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Thoughts on using a Miata IRS cradle?

12K views 30 replies 22 participants last post by  elcam84 
#1 ·
Its been forever since I posted on here(sold my one project car, but about to buy another 66 coupe next week). Anyway, in the time since I sold my last mustang I bought a 91 Miata I drove for a few months...and they of course handle extremely well...but more importantly the whole IRS system is based on a cradle that bolts to the chassis with 4 bolts...the control arms(upper and lower), carrier, etc. Obviously the miata carrier isn't sufficient for a torquey v8 or even a built inline 6, but the cradles are easily modified to hold a Ford 8.8 IRS from a T-bird or Explorer, or Cobra(I actually have a Ford 8.8 mounted to a spare miata rear cradle right now). With the Miata track width being 56.2" and the 65-66 Mustang being 56" it makes it a very appealing option. Miata hubs can be swapped for T-bird/Cobra hubs with minimal modification(though the 5 lug bolt pattern on the T-bird hubs needs to be redrilled to 5x4.5, whereas the far more expensive Cobra hubs already have the right pattern).

The only real problem I see(aside from cutting down an re-splining whatever CV axles best fits the carrier/hub combo you choose(Moser does this for a little over $100 an axle if I remember correctly)) is the physical fitment of the cradle in a mustang chassis(mostly shock mounting and whether the thing will fit in the space meant for a solid axle). Anyway, purely theoretical discussion...practical option? maybe with a shorter strut assembly?
 

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#5 ·
With enough patience and fabrication skills I am sure you can figure it out. I'll be interested to see how it goes.
 
#6 ·
probably would have cut out the spring pockets of a wrecked Miata to fab up some for the coilovers.
 
#13 ·
The Miata spring pockets are actually on the outside of the frame rail(see attachment I grabbed from GRM). Its hard to say whether you would be able to duplicate the setup on a vintage mustang. I would really need to measure the frame rail spacing on both cars to see where the upper shock mount would have to end up(I suspect the frame rail spacing is wider on the mustang by about 5-6" which would require mounting inside the frame rails and likely coilover shocks strictly for the smaller diameter spring) In any case, it is something I plan to look further into because it really doesn't seem to difficult...at worst it would require some floor cutting/reshaping for clearance and some new spring pockets/rear shock "towers" welded in, as well as some type of frame to hang the cradle...all pretty simple fabrication.
 

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#7 ·
That would be an interesting exerciser. This might give you some hints and pointers of whats needed. https://dazecars.com/dazed/JaguarIRSSuccess.html

It seems the Miata would need mods to the mod as I guess they all would. Im not sure why the Jag rears were so famous in so many cars, maybe since it was one of the available IRS rear but these days there seems to be more cars to choose from, then again people that want such a thing can more readily buy an engineered kit off of a shelf. These kits may have stymied junk yard experimentation. Some rears I would want to rule out first are T-Birds and Lincoln Mark?s that had the IRS, as long as the springs would fit under the car I think you could make up the width differences at the wheels:shrug:
 
#10 ·
A few people kicked around a Tbird transplant here a few years back. I recall one standout difficulty with it was that the center section would protrude into the fuel tank's space. A lot. To the point you would just about have to do like Jaguar and fab up a pair of "saddle" tanks. Most folks lost interest at about that point.
 
#14 ·
I also vote for the mod kit that accepts the IRS from... a '99~ish Lincoln LSC (which had Cobra motor and IRS but can be found much cheaper...) ;)
 
#15 ·
The only way to answer the question is drop the springs and third member from a mustang in the air and see what happens when you roll the Miata rear in and try to fit. Its interesting as I was seeking a cheap posi for a TR3 I have with 4.1 gears and Miata center sections can be had relatively inexpensively. SO I have spent a lot of time staring at photos adn exploded views of Miata rears.



THe down side I believe would be resale, Who wants to include the word Miata in any Mustang discussion. Would I purchase a muscle car that had a third member mounted in a Miata frame? that would be pause for concern...


Great idea and thinking...
 
