Hi everyone,
This thread is to show my process of designing and building a C3 rear driveline to hold high HP and retain the IRS using a Ford SUPER 8.8 differential and CV axles. All in CAD(Solidworks) so it should be a bolt together project when finished. Opinions and help are welcome!!! No massive tangents about completely different solutions please.

TLDR: Redesigning a rear suspension and driveline from the pinion yoke back on a C3 to provide 600+HP hard launch strength while retaining an IRS setup that is mostly bolt in for <$5,000 and retains a parking brake.


I am following in the footsteps of this wonderful thread and the amazing people who wrote it: https://www.corvetteforum.com/forums...he-irs-10.html

Background to the build and my decision making process is here: https://www.corvetteforum.com/forums...ith-600hp.html

My GrabCAD profile: https://grabcad.com/skinnypedalkicker-1/models

Goals and reasons:

C3 rear driveline parts are difficult to make live above 450 - 550HP. Options for upgrading the IRS are either drying up or getting extremely expensive($7k-15K). The first linked thread is an excellent path to follow but not everyone has that level of fabrication available to them. I am an engineer who used to be a technician and could do the fabrication myself. But I figured it would take me roughly the same amount of time and cost to design it all in CAD(SolidWorks) and just order the fabricated pieces. Which would be more fun and more challenging but most importantly, repeatable. If my current car ceases to exist (telephone pole or otherwise) I don’t want all my hard work to be lost in fabricated pieces that don’t have drawings.


COTS parts list:

1. Differential 2015-2023 Mustang SUPER 8.8 (front mounting ears need removed)

2. Rear Hubs 2014-2019 Corvette

3. CV Axle (2003-2016 Expedition) (will need shortened)

4. Coil overs (any brand) (size TBD)

5. Suspension links (size/length TBD)

6. Crossmember mounts (stock style replacement)

7. Brake Disk, 2pc Wilwood

8. Brake caliper Wilwood (120-14056 likely)

9. Parking brake calipers and cable kit Wilwood (mechanical cable style)

10. Pinion CV to Yoke adapter 1350 (Sonnax T35-ALFY-09 or T35-ALFY-10)

11. Driveshaft (1350-1350 custom length) (Adapt to Tremec TKX)

12. Bolts (McMaster)


Custom and Sheet metal fab parts:

1. Trailing arms

a. Mild steel plate. Laser, brake, weld, & powder

b. Johnny joints(front)

c. Design for OEM track width and ride height (offset style clearance for bigger tires)

d. Mounts for upper and lower suspension links

e. Mount for coil over

f. Possibly provision for stock leaf spring

g. Possibly mount for OEM brake caliper

2. Differential cradle

a. Mild steel plate. Laser, brake, weld, & powder

b. All original mounting locations to frame

c. Includes mounts for pinion, suspension links, and rear of Diff.

d. Clearance for diff cooler ports and bearing supported rear cover SKU: M4033G4

e. Possibly include adjustable geometry lower suspension link positions for drag, street, and autocross

3. Pinion snout hold down cap

a. Machined mild steel cap

to clamp pinion to front of diff cradle

b. Backup plan is a heavy-duty U bolt like the original thread used.

A friend wants to use the car in his November wedding and I am aiming for that so here we go……..

 

Suggestion:
If you are going to the trouble of building up a new trailing arm from scratch, don't.
Ditch it and go with two forward links. Like an 84 vette.
Then you can eliminate the excessive rear end on throttle drop C3s have since they do not have any anit-dive geometery.
It might look a lot like the early Riley designed Greenwood cars, or the stalled Hotchkis C3 suspension project.


Make the forward link mounting point heights adjustable, and you now have adjustable anti-dive.
Use two lower links and that controls the toe-in and you get rid of the weird C3 rear bump steer or toe-in on bump.
And then you get to design a smaller "hub" not an entire trailing arm. You need to hold the bearing and the 4 rod ends.
Win-win-win!

 

Ok you have my attention.

 

I love the idea. I really do. Having a setup that has adjustable anti dive or anti squat would help immensly. Hardest part is i dont think I can do that without doing "body off" modifications to the frame. Which i would love to do but i dont have the resources to do that currently. I know there are issues with the geometry that really dont make it handle great overall. At this point im still trying to do this mod without removing the body and doing minimal welding to the frame. At that point, if i was really cutting everything apart, it is probably just as easy to REALLY cut the frame and put in a subframe from a modern car. plus if i really changed the overall geometric style of the suspension the car would jump into 1 or 2 faster classes at most drag events for having "non factory style" suspension.

