Independent Rear Suspension for a 70 Mustang

[wantahertzdonut]

I just bought a beautiful and totally solid 1970 Mach 1 this past weekend and of course I've got a list of mods lined up for it to do over the next couple years. One thing I've considered for a long time is getting a conversion kit to transplant an IRS from a 99-04 Cobra onto the car.

DVS makes a kit to make this a bolt on job but I've not had any luck in finding someone who's done the conversion and can tell me how well it fit or how it affected the handling of the car. Has anyone on here done the conversion?

I know there are a number of shortcomings with using the Cobra IRS, and I've heard that there are other companies that make stronger and lighter IRS kits that can be used with the DVS kit or are stand-alone bolt on systems. However I have not been able to locate a website or obtain any information on these aftermarket IRS's.

Can anyone tell me their experience with an IRS conversion or get me some leads on an aftermarket IRS?

[MrQuick]

Hi Jim, I spoke to a coupl guys that have run the set up and they were fairly satisfied with it. Most complaints where minor, the threat of breakage was a common concern. One guy complaint of a loud bang during hard acceleration another had a bad hop sensation when accelerating and during fast shifts.

Most race fabricators can put a set up together for you using the stronger Dana 44 diff.ie: Wayne Due might be up to it.
Not sure of your area but we might be setting up a shop near the "Sea of Tranquility" but that won't be till we get that whole breathing in a vacuum thingy. :pat:

[jeffandre]

Not sure of your area but we might be setting up a shop near the "Sea of Tranquility" but that won't be till we get that whole breathing in a vacuum thingy. :pat:

Where in the heck is the "Sea of Tranquility" anyway??? You're not going to the Big House are you? It's all Allen's fault, I swear...

[sinned]

Jeff....The Sea of Tranquility is on the Moon. Note that Jim has "The Moon" for his location.

[wantahertzdonut]

Hey, thanks for the heads up. I forgot I put "the moon" in as my location. It's an old inside joke (stupid, I know!)

I live in Cleveland. (I don't brag about it either) I should probably make note of that since now that I have a car, my location actually helps. (I've been on car forums for over 2 years during my search for a car, since I never had a car, there was no real point in my location)

That being aside...

MrQuick- the issues you mentioned (wheel hop, worries about breakage) are what I've been hearing and are common concerns with the Cobra IRS. Not to mention the setup is fairly heavy too.

Who is Wayne Due? I've heard the name but mostly in Chevy circles. My only concern is a one off fabrication would wind up costing me a fortune, and I'm far from being made of money. Being an engineer I could design and fab a setup myself here at work and at home, but if there's something already in production I'd just as soon go with something that's already been tested (plus having a homemade IRS could lead to a lot of trouble!)

In any case does Wayne Due have a website? If so can you post up the link?

[MrQuick]

www.waynedue.com

[wantahertzdonut]

Well I emailed him but no response. It looks like he does Chevy stuff only so I guess I'll have to warm up the welder and get creative!

[kmcanally]

I was wondering why you want to put an IRS in this car? If you have not already you might want to cruise the late model Mustang forums....most of those guys hate the Cobra IRS. And alot of those guys have been pulling out the IRS and replacing it with a solid axle. Also did you notice Ford went back to a solid axle for the 06 Cobra?

If you are dead set on an IRS you might want to consider the kit from CTM at http://www.geocities.com/ctmengineering/mustanghome.htm.
This kit is based on an original Ford Design from the 60's that never made it into production. Because it was planned for production it is a complete bolt-in design.

My 2 cents....a Martz Chassis 4 link coil-over conversion...

[MrQuick]

cause its cool...he he

[wantahertzdonut]

I have read a lot of guys swapping the IRS for solid axles, and they're all drag racers. On a hard launch with slicks on a track you will break the IRS eventually. Plus there's some wheel hop in there.