#16 ·
You dont get much better than a Miata for handling for under $50,000...they handle far better than a Cobra or Mark VIII because of the suspension design...which is why they are a top choice for any event that requires a car to handle...besides, its not a Miata rear end, axles or hubs you are using...those would be Ford 8.8. If your goal is handling, you already have a healthy respect for the Miata name. Its not like even monster Miatas with 600+HP LS1s or SBFs ever break a miata IRS cradle...its the axles that fail first, then the carrier....but no one builds a monster miata with the stock Miata carrier and axles...its either the Ford 8.8" carrier or the GM getrag carrier and always custom CV axles for high HP builds. Or look at it a different way...the Miata rear upper and lower control arms are FAR beefier than the OEM vintage mustang front upper and lower control arms...why would stronger parts give cause for concern? 50 year old Mustang parts are going to be far weaker...automotive technology has come a long way in the last 50 years...even the cheapest econobox comes with 4 wheel power disc brakes and has better acceleration, stopping, and handling than a classic mustang as much as we may not like to admit any such thing. Take just about any new car and do a chassis rigidity test compared to a classic mustang...you can guess the results. I love classic mustangs(and other vintage cars for that matter) but it doesn't mean I think their quality is superior to a Miata(or anything else on the road today) 50 years of advancing automotive technology certainly shows.
 
#18 ·
Why include the word "Miata" in your ad? Just say it has IRS, enough said. The differential is still Ford, the rest is just metal.

I wouldn't hesitate to buy a muscle car that has been improved with parts that work better. Then again, I am not into the whole toxic masculinity thing most "car guys" seem to buy into. My masculinity speaks for itself, the suspension in my Mustang doesn't have to do it for me.
 
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#19 ·
I don't know about the Lincoln irs, but the Mustang irs geometry sucks. And it's way too wide.
 
#23 ·
I wouldn't hesitate to buy a muscle car that has been improved with parts that work better. Then again, I am not into the whole toxic masculinity thing most "car guys" seem to buy into. My masculinity speaks for itself, the suspension in my Mustang doesn't have to do it for me.[/QUOTE]


great information. thanks for sharing
 
#24 · (Edited)
I have a few things to add, since around 2005-2010 I was playing around with second gen (FC) RX7s (LS swapped) and was the first guy to offer a kit to swap in a Cobra diff into the FCs. It was used in this car and may still hold the FC IRS record:



What does this have to do with a Miata, you might ask. The fun fact is that the FC and the NA-NB Miatas used the same differential mounting points to hang the diff from the chassis. This is the part that is missing from the diagram shown on the first page.

Both the Miata and the FC have two giants (16mm I think) studs that are welded into the frame structure that the diff mounts to. The front diff support is very different between the FC and the Miatas. The FC used a large rear crossmember and used a trailing arm set-up much like the ones found on 944 Porsches (the Japanese have always been great at taking existing designs and making them better). The Miatas had the cradle that was pictures and used a superior double A-arm arrangement.

I think this is all doable, however, I don't think it will be easy or cheap. When I did the FC parts, I worked with Frank at The Driveshaft Shop for all the work on the hubs and stub axles. He may even have billet options already available, but again, not cheap.

At the end of the day, I guess I would want to know what problem you are trying to solve and why you think a IRS system would do it for you. If it's just a since project, I totally get that.

Andrew

P.S. I was living in BGKY at the time and the cradles were made in Franklin TN at a shop called Earl Owen's fabrication. Wonder if old Earl is still around. Getting old sucks.
 
#27 ·
The cobra rear uses the same big cast arms and spring pockets too. It would fit a Fox with 8” wide wheels but that’s no fun lol. As far a vintage car, I’m not sure.


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#28 ·
The MK2 and MK 3 FFR Cobras use a TBird IRS along with custom shortened axles and fabricated upper/lower control arms. The MK 4, I think uses a Mustang IRS with fabricated control arms and corresponding axles. I see no reason why this couldn't be applied to an Early Mustang.
Here's the MK 4 IRS assembly
https://www.factoryfive.com/galleries/mk4/independent-rear-suspension/
 
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#29 ·
Sounds like a great idea wicked93gs, and seeing that you have a spare miata cradle there, you're set to try this! From your picture it looks like the 4 mounting bolts are on top, correct, where it looks like they are on a flat plane? The underside of our vm's looks kinda rounded. Doesn't look like a HUGE fab issue, fairly basic. The miata cradle looks like fairly light-weight stamped and formed steel, so adding that weight to the substructure of a vm is probably a wash vs the leaf springs and full axle you take out? How much does your cradle assembly weigh?

If you fab, you might also think of jigging up a tubular replacement cradle like the Original Venice Crew did with their Shelby IRS that matches the miata upper and lower ca's and bolts right into our vm's for a stronger and solid connection? If this whole think could be made a 'kit' using lots of readily available jy and aftermarket miata and ford 8.8 parts, I don't see why it couldn't be done for a fraction of the cost of an OVC or Heidt's IRS kit? Pipe dreaming here too...

Yours looks a bit lighter weight than the Jag IRS parts that I've seen. Weigh that cradle for starters.

https://www.heidts.com/part/64-70-mustang-irs-independent-rear-suspension-racing-kit
 
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