FC Double A-Arm Conversion
 

Converting an FC to A-Arm suspension for F Prepared Autox. I've Attached the current mockup, current goal is -3.0 static with a camber gain of -1.0 per inch(comments welcome). I did a lot of googling and outside of the Locost community this hasn't been documented to my knowledge, so here ya go. Not a ton done yet, but it's accelerating. All Fabrication is courtesy of Frank Adams.

As an aside. I have a complete ISC Bilstein Coilover setup for sale as a result of the suspension change, pm/email for details. I bought functional camber plates for the kit instead of the shock shaft bending original design---

Very cool set up

cant wait to see finish result !!

Looks awesome! And the locost clguys ate usually pretty helpful, been thinking about building one for years!

Contact Tripoint engineering.

They build a FP (maybe EP back then?) FC in the 90s with double wishbones in the front. I'm sure you could ask them about it.

I've never seen any pictures of it though.

awesome project :)

its the holy grail! we have looked into it on and off over the years, and the only real "hard" part is that the FC uses the FWD style ball joint, every other RWD Mazda uses a tapered seat ball joint.

so you can't take a spindle say from and Rx8, fab up an upper arm and a couple of brackets and be done!

S4 control arms have bolt on ball joints so all you would have to do is fabricate a ball joint holder?

Yeah, would be awesome to be able to parts bin it, the reason you stated is why we are retaining the stock spindle. Still, I'm amazed at how relatively simple this is appearing, waiting for the other shoe to fall :). Currently waiting on a ball joint reamer to move farther, man those are pricey, any one know where to source a cheap one for two time use?

Lower Ball Joint is bolt on, we used the stock spindle-mcstrut attachments and my buddy fabbed an adapter to utilize a generic circle-track ball joint. We will then do some measuring, bend some tube, do some welding, cut some holes, weld some brackets, and call it done-

How much does your roll center move with that camber curve?

Thinking about what j9fd3s said... and knowing that the Miata and FC steering racks are the same except for width... I wonder if it'd be practical to get a Miata front suspension, widen the subframe by the difference in widths, use the RX-7 rack, and merge it with the FC nose.

Or tweak the strut geometry to get your desired camber curve. Mostly a factor of moving the strut towers inboard. If you need camber bolts to REDUCE negative camber, you're on the right track.

well since the FC uses a FWD ball joint that means 626 spindles go right on, use 626 struts, and go 4WD!

There would be additional penalties for modifications/substitutions of the subframe. If I stuck with the struts I would have just run a ton of static negative camber like I have in the past, but I've dealt with strut cars for the last 6 years and am going to try and eliminate that compromise-

Upper Control Arm laid out-

Looks good!

A guy I know did this on his v8 FC racecar. Very well done. He replicated the FD geometry as much as possible. Im considering doing something similar.

http://www.norotors.com/index.php?topic=8044.0

I remember seeing pics of a 510 with double wishbone front based off of FC parts years ago online. Wonder if I can find it again.

I think it was this car, but the links are down. FC lower arm and spindle with supra alum. upper arm.

Datsun 510 custom 5 lug and rotor conversion

Cool link, wish I had seen it earlier.

We are just planning on mounting the shock to the lower arm, and a circle-track weld-in balljoint on the uppers.

I like your design a lot better. maybe it's a good thing you didn't see it earlier :)

Thanks, hopefully it's strong enough :)

Current Mockup

Very nicely done! I am really considering doing this before the chassis is painted We have made the front end tubular, so This will be pretty straight forward.

However I am not a big fan of the lower ball joint that comes with the FC due to lack of adjustment. Are you doing anything in that department? It appears you are using the stock lower ball. I know AWR has a custom one built, but a local guy we know had some issues with it.

It might be easier/better on the long run for me to just buy an upright and build the double a-arms from scratch!

I just put the balljoint on top of the arm for the time being. If pish comes to shove, we may try to modify a stock one.

I was telling one of the guys I am working on a tubular lower arms which will mostly likely allow some adjustment for the lower ball joint. Will keep you updated on the progress.

Glad to see someone else trying this out. I'm doing the same thing, for the same class too!
Curious about the other specs, like roll center migration, caster, scrub, steering axis inclination etc. You don't have to give away ALL your secrets, but just wondering how you settled on the setup/dimensions that you did.

looks like the upper arm is longer than lower? that seems unlikely to have any kinda of decent camber gain unless you run a subterranean roll center

yeah, it does look very long....

if you're mounting it to the stock wheel well sheet metal it looks like you'll have pretty positive camber all the time also.

I am guessing he is going through the wheel-well and putting brackets on top of the stock frame which might get mighty close to the steering linkages!

What kingpin inclination angle are you running? Also did you put the control arm pickup points/lengths/balljoint loactions into any suspension software to check roll center at normal ride height and various other heights as well as the camber gain curve?

