C3/4/5 frankenstein frame

[mfain]

The criticism I've heard - and I could blame for an issue I have/had with my car is this:

Since the driveshaft is a loaded member, on hard corners the force spreads the carrier thus reducing the ability of the clutches to keep both tires spinning.

true? false? but given what you've said - would you achieve the same benefit as a C4 suspension by simply putting an upper bar and removing the c-clip?

please excuse the sentence construction, it's probably as stuffed up as my head...

I have not personally seen the posi clutch issue you describe. Several folks on this forum have tried various modifications to add an upper link - some better than others. I personally don't like sliding yolks in the axle shaft due to friction in the splines that sometimes causes unpredictable geometry changes. With big horsepower and big tires you put a lot of twisting forces on the splines. I prefer stout CV joints on the axle to absorb the minor in-out movement. An issue with an upper link using a C-3 frame and a C-4 upright is the lack of room for the link because of the position of the frame rail. Also, it is fairly difficult to adapt an upper link to the C-4 upright, although several guys have built custom uprights or modified C-3 stuff to accept the link. The same thing applies to using C5 control arms without moving the frame rails inboard (see the Jag/C6 picture above)

I have a rear differential from an 06 GTO that 1320gforce built with all the good stuff (and it howls like a banshee) - so this questions comes from "I have this on the shelf" and is probably one of those "you could, but it would still cost more than other options".... despite that I'll plow on -

why not maintain the C3 arms, and replace the differential with GTO, Viper, or even new CTS/Camaro center then adapt the CV joints?

The C-3 trailing arms introduce significant roll steer - that is the reason most guys (and GM as in the C-4) go with multi-link forward links and a toe control rod. The roll steer wasn't too bad with narrow, bias ply tires that you could overcome with throttle and just throw the car into the corner. A lot of fun, but not the fastest way around the track. With that said, I have been to several recent events and watched Danny Popp (72 Corvette) and Brian Hobaugh (66 Corvette) walk away with wins against some tough, late-model competition. Both are using basically stock C-3 IRS set-ups (stiff springs, modified roll center, and high dollar shocks), so it can be done. I watched Brian twist the end off one half-shaft in Scottsdale and fix it in time to make the final heat (which he won). Destroyed and replaced a $1000 JRi shock, but he still won. The apparent "trick" to both cars is minor modifications to the front control arm pick-ups, excellent caster gain with fairly high travel, big tires (315s front and rear), and superb shock valving.

BTW, the differential in the Jag picture is a late Camaro piece, and I am considering using a Strange Engineering S-60 (Dana 60) IRS differential that is used in the Art Morrison IRS. Very strong!

 

[Kid Vette]

The apparent "trick" to both cars is minor modifications to the front control arm pick-ups, excellent caster gain with fairly high travel, big tires (315s front and rear), and superb shock valving.

Care to share more details on the front control arms? Are they doing more than offsetting the uppers for more caster? Are they relocating upper and lowers?

 

[mfain]

The apparent "trick" to both cars is minor modifications to the front control arm pick-ups, excellent caster gain with fairly high travel, big tires (315s front and rear), and superb shock valving.

Care to share more details on the front control arms? Are they doing more than offsetting the uppers for more caster? Are they relocating upper and lowers?

Looks like they are going for more caster GAIN during suspension travel - spacing the front of the lower control arm down (putting a spacer block between the front of the control arm shaft and the frame), which causes more caster gain as the suspension compresses. Danny Popp's car had tubular lower control arms on it at the Goodguy's shoot-out, not the stamped lowers he had on it before. It also looks like they are using very high quality, adjustable shocks and, on the autocross, are using a slow rebound to pin the nose down through most of the corner. Watching Brian's car from the front, there is very little roll on corner entry and both front tires seem to be doing their fair share of the work. Brian in particular is dealing with a lot of scrub radius, which tells me he is probably compensating with lots of caster - it doesn't seem to be hurting him. Another thing I noticed, with the 200 tread wear rated tires are the very wide rims compared to tread width - not wide tires on too-narrow rims. I'll guess that some research has been done to optimize the tire contact patch with that tire. These are just observations on my part and do not reflect anything I was told by either driver. Oh, I almost forgot, the steering appeared to be very quick with the Borgeson steering boxes - so quick that during tight corners you could see the front wheels "twitching" like a dirt sprint car in response to the drivers' steering inputs. Looked perfect for a tight autocross track.

