300TD uneven rear tire wear - LCA bushings? Subframe bushings?
 

I have very rapid wear on the inner edges of both rear tires on my '81 300TD. My alignment shop found excessive toe-out on the right rear but said it was non-adjustable and "as good as it gets." Could this be bad LCA or subframe bushings? Or is something bent? The car actually tracks pretty straight although it does have a strong tendency to want to drift away from the high side of crowned roads.

non adjustable? I thought 4 wheel alignment was standard on these cars... anyway, the diff mount is likely shot letting the subframe sag.

Very interesting, I didn't realize the diff mount helped hold up the subframe, but now that I've looked at some pictures I can see it. I can swap it with everything in place, right? Just support the diff with a jack?

I think what I've heard is that on the W123 the rear is nominally non-adjustable, but there are aftermarket offset bolt kits to adjust it.

It's also possible I misinterpreted and they meant that it was outside the adjustment range.

yeah, it's only a few bolts to remove it.
I usually support the diff, drop the mount, and remove the rear cover and change the diff fluid while I'm at it.

yeah. I'd support the body on jack stands (more room to work)..use a floor jack under the differential, you'll use it to line holes up anyway. Its the same procedure on the W123 as my W116, and I have replaced the one on the W116.

You MIGHT have LCA bushings worn out....but try the diff mount first.

If you bent something odds are it would have been only one side that had the issue.

heh, good point... kinda hard to lower the mount without supporting the vehicle...

If he got a good look at what a good one looks like......a bad one is visibly obvious if its affecting alignment.

Maybe I'll take a look this weekend. I should probably change it anyway, though. Even if it isn't affecting the alignment it's probably hardened and transmitting more drivetrain noise to the cabin than it should. My experience with rubber drivetrain mounts on cars this age is even the ones that look good are usually bad.

The trailing arms are not adjustable out of the box so to speak - however you can buy bushes with eccentric holes in them (here on fastlane too).

But I'd follow the advice and inspect the rubber and if you think it necessary change the rubber before you get all involved in complicated eccentric bushing calculations.

One thing that might be of interest is that the washer bolts that hold the axles into the trailing arms are mean to be a fit only once item.

When I was messing about with my alignment I found that you could change the camber on the rear by as much as a degree by loosening and tightening these washer bolts...

(Sorry washer bolts isn't probably the official term - it is Friday - I'm knackered...)

These washer bolts probably won't be all of the problem but they might contribute at bit to the problem.

I should do the diff mount on my 240D. I had no idea that the mount was that crucial to the rear subframe geometry. My rear tires wear on the inside as well. New shocks did not help.

A out of spec Differential Mount Changes the Rear Tow In/Out? See Pic.

What are the symptoms when the Trailing Arm Bushing go bad?

I guess that would be a contribution to a saggy bottom... the trailing arms are meant to be set at a particular angle to the subframe (and chassis). If this angle changes over time due to a gradual weakening of the rubber (torsional moment - twisting) then that position will be changed. (If anyone needs this specification I'll dig it out the FSM for you)

If the rubber on the trailing arm bushings is cracked I'd replace them. However, the thickness of the rubber on these bushings isn't that great. So I guess they have less of an effect on the feel of the rear end when worn. There is more rubber and flexibility in the subframe bushings (as I'm sure you know). If these big bits of rubber along with the differential mounting get tired I imagine they would have a greater influence on the feel of the rear end. However, if we're talking about ride height all of the rubber parts on the rear suspension appear to place a role - that sounds obvious doesn't it?

Got a peek at the diff mount yesterday and it looks pretty hashed, so I think I'm going to start there. Hopefully the bolts won't fight me too hard.

Thought I'd follow up to note that I finally got around to changing the diff mount today (as part of an axle shaft replacement) and not only was the rubber pretty well shot, the whole mount was bent to the left. I don't know how that happened -- maybe someone tried to jack the car by it. Won't know if it helped the tire wear until I get a ways down the road.

Any chance of a photo of that damaged mount Orv?