Well I finally got tierd of "hearing" about it but no one actually trying it. So I finally got a hold of a rear subframe assembly to start the fab work on. Picking it up this weekend and I'll probably start the messurments and planning in a few weeks when I move into my garage. I seariously doubt it'll just "bolt right in" but I also doubt it'll be much harder than installing a V8.

Step two will be a fully independant double "A" arm set up in the front, but that's a good ways off for now. In the mean time, here are some teaser pics...

 

Good on you man. I'll be keeping an eye out for progress. Good luck.

 

Cool pics... gets the creative juices flowing

 

Bye bye suspension travel

- Pete (Three inches up... three inches down... I slow for *nothing*)

 

hey! those headlights kinda look like mine

(car is even the same color )

i'd like to have IRS.

 

HEH, thats what i was thinking when i saw it

 

good stuff. btw, like ur car man, lookin good.

 

i have been secretly looking for a rear for a little while now off and on to do the same thing. i have a friend at work that used to frabricate race cars taht was going to give me a hand. ill wait for you to do it though and see what you ran into. good luck man

 

Well, I think the biggest thing is going to be the shock towers that protrude into the passenger compartment. Just from looking at that pic, I can see your going to run into MAJOR problems with unibody parts getting in the way. I had all the stuff in the rear of my car apart for 6 months, so I know what it all looks like. The shock pockets (the part the shock goes up into) WILL NOT fit that strut assembly, so there will have to be some major modification to make a strut tower to work with that. Other than that, there shouldn't be much trouble.

 

yea thats gonna be the biggest concern, how to attach the dampers at the top. Your gonna have to somehow make the stock shock pocket like double the size. Good luck, I'll be interested in seeing it all come togeter. BTW, where and for how much did you get the subframe? also wheres the diff and half shafts?

 

Shouldn't be any harder than repairing bin rust. Except, instead of cutting out cancerous metal and welding in new, you'd be cutting out the shock towers and welding in something different.

Now, "major modification" would be something like trying to stuff the FC rear suspension under a 1st-gen... you'd have to do major structural changes to get that to work, since the crossmember would have to be right where the rear torque box area is.

 

You could probably just make a box of sorts out of thick metal, bigger than the shock mounts, and weld it in the place of those and mount the struts that way. Theres plenty of good stuff to weld to back that in order to brace it.

 

Or just integrate it into a roll cage.... Already considered. The "real" problem is not going to be bolting the rear in, it's going to be setting up the diff, driveshaft, and most importantly, the suspension IE aftermarket shocks and springs that will match the IT front shocks and springs.

 

Lining the shock mounts up and getting all the angles just right should be the most technically difficult part of the swap - unless you have laser alignment equipment you can hook the car up to while you weld the suspension mounts together. Otherwise the geometry could easily get botched.

 

diff and driveshafts aren't really a problem, you have all the mounts on the subframe to mount the standard Mx5 items and I'm sure i've seen kits to fit stronger diffs too.

getting the subframe correctly alligned in the car is going to be tricky, as is the strut tops. Had you considered the relationship between the roll centres front - rear ? The Mx5 rear end will be significantly different to that of a car fitted with a Watts Linkage

~Mike

 

Double "A" arms. The alignment is dictated by the arms mounting point, and the tie rods, all of which are already located in the sub frame. The shocks are floating, so their possition isn't as important as it would be in a strut design like the front suspension. The only thing to consider is the spring and shock forces on the wheel (wheel rate), which will be dictated by the distance off the hub (already fixed), and the Cosine of the angle of the shock assembly, which will be the only thing I need to get correct. As long as I can locate the shocks withing a degree or two of each other, than the wheel rates should be pretty close.

Getting the subframe squared on the frame rails isn't that hard. A few messurments and I should be good. Worse thing that happens is I'm like 1/8" off and I can just fix the thrust angle with an alignment. As for the diff, a miata rear end will not be used as I doubt it's ability to handle the 400+ft/lbs of torque the car will be putting to the ground in the near future. Driveshaft isn't all that hard, I've already done custom ones, however the tunnel is offset but the diff isn't, so that will be fun to figure out. I'm probably going to go with a Cobra diff and custom axels. We'll see what the Monster Miata guys are running.

As for roll center, that's one of the main goals of this project. The Watt's link has a rediculously high roll center. That's the main benifit to going with a pahnard set up. I still think the hardest part is going to be finding shocks and springs that will complement a miata suspension on a longer car with a V8. Not a whole lot of info floating around out there for a V8/miata rear end, 1st gen RX-7 race car.....

 

This is a good idea...at least the using the miata rear suspension in an FB.

