Thoughts on using a Miata IRS cradle? - Vintage Mustang Forums
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post #1 of 23 (permalink) Old 05-08-2019, 10:51 AM Thread Starter
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Thoughts on using a Miata IRS cradle?

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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post #2 of 23 (permalink) Old 05-08-2019, 12:46 PM
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Sounds completely doable. You'd need to strengthen the floor structure and tie that into the upper strut attachment. You'll probably lose some of the back seat, but that would be neat!
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post #3 of 23 (permalink) Old 05-08-2019, 01:47 PM
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I like the way you think wicked93!

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post #4 of 23 (permalink) Old 05-08-2019, 01:51 PM
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Never heard of this one, so I'm not sure what the pitfalls might be, but it sure sounds cool.

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post #5 of 23 (permalink) Old 05-08-2019, 01:51 PM
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With enough patience and fabrication skills I am sure you can figure it out. I'll be interested to see how it goes.

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post #6 of 23 (permalink) Old 05-08-2019, 05:49 PM
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probably would have cut out the spring pockets of a wrecked Miata to fab up some for the coilovers.

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post #7 of 23 (permalink) Old 05-09-2019, 11:25 AM
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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

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post #8 of 23 (permalink) Old 05-09-2019, 12:01 PM
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If you are going to keep it for a while I would consider doing it but its going to be LOTS of work, Lots of challenges along the way- but some people love that part of it!!
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post #9 of 23 (permalink) Old 05-09-2019, 01:48 PM
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Someone hand this man a beer please. This is a rad idea!
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post #10 of 23 (permalink) Old 05-09-2019, 01:53 PM
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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.
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post #11 of 23 (permalink) Old 05-09-2019, 02:32 PM
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A few years ago there was a guy that made the brackets to bolt in a cobra irs into these cars.
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post #12 of 23 (permalink) Old 05-09-2019, 03:23 PM
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Someone hand this man a beer please. This is a rad idea!
give him a full box! Cool idea
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post #13 of 23 (permalink) Old 05-10-2019, 11:37 AM Thread Starter
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Originally Posted by j persons View Post
probably would have cut out the spring pockets of a wrecked Miata to fab up some for the coilovers.


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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post #14 of 23 (permalink) Old 05-10-2019, 11:56 AM
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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...)

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post #15 of 23 (permalink) Old 05-10-2019, 12:05 PM
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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...

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