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Hello, I am building a body swap 1989 C4 corvette. I want to set the chassis at the correct ride height for the new body installation. because people run different size tires, etc. I was looking for the correct front LCA angle and rear driveshaft angle, for the best Roll Center for autocross and track days. This car will be set up with 315 square on 18"x 12" rims, which should be 25.5" tall. If you have ride height measurements from the frame, could you include your tire size so I can make the correct adjustments.
Sounds like an interesting project! I wish I still had my C4 to measure. I'd say it was about the lowest I would ever recommend going. The front on these cars has proper SLA setup, so I don't worry about the RC going too low with lower ride height up there. The rear, however, has nearly equal-length upper and lower links because the halfshafts serve as the upper lateral links. It can't gain camber with compression or maintain RC height as well at low ride heights because of this. My recommendation is to consider the After Dark Speed camber/strut rod setup. It comes with a bracket for the diff mount that allows five (I think) different mounting heights for the inboard end of this lower lateral link, and you might want to use one of the higher settings for a car with lowered ride height. It also has the very nice benefit of getting rid of the awful eccentric camber adjustment that slips and doesn't allow enough camber adjustment range. My seat-of-the-pants guess is that you'd want to aim to keep those links at least parallel to the ground at static ride height. Here's a pic of my car to give a vague idea of what seemed to work, and I've seen other autocross/track C4s set up similarly. I would not go lower.
I also have these drawings of an early C4 (84-87) suspension at factory ride height, which might be a little bit useful. You'll notice that the rear camber rods are angled upward toward the diff mounts significantly. I think the later suspensions reduced that angle somewhat, but if you lower the car you'll reduce it a lot more. Hence, the recommendation for the After Dark strut rod setup. Up front, the LCAs are very slightly inclined toward the center of the car if you measure from the center of the ball joint pivot to the inner mounting bolt (inside pivot axis). Again, ideally you would keep the LCA at least parallel to the ground plane, but that is harder to do up front with a low ride height. If you'll have latitude to place the LCA mount or the entire subframe mount, I'd shoot for a parallel LCA at whatever ride height you choose.
If you were starting to amass parts from scratch, I'd probably recommend using an 84-87 front subframe. The geometry is better for performance, especially in an autocross setting where the steering angles are high. The later setups have too much steering axis inclination (SAI), which causes a loss of camber on both sides as you steering into a corner. They also have a ****-ton of anti-dive that doesn't need to be there. With an 89 setup, I don't think you can realistically change the SAI, but you could modify the upper control arm to reduce the anti-dive quite a bit (use a lower front mount hole for the upper arms) and increase caster to remedy at least of the camber loss from the severe SAI (set the upper arm mount back 1/2"...but watch the interference with the shock tower!).
PS - You're close enough to a stock C4 wheel base (96" vs 97") to make this swap car work in SCCA's CAM T autocross class (if you can keep a back seat) or CAM S (if you can't). If the Optima and Good Guys events have a similar rule, then I'm sure this is part of the motivation behind the project. I actually considered a Gremlin body (96" wheelbase) for a similar reason: would still run CAM S, but gets a big 400lb weight break compared to a C4+.
Sounds like an interesting project! I wish I still had my C4 to measure. I'd say it was about the lowest I would ever recommend going. The front on these cars has proper SLA setup, so I don't worry about the RC going too low with lower ride height up there. The rear, however, has nearly equal-length upper and lower links because the halfshafts serve as the upper lateral links. It can't gain camber with compression or maintain RC height as well at low ride heights because of this. My recommendation is to consider the After Dark Speed camber/strut rod setup. It comes with a bracket for the diff mount that allows five (I think) different mounting heights for the inboard end of this lower lateral link, and you might want to use one of the higher settings for a car with lowered ride height. It also has the very nice benefit of getting rid of the awful eccentric camber adjustment that slips and doesn't allow enough camber adjustment range. My seat-of-the-pants guess is that you'd want to aim to keep those links at least parallel to the ground at static ride height. Here's a pic of my car to give a vague idea of what seemed to work, and I've seen other autocross/track C4s set up similarly. I would not go lower.
I also have these drawings of an early C4 (84-87) suspension at factory ride height, which might be a little bit useful. You'll notice that the rear camber rods are angled upward toward the diff mounts significantly. I think the later suspensions reduced that angle somewhat, but if you lower the car you'll reduce it a lot more. Hence, the recommendation for the After Dark strut rod setup. Up front, the LCAs are very slightly inclined toward the center of the car if you measure from the center of the ball joint pivot to the inner mounting bolt (inside pivot axis). Again, ideally you would keep the LCA at least parallel to the ground plane, but that is harder to do up front with a low ride height. If you'll have latitude to place the LCA mount or the entire subframe mount, I'd shoot for a parallel LCA at whatever ride height you choose.
If you were starting to amass parts from scratch, I'd probably recommend using an 84-87 front subframe. The geometry is better for performance, especially in an autocross setting where the steering angles are high. The later setups have too much steering axis inclination (SAI), which causes a loss of camber on both sides as you steering into a corner. They also have a ****-ton of anti-dive that doesn't need to be there. With an 89 setup, I don't think you can realistically change the SAI, but you could modify the upper control arm to reduce the anti-dive quite a bit (use a lower front mount hole for the upper arms) and increase caster to remedy at least of the camber loss from the severe SAI (set the upper arm mount back 1/2"...but watch the interference with the shock tower!).
Thanks for the answer, will look around and see if I can find an earlier subframe, or modify mine.
Setting correct ride height
