IRS vs. 3 link vs. 4 link

[Vogelsong]

Hi all... I’m new to the forum, and am looking forward to seeing everyone’s projects.

I have most of the plans worked out for my build and hope to start on it next spring.

The car is a 65 fastback mustang...

I’m staying very focused on the weight of the car, and trying to keep it no heavier than 2600 lbs.

It will be a touring, track car. The wife and I plan on doing road trips to shows, and track events.

Hot Rod power tour and such are the types of things we are looking to do, along with the occasional track day (she’s a bit of a gear head herself... I got lucky).

It’s a dream of mine to drive this car at places like Watkins Glen and the like.

The one thing I am still hung up on is rear suspension... I’ve talked to several builders, and suspension shops, and I get a mixed bag.

On IRS I’m told everything from, “you can’t go wrong with the performance” to “it’s heavy, I can’ build a 4 link that will handle just as well, or better”. The last quote was actually by a guy that built and designed a solid IRS system.

On 3 link... particularly with a torque arm and watts, I’m also warned about weight, and steered towards a 4 link.

So the question is...



From a performance stand point only what do you guys think of the 3 suspension systems?

Keeping money out of it... can a 4 link handle as well as an IRS?

Is the IRS really that much heavier?

Some people swear by a 3 link why?

I plan on trying to drive some cars that are built, but I’m wondering if any of you out there that are running these cars hard can help me on this so I can make up my mind on which way I want to go.

I know this is long, but I like to give all the background info so you know my plans, and my approach.

Thanks for the input.

[mikedc]

IRS -

I have often heard "A good solid axle can beat an IRS on the track."

That's sort of like saying "A good carburetor can beat EFI on the dragstrip." It's true but it's not the whole story. That setup only has to be a one-trick-pony.

You need a good suspension design if you are going to make it work. Flex. Travel. I mean while the driver is pushing it hard.

Hypothetical - Take a track car that drives only on very smooth pavement. The body motions are all pinned down. Low ride height and stiff springs/shock/sways, etc.
That suspension isn't moving very much because it's set up like an overgrown go-kart. (At that point it would probably respond to aggressive wheel alignment more than good geometry.) On that car an IRS may not earn enough to offset the weight penalty. But IRS shines in the real world where suspensions have to handle big bumps & potholes at all the wrong times.



Solid axle 3-link vs 4-link -

IMO a 3-link (combined with a panhard/watts) is a superior design in general. The lateral location is controlled separately from the longitudinal motions and you can tune it better. No binding bushings. Etc. The 4-link is popular with OEMs because most buyers won't be wanting to adjust anything and it's perhaps a little bit easier to package.



BTW, if you put much priority on being able to hook the tires in a straight line (drag racing) then this is a no-brainer. Stick with solid axles. IRS is inherently worse for that no matter how much optimizing/tuning it has. Having the differential mounted to the chassis makes it impossible to get the kind of anti-squat geometry that solid axles have.

[dhutton]

As someone who has had a four link and an IRS in a first gen F body I can say the IRS wins hands down on the street. My IRS Camaro is the best riding first gen F body car I have experienced.

Instead of worrying about weight, take a look at unsprung weight which greatly impacts ride quality. The IRS will shine...

Don

[dontlifttoshift]

Given your goals, IRS. But that answer does not take into account your ability to spend, build, tune, or drive.

Your weight goal is absurdly low for anything that will be an actual street car. You could shoot for 3000 but 3200 is more likely.

[Vogelsong]

Given your goals, IRS. But that answer does not take into account your ability to spend, build, tune, or drive.

Your weight goal is absurdly low for anything that will be an actual street car. You could shoot for 3000 but 3200 is more likely.

Thanks for all the input. Now I have to figure what direction to set on, and then determine the manufacturer.

I really want to go IRS, and more than likely will... choosing a manufacturer will be tough.

As far as the weight...

An original Shelby was 2600 pounds, so with some ideas I have to help with weight I think it’s possible, but we’ll see.

if anyone else wants to chime in with thoughts I’m all ears.

Thanks again.

[Vogelsong]

As someone who has had a four link and an IRS in a first gen F body I can say the IRS wins hands down on the street. My IRS Camaro is the best riding first gen F body car I have experienced.

Instead of worrying about weight, take a look at unsprung weight which greatly impacts ride quality. The IRS will shine...

Don

I’ll do that thanks.

I’m really digging your build thread.

Great car.

[dontlifttoshift]

I would do what ever it took to run the Detroit Speed DecaLink. It's narrow enough to fit under the sheetmetal and infinitely better than any double wishbone setup......especially power down.

You're not building an original Shelby. Everything you do to save weight comes at the cost of comfort, convenience, and reliability.

[mikedc]

I also agree about the weight goal being unrealistic. 2600 is almost Shelby Cobra territory. They built race cars that lightweight by making them only usable as race cars. They weren't all that safe by modern standards either.

