corvette spindles. Makes alignments easy.

Just a thought.

 

[MAP]

Thanks pwrhws,PCS74,zam70,

Good information. Putting this together as a list of pros and cons for IRS vs. solid axle, I get so far:

Solid axle:

Pros:

- better off-line traction due to greater range of practical anti-squat behavior.

- simplicity and low cost - chassis/frame mod's not required.

- more freedom of component choices = easier to make strong.

Cons:

- high unsprung mass

- as rear suspension came from GM, very high roll center = "weird" chassis dynamics over bumpy surfaces. This combined with high mass, means poor road-holding ability on bumpy surfaces while accelerating.

IRS:

Pros:

- lower unsprung mass

- more degrees of freedom for suspension behavior, essentially duplicating those available for the front independent suspension on the TR's. This, plus low unsprung mass, mean excellent road-holding ability on bumpy surfaces while accelerating.

Cons:

- Complexity and high cost - chassis/frame/brake mod's are a given.

- Limited range of component availability = harder to make strong.

- Poorer off-line traction due to limited anti-squat behavior.

Summary:

Putting this all together, it seems the one insurmountable disadvantage of the solid rear is its high mass. (The roll center problem can be solved with some effort by using a Satchell link or a Panhard rod setup (a la GNX,) for instance.) The IRS, by comparison, has several disadvantages, but none of the "brick wall" variety like that just mentioned for the solid rear. So, based on the information shared so far, it would seem that the solid rear is by far the more practical choice of the two, and the IRS becomes justified only when all-out handling on real roads (not smooth track surfaces) is desired.

Feedback? Thanks!

Best,
MAP

 

[Scottie-GNZ]

I know my car is not a Buick, but I just completed a C4 IRS swap into my 240Z. Couple of 1.55s with DRs and 5psi on a T-brake, so no issues with traction.

Take a look. If nothing else it will help to get familar with the IRS and its components.

http://www.scottiegnz.com/corvette_irs_swap.html

 

[MAP]

Greetings all,

Scottie - terrific link, and impressive work done to swap the C-4 rear into your car! Concerning the C-4 and C-5 rears, however, I have heard more than a few horror stories about broken axles and hubs, when going beyond (roughly) the 450rwhp mark. I think this prompts Powerhouse's recommendation for the Ford rear.

Scottie - did you weigh the entire C-4 rear before installation?

Best,
MAP

 

[Woody]

Heidt's Hot Rod Shop, Inc. (tele 1-847-487-0150) manufactures and sells what they call their "Superride IRS" setup using a ford 9" third member.......comes complete with inboard Wilwood disc brakes, corvette outer bearing assemblies, etc, your choice of gear ratio, etc.........for $3995......

you would have a bullet proof rear with irs and the ability to change third members pretty quickly....pretty cool

 

[GNVAIR]

Jon Meaney made well over 1,000 horses to the rear wheels on his C4 Vette. His only mod was a Dana 44 center section. I am not sure if he launches on slicks, but most of the Vette guys agree the 44 is plenty strong. The C5 Vette uses a 4L60e with a Getrag differential. Another excellent set up if you want to add more weigh to the rear of the car. The downside would be that you would have to add a torque tube.
IMHO, the IRS is a good idea, but may not be worth the effort. I have had discussions with others in the past when I was involved in Fox body Mustangs. There is still tremedous potential in a live axle. The William Mathis book (Mustang Performance Handbook 2) has a perfect example of this on page 71. That is the Baer Racing Blublocker World Challenge Mustang. It was one of the most successful Mustangs in road racing history and it utilized a 3 link rear suspension. It couldnt have been that bad as the car had many victories over many other sedan and sports cars including Corvettes and Porches. Just some food for thought there. What about a Watts link?

 

[Scottie-GNZ]

MAP, thanks. As you saw in the page, I had the IRS completely broken down and I weighed each component. I also had to break down the Nissan IRS to ship it to WA. I weighed both but cannot find my notes with that detail. Only thing I remember is that the C4 IRS was 80lbs lighter. It is all aluminum except for the rotors, stub axles in the hub and diff and of course, the R&P.

I know there are probably Buick folks who doubt the strength of the Vette IRS and they should look to the Rogue Vette. Some of you might have seen it at BG. It is a stageII-powered C4 running hi-9s. No traction problems or concerns about breakage there.

IMHO, if the TR is a street car that sees primarily 1/4-mile duty, it is not worth the effort/cost. However, if it is an all-purpose car looking for a better ride and sportscar-like handling, absolutely. Imagine explaining on the Vette forum how a TR hooked to the bumper of a C5 in the twisties then blowing by it in the straights .

 

[gnx7]

If you want a car that handles better and that is the main goal then IRS is a must. What supercars do you see in production that don't have IRS?

Like Scottie added..... the Dana 44 is the way to go. Here at the local salvage yards you can get the complete rear IRS from an '89 and up Vette (these came with the Dana 44 centersection). If memory serves me right they are about $1K-1.2K and this is complete from disc to disc with everything in between. Also of note the rear disc brakes are 13" I believe! Yowsa.

You can use Scottie's pics to see how he installed it in his 240Z. He had to narrow everything and did a killer job. The amount of adjustment you can get out of the way Scottie did his car with coilovers is incredible! He did away with the leaf spring.

My factory IRS works great also... but I'm not pushing 450hp yet. I do get 1.71 60ft times on 245/50/16 Nitto DR's though with only 5psi launch. My goal is some mid 1.50's as well.

Rogue Vette is a perfect example of big power with his single turbo Stage 2 running 9.70's@140mph+ with the stock Dana 44 and upgraded halfshafts with no failures to date .

345's on a GN will look awesome.