1. Ill make the upper and lower radius arms as long as i can to reduce the toe steer with bump.
2. im also going to try to make one of the link mounts adjustable so i can change the camber through travel from straight(drag), mild(street), and severe(autocross).


3.55 Diff is ordered and is in UPS! I'll need to scan it when i get it and i'll probably make a rough-in solid model of it for easier modeling.

 

Ok well you could just raise the front mount for the trailing arm a couple inches and gain a lot of antisquat.
Just move the bolt hole up. Most people would never even notice. The arm is level now...does not have to be.
I am even thinking of doing that on my street car. I can't stand the huge squat these cars have.

We did that with a 65 Corvair with a 455 Olds W30 in the back seat decades ago. Similar trailing arm style suspension.
Great traction. Wheel stands on street tires. Half-shafts were a different story....

 

Got the super 8. 8 today. 3.55s from a 2019 with 13k miles. ~$650 shipped. Now to scan it into cad. Still have to go buy the axles. I might weigh each because I'm curious of the difference. Hard to see in pics but the super 8.8 is really beefy compared to the 10 bolt.

 

I’m going to follow this closely. I may want to replicate this since I’m probably going to explode my Dana 44 soon. Since you are putting the CAD out there, I'm hoping it would be easy to use a service like sendcutsend to get parts cut out.

 

https://grabcad.com/skinnypedalkicker-1

A couple kits of parts are now on my GrabCad page.

Scanning and modeling the Super 8.8 is the next thing. if anyone has Ford drawings of it i would LOVE THEM. Then I could make all the dimensions exact instead of guessing.

 

If you have the super 8.8 already you can do some photogrammetry to get a decent 3D scan of it. You can do it with your phone, though I would double check critical dimensions.

 

I can use the scanner i used for the car. ill be able to make a great scan and CAD model thats very accurate but it just takes time....
https://www.einstar.com/products/pro...ble-3d-scanner

 

Very interesting. I love it when people really break the mold and do something new. Best wishes on this project!

 

Shock location question.
this is still super early as i am just generally laying out where my "non negotiable" components need to sit in the assembly so i can start designing around them.

I started messing around with a coilover and kinda had two ideas

1.
I was originally going to put the coilover in the VanSteel.com Coilover Conversion location becasue of compactness and welding on a bracket brace to the frame seemed easy and it is the original shock location after all. (LAST PICTURE)

2.
i thought of moving it behind the hub. this has several pros and cons. NOTES: Upper eyelet mount is roughly at the same height on the frame. I originally wanted to possibly retain mount for leaf springs in case anyone else desired it but the 8.8 diff is longer (shocker i know) and it really interferes with where the original spring mount was. i could move it back but i kinda figured it isnt worth the effort.
PROS:
-for space reasons so not everything is so compacted together
-I could design the upper mount into my Differential crossmember
-It can have a slight rearward tilt which will make it travel roughly tangent to the swinging arc of the hub rotating about the trailing arm mount which I would imagine is slightly better?
-It has more travel so the damping can be more easily controlled?
-more leverage so i can use a reduced spring rate?
-this will make it easier to mount the parking brake near the low forward part of the rotor.

CONS:
- bottom mounting point is physically farther from hub and will need more steel to support it. (more unsprung weight because more steel is needed)
- lower overall ground clearance becuase longer stroke shock is needed
- this technically would be rubber isolating the shock and spring from the frame now that i think about it. but maybe I just swap to Delrin diff crossmember bushings and i'll be good?

thoughts?

Another update. I got the CV axles, started drawing them up and they are BEEFY. 1.3" diameter shaft... I understand why the guy who did this originally never broke the factory units




he had.

 

The rear of a C2/C3 has precious little rear wheel travel already. IIRC it is 1.75" before trailing arm hits the rubber bump stop below the frame, at normal ride height.
Going much farther beyond that puts the half-shaft u-joints under an increased angle, specifically when they are maximally loaded at a standing start launch.
Ideally u-joints should be near 0* or neutral when under max load.
Tread carefully there.

