w123 bad rear end TWITCH - PeachParts Mercedes-Benz Forum

Zacharias · #1 08-01-2011, 11:10 AM

EDIT: hit post by mistake... title should say bad rear end TWITCH....

I had my 'new' w123, 92k miles and very well maintained, on a section of really poorly paved freeway on Sunday and got a bit of a scare.

I had noticed the rear end seems twitchy over bumps, more than I would like. However on Sunday I hit a stretch with longitudinal irregularities (a really bad paving job on a long left merging section), basically the pavement had a gap and was not level between sections.

Well at 75 mph the rear suspension reacted so violently that I thought I was going to loose control for second. Scared the bejezzuz out of me. When the right rear wheel hit the gap the rear end 'twitched' and swayed, then snapped back into line violently, like it had been pulled back and forth by a huge rubber band.

The suspension is otherwise tight and the car was just aligned to factory specs. It also just passed the (very intensive) Quebec safety inspection on its first pass, almost unheard of for a car of this age. Tires are almost new Michelins and are at spec pressure.

Any advice welcome.

winmutt · #2 08-01-2011, 11:50 AM

Sway bar links? Subframe mounts? Trailing arm bushings?

Stretch · #3 08-01-2011, 01:27 PM

All of the above in post 2

But are you sure the trailing arms aren't broken - they do rust through... I don't want to bring bad news but it does sound like something broken if you thought you were going to loose control...

sixto · #4 08-01-2011, 01:34 PM

My rule-of-thumb for rear subframe bushings is to check the gap between the bushings and the plates that go below them. There is an even pinkie width gap when the bushings are new. The gap becomes uneven as the bushing wears, as in the plate tilts relative to the bushing.

The typical symptom of worn subframe bushings is the need for steering corrections to keep the car tracking straight. I don't remember the violence you describe other than when a [edit] idler [/edit] arm bushing failed splaying the front wheels in severe toe-out. But that definitely manifested through the front, not the rear.

Sixto
87 300D

Stretch · #5 08-01-2011, 01:41 PM

Quote:

Originally Posted by sixto

My rule-of-thumb for rear subframe bushings is to check the gap between the bushings and the plates that go below them. There is an even pinkie width gap when the bushings are new. The gap becomes uneven as the bushing wears, as in the plate tilts relative to the bushing.

The typical symptom of worn subframe bushings is the need for steering corrections to keep the car tracking straight. I don't remember the violence you describe other than when a pitman arm bushing failed splaying the front wheels in severe toe-out. But that definitely manifested through the front, not the rear.

Sixto
87 300D

Ouch - that's got to hurt. Was that an idler arm bushing on a W123 / W126 or actually a Pitman arm bushing on a different car? (Or a totally knackered steering box?)

sixto · #6 08-01-2011, 01:46 PM

Ahh! Idler! Idler! Idler arm bushing in a 124. Same steering setup as 123 and 126.

Thanks, Army.

Sixto
87 300D

Zacharias · #7 08-01-2011, 02:11 PM

Quote:

Originally Posted by Army

All of the above in post 2

But are you sure the trailing arms aren't broken - they do rust through... I don't want to bring bad news but it does sound like something broken if you thought you were going to loose control...

Not possible... car is almost rust-free, everywhere.

As I mentioned, it just passed an extensive licensing inspection on its first go. Most cars over 7 yrs old need at least two tries.

Zacharias · #8 08-01-2011, 02:18 PM

Trailing arm bushings -- possible. How long do these normally last? I've owned five Mercedes and I never recall doing them (but my memory isn't what it used to be).

Subframe bushings -- I doubt it. Driven a car with them bad, before/after, and this isn't the same behaviour.

Swaybar links -- I can check but the government inspection was four weeks ago and worn links is a failure item. This is what I find bizarre: this inspection is a real killer.

This is a 92k miles, dealer-maintained car. If anything bushings-wise is worn then it's from age rot.

On a w123 are there rear adjustments for a four-wheel alignment?

Zacharias · #9 08-01-2011, 02:24 PM

Quote:

Originally Posted by sixto

The typical symptom of worn subframe bushings is the need for steering corrections to keep the car tracking straight.

Yes I had an SD that ate a set and I felt the difference before/after. This isn't the same feeling.

On normal roads this car requires a minimum of steering intervention to stay in a straight line.

One thing just hit me, however, that no one mentioned: loose rear wheel. Will check tonight.

Biodiesel300TD · #10 08-01-2011, 03:41 PM

Might have a look at the differential mount as well. Probably not likely but looking is free.

t walgamuth · #11 08-01-2011, 03:56 PM

Also could be shocks, especially if they happen to be KYB with 30K miles.

Biodiesel300TD · #12 08-01-2011, 04:44 PM

Quote:

Originally Posted by t walgamuth

Also could be shocks, especially if they happen to be KYB with 30K miles.

You mean the Keep Your Bilsteins!

Zacharias · #13 08-01-2011, 04:45 PM

Quote:

Originally Posted by t walgamuth

Also could be shocks, especially if they happen to be KYB with 30K miles.

Hmmm well I thought of that but not for long.... The rear shocks on the car pass the 'bounce' test with flying colours, but are admittedly way too soft for my tastes overall.

My previous w123 sedan had bilsteins all around and was really firm. I was assuming that this kinder, gentler setup was factory stock

.

I put Bilsteins onto the rear of my w116 SD, but it was never as soft as this car.

What I felt was a side-to-side yaw, however... how would shocks produce that? I've driven cars with bad rear shocks but it's more instability from excess bounce. I didn't notice much bouncy-boucy in this situation.

Orv · #14 08-01-2011, 07:42 PM

In my experience if the rear shocks are soft, or the rear end is squatting excessively, you'll have some disturbing rear end motions. These cars have a lot of camber change with suspension compression, so the more it compresses the less squarely the tires meet the road.

What's your tire tread look like? Is there uneven wear?

Zacharias · #15 08-01-2011, 07:48 PM

Quote:

Originally Posted by Orv

In my experience if the rear shocks are soft, or the rear end is squatting excessively, you'll have some disturbing rear end motions. These cars have a lot of camber change with suspension compression, so the more it compresses the less squarely the tires meet the road.

What's your tire tread look like? Is there uneven wear?

Tires have perfect tread wear all around. Rear end sits up like a newish car.