Bench Build- RX8 Rally Car

As always, I am scheming about the next big project.  I will eventually want to replace the XR4Ti with another RWD rally car- Mazda contingency and discounts are appealing, and FC/FB RX7s are extremely tight with a rally cage, not to mention there are already far fewer of them laying around (for spares) than there are RX8s.  I am not afraid of fabrication as long as the result is tough and serviceable.  Here is a basic outline of my thoughts, with questions in bold:

Cage/Shell: Seam weld, reinforce or replace front suspension, cut out and replace rear suspension entirely.  Retain use of the rear doors, the large and accessible cabin is one of the best features of this platform.

Front Suspension:  Must replace brakes to run 15" wheels, 8" travel should be attainable with just a custom coilover.  Has anyone broken this or NC Miata front suspension?  It looks flimsy but otherwise functional.  Other potential solutions include making steel control arms to replace the aluminum ones, or graft in an FC subframe for simpler strut suspension that I know is tough enough.

Rear Suspension:  Cut out and replace with 4-linked Toyota truck axle.  Links and shocks mount to cage, redesign trunk area for fuel cell since the tank will need to move.

Drivetrain:  Custom steel driveshaft to facilitate live axle and get rid of the flimsy carbon thing.  Is there any reason this transmission can't use a typical driveshaft?  Stick with the Renesis initially, possibility of standalone and ported engine in the future.

What am I not thinking of?  What other weaknesses does this platform have that would need to be addressed?

Well other than the obvious achey-breaky spinning triangle engine, they seem to do well as track cars. Do you have to change the rear suspension to accommodate a fuel cell? If not, why do it? The factory rear suspension is a good design.

In reply to GameboyRMH :

The factory rear suspension doesn't have much travel and looks even flimsier than the front.

Ah that could make sense then. Could steel control arms + increasing strut travel (maybe with helper/tender springs) + steel driveshaft + maybe limiting straps do enough to address those problems? It would be a lot of work but so is the 4-link live axle conversion.

In reply to GameboyRMH :

As far as I can tell, the rear suspension will do weird E36 M3 if allowed to move more than it already does.  I'd much rather do a (proven to work well on every Escort ever) 4 link once than spend a bunch of time iteratively chasing strength and travel in a Miata multilink suspension.

How does a rear suspension redesign affect car classification?

In reply to Indy-Guy :

Doesn't.  In the 2wd classes I care about (gr2, gr5, 2wd light, 2wd heavy) nearly everything is unrestricted- I can't make a FWD car into a RWD one or move the engine to the opposite end, but otherwise I can do whatever I want.

Have to admit this front suspension picture gives me concerns:  

Yeah that LCA could break just outboard of where the strut connects in a hard landing/pothole at speed. What happened in that pic was probably a baljoint failing simply due to excessive wear. A steel LCA would be a good idea, maybe with uniballs if you want to be extra sure.

In reply to GameboyRMH :

I think the failure in the picture would happen with a hard enough side-hit on the wheel (like a rock or a stump).  Agreed about the way the LCA would fail.  This is why struts are nice, one beefy tube, two beefy bushings, a strong control arm, and a strong balljoint and you're set.  With double wishbones you get to double all of those, although you get to remove the strut tube from the equation.

However, if all that stuff up front is strong enough that it will only break in a decently hard crash, it looks light enough to carry spares onboard.  Just awfully hard to tell the difference between that and will break if I land a little funny.

There's no good reason that those have to stay aluminum though. Buncha steel tubes welded into triangles would fix all of that. As a bonus, it would let you change the lower shock mount (like maybe an eye lower down, though the arm) which would let you run a longer shock. 

Now post pictures of the rear and let's reengineer that. 

In reply to mazdeuce - Seth :

OK.  Take this, with its' gazillion little links attached to a gazillion little tabs and dumb shock mounts:  

Sawzall a giant berkeleying hole in the car and do this instead, maybe with a panhard instead of the watts link:  

Yeah I agree that making radical changes to the rear suspension isn't a bad idea. After taking a close look at the design on the RX8's, those stubby arms are only acceptable if you don't need much suspension travel, and to fix that you'd have to build a custom subframe, which could cause you to run into packaging problems. Also there's not much room to make the strut longer by lowering the connection point to the upright because of where the axle shaft is. You'd have to do something serious like switching to pushrod suspension or moving the upper (and lower) strut mount location to fix that.

Four link all the things!

Alternatively, if I'm fabbing everything anyway, and I just want the RX8 for the Mazda contingency and nice roomy interior, what else has a nice box shaped cab to build off of?

I can shorten the wheelbase and move the radiator and stuff to the back...
 

What about a complete rear subframe/suspension transplant? 

BMW E32 is stout, compact vertically, lots of gear ratios to choose from, and LSD available.

In reply to Indy-Guy :

If I liked trailing arms I would be happier with my Merkur 

I thought you said (another thread maybe?) that trucks were terrible rally cars?

In reply to ¯\_(ツ)_/¯ :

Fair enough, but simple and self contained independent rear suspension would probably result in better driving dynamics?

In reply to java230 :

They are.  It's very hard to get good weight distribution out of one, and a regular cab doesn't have enough room.  Would need to do some math to figure out what the distribution for a B2200 ext cab with a 13B set as far back as possible, battery/radiator moved behind the cab, and spares hanging off the ass end would be.  My guess is "still not good".

In reply to Indy-Guy :

Not according to the old Escort, Celica, or RX7 rally cars that kicked ass back in the 70s and 80s.  Live axle with a 4 link seems to be the answer for RWD on gravel.

Rally truck gets my vote!  Sturdy, unique and cool.

In reply to ¯\_(ツ)_/¯ :

So rally truck is awesome. OK do that. 

I think there is something to be said about unique, but also parts could be a bitch. Desert guys have used them for years though, yeah not quite the same, but still race truck. Extended cab?

Before going away from what already works on the RX-8, I'd suggest calling PacoMotorsports and see what he has in the works for the NC/RX-8 platform.  He's a very cool guy and always looking to develop new things for the Miata aftermarket. 

In reply to KyAllroad (Jeremy) :

I wasn't under the impression Paco made anything that was actually competition oriented.