BGLeduc

First, the car is a US spec '90 C2 with Bilstein HDs and H&R sport springs (not the turbo springs, but the standard progressive C2/C4 springs).

The Question....

Once installed, are the perch heights on the rear shocks more or less the same, or is the left significantly lower than the right?

I was swapping my track tires back to my streets, and I noticed that the lower edge of the right side perch sits perhaps 2" or so above the bottom of the threads. The left side, however is perhaps 1/2 above the bottom.

I would have sworn that they were more or less even, but not having done the install, I can't say with 100% certainty. When the install was done, I did get a corner balance (I actually was there for the whole process) and nothing odd was noted.

And FWIW, the car does seem to handle fine on the autocross course.

But, I have been noticing a bit of a clunking sound coming from the rear, when going over moderate bumps a low speeds, almost like something is bottomming out.

I was thinking that something was not what it should be out back, and this perch difference really caught my eye.

A couple more details. I still own the factory original shocks, and I took a look at those and the perches are pretty well even.

And, I measured the ride height in back using the unscientific "ground to fender lip" approach, Left to right are within a 1/4" or so. Nothing obvously wrong there.

I have had some recent work carried out in the back (the oil line from the back of the oil filter housing to the back of the motor was replaced) and my wrench did day he had to take off the heat exchangers on both sides. I can't imagine they needed to touch the shocks.

FWIW, the perch does not look to have had a wrench on it lately, so perhaps its nothing, but the height on my old shocks, plus the bottomming I seem to be hearing has me very, very suspicious.

BGL

Adrian

Dear Brian,
Nothing unusual with this as long as the car is level. Remember the weight distribution from side to side in uneven in the 911 (most cars actually). When you have it corner balanced and the ride height set it all comes down to the installed length and postion of the suspension assembly.
The left side of a LHD 964 carries more weight than the right side.
Now with all that said it could be that the left side is carrying too much weight or the corner balance was not done properly. Please also keep in mind that stiffer suspension components are often noisier than standard.
No black and white answers I am afraid. Everybody forgets weight distribution and how it is adjusted. You may also be too low. I do not know how far you actually lowered your 964. I do not recommend anymore than Carrera RS ride height.
Ciao,
Adrian
964C4

BGLeduc

Adrian,

As always, thanks for the quick reply.

I took a yard stick to the car again this morning, and there are some small variations using the floor to fender lip approach, with the left rear about 1/2 lower than the right. It may have always been that way.

I did find the reports from when it was last corner balanced (about 14 months ago), and these where the numbers (note: I was in the car when this was done).

LF 700 RF 647
LR 990 RR 1000

The cross weight difference was 63 lbs, and we did not attempt to get it closer. This was with a full tank, tools/spare in the car.

Side note: I took a look in your book, and did not locate any detail on corner balance. Did I miss it?

I am a bit bothered by the fact that the old dampers did seem to have the perch heights close to even. Apples to oranges, perhaps.

I also note that when the last alignment was carried out (several weeks ago), there was a .4 degree difference in camber, with the left at -1.0, the right at -1.4. One year ago, they were both -1.2.

So, it does appear that something has shifted a bit, although it seems to be my experience that alignment measurements are never 100% repeatable....plus my shop does have new alignment equipment.

Just so I can sleep at night(!) I may make another appointment to put it up on scales, jut to make sure all is well.

Thanks again.

BGL

Jeff Curtis

Brian, from reading your 2nd reply, it sounds as if there's only a 1/2" difference??

If so, no big deal, this is what corner balancing is all about. Mine are fairly even as well, but that doesn't mean yours will be.

The only way to tell if something is off is to get your car on another set of scales...it's possible the perch could've rotated itself a few turns, but I would find that odd that they, or it, moved that much!

Let us know what you figure out, the bumping noise over low-speed bumps may be an indication that something is amiss??

BGLeduc

Jeff,

Sorry, I have created some confusion...

The 1/2 difference is in ride height as measured to the rear fender lip (the left side is lower, and thats w/o my weight in the car).

The big concern I have is that the rear perch heights are very, very different. The left side is maybe 1/2 an inch above the bottom of the thread, while the right side seems to be a good 2 inches.

BGL

Jeff Curtis

Okay then...I concur with your concerns...a 2" difference in perch height is a bit much - I would look for something amiss.

I read your original post, thought 2" and then read your reply and got confused...no worries, it's just me.

1/2" difference with your fenderlip measurement is:

1) of NO concern, as they will be different.

2) inaccurate way to measure...although I've done it that way MANY times.

Definitely let us know what you find, again, I believe this is worth the time to investigate.

Stephen Grellet

Re your corner balance, your rr/lf are carrying more weight than your lr/rf. The rr is the corner with the higher spring seat adjustment, and this also affects the lf. By lowering the rr a little and increasing the lr you should be able to bring your diagonal weights closer (on scales of course). Think of a chair with one short leg.

BGLeduc

Thanks for that info, Stephen.

I have since spoken to the wrench that did the install over a year ago (my normal mechanic), as well as the tech that takes care of my alignment work.

The wrench said he really does not recall, nor did he have any thoughs on whether similar cars (he just did the same suspension on a '90 C4) had dissimilar perch heights.

The alignment tech said that from his perspective, the difference is not all unusual, pretty much what Adrian said in post #2. Its all about getting the weights even.

At the end of the day, I may still get her up on scales again, just so I can sleep better! At that point we could try to shift things around to get the cross weights a bit closer, although I am a bit dubious that I will feel a difference in handling.

I don't know that the "butt-o-meter" is quite sensitive enough.

All that said, it felt damn good at the autocross this past weekend. While we don't have a lot of depth in the club, I did take top time of day of the Porsches that were there, and even beat up on a 993 C4S with MPCS tires.

Maybe thats a sign that I should pretend I never noticed the perch difference and just enjoy the car????

BGL

Jeff Curtis

Brian, I didn't give advice on adjusting your corner weights, using the info. given when you got the corner balance done because things have changed by now...after all, it's been a year!

Stephen is right on with how to adjust your weights though.

Either way, I don't agree with a 2" difference in perch adjustment from right rear to left rear whatsoever - I do think there's an issue...either one of the rear mounts has an extra spacer in it, the other does not or they're just out of whack and need another adjustment.

BGLeduc

The conclusion of this thread can be found here:

https://rennlist.com/forums/964-forum/151181-anyone-ever-blow-out-a-bilstein-hd.html

BGL