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I am doing some research on the rear C4 suspension and I have a couple of questions that I cannot answer.
First, you need at least 3x link bars between the wheel hub and the differential to keep the wheel steady in alignment despite hitting a bump. The C4 drive shaft seems to be one of these 3x link bars - am I correct with this assumption? As such, these drive shafts have no axial play, correct? They take axial compression and tension forces to align the wheel? Is this also the reason why they are sized this way? If you compare that drive shaft with a C5/C6 drive shaft, you notice the size and strength of it. Any insight here would be greatly appreciated.
Next - these 3 link bars - should theoretically connect at the same point if you extend them virtually. Is this the case?
Last, did the rear suspension geometry for the C4 change over the years and did the ZR1 have a different alignment than the base? What exactly were these differences?
I am doing some research on the rear C4 suspension and I have a couple of questions that I cannot answer.
First, you need at least 3x link bars between the wheel hub and the differential to keep the wheel steady in alignment despite hitting a bump. The C4 drive shaft seems to be one of these 3x link bars - am I correct with this assumption? As such, these drive shafts have no axial play, correct?
Correct
They take axial compression and tension forces to align the wheel?
Correct
Is this also the reason why they are sized this way?
Probably. The weak link is the U Joint.
If you compare that drive shaft with a C5/C6 drive shaft, you notice the size and strength of it. Any insight here would be greatly appreciated.
The C5/6 uses upper and lower rear wishbones to control camber (and squat). The half shaft has a slip joint and only transfers rotational torque to the wheel.
Next - these 3 link bars - should theoretically connect at the same point if you extend them virtually. Is this the case?
I don't think so. The half shaft and the lower strut rod control camber and are responsible for camber gain during suspension travel. The upper arm controls toe. The toe rod does have to be positioned so you don't get bump steer but I don't think those 3 links will converge at the same point if extended.
Last, did the rear suspension geometry for the C4 change over the years and did the ZR1 have a different alignment than the base? What exactly were these differences?
I believe the rear suspension remained the same throughout. But there was definitely a change to the front geometry and the wheel backspacing in the late 80s.
Also, the C4 uses a 5 link rear suspension. The 3 links you pointed out plus 2 trailing links (dog bones) in front of the half shaft which control torque and align the rear wheels longitudinally. The trailing links are also setup to converge at a point in relation to the center of gravity of the car and the tire contact patch to give the car anti-squat characteristics. I believe the C4 was the first Vette to employ anti-squat geometry.
What is the torque limit of that rear differential and were they all the same throughout the C4 years?
I haven't heard of the Dana 44s breaking. Everything I've read says the U Joints will break before the diff. My research suggests that high quality U Joints will hold up to about 600hp. But above that with sticky tires and they can break. Rear end seems to hold up fine.
The manual transmission cars came with Dana 44s. The automatics came with Dana 36. There may have been some exception in the first year of production - 1984. But I think this was the case from 85 on.
Does anyone know a project where the C4 suspension was used with a transaxle instead of the differential?
replies above
Last edited by C4industries; Mar 18, 2024 at 02:18 PM.
Last, did the rear suspension geometry for the C4 change over the years and did the ZR1 have a different alignment than the base? What exactly were these differences?
The main suspansion change is between 87 and 88.
I think the main differance in geometry is the camber rod under the driveshaft ( strut arm including the mounting bracket on the differerntial )
The knuckle also changes so mounting points can be different on that side also.
Yes, the halfshafts serve as the upper lateral links in this five-link design. As you surmise, they cannot have any plunge or else they wouldn't do that job. That's why they have U-joints instead of CV joints. In utopia, the halfshaft and lower lateral link (camber rod or strut rod) converge at virtual point (creating a front-view instant center or FVIC) and the toe link's virtual extension also intersects that point. That would mean there is no bump steer. In the real world we have to live in, though, the FVIC location migrates around when the suspension moves through extension and compression, and the toe link can't always intersect it. When it doesn't intersect it, there is bump steer. The idea is to set up the toe link height and angle to minimize bump steer in the range of suspension travel that's used the most.
The 88+ rear suspension generally lowered the mount for the lower lateral link. This extended the virtual swing arm length (the distance from the knuckle to the instant center), thereby lowering the roll center. Unfortunately, this also reduces the camber curve, but it's a better compromise than the older geometry because it reduces rear jacking during cornering. A better option is to not use the halfshaft as a lateral link, which allows a shorter and more angled upper control arm (like the C5+), so you can have lower jacking but still have a good camber curve.
OK - I wanted to make that C4 rear suspension work with a mid engine configuration. What you see here is an LS engine with a Porsche G86.20 gearbox. There is an adapter kit for mounting these units. From the first view it appears that the engine and gearbox height is too high to make this work. That is unfortunate. Any ideas on how I can make all of this work?
