How much should a w123 raise/lower in drive and reverse? - PeachParts Mercedes-Benz Forum

JHZR2 · #1 09-09-2018, 08:02 PM

Now that my 300cd is running again, my observation is that it raises/lowers excessively going in and out of drive and reverse.

Is there a specification for this value? A standard means to measure? Does it indicate any specific item needing replacement?

I feel like the cd has a little bit saggy rear but everything looks good and the car drives great. If I had one issue, it’s front tire wear, but my old ‘83 used to do the same thing.

97 SL320 · #2 09-09-2018, 11:45 PM

Anti / pro squat is a function of suspension design and , if you are anywhere near design ride height / nothing is severly worn , should remain constant. All the rubber in a bushing would need to be gone not slightly compressed for example.

Anti / pro squat for the rear and anti / pro dive in the front is measured in % as a function of suspension design not how many inches things move because the more dynamic load applied to the suspension the more action that will occur.

Spring rate ( lb per inch of compression ) plays a factor in how far the rear will drop however, spring rate does not change over time.

Look at some vids of your type of car accelerating and compare to yours.

JHZR2 · #3 09-10-2018, 04:02 AM

Quote:

Originally Posted by 97 SL320

Look at some vids of your type of car accelerating and compare to yours.

Sorry, I should mention, this feel/motion is at a standstill with brake applied, just manipulating the transmission selector...

97 SL320 · #4 09-10-2018, 09:31 AM

Quote:

Originally Posted by JHZR2

Sorry, I should mention, this feel/motion is at a standstill with brake applied, just manipulating the transmission selector...

Yep, holding the brake is the same as accelerating. Finding a vid of acceleration would be more likely than holding the brake and bringing engine speed up.

Junkman · #5 09-10-2018, 11:17 AM

How are your shocks? You shouldn't experience more movement than on any other vehicle. Springs spring. Shocks dampen.

SD Blue · #6 09-10-2018, 05:00 PM

Replace the rear differential mount. It may appear to be fine but closer inspection will show a shiny spot where the rubber has flexed too much.

97 SL320 · #7 09-10-2018, 06:55 PM

Quote:

Originally Posted by Junkman

How are your shocks? You shouldn't experience more movement than on any other vehicle. Springs spring. Shocks dampen.

Shocks ( dampers ) will only affect the rate at which the rear drops / rises.

The diff mount won't have any effect on body movement.

Junkman · #8 09-10-2018, 10:09 PM

Quote:

Originally Posted by 97 SL320

Shocks ( dampers ) will only affect the rate at which the rear drops / rises.

The diff mount won't have any effect on body movement.

And, the fact that the question is asked suggests that something is wrong. I've never driven a car where putting it in gear resulted in anywhere near excessive movement.

On second thought, when young and broke, an old Datsun would move around all over the place ... accelerate, stop whatever, the end of the car would move. Shocks were shot.

JHZR2 · #9 09-10-2018, 11:20 PM

Quote:

Originally Posted by SD Blue

Replace the rear differential mount. It may appear to be fine but closer inspection will show a shiny spot where the rubber has flexed too much.

Thanks for the hint. Can you advise where to look on the mount? I'll get under the car this weekend if not before.. thanks again!

SD Blue · #10 09-11-2018, 12:06 AM

Quote:

Originally Posted by 97 SL320

Shocks ( dampers ) will only affect the rate at which the rear drops / rises.

The diff mount won't have any effect on body movement.

I'll have to disagree on the differential mount from not only advisement of several others on this forum but also personal experience. A worn out and mushy rear differential mount will cause a rise and/or squat of the rear. It is most noticeable when changing from reverse to forward, such as backing out of a parking space.

Diseasel300 · #11 09-11-2018, 09:39 AM

Quote:

Originally Posted by SD Blue

I'll have to disagree on the differential mount from not only advisement of several others on this forum but also personal experience. A worn out and mushy rear differential mount will cause a rise and/or squat of the rear. It is most noticeable when changing from reverse to forward, such as backing out of a parking space.

Based on my own experience with my own car, I can also assure you that the contribution it makes is very small. If the rest of the rear end is worn out, replace the bushings and shocks to rebuild the whole thing (it needs it anyway by that point). The diff mount by itself won't make much difference, even if completely trashed like mine was.

Diesel911 · #12 09-11-2018, 10:17 AM

What is your idle speed? Sometime people turn up the idle speed to smooth out the idle.

If your idle speed is higher then what is on you emission sticker fix what is needed to lower the idle to normal.

When you shift into drive or revers is your engagement harsh?

My car does not rise of drop that I notice from the driver seat. And, I don't remember if it ever did that.

After owning the Car for a year or so I replaced the Differential Mount because it looked bad.

Also I am working on changing the Bushings in the rear trailing arms and when I lowered one end of the Subframe the Subframe Bushing fell out. So you don't know what evil lurks till you look.

97 SL320 · #13 09-11-2018, 10:28 AM

Found a pic on a web site of someone repairing a 123.

https://mercbenz123.com/posts/category/restoration-blog/page/2/

I've circled the diff mount in the pic. In this configuration it acts as one of the 3 subframe mounts that just happens to be attached to the diff. This makes terminology a bit fuzzy. The diff is solidly mounted to the subframe making it a stressed member.

Still, the mount would need to be very worn to affect suspension geometry.

SD Blue · #14 09-11-2018, 04:41 PM

True, the differential mount doesn't have a huge effect but it depends on how much we are talking about here. There are other components to take note. Usually, you will have other indicators.

Rear Differential Mount
Rear Subframe Mount Bushings
Rear Trailing Arm Bushings

Both the Subframe and Trailing Arm Bushings will give tire wear and a lot of looseness in the rear of the car. The Trailing Arm Bushings can be hard to detect how much wear they have. Even the slightest movement can indicate severe wear.

t walgamuth · #15 09-12-2018, 08:16 PM

I'll give it a stab. I'd estimate just from observing, not with any measurements taht the rear will rise 1 to 2" on a tight car and up to 3" on a well loved car. I wouldn't worry about it until it starts clunking or othewise causing performance problems.