Independent Rear Suspension

[jaybee]

I've been doing a bunch of research on IRS transplants from OEM cars recently. Not sure if anyone has interest in what I've found, as a semi-anonymous person on the internet seeking enlightenment from other semi-anonymous people on the internet, but I thought I'd throw it out there and see if it turns into a conversation.

First, there doesn't seem to be a lot out there that suits rear end less than about 59-60" wide, so small cars are out. That's not as narrow as it seems, as long as you're comfortable with the look of positive offset wheels. That's what they're built to work with, and most new cars have used such wheels for a long time now. Still, the options I've looked at are much better on intermediate and full size cars of at least the mid 50s. Not that a lot of them would look like much better than dog dirt on a car when you can get a really good look at them anyway, though Corvettes tend to use some very nice, forged aluminum parts. Seems like there's a truck down in Kansas that makes a late model Mustang rear look pretty darned good, too. Obviously pickup trucks are a natural home for wider suspensions.

Now, I'm already going to go off on a tangent. Having the whole engine, suspension, and steering drop out the bottom of the car is just about standard these days. As it happens, S550 Mustang front suspensions with brakes, steering and all can be found pretty easily at $1,000 or less. The rails are 30" apart and have risers on each side which can be cut down if needed to bolt it (4 bolts on each side) to the bottom of the frame rails of the car it's installed in. The K member has tails that take an S curve outward to take them out near the rocker panels, and the front track is 61".

It's a modified MacPherson strut suspension, but with great geometry, built for rear sump engines, and an electric steering rack that can be run without a computer.

Not for an early body style, but in the right car it could be a ready built solution to add modern brakes and suspension with the engine mounts already in place for a Coyote engine or an Ecoboost 4 or V6.

[jaybee]

The rear suspension off that S550 or S650 Mustang is a little harder to use. It's wide, at 67-69" wide. It's a really good unit, however, and if you don't happen to be building a truck, I found someone that cut it narrow enough to go under a 1980-era Toyota Supra by narrowing the cage and getting custom axles. Like the front end, the whole thing drops out of the car and is readily available used. Something about people wrecking Mustangs, I think.

A narrowed unit might make it hard to to keep the rubber diff mount bushings, or maybe use something a little different than the stock bushings that are pretty big. Converting to coilovers positions the lower mounts really wide for good motion ratio, allowing pretty light springs and offering naturally good control. This thing is solid, takes a healthy amount of power, has an available Torsen differential option, and it's a sophisticated, multilink setup that works very well.

[jaybee]

I'm trying not to be all Ford with this, and the Corvette IRS comes immediately to mind. It's purty, too.

The C4 runs right in that 61" flange to flange width, and it looks good, but it's not a particularly modern design, however, and like the old Jaguar rear end it uses the half shafts as suspension links. That's what gives them their reputation for handling that can bite an incautious driver. The center section is also built for 1980s horsepower, which is fine for a cruiser.

The C5/6 is a much better design, but it's also 6" wider and needs to be narrowed for most cars. These cars handle well enough to still be winning autocrosses today. I haven't looked into what it would take to put some other center section in place of the transaxle, or if the drivetrain can be lengthened to drive from the front seat of a longer wheelbase car.

[enjenjo]

There are a couple more choices. For one the Lincoln LC IRS. It is about 63" wide WMF to WMF. It was also used on some of the later Tbirds and Cougars, and XJS Jaguars. It mounts on a subframe with four large bushings with a spacing of roughly 32" bolt centers". The wheel bolt pattern is 110mm but there is a hub that will fit from a 03 Explorer with a 4.5" bolt pattern used with a bearing spacer on the LC axle. From the factory it had a factory air strut suspension suspension which can be replaced with a coil over shock They came with a 7.5" or 8.8" differential in the Tbird and Cougar, and with a 8" or 8.8" in the LC Lincoln. You can install a 8.8 diff in a 8"rear by using two left side axles, it bolts right in. If you mix and match axles with some other Fords you can narrow the subframe to get the rear WMF down to 59"

[jaybee]

That's good information to know, and information about some of these swaps seems to take a lot of work to find.