I'm not dead set on the IRS but it is an option I'd like to consider, partially for the smoother ride (not as much unspring weight bouncing around back there as you would with a solid axle on leaf springs), but also improved cornering on bumpy roads (which we have a lot of). Plus I'm finding used IRS's in junkyards from cars hit in the front for under $1500, so it's a relatively inexpensive swap.

KM- I think I've seen the CTM kit before but couldn't find anyone who's done the swap other than the guy who makes it. I'm just trying to get someone's opinion of how the IRS changed the handling of the old cars, to see if it's worth it basically.

I've considered 4 links but I'm trying to avoid permanent mods to the car because it is an original rust free Mach 1, and all the IRS swaps are bolt in. TCP has a nice looking laydown coil-over conversion but I've heard some pretty bad things about TCP's products so I'm probably ruling that setup out.

[race-rodz-inc]

my 2 cents. irs cars are a different animal when it comes to driving style, i dont think the unsprung weight should be a big deal, since the car isn't a race car. i would forget about the irs conversion, especially the cobra irs. if you are set on fixing the terrible factory rear set up.... go with the elenor style laydown coilovers. i dont think its 100% bolt in, but with the min of welding it requires.... i dont think it would be a big deal to "undo" someday.

no matter what direction you go with the car, subframe connectors should be the number 1 priority.

[wantahertzdonut]

Point taken.

Yes, subframe connectors will be going on the car, and I have a lot planned for the front, but at the moment there isn't much for the rear I can do other than 5-leaf aftermarket lowering springs. If the IRS is really that bad I'll start looking elsewhere.

[bnickel]

i doubt you'd be able to hurt a welll built irs with a mild 351w if you go to a stroker or a big block then it's a whole different story

[ProjectJ]

It's not exactly IRS (nor will the ride be as good) but have you concidered a Watts link setup. Many would argue it's the best way to go if you're gonna stay w/ solid axle. I know a guy in AV8SS (Brion Gluck) that runs a bolt-on kit from Fays2 (I think that's the spelling). It seems to work really well, and if he can't break something... it's unbreakable.

Just food for thought.:3gears:

[CAMAROBOY69]

Cool another IRS idea for an older car. :cool: Keep us updated on which rear end you decide to buy.
Protour Pinto was able to find an IRS out of a Jag for only $250.00 You might want to shoot him a PM if you have any questions.

[wantahertzdonut]

bnickel- at the moment the engine is pretty mild, but I plan to build another 351W and the goal is 450HP. Your thoughts on how the IRS will hold up to that?

ProjectJ- Watts links are the rear locating mechanism, correct? (I know what they are and how they work, but can't remember what they look like since I left my Vehicle Dynamics book at home, crap!) Can you get me a website with the setup you mentioned? I'd like to get some more info on this.

CAMAROBOY69- I've heard of guys putting Jag IRS units into Cobra kit cars but I haven't heard of anythign else. I'll PM the guy you mentioned.

[DarkBuddha]

I'm finding all your threads a bit late, but as an alternative to a Cobra IRS, and since you mentioned that a 4 link might be tollerable, have you checked out Air Ride's bolt-in airbag 4 link kit? Bret Voelkel bought a '69 fastback just to do the R&D on the project and now has a very nice, completely bolt-in, no mods required, airbag system that supposedly handles fantastic and rides even better. Bret has been taking that '69 all over and getting tons of track time in it, so it mush handle pretty good. Plus you'd have instant ride height adjustment. Just a thought... check it out at www.ridetech.com

[bnickel]

wow i don't know what's up with my topic notification so i didn't know about your question. but yes the cobra irs should hold up fine behind 450hp 351w it's the torque that will kill at actually that's why i said it wouldn't handle a big block or maybe even a stroker.

[ProjectJ]

ProjectJ- Watts links are the rear locating mechanism, correct? (I know what they are and how they work, but can't remember what they look like since I left my Vehicle Dynamics book at home, crap!) Can you get me a website with the setup you mentioned? I'd like to get some more info on this.

