Pro’s and con’s of solid axle vs IRS

Daryl

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In Layman’s terms (please!)…. Does IRS vs solid axle rear surely come down to ride quality? Cornering/grip performance? Intended use of the car?
Overall comparisons….etc

Does switching from one to another require total redo or are some components interchangeable?
 

shovel

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In some circumstances IRS provides better ride quality but it's not as though the solid axles ride poorly by design.

The summary is: IRS provides more consistent traction over a greater number of conditions.

The moving parts weigh less than a whole axle so they move faster in response to the road surface and a disturbance on one side doesn't automatically disturb the other side so a driver can deliver power, braking and steering input over a wider range of surface conditions like when the road is rough, when the car is going around a corner, when the road is rough while the car is going around a corner.. etc.

In the case of the SN95 platform I believe the complete IRS assembly (including the parts that don't move) weighs more than the solid axle and also it's easier to build a solid axle for absolute strength, so solid axles are preferred for drag racing in particular where they don't really have any substantial disadvantages. The ground is flat and level and the car shouldn't be going any direction other than forward.

For other competitive racing in a SN95 there may be some merit to a well set-up 7.5" axle (the one from a V6) because they are still plenty strong for ~300 horsepower and they weigh less than either a GT solid axle or a complete IRS assembly.

If cost was no object I'd want IRS on both of my Mustangs just to modernize them and get the warm fuzzy feeling of knowing I have the best stuff, but if you asked me for an example of when the solid axles let me down the fact is they haven't. Most of us aren't driving our 'stangs at the limit and that's the only place the difference really matters.

It is necessary to have IRS specific exhaust, either side exit or formed to pass under the differential because the over-axle route used on solid axle Mustangs is not available with IRS . It's also different shock absorbers and brake hoses. The rear "Cobra" calipers have the same displacement as GT calipers so you don't have to change master cylinder and as I understand it the tone rings are the same so your ABS should work with it just fine.
 

Makoto

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IRS is vastly superior in every way except weight savings. my cobra had everything from MM done to it and was amazing. absolutely predictable in every situation.
the main things are

1. brake line conversion
2. making absolutely sure that sucker is square (I had to do a lot to make my IRS perfect in my car)
3. replace all the soft factory bushings with poly or delrin variants.
 

white95

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IRS Pros: What they said ^

IRS Cons: They’ve gotten pricey these days. I’ve seen them going for 1,500+ and not necessarily complete. Properly set up IRS can get quite expensive when you start going down the rabbit hole.

Personally, I wouldn’t consider an IRS unless it had a differential cooler for the abuse I put my car through.

That being said, I love my SRA with the panhard bar and torque arm set up. It does everything I ask it to do except ride like a Lexus. Simple and effective. Wise man (@MadStang) once told me it was cheaper to modify what you already have. Which “cheaper” is subjective to your own personal perspective.
 

cobrajeff96

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If you want a simple, quick answer, just ask yourself if any super or hypercar has ever had a SRA in the back. The kinematics (theory) of the IRS alone has it over the SRA in spades.

Now as it relates to SN95, just keep in mind that Ford shoehorned an IRS subframe into a car that had no intentions of ever having one. And while they did an excellent job of achieving that, it still is not the best IRS as a system in itself. Really the tradeoff comes in the form of increased weight because the IRS subframe for SN95 needs to be very beefy due to the mount point locations being so far apart (and because of this the control arm geometry is not the best either).

The bottom line is that a solid rear axle with some precision components will get you into drive-fast-but-confidently territory in well under a grand spent. It will transform the Mustang and you'll want to drive around like Mario Andretti all the time with the newfound confidence. But it'll still kinda ride like a lumber truck and wear you down after a while. But even with just minor upgrades to an IRS retrofit (things like better bushings, springs/shocks, etc) the IRS will always edge out the SRA car in every category except abuse-taking-potential at the drag strip. It all comes down to that.

My advice to everyone I talk to on this subject (having gone down both rabbit holes during the course of a decade) is to modify what you already have as mentioned above. Wade into the ocean, don't go for the deep sea dive right away. The SRA modified to a substantial degree can give you benefits of handling in the curves while still being built for drags and spending little money while doing it.

Me, I have no interest in drags anymore so I went full retard with the IRS.
 

lwarrior1016

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But I drag race with my irs and it does surprisingly well. Launching at 6500-7000 rpm on slicks and it’s always driven me home from the track.
 
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jeez this topic is beat to death...

it all depends on what you do with your car

if you just want a street car - irs
if you just want to drag - sra
tracking? - a fully built irs/sra are nearly identical

if you really want to blow money, get a spherical/cambered sra and be done with it
 

cobrajeff96

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I'm pretty sure a fully built IRS will edge out a fully built SRA on the road course / autocross. I mean, if SRAs were just as good or better than IRS cars, why aren't any supercars or hypercars running SRAs in the back? There's your answer.
 

shovel

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For me the answer is real easy, I don't have $2-3k to casually spend on it so I won't.

I think if you're not racing competitively and don't have that kind of money to spend casually on a whim, then there's just no point in going to the effort. Buy $2k worth of gas and hotel rooms and have a road trip you'll remember that on your deathbed, not that time you had slightly better suspension on that one road that one time.

If you do have plenty of money and want the safety and handling of IRS you know what's even safer and handles even better? A car that's a quarter century newer.
 
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Daryl

Daryl

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Thanks everyone for reply. Topic closed.
 

white95

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I'm pretty sure a fully built IRS will edge out a fully built SRA on the road course / autocross. I mean, if SRAs were just as good or better than IRS cars, why aren't any supercars or hypercars running SRAs in the back? There's your answer.

My car identifies as a Ferrari.

A redneck Ferrari.
 

r3dn3ck

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I've done the conversion twice in the past and have a IRS cobra now. My first one had factory 4-link (6-link) suspension before the IRS. Night and day, no comparison. My 2nd one had the IRS but I kept breaking expensive parts so I went SRA with MM TA/PHB, coil over, control arms and that was a monster of a good setup other than high NVH and a fairly jarring ride. In stock form the IRS is a bit of a wobbly goblin and it's expensive to iron out all the little bits just in parts. If you can't install the parts yourself it's bloody near cost prohibitive for what you get out of it. If you replace all the squishy bushings with harder ones and do a few other mods then it's pretty hard to beat for handling but I haven't seen either my 3-link or my IRS cars being very different in handling prowess when fully kitted out.
 
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Daryl

Daryl

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Because apparently there’s plenty of discussion on it from the past. My fault for not checking first.
 

Musturd

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Sold my 04 cobra IRS for 2500usd that I had in my mystic .
 

ttocs

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it is one of those topics that is relative to what you are wanting to use the car for and your checkbook and when it is all combined then you have an opinion one way or the other.
 

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