jhalsey

90 GT: My rear tires are worn on the outer 1/3 edge after only 2000 miles. I thought the common problem was too much negative camber not positive. Other than a visit to an alignment shop, which i will eventually do, where / what should I inspect to determine if its something that needs repair?

SwayBar

You may have an alignment issue, too much toe-in.

icsamerica

Most likely you need rear upper control arm bushes. Unless they've been replaced, at 34 years old they've given up. Before you spend good money on an alignment consider replacing the rear upper control arm bushes. Over time they take an oval shape and allow for both additional toe-in and positive camber, both of those conditions are typical for outer edge tire wear.

Powerflex make them now and there are a few venders here that sell them.
https://www.powerflexusa.com/shop/po...1479#attr=4129

Mrmerlin

I didnt know they made aftermarket dog bone bushing kits.
the price is almost the same and these are easier to install,
just make sure you mark everything so the arms go back the same way.

For the OP has you car recently been towed or shipped ?

If so its quite possible the rear LCA arms are bent ,
as are the dog bone bushings worn and sagged as pointed out above.

First order, post some pictures of the rear suspension parts,
include the blades as these are common anchor points.
These are easily deformed from an errant towing operator with a ham fisted winching hand.

Also before lifting, pictures of the cars stance from the front and back along the sides

icsamerica

Dog bone? I thought it was a banana.

Mrmerlin

I have never heard them called anything else besides dog bone bushings,
or rear upper control arm bushings

icsamerica

There is some precedent for the banana moniker. All the same to me call it what ever you like. Carl's design doesnt use the conical part. The Powerflex does have the conical part which is critical if you are to retain the normal function of the rear suspension as designed.



In the C4 world we call these trailing arms 'dog bones' for some reason, perhaps becasue of the resemblance.

jhalsey

The car was shipped cross country in 2017. I have since replaced the tires. Pics to follow.

jhalsey

Ok, I have attached pictures as requested. To my untrained eye, none of the rear suspension members appear damaged. The upper control arm bushings appear crusty. I did find a split CV axle boot on the left side which I will repair with a rebuilt from 928 Int’l.

Any DYi”s out there for the upper control arm bushings? Special tools required other than a hydraulic press?

thanks for the help









Split boot




Straight


Looks straight to me

Mrmerlin

based on your pictures the upper bushings are sagged.
Replacing them will give you back the adjustment range for camber.

NOTE if you notice the rear of the car changing direction like its being steered,
from a full power application to letting off then,
it would be wise to replace the eccentric bushings at the cross member.
Even though they may appear hard,
these 2 bushings can let the control arms move enough to make the rear end feel loose.
These eccentrics get torqued to 140 ft/lbs

The good news is the blades appear to be flat,
that means they are still in good condition.
I would Replace the rear tires and drive the car for 50 miles then get it aligned.

jhalsey

Thanks Mr Merlin, I am going to attempt a complete rear suspension overhaul, all bushings, not sure about the bearings or the shocks. Wish me luck, I’ll need it.

SwayBar

The shocks are 35+ years old, they need replacing. Since you're doing all this work, now is the time to do it.

FredR

If one of the half shaft boots has gone chances are the others are not going to be far behind. You do not need re-built arms unless the jointsinside are shot. Replacing the rubber gaiters is a messy job but it is not difficult and gaiters are cheap enough.

General bushing degradation can lead to not being able to get the rear aligned without excessive camber. Another problem is that if the adjustment bolt is not tightened correctly the adjuster can bounce around knocking out the side walls the cam plate pushes against making adjustments nigh on impossible during alignment. Carl of 928motorsports offered a steel insert for both sides- no idea if they are still available- it was a simple concept, easy to fit with a Dremel used to form the necessay cut out to fit them- see below link to my thread

https://rennlist.com/forums/928-foru...nt-pocket.html

Mrmerlin

I dont recommend that you replace the bushings that the lower link pin passes through in the control arms.
NOTE Special tooling is necessary to properly align these bushings or they can be put in cocked.

NOTE if your replacing the Weissach joint,
I suggest before you remove the old joint from the LCA ,
first test fit the new parts into the aluminum mounting plate.
If the joint is not close to a precision fit then your going to have to get large flat washers to shim it.
What you dont want to do is have the joint fit loose in the plate then when tightening it ,
the bolt bends the plate and cracks the aluminum.

NOTE I suggest that you replace both of the halfshafts with new units available from 928 INTL,
this will give you the longest duration fix to the drive line, ( while they are still available)

NOTE get a new set of 4 Red Boges while they are still available Roger might have some left.
Not replacing the shocks would be the biggest mistake on this repair.

NOTE you will need a big press to remove and install the LCA eccentric bushing,
but it will make the biggest improvement on the solidity of the handling.

hacker-pschorr

What tire pressure are you running?