Yeah, it's basically just a different way of locating the housing. It seems to be the best compromise to help eliminate bind. here is a link to a company making them for those cars:
http://www.fays2.net/

This is a bit more of a race setup than street (rod-ends can be noisy and transmit alot of road noise), but it's certainly good for handling.

[wantahertzdonut]

Yeah, it's basically just a different way of locating the housing. It seems to be the best compromise to help eliminate bind. here is a link to a company making them for those cars:
http://www.fays2.net/

This is a bit more of a race setup than street (rod-ends can be noisy and transmit alot of road noise), but it's certainly good for handling.

I'm glad I checked back on this thread, thanks for the heads up on that watts link setup, I think I may be looking into that setup, or making one of my own even. It's a lot more simple than I would have guessed.

After driving my car more (going on 6 weeks of ownership now) I'm beginning to notice how much the car tends to want to kick the rear of the car out in turns. Even when going in a straight line on bumpy roads, the rear of the car tends to kick to the left. Am I safe to assume this is due to the rear axle shifting due to flex in the leaf springs? I know my bushings are probably shot and I need new springs and probably shocks, but I'm can I assume the remaining "drift" that can be eliminated with a watts link?

[funcars]

I have a 1990 C4 track car with an IRS and a 1970 mach I with leaf springs (currently) and a heavily modifed front end. I've taken both cars road racing a lot and the solid rear isn't that bad. I'm also running a black gold torque sensing diff with global west spring bushings and they help a lot. The only time there is really a substantial difference is cornering on really bumpy roads. Otherwise the IRS isn't really a large benefit.

Leaf springs for lateral location still are not good for the track so I'm going to a torque arm with a watts link and a cambered rear. For the street it's probably not worth it. I'd try some simple bolt-on solutions with the solid rear.

Good luck

[wantahertzdonut]

That's good to hear. I know the solid axles can work very well on a track, but some of the setups that I've seen (like the TCP coilover conversion, Global West's Stage 5, etc) just cost a damn fortune. My main interest in a rear suspension modification was started when I saw DVS's IRS conversion kit and then everything went from there.

But as you (funcars) said, for the street some of this stuff isn't really worth it. The more I drive, the more I realize I'm not going to be doing much racing simply because I just enjoy the car in the everyday. I was also thinking that with everything that I will do to the front (tubular control arms, Shelby Drop, bushings, steering, etc) that it would make sense to also do some work in the rear, but spending thousands of dollars on what everyone says are inferior or problem-ridden systems doesn't make much sense.

I really think I'm going to pursue the Watts Link setup if anything. It's simple enough that I can make it myself (big plus!) and it takes care of one of the biggest issues with a leaf spring rear (lateral location). At very least I'll try a Panhard rod for the most simple mod, and see how I like that. I will eventually autocross the car a few times a year so having some work done to the rear will be a help, I hope!

[Maxoctane]

Late model Crown Vics, Marquis' and Town Cars have 8.8s with a watts link. Some even come with disc brakes and traction lok. These cars are all over the salvage yards. Check one out, maybe with your engineering prowess you could fit one under there! Let me know cause I also am working on a 69 Mach One.

[4MuscleMachines]

If you really want to improve the rear end of your car and not go the high-end way, try composite springs from www.flex-a-form.com (http://www.flex-a-form.com). They are much lighter than your steel springs and work excellent on street and especially on a track.

Try this link too:

http://forums.stangnet.com/showthread.php?t=569700&highlight=composite+springs

[wantahertzdonut]

I'm going to investigate the feasability of carrying over a crown vic watts link into my car. I don't think the entire rear will be compatible because the CV/TC's are 4 link setups, not to mention probably wider. I am not going to carry over an entire 4 link setup and if I did, it would be off a Mustang.

The composite springs are very interesting for a number of reasons. Do you know if anybody using them has to use a sway bar?

[HsvToolFool]

Wantahertzdonut, I apologize for digging up an
old thread and I don't know if you even care
about this subject any more, but I wanted to
comment on installing an IRS on early unibody
cars.

