It is my personal opinion that more than two-thirds of the 928s on the road have misaligned front suspensions, causing the inner edges of the front tires to wear more quickly than the outer edges.
There are two reasons for this - the first is that the toe changes with
suspension movement on the 928. [If this is not properly compensated for in
the suspension design, it will cause "bump steer", where the car darts
sideways as the wheel turns in or out as the suspension moves up or down.
Apparently, the Porsche engineers knew what they were doing, since the car's cornering limits are MUCH higher than most drivers will ever reach.] The suspension design is such that toe moves from "in" to "out" as the chassis moves down, and 1/4" of ride height reduction moves the toe 1/8" out.
This means that if your car was perfectly aligned, and the ride height has
settled 1/4", you now have toe out, not toe in. A little bit of toe out can
have big effects with high-performance tires. The better the tires ore, the
more they are affected by incorrect toe. The tires get such good traction
that a little toe out will pull the tire tread outwards, with the result
being the commonly-seen inner edge tread wear.
The second reason for misalignment is the unusual quirk in our suspension systems that causes them to bind in the raised position after the car has been lifted. It can take many miles of driving before it settles fully to its final position.
Earl Gillstrom (Capt'n Earl) has developed a very precise, laser-based home
alignment system for the 928. He has repeatedly checked toe measurements,
and reports that it may take as much as one hundred miles of driving before
the suspension settles fully, depending on how rough the road is - if the ride height was adjusted it may even take more.
According to Earl:
The sequence for wheel alignment of a 928 should be:
1) Check and adjust tire pressure.
2) Check ride height
3) Jack up and check for and replace worn/damaged parts.
4) Adjust ride height if necessary.
5) Drive 25 to 100 miles over bumpy roads.
6) Repeat 1,2. If jacking is necessary, repeat 5 and start over. DO NOT raise car before or during alignment
7) Adjust camber and toe in on rear wheels
8) Adjust camber and caster on front wheels.
9) Install rack centering bolt.
10) Adjust front toe in.
11) Check for level steering wheel and pull and adjust on splines if necessary.
12) Remove rack centering bolt and replace with plastic plug.
All of this means that if your alignment shop lifts the front of the car, then sets the toe to the proper measurement, after a few miles the suspension will settle and you will have extreme toe out, and very rapid inner edge wear.
There are only two safe ways to align the front end of the 928.
1) Don't lift the front end at all. This creates some very serious problems.
a) Alignment shops don't believe this. They are experts, and they aren't about to listen to you. They know that all cars have to settle after being lifted, but rolling other cars a few feet allows them to settle.
b) Most of the alignment equipment in use requires that the sensors be
placed on the wheels, and then rotated. The common way of doing this is to
jack the car and spin the wheels. In some cases, the car can be pushed back
and rolled forward to set the sensors, but may shops don't have room or
don't have long cables on the sensors.
2) Use Porsche Tool 10-222A (or an equivalent) to pull the front end down 60-70mm for one minute. That is 2 3/4". Try to push your car down that far - no way are you ever going to settle this car by bouncing on the front fenders!
[Ed R. reports that his mechanic can properly align the car by lifting the
front at the ball joints so that the suspension doesn't unload, so the car
shouldn't be high afterwards. First problem is that this takes special
fixtures on the jacks, as the ball joints are well inside the wheels.
Second problem is that the ball joints are still about an inch inside the wheel centerline, so there may still be some unloading. Might work, might not. Earl tried to see if there was any movement, but couldn't lift the car by the ball joints to see.]
If you want to do a "quick'n dirty" check on your car, get two
boards or pipes about six to eight feet long, and two assistants. Put the
front wheels as straight ahead as possible, and have the assistants hold
the straightedges against the front wheels so that they are on the
sidewalls of the tires, and protrude ahead of the car. Measure the distance
between the straightedges just in front of the tires, then four feet out.
The boards should be ~ .210"(5.3mm) closer together at the outer ends. If they are farther apart at the ends, your suspension is misaligned. You can move the car just far enough to rotate the wheels 180 degrees and repeat to
eliminate the effects of crooked tire sidewalls if you like. Using the same method but with the boards out the back of the car, you can check the rear toe in. The boards should be ~ .279" (7.09mm)further apart four feet out, behind the car. This isn't a very precise method, but it will usually show how close your toe setting is.
Thanks to Earl Gillstrom for his contribution to this.
Great post and Cap'n Earl is definitely a great source for the home alignment technique. I would encourage people to just buy a cheap laser ($6.99 from Harbor freight is what Earl and I both use) and not bother with the board on the tire routine. It only takes one raised letter to give a bad measurement and panic someone. Also, rims/tires are not necessarily true running. Wobble will mess up alignment measurements. Earl uses the center cap journal to try to ward off the wobble variable. I use the rim edge but check to make sure it is true or average out the wobble by rolling the car through 180°.
I concur on the 100 mile settling. Boge gas shocks seem to be the worst.
Konis the best. Cars that are regularly flogged seem to settle quicker than ones that sit a lot. Some cars settle for a month after lifting. Cars that are lifted and sit up on the stands with their suspension hanging can take a set and not settle all the way till properly flogged. Temperature also seems to affect this.
Ride height is key. An aging gas shock or spring can completely mess up a good alignment. Fix the shock/spring issue and the alignment comes right back to true. As you said 1/4" ride height issues can wreak havoc with toe measurements as the suspension is designed to change toe off of its neutral height. Lowered cars will have different characteristics than stock for this same reason.
For anybody coming to Mass for the gathering this Sunday we can go over this stuff in nauseating detail :) Plus the original alignment guy will be there, Kurt Gibble! And another legend.... LL!!!!!!
79 US 5ish speed.
I know that this is what Earl Gilstrom specified on his website detailing alignment procedures: