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> Jensen Healey & Jensen GT Tech > Suspension > anyone know if a miata rear IRS subframe can be mounted to a JH?

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anyone know if a miata rear IRS subframe can be mounted to a JH?  Rating:  Rating
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 Posted: 09-12-2006 06:08 pm
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tweedpower1
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Looking for an IRS that can handle a rover v8. Yes, I know it not stock but the car has no engine anyway.... So no grumbling please.

 

Marc

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 Posted: 09-12-2006 06:19 pm
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Jensen Healey
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I hear the Nissan 300 ZX has the proper width but I have never checked the rumor.

 

Kurt

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 Posted: 09-12-2006 06:24 pm
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tweedpower1
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What I was really interested in was the underbody "geography".

If there was sufficient flat area and mounting possibilities to the rails.

 

Marc

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 Posted: 10-01-2006 11:22 pm
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Ron Earp
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Have you got the Miata diff in hand?  We've had a lot of experience with them in our SCCA SMs and the diff is a little odd in how it bolts into the subframe assembly in a Miata. I don't know how to duplicate this in a JH, although you could do it with a lot of cutting and torching in the area where the stock gas tank is now. You can move that around though, so no huge issue.

Be a lot of work for sure, but it'd come out fine.

You might also take a look at the Ford 7.5" IRS from the Thunderbirds (also a 8.8" version too but that would be way overkill).  It is a little more conventional and might fit better, but on either one, Mazda or Ford, I think you'll need the entire thing - uprights, arms, etc. and weld that joker in there.

R

Last edited on 10-02-2006 01:24 am by Ron Earp

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 Posted: 03-30-2007 07:16 pm
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Jensenman
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I have a JH5 and I also have a rotary powered Triumph Spitfire which uses Miata and RX7 drivetrain and suspension components including a home built IRS. (Look for an article on the Spitfire in an upcoming issue of Grassroots Motorsports.)

 The short answer to the Miata subframe question: no it won't fit under a J-H. That's not to say a custom IRS couldn't be built; in fact that's on my list of projects. No it's not simple, particularly since one of my criteria is that it be 'bolt in' with no cutting of the J-H.

The 300ZX, second gen RX7, third gen RX7, and 240 SX are all similar in track width to a J-H but again major chopping would be required to get their subframes under the car. Those are 'compromise' IRS's with weird toe changes for rear wheel steering so you probably wouldn't be happy with the results.  The Miata's geometry is a much better design.

 

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 Posted: 04-02-2007 03:03 pm
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Judson Manning
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Marc,

An IRS would be pretty cool, but a solid rear end would be a lot easier to engineer.  The Chevy S10 2x4 has a similar track, disc brakes and limited slip.  Weld in upper and lower arm mounts, a Panhard rod, and you're done.

Jensenman does have a valid point in that a lot of IRS are 'compromise' suspensions.  If I remember correctly, even the guys at Saleen retrofitted a solid axle into the first generation of Mustang that had IRS (SN95?).  If Saleen couldn't make an IRS work, I know I surely couldn't do it!

Of course all this depends if you are building a 'show' car or a 'go' car.  IRS will win hands down for a show car, however, if you are building a competition car stick with a solid rear end.

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 Posted: 04-02-2007 10:55 pm
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Jensenman
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The Saleen guys did swap a stick, or solid, axle in place of a Mustang GT IRS. The story goes that it was simpler, not necessarily better. Lots of the Mustang guys drag race and IRS 'anti squat' is very limited, creating weight transfer problems at launch.

I autocross and for that reason prefer an IRS, it's easier to tune out roll steer and engineer in camber gain.

 

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 Posted: 04-03-2007 12:26 am
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Judson Manning
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I always thought the Saleen guys kept the solid rear axle because they didn't want to re-invent all of that geometry?  I'll agree a lot of Mustang guys are drag-racers, but didn't a famous TV comic campaign a Saleen for years in SCCA/IMSA road-racing?

And to confuse matters further...When Trans-Am allowed IRS they found negligible gains over a solid rear axle or De-Dion tube.

In the conventional (factory) sense, a solid axle is simple, but when you get into de-coupled torque-arms, bird-cages, 6th Coils, pre-loaded panhard rods, etc., the circle track guys have some truely amazing set-ups.

 

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 Posted: 04-03-2007 07:04 pm
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Jensenman
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A solid axle setup, correctly designed, is as good as an IRS in many applications. A circle track car, since it only turns one direction, is a perfect place for a stick axle.

A lot of the solid rear axle setup secrets lie in the suspension's roll stiffness; the less body roll the better since that minimizes the roll steer of a four link setup. Of course that brings up its own set of problems. I battled that whole thing with my 1979 RX7.  The best set up early RX7's are using a 'tri link' setup which minimizes the roll oversteer. Thing is, by the time you do all that, it's just as easy to build an IRS. I am currently building a rotary powered Opel GT which has a stick axle stock. I have a complete 1st gen RX7 limited slip disc brake axle, my first thought was to narrow and use that. After crawling all over, measuring everything, and plotting suspension mount points for best rear geometry, heck I'm going to build an IRS for it, too.

Where an IRS shines is over uneven surfaces and in its tuneability. Being able to add and remove camber and toe in or out without having to replace the axle housing is a good thing. For instance, the Kumho V700 tires I was using on my RoSpit are discontinued, and when I went to the new ECSTA V710 it was easy to set it up. It took one afternoon, a set of toe plates, a bubble camber gauge, a memory pyrometer and a handful of shims.

Last edited on 04-03-2007 07:10 pm by Jensenman

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