Silly question... is there a way to get the later 88-96 rear suspension geometry while making use of the earlier 84-87 rear knuckle?
Can I just swap only the lower lateral strut camber arm rod and the camber bracket?
I have a complete 1985 D44 rear end and a complete 1991 D44 rear end.
I really don't know if you can mix and match those, Yak. I'm thinking you probably can't, just because the lateral link lengths are probably different and the spindle/hub for the early setup is probably different. The hub ultimately sits about 3/4" further inboard on the early cars, so I'm basing my guess on that. But maybe someone else here knows.
OK - I wanted to make that C4 rear suspension work with a mid engine configuration. What you see here is an LS engine with a Porsche G86.20 gearbox. There is an adapter kit for mounting these units. From the first view it appears that the engine and gearbox height is too high to make this work. That is unfortunate. Any ideas on how I can make all of this work?
I can't recall anyone every having done anything like this. If you are already locked into that gearbox, this will be tough. If not, maybe there's another option out there. Honestly, I would be surprised if a Boxter transmission would hold up to an LS anyway. There is a Factory Five GTM rear-mid-engine car that uses a 911 transaxle. But then they build a cage to mount a C5 rear suspension to it. I suspect it's going to be really hard to make this work while using the halfshafts as lateral links. You could fab some mounts for shorter upper arms, but then you'd have to make halfshafts that use CV joints to accept the plunge they would have, and by then you might be better off just buying parts from Factory Five and using a C5 doner.
I suspect it's going to be really hard to make this work while using the halfshafts as lateral links.
I agree with this assessment. (Also, nice explanation about the FVIC and bump steer 👍
It seems like the rear camber control on the C4 is a limitation in high G force cornering. The half shaft and lower strut rod are mounted relatively close together so the tension and compression forces through those members are high. And the stock strut rod is rubber bushed on both ends. AND there's only one attachment point (as opposed to 2 in a wishbone design) at the frame to handle all that load. I believe this poses a camber control problem under heavy cornering. This is also why people serious about tracking and autocross switch to heim joint strut rods - which is next on my list of mods.
Adding an additional 700 lbs of engine and transaxle mass for this design to cope with is probably not advisable.
Rear knuckles are the same 1986+ the difference for the 1988+ is the brake caliper bracket that also acts like a thick spacer. Brakes and parking brakes are different also 1988+
the geometry in front view, is improved with regard to camber change with jounce. This was done with inner camber rod brackets lowered, which meant a slightly different camber rod. The early years had excessive neg camber gain, and there are aftermarket brackets to adjust to your liking either way.
the diff was changed sometime before 1989, probably with the 1988 changes. The bottom holes for the brackets used a bigger bolt in the later models, and the casting was structurally different.
Silly question... is there a way to get the later 88-96 rear suspension geometry while making use of the earlier 84-87 rear knuckle?
Can I just swap only the lower lateral strut camber arm rod and the camber bracket?
I have a complete 1985 D44 rear end and a complete 1991 D44 rear end.
Use the After Dark Speed adjustable camber brackets, works best with heim joints on an adjustable aftermarket camber rod. In the center position, they replicate the 1988+ geometry. Most people with more power, run them in the position one lower than that even.
I can't recall anyone every having done anything like this. If you are already locked into that gearbox, this will be tough. If not, maybe there's another option out there. Honestly, I would be surprised if a Boxter transmission would hold up to an LS anyway. There is a Factory Five GTM rear-mid-engine car that uses a 911 transaxle. But then they build a cage to mount a C5 rear suspension to it. I suspect it's going to be really hard to make this work while using the halfshafts as lateral links. You could fab some mounts for shorter upper arms, but then you'd have to make halfshafts that use CV joints to accept the plunge they would have, and by then you might be better off just buying parts from Factory Five and using a C5 doner.
These are the Boxter S gearboxes - not the Boxter gearboxes - and these G86.20 are the same internally as the 911 gearbox G96 - just "swapped around" for a mid engine application. These G86.20 were only built between 2000-2004 and in my opinion, they are one of the finest gearboxes for a mid engine car that you can find for little cost.
I understand the C5/C6 suspension argument - much easier to get that working as the drive shafts are independent of the suspension.
I came across this thread by chance. I have a mid-engine project car (started by someone else) using a C4 suspension. They made an upper control arm for it.
Thanks folks for the feedback. So this is how it would look like:
No welding on the aluminium hub required. 3x adjustable rods outwards and 2x adjustable rods backwards (those 2x are as stock). A custom bracket with an M14 bolt welded to it (green part) is used to mount the additional upper link bar to the upper trailing arm mounting point.
C4 rear suspension