 

[SuperBuickGuy]

You point out what really got me thinking - Danny Popp's car. I've not seen it in person, so your observations are quite helpful. Seeing him spank the high-dollar muscle-car suspensions with the "old" Corvette system really does make me smile.

I know a guy with an 06 GTO that has a 1000 hp LS motor in front of his rear differential, and he's settled on a based-on-ford-9-inch designed center section and he has kept the IRS....

you've given me lots to consider - thank you.... we now return to our OP's thread :lol: sorry for the hijack

 

[vette427sbc]

.... we now return to our OP's thread :lol: sorry for the hijack

Carry on! All valuable info :drink:

 

[mrvette]

You point out what really got me thinking - Danny Popp's car. I've not seen it in person, so your observations are quite helpful. Seeing him spank the high-dollar muscle-car suspensions with the "old" Corvette system really does make me smile.

I know a guy with an 06 GTO that has a 1000 hp LS motor in front of his rear differential, and he's settled on a based-on-ford-9-inch designed center section and he has kept the IRS....

you've given me lots to consider - thank you.... we now return to our OP's thread :lol: sorry for the hijack

WELL, that makes me feel better about my lo-budget build, maybe not ALL the fun is lost.....

:harhar:

 

[mfain]

Here is a modification I have been planning on testing for some time - just haven't gotten around to it. It is a way to eliminate the adverse C-3 rear roll steer problem without making major modifications to the car's frame. One solution is with the C-4 suspension (or Guldstrand mod), but that requires surgery to the frame at the front trailing arm mount, and the forward links are shorter than I would like to see. The forward links provide an "instant center" of motion (at the intersection of their extended centerlines), but unlike a 4-link set-up on a solid axle, do not transmit torque from the axle to the chassis. The C-4 forward links merely transmit forward force through the instant center, although they do transmit braking torque since the caliper is mounted to the bearing support. The forward links of the C-4 suspension can therefore be replaced with a "ladder bar" that uses the original C-3 trailing arm forward pick-up point. A C-3 trailing arm can be modified (if you prefer the C-3 bearing configuration) to do the same thing. See the attached "crude" sketch, for which I apologize.
Finally, a toe control rod can then be added to either the C-4 upright or to a modified C-3 trailing arm as shown in the sketch. Note: you cannot simply add a toe control rod to the back of an unmodified, rigid C-3 trailing arm because the suspension will bind (too many things swinging in different arcs). All that remains is to add a pick-up fixture, either on the differential or the differential cross-member, to pick up the inboard end of the toe control rod. I have a couple of extra C-3 trailing arms and I will build an example to test.

 

[Sam Cogley]

Something is wonky with that image. It won't show up, and when I try to open it in a different tab, I get a page showing that I'm not logged in to VM. :bonkers:

 

[mfain]

Something is wonky with that image. It won't show up, and when I try to open it in a different tab, I get a page showing that I'm not logged in to VM. :bonkers:

Sam,

I'm logged in and the image is presented as a thumbnail. When I click on it, it just expands it to a larger view. The only time I've had the problem you're encountering is when I am not logged in, and then it shows "attachment" at the bottom and it won't let me open it. If you can't get it, PM me an e-mail and I will send it to you that way.

