IRS Shortcomings?

[Teej281]

Hey guys,
Something that I've been quite curious about lately, as there has been a lot of interest in my community in IRS swaps into GT's, is that people keep saying that the 99/01 and 03/04 Cobra IRS designs suck and that they are terrible rear ends. Aside from the halfshaft problems that are encountered, what are some of the design flaws that make the Cobra IRS so terrible? All we ever hear is that they suck and swap for a solid. No reasoning, no hard evidence(other than halfshaft design) as to why they suck. Some say that they are setup wrong, and give no numbers or comparisons as to what makes it sub-par compared to say a GTO IRS, or some foreign IRS's.

I'm not trying to ruffle any feathers about this subject, I'm just looking for concrete evidence that these rear ends are of terrible design and numbers and facts that support what everyone says that the 03 cobra IRS is indeed terrible and that its not just improper setup from owners that make them terrible.

Thanks guys! Looking forward to learning a lot from this! :beer:

TJ

 

[Bruha]

I don't think anyone would try to convince you that it is a state of the art design, but with that being said, you can make them work very, very well. The biggest problem is the type of bushing the factory put in there for NVH.

You need to read everything you can about this. Here is a great place to start!!

FTBR Frequently Asked Questions

I just installed his whole kit about two months ago, and it has completely changed my car. I have a new found love with the 2003-04 cobra. It eliminated wheel hop, and handles better to boot.

Whoever you are talking to is not informed on the subject. They are ignorant and need to educate themselves.

Talk to bruce. He will get you going in the right direction.

 

[SlowSVT]

Most people who buy these cars tend to be younger, are only interested in straight line performance and have little understanding on the dynamics of a rear suspension. Just blasting out of the hole can get pretty :bored: after a while but a car that can do that and still hang a corner is :banana: A well sorted car set-up for drag racing :xpl: when you want to change direction hard.

It's kinda saddening to me seeing a Cobra with a stick axle :nonono:

 

[UncleSAm]

Most people who buy these cars tend to be younger, are only interested in straight line performance and have little understanding on the dynamics of a rear suspension. Just blasting out of the hole can get pretty :bored: after a while but a car that can do that and still hang a corner is :banana: A well sorted car set-up for drag racing :xpl: when you want to change direction hard.

It's kinda saddening to me seeing a Cobra with a stick axle :nonono:

thus my irs swap

 

[UncleSAm]

from FTBR:

The more complicated answer is this: If you're a very sharp mechanical engineer, and you've spent many years researching, designing, developing, testing, racing and executing the production of motor vehicles and suspension systems, there is still a very slim chance you'd be more knowledgeable than a team of SVT engineers. SVT engineers came to the ultimate conclusion that the SVT product line would be better served with an IRS under it. It unfortunately came quite compromised due to NVH requirements which has led to many of it's issues and unpopularity, particularly as the rubber suspension bushings age and wear.

Numerous centuries ago when wagons were built to be pulled behind oxen and horses, the SRA was axle of choice. Roman's put SRA's on chariots. The first motor vehicle produced back in the late 1800's had solid axles on it as well. Both front and rear. As you may have guessed suspension systems, just like anything else have improved in design over the years. Almost every vehicle on the road now has an independent FRONT suspension on it with the exception of four wheel drive vehicles. Independent REAR suspension systems have become so popular due to improved ride and handling that they don't just come on high performance sports cars any more. They also come on pony cars, two and four door sedans, sport utility vehicles, mini vans and many other vehicles as well. There is a reason that every high performance sports car manufactured today, comes from the factory equipped with an IRS. The SVT Cobra Mustang, considered a pony car, is one of many high performance cars that came with an IRS. The Corvette, which is without a doubt one of America's most successful sports cars, came with an IRS starting in 1963. That's more than forty six years! These cars all came with an IRS for a reason. Their manufacturers decided these car would be better served with an IRS under them for improved ride quality and superior handling.

