ScaryFast

Guys, there is no proof that the coil overs caused this. Logic would suggest it, but the pothole I hit caused a rearward load on the knuckle at the attachment point of the rear shock. Who knows if a non coil-over car would have survived the failure. I still feel the largest contributer was the stiff sidewall on the R compound tires as compared to street tires.

I was trying to give DRM a plug because I've had nothing but great products and service from them (and Randy specifically), but it came off like I blamed them for my car breaking.

I tear things up all the time (look at the 50 threads I've started in the past month), hell, I even set my car on fire three weeks ago. It's an old, tired C4 with a bumload of extra HP that I thrash around a racetrack every freakin' weekend. Stuff breaks. It's my fault things fail, not the vendors who sell me their products. If I simply drove it around the street and waxed it every night (the car, guys...sheesh) like some folks here do then I'd probably never have to fix a thing. But that would make me a pansy.

CentralCoaster

Why does anyone want the thread locked?



I want to see pics of all 3 designs.

It appears that 87-91 had the same weak knuckle attachment as 84-87, and a LONGER bolt that would put even more load on it.

The wheel offsets or knuckle width means nothing, anything bolted to the knuckle will move up and down the same amount relative to the wheel, relative to the car, no matter what year.

jhammons01

I think the thread has surpassed that question and is now more along the lines of.....

"What was the Original angle of the Shock lower mounting bolt for three separate eras"

I think....

84-86
87-91
92+

The debate seems to be a two headed dragon.

On one side people are saying this is the way it was made. and the years are different vs those that say all were at 90°

On the other side people are saying It was bent by any old shocks vs those that are saying it was the fault of using coil overs.

We need to see if all the years were 90° first.....if so then we need to move on to what would cause it to bend.

Agreed?

I'd really like to know the answer to all this and I keep reading contradicting post with no real proof. Not that I'd run out and replace mine for one reason or the other........but this is part of learning about a subject that interest me.

Aardwolf

Easy enough, I took two pix of my '88. It is sitting normally on the ground in these pictures. If you want pix of anything else just ask.



And a closer pic:



Note, where the shock is mounted there is a smooth section of the bolt there that is larger then the treaded end part.

jhammons01

Ardwolf, Be specific, are you saying that yours are bending down?

They are slightly off of 90° but nothing like what has been posted and determined bent.

todd_vette

Ok I have one pic of the 88 rear BEFORE i installed it in my 87.
http://todd-vette.tripod.com/mycarpi...e-rearend?i=10
When you go to the enlarged version you will clearly see that the lower shock mount/bolt is bent to change the angle to match the inboard upper mount, this is a stock 88 rear(polished & chromed, but still stock). Now also in the enlarged pic it looks like the knuckle mount the bolt goes through is also at an angle... that is an optical illusion. It is perpendicular to the rim mount. It is almost exactly the same as the 87's. I am sorry that i don't have any pics of the 87 out also, but i assure you that the knuckle mount looks identical but the bolt is straight and about 1/4" shorter(didn't need clearance for the angle at the neck of the mount). And i can confirm that they interchanged quite nicely. I used the 87 bolts/mounts on the chromed 88 rear for perfect alignment of the shock.
Now later (others on this thread are saying 92 but i think it was actually 91 so i am uncertain) GM changed the knuckle to a thicker ear at an angle to the rim mount for the bolt to go through...and used a straight bolt that was about 1/4" longer on the thread side to reach through. That should clear up the mount changes question and let me read about muzings on coil-overs. I am also wanting to learn.

todd_vette

Aardwolf, Are you sure you don't have 91up knuckles???

Now i am confused and i have worked on both! The bend in my pics is deliberately that way from the factory.... WTF!

Aardwolf

Back in post #70 I wrote that there is a slight downward angle but not as much angle as the 84-87 pix. As far as I know the knuckles were not changed by a previous owner, and I have not changed them.

Does that angle change with alignment?

Dale1990

Comparing the 86 to the 90 (since they are both in my garage) - the 90 is identical to Aardwolf's pic, slight downward angle and a large smooth space between the knuckle and the shock. The 86 has a more pronounced angle and no smooth space between the knuckle and the shock.

69427

Nonsense. Unless you've gone to all the trouble to fusion weld your rear suspension into one solid piece, you are mistaken. And even if you did have a completely solid suspension, there would still be stresses and bending moments.
There are several pivot points in that suspension that are isolated with rubber. A squishy, compressible material. Any time the wheel moves, there will be a difference in angle and acceleration rate between all the parts. Any time you have adjacent parts undergoing different acceleration rates, you will have differential movement or bending going on. Additionally, the wheel will try to move, but will be resisted by the fluid/valving of the shock (remember, it's a speed sensitive dampening device). The fact that these two items are inputing opposing force vectors causes deflections in the bushings, the knuckle, the shock bolt, and likely a small amount of negative camber as the wheel goes up. This can also cause a slight bending moment on the shock mount or bolt. A wider trackwidth will cause a greater moment arm, increasing the compression on the lower strut bushing, causing potentially more negative camber.
There's tons more that can be written about what each suspension component does with any number of inputs. But there's probably only a handfull who can stay awake reading through all of it.
The net effect is, show me what was wrong in the original picture, or show me data (not speculation) that shows a design weakness in some of the years, or just kill this silly thread. Please!

rickneworleansla

Good discussion here. Let's see how long we can keep it from getting locked.

rocco16

I agree: no need to kill the thread.
There are still answers to be found here.

Here are some things as presented in the thread:

The C4 owners of some years report that their rear/lower shock mounts appear to be angled down.
The C4 owners of other years report that their rear/lower shock mounts appear to be parallel to ground.
This would appear to be consistent with the relocation of the top mount, at some point in the model run.
The stock rear/lower shock mounts appear to be easily strong enough to withstand normal shock loading, as no one has reported bent mounts from other than crash damage.
One coilover user reported mount breakage after hitting a pothole.
The stock rear/lower shock mounts appear to be strong enough to withstand, in most cases, the additional loading from coilovers.

I see the only question remaining as being: during what year did the mount change?



Larry
code5coupe

__________
not easily impressed....

jhammons01

Exactly.

And surely there is someone out there that has a "new" knuckle or shock mount that we can get a picture of for verification.