Pappy

 

[vette427sbc]

Pic working on my end... Seems like a good idea :hi:

 

[69427]

Here is a modification I have been planning on testing for some time - just haven't gotten around to it. It is a way to eliminate the adverse C-3 rear roll steer problem without making major modifications to the car's frame. One solution is with the C-4 suspension (or Guldstrand mod), but that requires surgery to the frame at the front trailing arm mount, and the forward links are shorter than I would like to see. The forward links provide an "instant center" of motion (at the intersection of their extended centerlines), but unlike a 4-link set-up on a solid axle, do not transmit torque from the axle to the chassis. The C-4 forward links merely transmit forward force through the instant center, although they do transmit braking torque since the caliper is mounted to the bearing support. The forward links of the C-4 suspension can therefore be replaced with a "ladder bar" that uses the original C-3 trailing arm forward pick-up point. A C-3 trailing arm can be modified (if you prefer the C-3 bearing configuration) to do the same thing. See the attached "crude" sketch, for which I apologize.
Finally, a toe control rod can then be added to either the C-4 upright or to a modified C-3 trailing arm as shown in the sketch. Note: you cannot simply add a toe control rod to the back of an unmodified, rigid C-3 trailing arm because the suspension will bind (too many things swinging in different arcs). All that remains is to add a pick-up fixture, either on the differential or the differential cross-member, to pick up the inboard end of the toe control rod. I have a couple of extra C-3 trailing arms and I will build an example to test.

View attachment 1814

Hey Pappy, we're on the same wavelength. :thumbs: Prior to putting in the C4 stuff I kicked around a modified trailing arm similar to your drawing. Went the C4 knuckle route though when I realized I had enough spare C4 stuff laying around from my street rod suspension conversion, but still had to make a bracket to adapt the toe arms to the C3 differential cover.

 

[Kid Vette]

The apparent "trick" to both cars is minor modifications to the front control arm pick-ups, excellent caster gain with fairly high travel, big tires (315s front and rear), and superb shock valving.

Care to share more details on the front control arms? Are they doing more than offsetting the uppers for more caster? Are they relocating upper and lowers?

Looks like they are going for more caster GAIN during suspension travel - spacing the front of the lower control arm down (putting a spacer block between the front of the control arm shaft and the frame), which causes more caster gain as the suspension compresses. Danny Popp's car had tubular lower control arms on it at the Goodguy's shoot-out, not the stamped lowers he had on it before. It also looks like they are using very high quality, adjustable shocks and, on the autocross, are using a slow rebound to pin the nose down through most of the corner. Watching Brian's car from the front, there is very little roll on corner entry and both front tires seem to be doing their fair share of the work. Brian in particular is dealing with a lot of scrub radius, which tells me he is probably compensating with lots of caster - it doesn't seem to be hurting him. Another thing I noticed, with the 200 tread wear rated tires are the very wide rims compared to tread width - not wide tires on too-narrow rims. I'll guess that some research has been done to optimize the tire contact patch with that tire. These are just observations on my part and do not reflect anything I was told by either driver. Oh, I almost forgot, the steering appeared to be very quick with the Borgeson steering boxes - so quick that during tight corners you could see the front wheels "twitching" like a dirt sprint car in response to the drivers' steering inputs. Looked perfect for a tight autocross track.

So the caster gain on compression would equate to camber gain on hard cornering but no camber gain on hard braking, correct?

 

[mfain]

"So the caster gain on compression would equate to camber gain on hard cornering but no camber gain on hard braking, correct?"

That is essentially correct - very minimal camber gain on compression (less that 1/4 degree per inch of compression), but lots of camber gain as the wheels are turned due to the high caster. I end up with about 3 degrees of negative camber on the outside and about 2 degrees of positive camber on the inside with 25 degrees of turn and at 4 inches of compression. All good!

Pappy

 

[vette427sbc]

Minor progress... Cut the top of the C-channel mount off. Even after this I think Im still going to have a little interference with the storage compartments
Before:


After:
I tried to maintain the center rib as best I could.