Yes, handling can be improved to a SRA adding things like a torque arm, panhard bar or a watts link. But they will never be an IRS. Some people like the idea of swapping to an SRA because they will reduce their vehicle's weight by about 150#. This is fine for drag racing but for street manners you are making a vehicle that already has a lopsided F/R weight distribution of 57/43 (for an '03-'04 Cobra Mustang for example) even MORE unbalanced. One other thing to keep in mind is a SRA will add a substantial amount of unsprung weight to the rear suspension. Some people run these GT axles with GT rear brakes. Brakes are a safety item and for people that are making their cars faster with aftermarket parts should not be downgrading their brakes. They should be upgrading them.

Cobras are more sophisticated vehicles than a Mustang GT for numerous reasons, making them considerably more expensive . They have a higher performance engine, sometimes equipped with a supercharger, better transmission, improved brakes, wheels and SUSPENSION including the IRS. These cars are already depreciating fast enough without downgrading the rear suspension to that of a Mustang GT. Be certain what you are deciding to do with your car.

In closing I'd like to say the ultimate decision is in your hands. It's your car, do with it what you want.

 

[Tractionless1]

From an internal hardware standpoint I've read about failures due to the crush spacer on the pinion gear resulting in cap damage and cover damage. I personaly don't like that the trac-lock has plates that were. I just received my built IRS consisting of a Ratech soild pinion spacer, Mark Williams billet caps, Detroit true-trac, and will be topping it off with an LPW cover. It will be installed with MM poly bushings, rear subframe welded ears, and proper size front mount bolts. All of which have served me well to date with the stock IRS pumpkin assy.

I do not roadrace the car, I save that for the bikes. So any advantages to be had by $400-$600 bushing kits I'll never know.

 

[Teej281]

Ok...maybe I need to let you guys know a little bit more about me. Lol I have an 02 Gt convertible that has an 03 cobra IRS in it with the full tilt boogie racing catalog thrown at it, sans lowering diff mount, and maximum motorsports coilovers with bilstein hd's. I just really would like to know where everyone is getting these ideas at that the IRS sucks and other IRS's are better without justification. Just because the suspension was designed around the car doesn't mean the suspension sucks.

 

[kingcobra8541]

An 03 Cobra IRS "with the full tilt boogie racing catalog thrown at it" is probably the best all around rear end that you can put in a mustang. With that being said that is one expensive rear end. I just did a SRA swap in my car only because I got a sick deal on a built rear with all UPR suspension for half of what I sold my IRS for. If I wouldnt have found that deal I would have kept saving for the FTBR kit and bought when he had his yearly X-mas sale. I would have loved to keep my IRS is storage but just didnt have the space and needed the $$$.

 

[Teej281]

Well I have a friend that gave me a good price on it all as he is a dealer. And yea, it's a wonderful setup. This rear launches well and handles even better. I cut a 2.1 in my GT with street tires launching at idle with 3.55's.

 

[1wild-horse]

The suspension doesnt suck, depends on the application though. I dont think anyone would disagree that for drag racing a solid axle is a lot more commonplace and easier to dial in. An IRS setup lends itself better to road course action for obvious reasons, more individual travel out of each wheel will give you better handling characteristics in the corners.

One short coming or design flaw I believe is that the aluminum case is a little weak and causes some deflection you wouldnt have in a cast case. But then again, it wasnt designed to put 400+ hp to it either. But I know for a fact that there were a lot of these cars, in their stock state that had carrier bearing failures under warranty.

 

[Teej281]

See, thats the kinda stuff I'm talking about! Something that's wrong with the design that makes the IRS sub par that could be verified if you would put the work into it. But most people just say it sucks and don't say why. Let's get the real story out there so people quit talking trash on it when they usually have no ground to do so.

 

[mu22stang]

See, thats the kinda stuff I'm talking about! Something that's wrong with the design that makes the IRS sub par that could be verified if you would put the work into it. But most people just say it sucks and don't say why. Let's get the real story out there so people quit talking trash on it when they usually have no ground to do so.

It is out there.

http://www.svtperformance.com/forums/new-edge-cobras-51/213384-irs-tech-article.html

FTBR Frequently Asked Questions

SVT has always combined comfort with performance in the Cobra. Heck, the Fox Cobra has SOFTER springs than the GT. The Cobra is supposed to represent the refinement of the standard Mustang with an added bonus of power. The apparent weakness of the IRS illustrates how this refinement has compromises. I can imagine the overall design sequence read something like this:

-IRS from previous Cobra model, check
-Add MOAR powa!!1!1!! (and weight), check
-Stiffen springs, shock, struts, bushings to resist weight and improve performance, check
-Test on the track, check
-Satisfied with performance, check
-Hand over to NVH department at Ford, check
-Princess and the pea NVH engineers think it's too rough... hmmm
-Add weak, squishy rubber to solve "problem"... check
-Sell to thousands of straight line drag racers, check
-Boom, check

 

[Crackerballer]

And most people on here have never driven a well setup straight axle... and I don't necessarily mean setup for drag racing.

IMO, the problem with the IRS in the new edge is that it was an after-thought. They had to compromise it for several reasons, including the car not originally being designed to have IRS.

I did the solid swap and will be adding my torque arm/panhard setup soon. Even without the 3 link installed, I will never own another IRS new edge. It sucked in my 99 and sucked in my 04, especially when you consider the costs of building the IRS to make it tolerable. The same money spent on a solid will yield very nice results. Also, I never plan on visiting the drag races. It's a street car.

To each his own though.

 

[SlowSVT]

And most people on here have never driven a well setup straight axle... and I don't necessarily mean setup for drag racing.

IMO, the problem with the IRS in the new edge is that it was an after-thought. They had to compromise it for several reasons, including the car not originally being designed to have IRS.

I did the solid swap and will be adding my torque arm/panhard setup soon. Even without the 3 link installed, I will never own another IRS new edge. It sucked in my 99 and sucked in my 04, especially when you consider the costs of building the IRS to make it tolerable. The same money spent on a solid will yield very nice results. Also, I never plan on visiting the drag races. It's a street car.

To each his own though.

You stated "they had compromise it for several reason" but never accually stated what they were :shrug:

 

[SlowSVT]

Ok...maybe I need to let you guys know a little bit more about me. Lol I have an 02 Gt convertible that has an 03 cobra IRS in it with the full tilt boogie racing catalog thrown at it, sans lowering diff mount, and maximum motorsports coilovers with bilstein hd's. I just really would like to know where everyone is getting these ideas at that the IRS sucks and other IRS's are better without justification. Just because the suspension was designed around the car doesn't mean the suspension sucks.

Teej

Help me out over here.

It's nice to hear from someone who appreciates the advantages of the IRS. But if I recall correctly weren't you beating-up on me on a thread regarding swapping out an IRS for an SRA? I was the lone wolf on that one and taking repeated body blows from a crowd suffering from "Straight Line Fever" when I pointed out to the OP the fact you saw fit to swap out and SRA for an IRS in your GT yet you were telling him "the SRA all the way" :dw:

To this day I'm still scratching my head over that one. What gives :shrug:

op:

 

[Teej281]

Well I'll elaborate a little bit. At that point in time, my friend had just gotten back from a deployment in Afghanistan and was getting ready to leave the marines. He was coming out of there with a lot of money and we had plans to turn his car into a very high hp car, with the primary focus being on drag racing. We're talking like 800+rwhp and launching on slicks. So for that situation, I found it acceptable to go the route he was choosing. He is currently waiting to get a job to even put the new sra in and get his car tuned for e85. I think with his current plan that he should leave the IRS, but he put nearly $3k into parts for his sra. So I can understand if he decides to use the sra. I helped him pick good parts for it. Lol

 

[P49Y-CY]

i don't think the irs sucks, and it serves me well in many road situations (better i am sure than a stock 99-04 sra), but to date i have never had a car with worse behavior when going full-throttle from a standing-stop situation. (i mean just standing on it from a stop, expecting to obliterate the tires) occasionally that is my idea of fun :rockon:

sometimes it works out ok, but for the most part it doesn't work out too well. its like i never know what its gonna do when i try it.

yes i have a vert, but my 96 was also a vert and although not as powerful as my terminator, i could just sit there all day and burn rubber if i wanted to, never once with wheelhop or any other adverse reaction.

 

[delvin.a]

^ I'd stop having that kinda fun until you sort that thing out. You don't wanna break it and be out all that cash as well