WVZR-1

OP - show us some snapshots. I've been around some nice, some very well engineered C2 and mid-year resto-mod projects all done with C4 components - "components", I've also seen "other than" a couple times.

Nearly every "other than" if the owner had it to do over they would have likely spent the money for an SRiii or similar chassis. I don't know that any of the buyers of the "other than" kept the cars, I believe they were sold and one I know did an SRiii later.

I believe one of the guys is doing a Street Shop chassis mid-year with C5 or C6 components now. It may not be Street Shop but it's damn nice. I don't believe it's SRiii.

I don't quite understand "hanging the body" doing this the way you mention. I just don't -

RyanB88

Ok sorry I was out of town this week and wasn't able to get any pictures.

Below are a couple pictures of where the suspension sits right now. As I stated in the beginning of the thread, I would like to lower the car about 1.5", and I'm not sure whether to do it by just buying a lowering kit/coilovers, or raise the suspension mounting points in the rear.

MatthewMiller

So this may be a dumb question, but how do you know how high the car will sit at each end once it's finished and setting at full weight? Unless these are old pics and the car is mostly finished now, and you've actually tried it? Anyway, rear C4 suspensions are trivially easy to make adjustable for ride height, so I'd just go that route and set the ride height wherever you want it once it's finished and you can see the actual static ride height.

Churchkey

Theory is well & good however for this issue I suggest practical application.

With the rear suspension @ the current ride height ( to high) & no spring use a camber gauge or digital level to zero the camber. Raise the corner 2" & record the change.

Rear corner 1.5" higher zero the camber then repeat as above. SWAG the camber will
go more negative in 2" of bump = good.

Top dog bone @ or near level will move the instant center down = good.

If toe change during bump / droop is a concern use toe plates & dial indicators to check. See pic.
Heims & spacers @ the rod ends can be used to eliminate toe change.

Good luck
jpg.gif 8-16-13.JPG (130.2 KB)

RyanB88

Well that's a good question... These aren't old pictures, but we have 90% of the parts. When we put the body/engine/trans/bumpers/glass on, the car came down a bit, but still needs to come down a little further. Things like the interior still need to be added, but I don't think it will take the car down to where I want it. The car is lighter than the stock C4, so spring rates may need to change as well. However, I can weight the car and come up with reasonable spring rates. I feel pretty comfortable with that part of it. I didn't mention that part because I didn't want to muddy the waters. My main concern is geometry.

Hopefully that makes sense.

RyanB88

Ha, I guess I could always measure it and see what I come up with.

If the camber gain is reasonably close to stock ride height, I won't worry about it. I guess I wanted to make sure I wasn't overlooking something that was going to come back and bite me.

MatthewMiller

It makes sense, but I'd wait to put everything together and then see how it sits. Let's reframe how we're looking at the possible problem.
There's a big difference between: 1) it's only sitting higher than a stock C4 because it's lighter; and 2) it's sitting higher than you want it to because one or both subframes are mounted too low on he chassis.

In the case of #2, we're saying that the suspension geometry at static ride height is similar to a stock C4's, so that the suspension has the right amount of preload from the unsprung weight. In this case, the problem is that the chassis was mounted a bit too high on the suspension. But I'm guessing this is not the case. I'm guessing you measured for this before setting up the mounting points for the suspension: e.g., by measuring the height of the pumpkin relative to the frame rails or the proposed fender openings, or something like that. but if I'm guessing wrong and #2 is the issue, then the truly proper fix is to change the mounting points of the entire front and rear subframes.

In the case of #1, we're saying that the car is lighter than a stock C4 and therefore the preload for the stock springs at their stock ride heights is wrong. In this case, the suspension geometry at static ride height is actually off: the arms will all be angled too high going from wheel to frame because there isn't enough weight pushing the springs down to stock C4 ride height. In this case, the correct fix is to lower the car on the springs to regain correct ride height and suspension geometry. This is no different than if you took 500lbs off an otherwise stock C4: the C4 would ride too high as well.

So this all comes down to two things. First, when you loaded everything onto the car and let it sit, did you roll the car around to let the suspension settle? The contact patches go through an arc in front view as the suspension travels, so they will hold the car up if you don't roll the car to let the suspension settle. If you didn't, then you didn't get the true ride height measurements. Second, when you did this, were the control arm angles (or the relative heights of each control arm's two ends) close to those of a stock C4? It sounds to me like the answer is "no," but that's just a guess on my part.

WVZR-1

I don't recognize the beam used at the pinion snout. Where was that sourced and how is it attached at the other end. I'd think that could potentially create issues. I'm more accustomed to a bracket that provides fixed height support and makes the differential fixed in position. I'm also accustomed to seeing modifications between the frame rails and adding an appropriate cross-member for the transmission that provides appropriate drive-line angle.

Perhaps I'm missing something but I don't think so.

Have you ever had this "rolling"?

I believe I understand the front but dimensions are the tell all. Did you use a chassis builders template for the front or is this all your creation?

MatthewMiller

I'm glad you brought this up. It looks like it would work fine in place of the stock C-beam, since it only has to resist vertical bending. But it reminds me of something important about pinion angles. The C-beam setup fixes the pinion angles, no matter the relative heights above ground of the trans tailshaft and the diff center section. So if the OP "raises" the rear subframe in the frame, it will change the angles of the center section and retain proper pinion angle. It will also change the angle of the engine/trans combo, if that matters. I'm not saying this changes the approach the OP should take, just pointing out that a C-beam setup is different in this respect than a traditional crossmember and rear subframe mount setup.

