Wondering if anyone has successfully swapped in an IRS into their first gen. Im toying with the idea for now, looking specifically into a miata rear end or a ford 8.8 IRS.

 

Its a lot of work and not a bolt in.

 

several have tried, none have succeeded. to put IRS into a 1st gen requires basically back halving the whole car, which is a huge job. everyone also seems to think the FC suspension is an upgrade, and it isn't, at least the miata is ok.

 

I want to go the other way, put a solid axle in a Miata. The Miata rear suspension does some funky things with toe that make the handling unpredictable if you have enough power to make the suspension squat heavily. Unless you stiffen the suspension to the point where it might as well be bolted solidly to the tub.

 

we haven't run into that, but the miata doesn't have a lot of travel* so its very easy to have it hitting the bump stops, which make it skittish.


*for reasons unclear to me, the miata, Rx8 and newer Mazda's have 2-3" of bump travel and like a foot of droop/jounce/rebound travel. anyone who has tried to jack up an Rx8 knows what i mean, the car needs to be way high up, before a wheel comes off the ground, but put two people in the back and its on the bumpstops. miata is not as extreme, but its got more droop travel than bump travel.

of course the NC miata is backwards, the rear shocks top out, and it causes some really weird behavior

 

When my friend got a GoPro courtesy of the SCCA, he stuck it on his Miata and went autocrossing.

The toe curve looks like a hockey stick... under droop it stays neutralish but under bump it toes in massively.

 

Wow, it does change a lot!!

could it be intended to keep to tail of the car tucked in while cornering?

 

rear toe is stable, so yeah. every IRS toes in with squat, and usually adds a ton of negative camber too.

actually the rear from the factory is usually setup for max grip, with lots of camber and toe gain, and then the front is setup with less camber gain, so the car understeers. if you look at any car, you'll see that the rear has negative camber, and the front doesn't.

 

Having the rear suspension steer to the outside makes for a negative feedback loop, makes handling more stable. Mazda's rear suspension tweak for '84 did the same thing, lowering the front of the lower links made for more roll understeer by pulling the compressed side forward.

VW practically invented suspension kinematics for this... the twist beam rear suspensions they have traditionally used will have toe out under side load, which is sketchy at best and makes your Porsche spin out easily at worst, so VW realized that if they designed the suspension bushings with engineered in compliance and little ramps, they can cause toe IN under side loads.

And then there's Mazda's FC rear suspension design which took kinematics to the extreme, with toe-out and toe-in under side load depending on how MUCH load there was.

But all that usually only works right when the bushings are new, so it is more popular to do it with geometry. Downside with that is when some doofus lowers the suspension and puts it into the "cornering" regime...

 

don't forget the british! their cars didn't roll over like the VW's, but they still contributed to the kinematics