[Vogelsong]

I would do what ever it took to run the Detroit Speed DecaLink. It's narrow enough to fit under the sheetmetal and infinitely better than any double wishbone setup......especially power down.

You're not building an original Shelby. Everything you do to save weight comes at the cost of comfort, convenience, and reliability.

I thought the decalink was geared toward vettes only.

[CSG]

I pretty much agree with what Mike says above. My addition to the discussion is about balancing the front and rear. If you want the car to handle well on the track, Make sure you go with parts that will play nice together.

[andrewb70]

For an IRS solution that is narrow enough to fit a 65 Mustang, be prepared to spend cubic dollars...

Andrew

[dontlifttoshift]

I thought the decalink was geared toward vettes only.

Given your goals, IRS. But that answer does not take into account your ability to spend, build, tune, or drive.

Bolding mine. Nothing is impossible, sometimes it's just difficult.

[Bruce1968]

I also agree about the weight goal being unrealistic. 2600 is almost Shelby Cobra territory. They built race cars that lightweight by making them only usable as race cars. They weren't all that safe by modern standards either.

FWIW, I Googled this out of curiosity and found several resources that put the weight of this car around 2600 lbs when new.

[dontlifttoshift]

No one is questioning that. It's the idea that you can take a 2600 pound car, make a bunch of modifications that only add weight and still end up at 2600 pounds.

[Vogelsong]

FWIW, I Googled this out of curiosity and found several resources that put the weight of this car around 2600 lbs when new.

Exactly...

No one is questioning that. It's the idea that you can take a 2600 pound car, make a bunch of modifications that only add weight and still end up at 2600 pounds.

I didn’t say it would be easy, but if I add here, and take away there, I’m hoping I can get close.

[Vimes]

If you wanted to get fancy with it, fleabay has rear end parts for Mercedes SL500s. They have IRS, and the rear track is 60 inches as opposed to your Mustang's 56 inches. They also came with supercharged V8s and V12s pushing out as much as 800HP and weighed in over 4000lbs. Course, there's a lot of leather and comfort features as well as an electric convertible roof on those cars to make up that weight. I'm pretty sure they are unibody cars as well, like your Mustang, so you'd have to modify the parts to match your mounting points. That was a given anyway.

[mikedc]

I didn’t say it would be easy, but if I add here, and take away there, I’m hoping I can get close

I wish you luck just like everyone else. But I'm guessing you will end up at more like 2800 lbs. That will probably be difficult enough to hit.

Look at it this way: The factory wasn't selling finished cars.

If you were building your own DIY custom car, would you stop at that point? When the chassis was so floppy that normal radial tire loads would flex the shock towers out of shape? No door beams at all? A pretty flimsy roof & cabin cage? Flimsy seat frames and not much for seatbelts? No metal trunk floor to protect the passengers from the gas tank?

No. You would think "Dammit, I've reached my weight goal but this thing just isn't finished yet. It's still missing some big necessary pieces of steel."

[Tsaints1115]

Debate aside of which rear is better ask yourself this. When all is said and done, the way you plan on doing the majority of your driving, will your "butt dyno" really be able to tell the difference enough to make it worth it?

[Vogelsong]

Debate aside of which rear is better ask yourself this. When all is said and done, the way you plan on doing the majority of your driving, will your "butt dyno" really be able to tell the difference enough to make it worth it?

The way I see it it’s not about the butt dyno so to speak.

It’s more about it being easier to make a 2600 pound car go around a corner fast, than it is a 3200 pound car.

I know 2600 is aggressive... If I’m over I won’t kill myself.

It’s my way of keeping the weight under control and not putting every frivolous luxury in it that i really don’t need.

- - - Updated - - -

I wish you luck just like everyone else. But I'm guessing you will end up at more like 2800 lbs. That will probably be difficult enough to hit.

Look at it this way: The factory wasn't selling finished cars.

If you were building your own DIY custom car, would you stop at that point? When the chassis was so floppy that normal radial tire loads would flex the shock towers out of shape? No door beams at all? A pretty flimsy roof & cabin cage? Flimsy seat frames and not much for seatbelts? No metal trunk floor to protect the passengers from the gas tank?

No. You would think "Dammit, I've reached my weight goal but this thing just isn't finished yet. It's still missing some big necessary pieces of steel."

I could live with 2800. 👍🏻

[Vogelsong]

If you wanted to get fancy with it, fleabay has rear end parts for Mercedes SL500s. They have IRS, and the rear track is 60 inches as opposed to your Mustang's 56 inches. They also came with supercharged V8s and V12s pushing out as much as 800HP and weighed in over 4000lbs. Course, there's a lot of leather and comfort features as well as an electric convertible roof on those cars to make up that weight. I'm pretty sure they are unibody cars as well, like your Mustang, so you'd have to modify the parts to match your mounting points. That was a given anyway.

I’ve considered this very thing.

My problem is I need to be able to see stuff already done to feel comfortable with it, and I don’t see a lot of mustangs with benz underpinnings. 🤷*♂️ LoL