Fairly sure it was a Chevy 2.8 or VG30 engined car.

I remember seeing it, and thinking "Bad Idea." when I saw how he put the springs and shocks on the lower arms. I don't think the FC ball joint would handle tension loads very well, usually suspensions that load the control arm like that will have ball joints at least twice as large.

Where did you go Chops? We need more details! ;-)

Sorry, still trying to get caught up after the El Toro Pro, trying to tune an FRS for STX this season. As suspected, it would appear we have an upper arm length issue, working on a fix, will post pics when we figure it out :)

What you said. Not too worried about steering linkage, thats easy to move, more worried about motor.

As for geometry, looking for -7 caster, with the aforementioned camber gain profile. Shortness of the arm is going to make that tricky, although I had a discussion this weekend with a top flight STC driver who goes from -6 to -11 in 2 inches up front, so maybe it will work ok-

I estimated that 2" of bump/droop at the tire is about 5 degrees of chassis roll. Feel free to double check my numbers though.

Need some dimensions, pulled the motor before I decided on this route. Can anyone provide a rough dimension of the distance from the frame-rail to the engine? Bonus points if you can provide the dimension from the firewall to the spark plug locations and the amount of intrusion the plugs have- :)

PM sent

Movement again, but no pics yet. Strut tower is getting cut out, and a a tubular coilover/upper arm pickup structure is being attached to the frame rail. The thought of just cutting the front of the car off has crossed my mind...

If you still need some dimensions and cam_ron didn't supply them, PM me since I have a CAD model of the front chassis with turbo engine installed. Presumably you don't have a turbo to worry about.

Interesting you say this. The first time I sawa a double A arm setup on the front of an FC was on an RX7 out of Quebec about 10 years ago. They had cut everything off above the lowest frame channels and built up the whole front of the car like a Late Model Stock Car with a half tubular front end. The car worked well, but they told me that they had issues with cracking welds on the firewall and they were thinking of tieing everything together with the 8 point cage. To my way of thinking, this was way to much work and almost like building a tube frame car inside of an FC shell.

Eric

That's what I ended up doing. 2x2 frame rails to the front and cage the front tower and tied it into the rest of the chassis. One of the main reasons behind this was the future double a-arms that I had planned which might happen earlier!

You can see it in the following pictures:
http://www.lamanracing.com/wp-conten...7/img_0799.jpg

http://www.lamanracing.com/wp-conten...7/img_0800.jpg

Sorry for the second post: let me warn you that if you continue on this path, you will wish you just did a tube-chassis and put an FC body on it. All that is left of my car is 1/4 of the original firewall and the roof... haha

So, this is moving slow. Project is active again, pics pending. Got the drivers side tacked on, pic to be posted shortly.

Rethinking the FP thoughts as I'be been spoiled by 5.0 coyote power in ESP this year, debating whether or not to go with a renesis or get killed by Fred with a turbo 13b or LSX.

Tacked up to check geometry

I was looking at a Miata being scrapped out and saw that their front subframe holds both upper/lower arms and steering rack.

Seems like Miata would be good candidate for double wishbone conversion.

http://i54.tinypic.com/14vod8k.jpg

Looks like you would have to chop the FC front unibody to clear the upper arm mounts.

the miata subframe doesn't hold the rotary that well either, maybe FC subframe, and miata arms/hubs/etc?

Chops, how are you attaching the coilover on the bottom? Hopefully not to the lower control arm, like before.

Lol, why yes we are. We think it will be strong enough. If it bends we'll have to rethink the mount, mebbe build a mount structure off of the spindle adapter, which means shortening the shocks. Could build lower arms as well I suppose.

It looks really similar to how the stock miata suspension is laid out above... the aluminum arms were already built with withstand the sway bar forces at that point, I don't think it should be a problem.... (but I'm not a mechanical engineer)

No way will the FC's lower ball joint handle all suspension loads, you'll definately want to rethink that joint. Its a section of ~1/2"x1.5" flat bar... Relatively low resistance to bending. If anything, you'll want to weld a vertical web lengthwise along the ball joint, or something like that.

Switching to a tubular arm with (larger) ball joint would make more sense. Good 'ole circle track parts ;-)

I'm not worried about bending the ball joint holder, where it connects to the lower control arm. I'm worried about the ball joint stud connection to the spindle. It only held in place by a small amount of clamping force from a single bolt... what maybe a 10-24 bolt? The only vertical forces it has to withstand in the stock configuration is from the sway bar. But now you're running ALL vertical spring/damping forces through a joint that's meant to handle mainly lateral forces. I'd put money on the ball joint pulling out of the bottom of the spindle.

That's easily fixed :)

https://lh3.googleusercontent.com/-y..._164758417.jpg