But I have to ask WHY?

Why got through the trouble?

I once had the exact same idea. I thought whw how cool would it be to have an IRS in an FB.

You are putting a V8 into a chassis and adding an IRS from another low powered car. Why not go to a Ford 9" and be done with it. It seems like it would be a lot easier and probably more effective with less work.

However, I think the idea of the IRS is a very good one. I guess the real question in my mind is, "why put a V8 into the car?"

To each his own.

I am actually very interested to see the progress you make.

 

You nailed that one. I was going to post the same thing. Your geometry is already figured out. Now about the wheel rates. If the shocks could fit within the current shock towers with a new top mount machined. You could calculate the original angle of the shocks verses the angle they are mounted in and calculate a spring rate change to give you the same wheel rate. Besides, I would figure the Mata has differing weight, weight distribution and weight transfer than the FB so you will need differing spring rates anyways.

 

unrelated question: what headlights are those, and how do you have them mounted?

 

Well I picked up the subframe and took some quick messurments myself. The initial messurments look extreamly promising. It really does look like it'll line up with the RX-7's frame rails. As for shock mounts, I can't be possative, but it apears that they will also line up directly with the stock spring mounts, meaning that cutting into the 7 should be minor. Also unknown to me was that the Miata already runs an offset differential, so I don't have to worry about my offset driveshaft, I can just get the axels cut to the correct length. I should start getting into the project hard core around mid Aug when I get my garage. Expect pictures....

As for why, anyone that knows enough about the FB rear suspension, knows it's the biggest draw back in the car. The solid rear doesn't lend itself to cornering like an IRS will. No camber changes, high unsprung mass, lack of toe adjustment, high roll centers, and a binding problem that is the result of poor geometry. Running say a Ford 9" would mearly give it more strength, however it wouldn't lend itself to high cornering forces. I've built this car as a track car, not a drag racer, and not a drifter. I only use it to drift because that's basically all it's good for right now until I can afford better wheels and tires. Anyone that has seen a Miata suspension knows what an awsome design it truly is. The IRS is light, is fully contained in a subframe, and basically shares all the dimensions of the FB. On paper it's an awsome idea.

Stop thinking about this as an FB RX-7 and start looking at it from an outside perspective of what it's going to be. A 2500lbs V8 race car with fully independant suspension. Pretty hard to top that IMO.

 

Good luck on your project, I hope you get the suspension dialed in nice! deffinately be watching this thread. I think you could top it though with A 2500lbs 13BT race car with fully independant suspension. Pretty hard to top that IMO. V8 in a rotary just gives me chills but good luck, if it's what makes you happy, go for it!

 

The solid axle is a drawback but not that bad of one. With the G-Force Engineering Tri-Link and Panhard bar, my car can sustain 1.2gs in the turns and it took about a half a day to weld into the car. All the stock parts are useable still...therefore servicing it will always be simple. I should note though that I have anything but stock spring rates. I have the ground control coil overs up front and the caster/camber plates also. The car is set up for autocrossing. It does better than I do.

Again, I love the idea of putting a miata rear suspension into an FB. But, please don't think right out of the box it will run better or even as good as a well sorted standard FB.

You sound like you know what you are doing with setting up the suspension. If you ever need any engineering help look me up. I have a decent model of the FB and the 1st gen Miata rear suspension.

I will be very interested in seeing how it turns out. Good Luck.

 

I'll be watching this thread closely, it sounds as if you have all the planning sorted nicely. I still don't know why everyone gets in a twist about the whole v8 thing, I like his car. On a side note, does anyone have pictures of a trilink and panhard bar installed?

 

So....you want a gutted FD? Because thats basically it.

And for those who are saying that this car won't handle as well as a well setup FB, all I have to say to you is "Yeah right". Have you ever seen a Miata on the track? Because if you have you'll know that they're all over the FB's. Same HP, same weight, better suspension.

 

I highly doubt that the day he gets this car rolling it will handle anywhere near as well as a completely stock Miata. That is the point I was making.

I don't disagree that this will be an improvement over a solid axle. But there are far more things that seperate the FB from the miata hanlding wise than there are things that are similar. I can think few right now, struts up front (miatas are double A arms), a much more flexible body than a miata, longer, recirc. ball steering, no PPF (power plant frame), weight up high (hard top vs. convert.) and he is putting a V8 in the thing that will change the front to rear balance even more than the stock FB vs. the miata.

This will take a lot of tuning and tweaking to make work. That is my point.

It can be done. I'm just not convinced it is worth it. Sounds like a fun project no matter what though.

Good luck.