Positive caster, with a little help from the idler arm bushing.

Thanks; so when I look at the W124 alignment specs and see caster listed at just over 10 degrees, that's assumed to be negative? I think mine measured at just under 10 degrees (9.7 or so as I recall) after my last alignment, and I am having centering issues. I have tried loosening the steering box already, and while there is now more play, it still doesn't self-center.

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)

mine centered very well until I put wider wheels and tires on, do you have stock tires and wheels?

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1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)

Mine are a bit wider, but it did the same thing with the previous (stock) size tires. Does yours not center at all now?

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)

it centered perfectly with my previous ones. now it get close but is usually off to one side or the other depending on which way it is returnng.

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1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)

Thats pretty much what I have as well. I assumed it was time for a pump rebuild. Happened like this before and after alignment.

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http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

I should have said "positive" caster in my original post, which has now been corrected.

It's the caster angle that is primarily responsible for steering wheel return. There is no other mechanism that causes it to happen, although there are plenty of opportunites for problems in the steering system that could interfere with the return to center.

Makes sense -- thanks for the help, and for the follow-up!

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)

Right. Its the same idea that keeps shopping cart wheels following the path you want the cart to go.

It probably would not hurt anything for someone to have that image as an impression of positive camber... but technically I do not think that applies.
The pivoting axis is vertical to the rolling surface...or you have great trouble turning... and the axis for the wheel is 90 degrees to that axis but offset...and following....
If you keep that relationship and apply it to the front of a bicycle you could not balance on it... I saw something on PBS showing people trying to ride on made that way....

>>but technically I do not think that applies.

For a 4 wheeled vehicle, you can implement castor with a vertical swivel axis and/or a tilted swivel axis. As long as the projection of the swivel axis on the road surface is ahead of the centre of the contact patch, there will be self centring action.

For the OP, I would jack the car up, and temporarily disconnect the inner track rod ends. Then, try to swivel the wheels by hand - if, for example, you have a ball joint which is seizing up, this could hinder self centreing action. By disconnecting the inner, you can also assess the friction levels in the outer track rod ends using a similar method.

If the claim is that that is one example of positive camber...in this discussion of SELF centering ability.... you can see that a supermarket basket wheel has no self centering ability because it is simply a follower of the forces placed on the upper part of the basket....
It has a ' passive reaction centering' ( or following ) ability only.
Greg

This post reminded me of a site I had seen before so I started looking and found it. This is info that is good to read even if you are having work done by a shop just so you can understand what they are talking about.
http://www.familycar.com/alignment.htm

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1983 300SD
200000miles

>>is simply a follower of the forces Yes, but on a car, that amounts to being a self-centreing action. To see this, probably the best way is to draw out the force vectors acting at the contact patch. In a car (not a bike), whether you choose to implement trail by moving the "king-pin" axis forward, or tilting it, as long as the projection of the swivel axis is ahead of the contact patch, you get self-centreing action.

Dumb question, but -- how "freely" should the ball joints move? I did a little tinkering over the weekend and found that the wheels will swivel, but not terribly easily. I know "not terribly easily" isn't a very specific term, but I'm not sure how else to put it. The wheel was off the hub, and I was able to swivel it with some effort by just grasping the rotor. I would think that the car ought to be able to overcome that in order to self-center, no?

I did discover that one tie rod end has a torn boot, so the exercise was not in vain. I have something to replace!

__________________
1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)