My stock diff was solid mounted to the frame at the sombreros and the pinion... Now this one has the rubber bushings on the batwing and I would think Im now going to lose a pretty significant 'crossmember' in my frame. Not sure if I feel comfortable using a solid bushing in the batwing mounts. Maybe poly? Opinions? Do the C4 guys change these out?

 

[renevette]

Well, at least you have a welded steel crossmember behind the batwing. My '80 has only the batwing diff with rubber mounts to connect the rear kickup frame members so i assume your setup is still stronger than a '80-'82 C3 frame which has no extra steel crossmember in the back, only a flimsy steel strip to support the gas tank.



I would love to weld in a '79< steel crossmember behind the batwing in my '80. Given the fact i have stripped my Vette to the bare frame i might even do so.

Great topic to follow! :thumbs:

 

[vette427sbc]

Wow I didnt know the later vettes did not have that crossmember... Yikes!

A lot of head scratching went on in the shop today... Here is where packaging all of this stuff turns into a nightmare.
This is ride height. The upper forward link is going to hit the underside of the frame rail. Im going to have to clearance the underside of the frame rail about 2.5" to give me 2.5" of compression.


the toe control rod is also very close to the batwing, but it should clear if I mount a rod end to the bottom rather than the top.


The spring perch and sway bar mount were interfering with the frame and the batwing, so off they go. Im likely going to trim a bit more off the back side where the perch was. This is a rough cut



Since the original swaybar mount is gone, I think Im going to use the lower shock mount as my new swaybar attachment point and run the swaybar along the crossmember before the frame kicks up. Im not sure where Im going to mount a coilover in here since using the stock lower shock mount would put the top of the shock inside my storage compartments.



Heres what I have so far into the suspension analyzer...
This is at my ride height and diff placement. I think I might move the diff up another inch or so. The frame mounts for the forward links are just dummy numbers for now until I can see where I can really put them. I would like to keep the misalignment (angled in) if that isnt going to cause me any problems.



Also after doing a bit of reading in the C4 section of CF it seems as if the D44 batwing is a bit weaker then the early D36 batwings. I see someone makes an adapter plate to mate the D36 wing to the D44 chunk but I dont really like the ovaling of the holes to get the bolts to line up. Can anyone confirm that these batwings are cast and not forged? Im going to have some braces welded in (assuming its not forged) so I feel a bit more comfortable solid mounting it and making it a structural member of the frame

 

[Bfit]

Vette 427
That wheel looks like a 17" rim , so I'm thinking this car is going to be very low at the ride hight you have set up.
I have a number of things worked out for this swap but my car is set up at 1" lower than stock?
What is the underside of the side frame rail to ground dimension.
Bfit

 

[Bfit]

photos






two photos of the swap I'm doing
frame is set at 6 3/4" from ground level to bottom of frame.
I have purchased a later Viper diff to see if it will work better than using the C4 Dana 44 for mounting reasons.
Bfit

 

[vette427sbc]

Im not at my shop currently so I dont know what mine is at that position. Im measuring frame height at the top of the #4 and #6 body mounts. My ride height puts them both at 10.5". This should put my rear fender lip at right around 27.5"

My current wheels are 18" with a 27" tall rear tire. I am pushing the track width out about 1" from stock C3. Im worried if I push the diff up any higher it will hit the floor, and I really dont want to cut that up. Im going to try and work out the other suspension link pickup points to compensate for the halfshafts being angled up. We'll see if I can get that to work or not. The screen shots I posted are just the stock pickup points as it sits now. I havent messed with it yet to get my desired geometry.

Keep us posted on the Viper diff... Id be interested to see how that works out!

EDITED

 

[Bfit]

My frame measures 12 1/2' from ground to the top of the body mounts on the side rails, mounts 2 and 3
I have 7" from bottom of frame to ground .
I will set suspension travel at total 4" and use Ridetech TQ shocks,

one of my other C3's ( 70 Model ) measures 28" to the lip on the rear and 26" to lip on the front .
I have 6 3/4' ground to the bottom of the frame just in front of the rear wheel
Bfit