Churchkey

Looks similar to a Firebird torque arm set up to me it should work ok.

IMHO Guesstimating finished ride height is like picking lottery #'s.

Suggest this method: Frame level side/side & front/rear, springs removed make tube or threaded rod spacers for each corner. I like 1/2 threaded rod to adjust suspension heights.

Operational (driving) bottom frame height from the ground lets use 6" for this example.
Jack stand height: Set them @ 16" = 10" datum line.

Nominal hub center line to ground 13" this figure includes a 1" tire squat radius.

Adjust corner heights using the hub center line of 23" IE 13" squat radius + 10" datum line.

Set camber / caster, check / set toe, bump steer etc in the above configuration.

When the car is built & on the ground adjust or change springs for desired ride height.
When you get it where you want it do a final alignment.

Tom400CFI

All of this^^

I would ensure that the diff pinion is at the same elevation, relative to the frame rails, as the original C3 diff was...then adjust ride height with the spring bolts and/or spring rate.

I just went through this on my 'Vette Kart project; I removed ~1000 lbs from an '89 and no surprise, it sat way too high. I made longer spring bolts for the rear to get the rear back down where it belonged and inIN THE FRONT, I moved the spring UNDER the control arms and installed bolts so it functions similar to the rear. Car is back to stock ride height, now.

MatthewMiller

Yes, that's the perfect example of what I was talking about. I don't know why I didn't think to link to your thread!

RyanB88

Well it is case #2. I originally set the suspension so that the desired ride height would have stock C4 suspension geometry. This way the suspension arms would be at the same angle as a stock C4. However, now I have decided that I would like the rear to be about 1.5” lower than I had originally intended. This means either lowering the suspension so that the geometry would be similar to a C4 Corvette that had been lowered, or remounting the entire rear suspension/differential to maintain stock geometry at the new desired ride height. Obviously the second option is a lot more work, but I would rather spend the time if that is the better option.

That is why I wanted to opinion from those running lowered C4’s to see if I would be advantageous to just lower the car 1.5” (on coilovers) and keep the suspension mounting points where they are.

BTW: I am using a template from Newman Car Creations. They also provided the torque arm and the tie rod location kit in the rear. Unfortunately, they are now out of business, so I can no longer consult them.

MatthewMiller

Okay, well that clarifies a lot. So, this really a matter of just the rear ride height relative to the front. Conversely, we can think of it as an issue of the angle at which the frame and body sit on the subframes. I THINK the proper textbook solution is to change the mounting height for the rear suspension if you're really sure you want it that much lower in back. BUT, I know from my own experience that there is no problem running 1.5" lower than stock on either end of a C4 in terms of geometry. The only wildcard I can think of in just lowering the car on the springs at the rear is that you are only really lowering one end. But in that case, I can't really see any harm done to the geometry. As I said originally, you're actually going to lower the roll center and anti-dive in the back by doing this, which should be a good thing overall, especially for a lighter car. It will actually make for more stable handling with less tendency to snap-oversteer. Just make sure that you don't cause front-view or side-view instant centers to go below ground, as that would create pro-roll or pro-squat, which you don't want.

RyanB88

Ok great. Sorry for being unclear on the original question.

As far as the front goes, I could actually lower the front suspension 1" as well without a problem. Either way would work aesthetically and I would still be fine on ground clearance.

RyanB88

I'll measure and make sure the IC is above the ground. That's exactly one of the things I would have overlooked!

MatthewMiller

No need to apologize - this is not a simple project or a simple question!

Even more reason to use the springs to lower each end to your preference. Basically it sounds like you will have the equivalent of a stock late C4 with it's overly tall ride height, especially in the rear (see pic of stock 96 GS below), and you're going to want to lower it by about the same amount as many people lower their C4s.

MatthewMiller

To simplicate this part, if the lower lateral control arm (camber arm) is parallel to the ground or angled up toward the center section, then the IC is plenty above ground. There are good drawings of an early C4 in post #2 of this thread. As you can see, it would be hard or impossible to lower the rear enough get the IC below ground (not sure why they don't include the drive shafts here, as those serve as the upper control arms).

I'm not sure if you're using the early or late version of the C4 rear suspension - if you mentioned it, I missed that. But the later version will have a lower IC from the factory, as well as less camber gain, because the camber arm mount by the diff is lower (lower IC and longer virtual swing arm). But as a reference, I just looked under my 96, which is lowered about 1.5" from stock, and the camber arm is still inclined upward from the spindle toward the diff. I'd guesstimate that the inboard end is 1" higher than the outboard end as it sits. I was going to take a picture, but the exhaust blocks the view of the camber arms.

Also, the same rule applies to the trailing arms vis-à-vis anti-squat. C4s have a quite a bit of anti-squat built in from the factory. Again, as point of reference: even with DRM lowering brackets for the trailing arm frame mounts and even being lowered ~1.5", my 96 C4 has significant positive anti-squat (the lower trailing arm still inclines upward from spindle to frame mount). I think the later C4s like mine might have come with more anti-squat than early cars because they had softer springs, but at most it's probably a wash compared to mine because of the lowering brackets I'm using. But this is all probably trivial because you can mount your trailing arms at whatever height on the frame you want.

Kubs

I had the same issue with my race car. I took so much weight out the car sat too high. Did the same thing ~5-6 years ago. Still works well.

Tom400CFI

Right on! I wondered if it would work....seems to work great so far. Thanks for sharing; it's sort of "Confirmation" of an idea.