 

So the general layout is starting to come together. the hardest part was the brakes. figuring out how to use aftermarket brakes but keep stock 15" clearances as well as making a mount for the stock caliper that mounts on the same mounting bracket but different holes. there were so many other small things that went into it. not super happy with how many pieces it is but I think its going to be cost effective. There is still a tall aluminum spacer that will get pin aligned to the steel. Going with round tubing would have been easier to design but oh my gosh that tubing is expensive! getting some laser cut and some light milled parts are way cheaper! and it makes other stuff easier to mount to because i dont have to interface to a round surface.

anyway here are a bunch of pictures of where im at. The wheel in the photos is a 18x10 @ 5.5"BS because that is about the most extreme backspacing ive heard being used.

Retaining the Leaf spring mount is kinda up in the air still. Im probably going with Ridetech's approach to Coilover mounting and ditchnig the oem style "forward of hub" setup.

Black piece is the CV boot
[img alt="top View. OEM trailing arm overlaid in Grey.
WHeel is 18*10 @ 5.5"]https://cimg0.ibsrv.net/gimg/www.corvetteforum.com-vbulletin/1938x1114/screenshot_2024_11_02_180842_b008e330684 c9c89feaa00d0834e0ed2fddc5310.jpg[/img]
top View. OEM trailing arm overlaid in Grey. WHeel is 18*10 @ 5.5"BS

OEM Caliper scan and disk. not original clocking location but they will fit.

Again OEM brakes
[img alt="1.960 BS hat
0.81"]https://cimg6.ibsrv.net/gimg/www.corvetteforum.com-vbulletin/2000x1149/screenshot_2024_11_02_175645_7184385ac67 dada4d22969620853d85b6e59c463.jpg[/img]
1.960 BS hat 0.81" Rotor Dynapro 6A Caliper (model is not quite to scale as it is only an artist rendition ive been using till I get to scan the real thing) Yes i know this is INSANE for a rear caliper but this matches the kit im going to get for the fronts and the only other calipers i could get looks DRASTICALLY different and i didnt care for that. This one is still the correct in^2 of piston area so it shouldnt ruin the balance.

Blue piece mated to the brown hub flange is the aluminum spacer. theres a steel backerplate(pink) then a 10Ga boxy piece(teal) and the dual caliper mount(red).

 

Reposting pic that got eliminated in last post.

 

Any progress Skinny?

 

Yeah some. I didn’t get to work on it during the holidays and am just now getting back into the swing of it.
1. Last night i got the anchor point bolts fixed into the 3D scaned model.
2. I also chose the Coilover location finally. It is going where the original shock mount was.
3. Next is to start mating items together more permanently so I can really start defining the individual dimensions of each piece. The hub is in about the right spot.
4. I THINK I’m going to go with Viking Warrior C218 shocks. I haven’t calculated the spring rate yet.
5. I have not started on the Diff mount yet. I started considering not doing the pinion mount and just bracing it all onto the rear crossmember that is part of the Frame that runs just behind the differential cross member. Then the Diff Bracing would all just be bolted to the Diff housing cover and the rear upper mount ears that come on it. I saw somewhere that some other company made a mount that worked like that. it would probably requrie welding and drilling though.
6. NOTE: the picture that includes the trailing arm has MANY things wrong with it. most of it isnt mated together correctly as i have just been dragging things around to ROUGHLY get them into place. The Green piece at the back was for mounting the coilover in the back but i have since abandond that. Also, it is shown it the "fully compressed against the bump stop" position.

 

I believe the way to eat an elephat is one bite at a time... Tomorrow might have some more progress too. We'll see.

 

Looks like you are making progress! Can’t wait to see what you come up with and how it all comes out (finished product). Once parts are ordered and made, please keep track of who the supplier or fabricator is/was so we can more easily duplicate. Thanks!

 

Today was a lot less "design" and a lot more "making the model work". really does feel like a big accomplishment though. starting with the trailing arm first is definitely biting off the biggest piece first but it was kinda necessary. The Diff mount has a lot less moving pieces. technically infinitely less.

check out the clip of the suspension links working in the model below. this was pretty much proof of concept. Anybody have any good info on good targets to hit for alignment through the sweep? or spots that would be good to put multiple mounting holes for the links? Wheel and tire shown is a 15x8x4BS on a "small tire" 275/60
Suspension movement first test.mkv