I know this isn't a diesel-specific question, but you guys are so much better about answering my questions than are the other forums!

(On a W124 specifically,) what causes the steering to self-center after a turn? Is that a function of suspension geometry (camber, caster, toe, etc...), the steering box, a combination thereof, or something else entirely? I am trying to wrap my mind around how centering works.

Thanks!

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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)

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)

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

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!

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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)

toe in is critical.... follow all the rules... use search ... many good threads on subject...

That's the off thing -- my toe adjustment is fine. I have searched and searched (for better than 2 months now), and while there are indeed many threads, the vast majority have to do with the steering box adjustments, which I don't THINK are my problem. That's why I asked whether self-centering was a function of the steering box itself; my assumption is that it's not, but that an overly-tight steering box can prevent self-centering. Am I wrong?

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