I just put 245/45/16 Potenza S-03's out back on my car and coilover suspension with KYB AGX 4 way adjustable shocks, caster camber plates too. I'm trying to make a corner carving warrior that kicks butt on the straightaway also.

-GNX7

 

[MAP]

Greetings folks,

These are excellent posts! Very informative. I'm a bit confused about the C-4 rear situation, though - I get the impression from some posts that some need to be converted to a Dana 44 center section to handle a lot of power, while I read in others that they were stock GM issue for certain year C-4's. Please clarify - thx.

Also, does anyone know where I can get detailed information about these rears, especially as it would pertain to high-performance applications?

Gnx7: good point about the super cars never using solid live rear axles - I think I come back again to the "brick wall" solid's problem of having lots of unsprung mass (for the rear of a typical TR, the ratio of the sprung/unsprung mass is in the vicinity of 4:1, which is not very good at all. This creates a rough ride, and a tendency for the tires to lose traction over bumpy surfaces under acceleration.) An IRS like the vette's probably changes this ratio to something more similar to the front of the car, which is on the order of 10-12:1 - a vast improvement.

For me, perhaps, the very high stock rear roll center is probably the worst problem of all with these rears. As the wheels travel over bumps, the rear of the car literally twitches from side to side - which always creates, for me, a sense of vague apprehension about controlling the car's exact direction of travel: re, the "hip out of joint" syndrome. One never fully relaxes driving the G-bodies for this reason, I believe. And the problem is amplified by using stiff suspension bushings and/or low-profile, stiff-sidewall performance tires. The stock low-performance tires are very forgiving in this regard.

But the high roll center problem is surmountable - and a small reduction in roll center height can create a relatively large reduction in this undesired transverse component of motion of the rear of the car while traveling over bumps.

And so we come back yet again to high mass as the clincher against a solid axle.

About traction with an IRS - yes, it does purportedly have limited opportunity for anti-squat, but this can be compensated for with wider tires. For Scottie and Gnx7, I'm not very familiar with your cars, but I'd venture a guess that your weight distribution is far more favorable than a TR's, and that's partially why you get favorable 60' times. The TR, on the other hand, needs all the help it can get in this regard, given its nose-heavy condition from the factory.

Best,
MAP

 

[GNVAIR]

C4 Vette rears are Dana 36's on all automatic cars and Dana 44's on all stickshift cars. That holds true for all from 84-96
The ring gear is about 8.5 inches on the 44 and I believe 7.5 on a Dana 36.
The Vipers use a Dana 44 similar to the Corvettes.
As someone previously noted, the difference in price when obtaining a differential is substantial when comparing a Dana 36 to the 44. Most places get ridiculous $$ for the 44 because everyone wants them.

 

[monte_383]

I've got an 8.8" IRS out of a '89 T-bird for sale if any one wants it. Been setting a while, but complete disc-disc. They are about 1/2" narrower outside to outside of the tires than stock MC/TR. Will fit inside wheel wells with 245/50R16. A little fabrication of mounts would make it a bolt in deal. I was toying with this idea too. -Chuck

 

[MAP]

Greetings all,

Here's an update after some further research on this topic, and after learning from a friend's traction-limited struggles with an '02 ZO6 Corvette making 550 hp or so to the ground. He just can't hook-up, even with drag radials, he says - he's run a best of 11.83 so far.

After pondering what to do with the TR's existing live-axle rear, I think I'm coming to the conclusion that it's just too much trouble and money to switch to an IRS. The live axle's high roll center problem is solvable, as I mentioned in a few previous posts. The one fundamental inherent disadvantage is its high unsprung mass.

But a live rear's good potential anti-squat behavior gives it a significant traction advantage over an IRS. This is one of the reasons GM went to mounting the trans in the rear of the Corvette: it wasn't just for better overall handling; it was for better rear traction too, in a car that needs all it can get. If I'm not mistaken, there seems to be a pretty good correlation between engine output and weight distribution for the IRS "super cars." The basic configuration, if one could use such a term, would be the front engine/tranny - rear IRS setup such as the C1-C4. As power to the ground increases, we have the C5 with a front engine/rear tranny - rear IRS setup. And then we have the all-out super cars, with all of these components in the rear, so as to create a tail-heavy condition to improve traction. The progression is always to put more weight over the rear with an IRS to overcome an IRS's poor anti-squat limitations.

The G-body cars, however, are far more weight-distribution "challenged" than Corvettes. From the factory, these cars had roughly a 60/40% to 56/44% front/rear weight distribution, which doesn't bode well for rear traction. I think a live axle's potential for more than 100% anti-squat is a key factor in getting these rears to hook-up with such a poor weight distribution.

So, ultimately, the matter of an IRS versus a live rear seems to come down to a choice between limited traction, and high unsprung mass. If we now add the high differential of cost and complexity of the IRS swap, I think the live axle becomes the all-round winner for the G-cars. Opinions?

Best,
MAP

 

[TTA89]

I know a guy with a TT 01 Z06 6-speed that has been 10.6 on the stock rear with Drag Radials. Car has been 140mph in the 1/4.

Just adding some info

 

[Scottie-GNZ]

MAP,

you make good points on your decision, but I think your buddy's traction problem is not norm for an IRS, especially a Z06. Based on my experience I believe his rear shocks are rebounding too quickly and unloading the rear. Either that or he is simply trying to launch too hard on DRs.

 

[MAP]

Greetings Scottie,

Your signature stat's speak impressively for your combination. Do you know your car's static front/rear weight distribution? And what tires do you use to get those 1/4-mile times? If they're not the ones you use for daily driving, would you happen to know how well you do with the tires you normally use?

Thanks and best regards,
MAP