I have a 66 Mustang convertible and I plan to
install a polished Kugel IRS (bling!) as part
of my restomod. I'm doing a lot of research on
this subject (how I found this thread) and even
discovered a few brave souls who've done this
extreme mod to early Mustangs...

www.reenmachine.com (http://www.reenmachine.com)

www.stang-aholics.com (http://www.stang-aholics.com)

Both of these builders appear to have boxed the
rear frame rails around the mounting points and
then welded in brackets for the IRS. In other
words, they mounted the IRS to the existing
stamped frame rails with some minor reinforcing.
I was very gung-ho about this approach until I
began calling local Pro rod-builders to get estimates.

Without exception, the consensus was that old
unibody cars cannot support the focused pressure
points that an IRS demands of a chassis. Once I
considered their arguments, I was forced to agree.

Leaf springs attach to the rear rails at four
points which spreads the weight over a large
area of the rear unibody. A leaf spring also
directs force forward and aft along the frame
rail's length, rather than entirely upward
against the sheet metal.

Most IRS (Jag/Heidts/Kugel, RCC, CWI, Cobra)
have a single crossmember directly above the
differential which carries the car's weight. This
focused weight isn't a concern for body-on-frame
suspensions because the IRS crossmember carries
the load to frame rails made from heavy steel
tubing which is strong enough to carry the load.
The Pros I spoke to believe (and I agree) that sheet
metal frame rails, even with moderate reinforcement,
will eventually fail around these critical mounting points.

Obviously, I've not given up hope. I plan to
remove the stamped frame rails and replace them
with custom mandrel-bent 2x3 (or 2x4) steel tubing
and then add fore and aft crossmembers. Once
this rear clip is welded into the unibody, the
car's weight can be carried by a single IRS
crossmember. The weight will spread through the
entire rear unibody and no stress fractures will
form. I hope.

I've spoken to Art Morrison's tech line and they
can bend the tubing to my design and put together
an entire kit (unwelded) for about $600...

www.artmorrison.com/frames/framekit.cfm (http://www.artmorrison.com/frames/framekit.cfm)

Perhaps it's over-engineered, it's certainly
unreasonable, but it sounds like a fun project
and a very cool ride. Now just give me a year
or two and I'll check back in with a progress
report.

[Stangonline.com]

Ask and you shall receive. :)
http://www.jktaylor.org/gallery/70-03

[468mike]

I plan on doing the same thing. However I am going to use the complete eng/trans, air ride, steering rack, K frame, etc., from a lincoln Mark VIII.

The reason for doing this is because I am a cheap.
I got a 68 coupe rust free roller body for $500, and the 90K mile lincoln was $500.

I am a painter so the body/paint work is no problem. I all I should need is some tires/wheels, and a interior kit. The total investment should less the 5 grand, including some shelby fiberglass parts for the body.

Mike

[ProTouring442]

I plan on doing the same thing. However I am going to use the complete eng/trans, air ride, steering rack, K frame, etc., from a lincoln Mark VIII.

The reason for doing this is because I am a cheap.
I got a 68 coupe rust free roller body for $500, and the 90K mile lincoln was $500.

I am a painter so the body/paint work is no problem. I all I should need is some tires/wheels, and a interior kit. The total investment should less the 5 grand, including some shelby fiberglass parts for the body.

Mike

I wish I still had my T-bird, I'd take a few measurements here... Seems to me that this approach makes a lot of sense. The Mark VIII is a unibody, take a whole bunch of measurements, cut and weld, and you have a nice hybrid.

Shiny Side Up!
Bill
'72 442 "Inamorata"

[468mike]

Thanks Bill, I think I will get the plasma cutter out and take part of the shock towers from the lincoln and weld it on to the stang, same with rear (shock mounts).

True, the stang will not be G-machine for killer handling but I don't really care that much, I intend to use it for a daily driver. And being the car is coupe it just isn't worth sinking alot of money into.
I will put the money into the camaro after I decide on the suspension/chassis setup.

I am going to visit Matt at AME next week and check out their stuff.

Mike