The earlier Thunderbird IRS and Mustang Cobra IRS are reasonably well known, but I'd think those are getting pretty hard to find by this time, especially since only certain models of Thunderbird/Cougar came with 8.8" center sections. The Thunderbirds are also mostly 4 lug wheels.

IRS Explorers and Explorer Sport Tracs through 2010 have a pretty nice setup with upper and lower control arms, but it isn't on a subframe. As a result, using one requires finding a frame you can cut apart in order to use it, or doing a frame swap to a near stock-height truck. Explorers 2011 and later are a unibody construction and significantly larger.

[jaybee]

Would it be worth considering something clear out of left field? Ford Escape.

It would be necessary to fit a different center section, because the stock rear diff would snap like it's made out of toothpicks, but they're decent, multilink suspensions that got stronger and more sophisticated with each generation, and mount to the unibody with a subframe. The hardest thing to deal with might be the use of trailing arms carrying the hubs and wheel bearings. These are allowed to flex longitudinally so the other links can control the camber curve. Attachment of those trailing arms might be the hard part of using this suspension. 

[kb426]

If you look at other brands such as Lexus, I think you will find more choices. I haven't researched them to know if that is good or poor. 2006 and newer Chrysler big cars are a possible.

[enjenjo]

The escape uses 23 splines on the differential and 26 splines in the hub. The whole rear end is Mazda parts and interchanges with nothing.

[Bruce Dorsi]

Don't know if current offerings are comparable, but I was impressed with the IRS on my Dad's 1983 Mercedes 300D.  ...1979 to 1985 were basically the same.
 
Rear track width is listed as 56.9" and has a "drop-out" center section.  ....The IRS unit is mounted in a cradle which mounted with 4(?) bolts.  ...Rear disc brakes.

I don't know if gas-engine models used the same rears.

https://www.encycarpedia.com/mercedes/79-300-d-w123-saloon

[jaybee]

That's quite a few possible donors. Now, how to get it into the car.

One option is to just weld whatever cage/cradle/K member right to the recipient. Nothing wrong with that. Most of the suspension links have some sort of rubber bushings on them, so their NVH characteristics surely won't be worse than the original cars of the 50s, 60s, and 70s. Especially if it's a full frame car, in which case the frame could probably do with an additional crossmember or two.

It is nice that the units can be serviced by dropping them out the bottom of the car.

What about unibody cars, like Camaros, Firebirds, Novas, and most of the Mopar and Ford cars of the 60s and 70s as well as just about everything built since then? They could use a little shock and vibration isolation and could certainly use the serviceability of a drop out suspension package.

As long as the attachment points are nice and solid, how much of a difference does the point of attachment make? Some parts of a unibody, and particularly with older unibodies, are stronger than others. Normally there are torque boxes and other reinforcement at the front leaf spring hanger. Is it important to tie the subframe to that point? What if you went to the kickover area at or slightly back from the bump stops, on the frame rails down low where they turn forward, and maybe a 5th point where a pinion snubber would hit?

What kind of mounts? The OEMs tend to use rubber bushings that are very large and also tend to promote wheel hop. Something more compact makes more sense. Poly bushings used as they often are for engine mounts, with a horizontal through bolt would seem to me to have a problem. They're perfect in the engine mount example because you want the engine to be able to rock but not to move fore and aft. Not so much in this application because you need to contain side forces for cornering as well as acceleration and braking forces. But mounts oriented to control one will be wrong for another.

How about shouldered bushings with a vertical through bolt? That would be most similar to stock-style bushings.

[kb426]

I have done 3. All have been bolt in. on the last unit, I used poly suspension material to make spacers to reduce the bore size down to bolt size. They were a light snug fit on both inner and outer dimensions. The older Ford 8.8's have the carried rubber mounted so mounting the cradle with isolators isn't important to me.

[jaybee]

I suspect the poly is enough to give a little isolation without letting the unit move very much at all.

Btw, that S550 Mustang unit you used most recently has the shocks mounted way out to the end of the control arm, which is good. The leverage ratio is excellent, which allows the shocks to be more supple and responsive. Converted to coilovers like you did, I see why you needed to reduce your spring rate. The spring has much better leverage on the wheel than with coils in the stock spring pocket, which doesn't make as much difference to function of the springs as it does the shocks, but it does require a different spring rate.